Understanding Lighting

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Just a little stuff ive found on lighting that can help us all.

Facts of Light
by Sanjay Joshi

Part I: What is Light?

Introduction:

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The choice of lighting is one the most important decisions to make when setting up a reef tank. The light fixtures and related equipment are some of the more expensive pieces of equipment both at initial setup up as well as in their contribution to daily operating costs. In addition to being necessary for the photosynthetic organisms we keep in our aquariums, light also provides the visual element of color. From talking to aquarists and perusing the various reef-related bulletin boards, it has been my experience that lighting and color are often a very misunderstood aspect of aquarium keeping. Given that lighting and color are important in the functional and aesthetic elements of reefkeeping, I feel it is important that hobbyists have a good understanding of light and color. The purpose of this series of articles is to provide beginning and intermediate reef aquarists with a comprehensive understanding of lighting concepts and terminology, and the ability to understand and comprehend lighting related discussions and data. The information will be presented in a series of short columns focusing on a few concepts at a time and build to a comprehensive understanding of light, especially as it relates to reef aquariums.

Light is a form of energy, and to understand light we begin with the electromagnetic spectrum (Figure 1: ) which is basically a grouping of all electromagnetic radiation arranged according to the amount of energy contained in the radiation. Visible light is a part of this electromagnetic spectrum that creates the sensation of light when it falls on the human eye.


Figure 1. The electromagnetic spectrum.

The properties of all electromagnetic radiation can be described by three inter-related terms. These are wavelength, frequency and energy. Since light is a part of this spectrum, it too can be described by these terms. Hence, it is important to understand these terms as a first step towards understanding light.

1) Wavelength:

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Simplistically, we can think of light traveling as a wave. A typical wave form (e.g., ripples on the surface of water) has crests (or peaks) and troughs (or valleys). The distance between two consecutive peaks (or troughs) is called the wavelength, and is denoted by the Greek letter λ (lambda). Because the wavelength is a measure of distance, it is measured in units of length (meters). Since these wavelengths of visible light can be quite small, they are measured in nanometers (nm) where 1 nm = 1 billionth of a meter (10-9 meters). The wavelength of visible light is between 400-700nm. Incidentally, these also happen to be the majority of wavelengths of light that are relevant to photosynthesis. The combined effect of the complete range of radiation between 400-700nm appears as white light to the human eye. Radiation with a wavelength of 400 nm generates a response in the human eye that makes it perceived as violet, while radiation with a wavelength of 700nm appears red. The different colors of the rainbow (ROYGBV - red, orange, yellow, green, blue and violet) are arranged in descending order of their wavelength. Roughly, we can break down the various colors into wavelength bands as follows:

Violet - 400 to 440nm

Blue - 440 to 490nm

Green - 490 to 540nm

Yellow - 540 to 590nm

Orange - 600 to 650nm

Red - 650 to 700nm

Radiation below 400 nm wavelength is called ultraviolet (UV) radiation, and is typically divided into three segments: UV-A (400-315nm), UV-B (315-280nm) and UV-C (280-100nm). UV radiation is not visible to the human eye, but it can have a damaging impact on humans (as well as corals). The UV-A segment, the most common in sunlight, overlaps slightly with the shortest wavelengths in the visible portion of the spectrum. UV-B is effectively the most destructive UV radiation from the sun, because it penetrates the atmosphere and can injure biological tissues. UV-C radiation from the sun would cause even more injury, but it is absorbed by the atmosphere, so it almost never reaches the Earth's surface.

Infrared (IR) radiation has slightly longer wavelengths than visible light. The IR region of the electromagnetic spectrum is also divided into three segments: IR-A (780-1400 nm), IR-B (1400-3000 nm) and IR-C (3000-10600 nm). Infrared radiation is thermal and is felt as heat.

2) Frequency:

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The number of waves that pass a given point in space during a specified time interval is the light's frequency; consequently, frequency is a time based unit. Frequency carries the units "per second," but we use a special term for the unit called - Hertz (Hz), where 1 Hz corresponds to 1 wave/second, so 50 Hz would mean 50 waves/second.

As seen in the figure above, the wavelength and frequency are related to each other. If we take any two points on the waveform labeled "start" and "end," and count the number of waves in between, we can easily see that we will have more waves if the wavelength is smaller. More waves imply that the frequency will be higher. Thus wavelength and frequency are inversely related: the shorter the wavelength of the wave, the higher the frequency of the wave.

Since all the waves travel at the same speed - the speed of light - the relationship between wavelength and frequency is determined by the following formula:

Wavelength = speed of light / frequency

In the typical notation that you will see in most articles and books:

λ = c/ν

where:

λ = wavelength
ν = frequency
c = speed of light

The speed of light is 299,792,458 meters per second (approximately 3.0 × 108 meters/second). To be precise, what we usually call the "speed of light" is really the speed of light in a vacuum (the absence of matter). In reality, the speed of light typically varies depending on the particular medium that it travels through. Light moves more slowly in glass than in air, and in both cases the speed is less than in a vacuum.

If we look at the colors of the rainbow from blue to red, we can now understand that the blue light (400nm wavelength) will have a higher frequency than red light at 700nm, with the other colors of the rainbow falling in between. In fact, the frequency of blue light will be 57% (400/700 × 100 = 57.14%) higher than the frequency of red light.

3) Energy:

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As mentioned earlier, light is a form of energy. According to the quantum theory, all energy is transmitted and absorbed in discrete particles called quanta or photons. Thus, the smallest amount of radiation energy that can exist is one photon.

If one thinks of the photon as a small packet or ball of energy, it is most useful in understanding light, especially for our purpose of reefkeeping. For our purposes, let us take a simplified, unified description that says that light travels as discrete photons along a wave. Visible light is a mixture of many photons with different wavelengths. The photons are reflected and absorbed by various surfaces, and when they reach the eyes, they create the sensation of sight and resultant perceptions of color and brightness. These photons are also directly responsible for photosynthesis in plants and corals. The energy from the photons is used during photosynthesis to convert CO2 into sugar, which is a primary energy source for the photosynthetic endosymbiotic zooxanthellae living within corals.

As discussed earlier, the energy carried by electromagnetic radiation is contained in the photons that travel as a wave. According to quantum theory, the energy in a photon varies with its frequency, according to the equation:

Energy = Plank's constant × Frequency
E = hν = hc/λ
Where h = Plank's constant is 6.626 × 10-34 joules per second


Energy is measured in units called joules.

As the frequency of the radiation increases (wavelength gets shorter), the amount of energy in each photon increases. Now we can begin to understand why the red light gets absorbed quickly in water as a function of depth.

These basic equations provide us with the relationship between wavelength, frequency, energy and photons, and can be used to go back and forth as seen in the following examples.

Example: What is the energy in a single photon of light at 500nm?

E = 6.626 × 10-34 × 3.0 × 108/(500 × 10-9)
E = 0.039756 × 10-17 J


Example: How many photons per joule exist for light at wavelength λ = 500nm?

E = Energy/photon, so to create 1 J of energy we will need N photons.
N × E = 1 joule, hence N = 1/E
N = λ/hc = 25.15 × 1017 photons

As seen above, to produce 1 Joule of energy by light at a wavelength of 500nm requires a very large number of photons. To avoid having to deal with such large numbers, we can measure the number of photons in "moles" where 1 mole = Avagadro's number = 6.02 × 1023. So 25.15 × 1017 photons would correspond to .000004177 moles. Now, this number is too small, so instead we will measure in "micromoles," where 1 micromole (denoted as µmol) is 10-6 mole, giving us 4.177 micromoles of photons.

What about watts? Energy is measured in joules, and the "watt" is the unit used as a measure of power. Power is defined as the rate of flow of energy. By definition, 1 watt = 1 joule/second. So, one watt of power from light at 500nm would need to provide 25.15 × 1017 photons per second or 4.1769 micromoles/sec. The figure below shows the relationship between watts and micromoles of photons to generate 1 watt of power.

Summary:

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This column has focused on providing the basic terminology required to understand light. Light is a form of energy, and can be simply described as a stream of photons traveling along a wave. Photons are discrete particles of energy. The characteristics of light and the photons are specified by three terms: wavelength, frequency and energy, which are mathematically related. Photons with wavelengths of 400 nm carry more energy than those with larger wavelengths and will appear violet to the human eye, and photons with wavelengths of 700nm carry less energy and will appear red. White light is a mixture of photons in the wavelength range 400-700nm. This range is what the eye can see and is also useful for photosynthesis. The photons carry the energy and the number of photons is measured in units of "micromoles."

The next column will discuss how light sources generate photons, the distribution of the photons in a light source and how this distribution is represented as a spectral plot.

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Part II: Photons


As discussed in Part I, we need to think of light in terms of photons. A photon is the smallest discrete particle of energy that travels along a wave defined by its wavelength, and the amount of energy contained in the photon can be mathematically determined. For the purposes of reefkeeping and human vision, we are interested in photons that have a wavelength in the range 400-700nm. In this article, we will look at how photons are generated by light sources, determine how they are distributed according to wavelength, how this distribution is represented as a spectral plot, and the correct terminology used to characterize photons.

How are Photons Generated?



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Any source of light is basically a source of photons. Atoms emit light as a release of energy, in the form of photons. Atoms under normal conditions are in a ground state, with their electrons (the negatively charged particles) moving around the atom's nucleus (which has a net positive charge). An atom's electrons have different levels of energy, depending on several factors, including their speed and distance from the nucleus. Electrons with different energy levels occupy different positions within the atom. Electrons with greater energy move in an orbit farther from the nucleus. When the atoms are excited (by the addition of energy) the electrons jump to a higher energy level. This is an unstable state, and the electron quickly returns to a lower energy state by releasing this energy as a photon. Because the jump from one energy level to another is discrete, the photons carry a discrete amount of energy. If this released photon has a wavelength that is within the visible range of the electromagnetic spectrum it appears as light. The light's wavelength depends on how much energy was released which, in turn, depends on the electron's position. Atoms of different materials have electrons at different energy levels and hence release different 'colored' photons. This is the basic mechanism for the generation of all light.

The following picture (Figure 1), taken from How Stuff Works, helps explain the process.


Figure 1. How atoms emit light.

What differs in the various light sources is the mechanism by which the electrons are excited and the composition of materials used to provide the atoms. In an incandescent lamp, atoms are excited by heat created by a filament's electrical resistance. In a fluorescent lamp free electrons are created between a cathode and anode, and these free electrons are used to energize atoms of mercury, which give off photons in the UV range. These UV photons then strike the lamp's phosphor coating, pushing its electrons to a higher energy level and emitting visible light in different wavelengths, depending on the mix of phosphors used. Metal halide lamps use a different approach, in which atoms of metal halide gas are used along with mercury, and are energized by a plasmal arc between electrodes.

What is important to note here is that a photon is a photon is a photon… no matter what source is used to generate it. In other words, a yellow photon from a candle's light is the same as the yellow photon from the metal halide lamp. The only difference is that the metal halide lamp generates a lot more photons/second than the candle light.

Characterizing the Photons



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A light source is basically a continuous source of photons, in our case converting electrical energy into visible photons. So when we characterize a light source, we are interested in determining how many photons it generates per unit of time. This is called its photon flux. These photons are generated and spread in all directions, and ultimately land on some object of interest (often in our case, the corals). A light source generates photons at a constant rate, and as we move away from the source, the photons will spread over a larger area, hence fewer photons land on the target area the further we move from the light source. We are interested in how many photons land on a given area, usually 1 meter square, and this number is called the photon density. Additionally, we are interested in the photons that are available for photosynthesis, which happen to be photons in the range 400-700nm (the same as visible light). These are called photosynthetic photons. These three entities of interest combine to comprise the Photosynthetic Photon Flux Density (PPFD), which is a measure of the number of photons in the range of 400-700nm falling on a 1 meter square area per second. PPFD is a measure of Photosynthetically Available Radiation abbreviated as PAR. Recall from Part 1 that to generate 1 watt of power we would need 25.15 × 1017 photons/sec at 500nm. This is a lot of photons!!! Since we are dealing with a large number of photons, the number of photons are measured in units called micromoles (1 mole = Avogadro's number = 6.022 × 1023, hence 1 micromole = 6.022 × 1017). Hence the units of PPFD are micromoles/m2/sec, so, a PPFD of 1 corresponds to 6.022 × 1017 photons falling on a 1 meter square per second. In the aquarium hobby we often refer to light output in terms of PAR. Technically, this is incorrect. PAR is typically measured as PPFD.

Different light sources have different distributions of photons in the 400-700nm range. The light source can be characterized by determining this distribution of the photons, and this is done using an instrument called a spectroradiometer. A spectroradiometer simply is an instrument that has a sensor and associated hardware and software to determine the distribution of energy (measured as power density in Watts/m2) at different wavelengths of the electromagnetic spectrum. This is usually displayed as a graph with the wavelength on the X-axis and the power density on the Y-axis, and is called the Spectral Power Distribution (SPD) plot. One such SPD plot is shown in Figure 2 below. This is the most important piece of information about a light source, and all relevant light measures can be derived from it.


Figure 2. Spectral Power Distribution for a 400-watt Ushio lamp on a Magnetek (M59) ballast - 18" from the lamp.

Note that for each wavelength the spectroradiometer measures the power density in watts/m2. This is termed the Spectral Irradiance. You may recall from Part 1 that there is a direct relationship between power/energy at each wavelength and the number of photons. For example, as seen in the graph above, at 420nm the lamp produces 0.4 watts/m2 of power or 0.4 joules/m2/second of energy. Using the relationship between energy and wavelength, it can be determined how many photons/m2/sec at 420nm will be required to generate 0.4 joules of energy - 1.46 micromoles. Thus, we can easily convert from watts/m2 to micromoles/m2/sec. If this is done for all wavelengths, we would get a plot that shows the distribution of the number of photons at each wavelength per meter squared per second.


Figure 3. Photon Distribution (measured as PPFD) for a 400-watt Ushio lamp on a Magnetek (M59) ballast - 18" from the lamp.

Adding all the photons over the range of 400-700nm will provide the measure of the photosynthetically available radiation (PAR) measured in terms of PPFD. Technically, the photosynthetically available radiation would be the area under the curve shown in Figure 3. These computations are often performed by software that is available with the spectroradiometers. Since the power distribution and the photon distribution are mathematically interchangeable, either of them can be used as the basis for comparison of light output from different light sources.

On my website, www.reeflightinginfo.arvixe.com, which catalogs the light output from various metal halide lamps and ballast combinations, I have been using the spectral power distribution to show the light output. By using the data available, comparisons can easily be made between different metal halide lamps based on their spectral distribution. The plots depicted show the spectral irradiance at each wavelength. The values indicate the amount of power density (Watts/m2) at each wavelength. So, a lamp with higher power at a given wavelength will also have a larger number of photons at that wavelength.

What is important to note is the following:

1) Because each photon's energy is different at different wavelengths, a different number of photons will be required to produce the same amount of energy at different wavelengths. To produce the same amount of total energy at 400nm would require 57% less photons than at 700nm, because the photons at 400nm have higher energy.

2) Because the PPFD is a summation of all photons in the 400-700nm range, two very different spectral distributions can have the same PPFD. What this means is that there is not a one-to-one relationship between PPFD and spectral distribution, so knowing a light source's PPFD does not tell us anything about how its photons are distributed. Different light sources with similar PPFD values can have very different spectral distributions. As seen in Figure 4 below, the two lamps have very similar PPFD values, but their spectral distributions are very different. The independence of PPFD and spectral distribution is one reason that we must consider spectral distribution data as well as PPFD when comparing light sources.


Figure 4. Comparison of the spectral distribution of two lamps with similar PPFD values.

3) Also note that PPFD measures photons falling on a given area; the number of photons falling on this area changes as its distance from the light source increases. Hence, when comparing lamps' PPFDs it is very important to know the distance at which the measurements were taken, and only PPFD values at the same distance can be compared.

The spectral distribution of the lamps is quite different when compared to sunlight. Figure 4 also shows the spectral plot of sunlight at the surface of the water in the tropics at noon time during summer. For a more detailed comparison of the underwater light field to natural light underwater, the reader is referred to "Underwater Light Field and its Comparison to Metal Halide Lighting."

Inverse Square Law of Light



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The relationship between PPFD and distance from the light source follows what is called the Inverse Square Law, as long as the source is a point source of light.

According to the Inverse Square Law:

PPFD1/PPFD2 = (D2/D1)2

D1 and D2 = distance at which PPFD1 and PPFD2 are measured.

This rule basically says that if you know the PPFD at a given distance from the lamp, then you can compute the PPFD at any other distance. It will vary as an inverse function of the square of the distance.

For example, if the PPFD is 100 at 1 meter, then at 2 meters it is 25. If the distance is doubled, the irradiance is reduced to ¼ of the value at the original distance. This effect can be easily visualized by shining a flashlight on the wall. Stepping away from the wall increases the size of the light spot and decreases its intensity.

This rule is applicable only to point sources of light (or lights whose source can be approximated by a point). The "five times rule" is often used as the rule of thumb. As long as the distance from the source is five times the size of the emitting source, we can consider it to be a point source of light. For a clear metal halide lamp, the size of the point source can be considered to be the inside envelope that contains the gases. If we wanted to consider a 4' fluorescent lamp to be a point source, it would have to be at least 20' away! Similarly, a 2' reflector would have to be at least 10' away to be approximated as a point source.

Summary:



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In this article, I have described how the light from a source can be characterized by the distribution of the photons that emanate from it. Two mathematically equivalent plots - one using the power density distribution at each wavelength and the other using numbers of photons - can be used to show the distribution as a spectral plot. The light available for photosynthesis is termed PAR, and is typically measured as PPFD (photosynthetic photon flux density) with units of micromoles/m2/sec. Using just the PPFD number gives us information only about the number of photons in the 400-700nm range but does not tell us anything about their spectral distribution. Two lamps with the same PPFD can have very different spectral distributions. Additionally, the PPFD measurements can be compared only if the distances at which the measurements are taken are the same. However, given that light follows the Inverse Square Law, we can compute PPFD at different distances if we know it at one particular distance.

The next article in the series will discuss other measurements of light that you may have seen such as Lux and Lumens and how they relate to measurements of light for photosynthesis.
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Part III: Making Sense of Light Measures


A large amount of confusion about light measurements comes from the fact that there are several different ways of measuring light. Many terms are used to measure aspects of lighting, and a simple cruise on the Internet will take you through a plethora of terms such as: lumens, lux, candlepower, foot-candles, lamberts, phot, nit, irradiance, illuminance, Color Rendition Index (CRI), Kelvin and Photosynthetic Photon Flux Density (PPFD), among others. Why are there so many measures to describe light? This article will sort through this abundance of lighting measures.

There are basically three categories of light measures based on the particular application and interpretation, each with its own set of terminology.

1. Recall that light is a form of radiation, and hence can be measured as radiant energy using energy based measures. These units of light measurement are termed radiometric measures of light.

2. Light is also used to illuminate for visual purposes, hence light can also be measured for this application based on how the human eye perceives light. Measures of light based on visual perception are called photometric measures, and are by far the most commonly used and available metrics because they are used by the large lighting industry.

3. The form of measurement that we, as reefkeepers, are concerned about is the Photosynthetically Available Radiation (PAR), which measures the number of photosynthetically useful photons, and has been discussed in detail in Part II.

There are two main ways light can be measured: 1) at the source, and 2) at the surface of the object being illuminated.

The quantity of light at the source is termed flux, and is measured as "quantity" per unit of time. This is very similar to measuring the flow of a pump in gallons/hr or liters/min. We can think of a light source as a pump emitting radiation and measure this pumping capacity over time. It represents the total light output from the source per unit of time. What this measured "quantity" is depends on whether the light is interpreted using radiometric, photometric or photosynthetic standards.

The light radiating from the source ultimately falls onto an object, and we can measure the amount of light falling onto a given area of the object. This quantity is measured per unit area, and measures the light's density on a unit area.

The light emanates in several directions and we can either measure this without regard to the direction from which it comes, or measure it in a given direction. When measuring light in a given direction, it's helpful to visualize the light as radiating from all directions in a sphere. This sphere can be broken down into cones whose apex is at the center of the sphere with the cone specified by the solid angle at the apex. Thus the light can be measured as the amount of flux contained in such a cone and measured per unit angle. The unit angle used is called a steradian (similar to a radian in three dimensions). A complete sphere has 4p steradians.

The different quantities used in light measurements and their units of measure are summarized in the table below, and will be discussed in further detail.



Radiometric Measurement of Light



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Because light is radiant energy, the energy is measured in typical units of energy - joules (J).

Radiant Flux, also called radiant power, is the flow rate of radiant energy. It is measured in terms of power units called watts, which are basically a measure of energy per unit time. 1 watt = 1 joule/sec.

Radiant Intensity is the radiant flux per unit solid angle and is measured in watts/steradian. The radiant intensity is independent of the distance because it measures only the amount of radiant flux contained in the cone with an angle at the apex equal to one steradian.

As we move further from the source, the cone's spread increases so the radiant intensity falls onto a larger surface. Thus, the density of light falling onto the surface decreases as the area increases (following the inverse square law for a point source), even when the angle at the cone's apex does not change. This is measured as radiance. Radiance is the radiant flux density per unit solid angle and is measured in watts/m2/steradian.

If we do not care about the light's direction and want to measure the light falling onto a source from all directions, then we measure this as irradiance. Irradiance, also known as radiant flux density, is the radiant flux per unit area at a point on the surface. Hence, its units are expressed in watts/m2 or joules/sec/m2. It is denoted as E.

Spectral Irradiance is the irradiance per unit wavelength interval at wavelength λ. This is denoted as Eλ and its units are expressed in watt/m2/nm. Recall that the spectral power distribution plot discussed in Part II is plotted using spectral irradiance values.

Photometric Measurements



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Photometric measurements are geared toward how the human eye perceives light. The sensitivity of human eyes is different for different wavelengths. In the late 1920s the Commission Internationale de L'Eclairage (CIE), based on experimentation using human subjects, established how the human eye responds to light at different wavelengths. The human eye is more sensitive to light at 555 nm (green) and less sensitive to blues and reds. This characteristic of human vision established the standard observer response curve known as the luminous efficiency function to represent how the human eye responds to light at different wavelengths. Per this standard, detectors in the eye respond differently to different regions of the spectrum, and the response is scaled with respect to the peak values.

The change in the eye's spectral response can be explained by the presence of two types of receptors, rods and cones, in the retina. Cones are active at high light levels and are most densely situated in the central part of the field of view. The cones' spectral response corresponds to the photopic sensitivity curve. The rods are responsible for human vision at low light levels and are prevalent in the peripheral field of view, away from our direct line of sight. As light levels are reduced, cones become less active and rods become active with established spectral sensitivity gradually switching toward the scotopic response curve. The peak spectral sensitivity for photopic vision is 555 nm, and 507 nm for scotopic vision. From this it is quite clear that the human eye finds light at 555 nm to be the brightest, with the blues and reds tending to be less bright. The luminous efficiency functions are shown in Figure 1.


Figure 1. Luminous Efficiency Functions.

All photometric light measurements evaluate light in terms of this standard visual response described by the luminous efficiency function and, hence, all are weighted measures. Not all of the wavelengths are treated equally. The wavelength at 555 nm is assigned a weight of 1, and the others are scaled according to this function. According to this function, light at a wavelength of 450 nm is given a weight of 0.038. This explains why a light source with large amounts of radiation in the "blue" region will have a low reading when using photometric units.

The quantities used for photometric measurements correspond to those used for radiometric measurements, with the main difference being that the measurements are evaluated with respect to the human eye's response.

Luminous Flux is the amount of radiation coming from a source per unit time, evaluated in terms of a standard visual response. Unit: lumen (lm). You will see most data from lighting companies refer to light output in terms of lumens. Think of this as the amount of light produced by the lamp as perceived by the human eye.

Luminous Intensity is the luminous flux per unit solid angle in a given direction. Unit: candela (cd). One candela is 1 lumen/steradian.

Illuminance is the luminous flux per unit area. It is measured in lux (lumen/m2) or footcandles (lumen/ft2). The light emanating from a lamp is used to illuminate objects and the amount of light (measured in lumens) falling onto a specific area of the object, usually one square meter, is termed lux. When we measure this same area in square feet, the unit is footcandles. These units are often used in photography, where we are interested in how much light is falling onto the subject.

Conversion from Radiometric Units to Photometric Units

The following method is used to convert between photometric units and radiometric units. As defined, 1 watt = 683 lumens at 555 nm (peak photopic response), and it is scaled for other wavelengths based on the Luminous Efficiency Function V (λ) shown in Figure 1.

To determine a lamp's lux values, the spectral irradiance at each wavelength (taken from the spectral power distribution) in the spectral range (380-780nm) is multiplied by the luminous efficiency function at the equivalent wavelengths. Then, all of these multiplied values are summed and multiplied by 683 to find the total lux output. As you can see, the conversion requires knowledge of the spectral power distribution and cannot be done without it.

So far we have been dealing with metric units. To convert to English units, or to other measurement systems, appropriate conversions need to be made. These converted units are often given different names (thereby adding to the confusion)! As an example we can look at the different terms and units used for measuring luminance.

LUMINANCE:
1 lm/m2/sr (lumens per sq. meter per steradian)
= 1 candela/m2 (cd/m2)
= 1 nit
= 10-4 stilb (sb) (or 1 candela/cm2)
= 9.290 x 10-2 cd/ft2
= π apostilbs (asb)
= π x 10-4 lamberts (L)
= 2.919 x 10-1 foot-lamberts (fL)

Units for Photosynthesis Measurements



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In keeping corals and plants we should not be concerned about light as humans see it, but rather as the plants and corals see it. For the purpose of photosynthesis, light is termed Photosynthetically Available Radiation (PAR). This radiation's range is identical to what humans can see in the 400-700 nm range, but each photon is treated uniformly in this measurement (unlike the photometric measurement, which weights the photons according to how the human eye sees them).

The reason for expressing PAR as a number of photons instead of energy units is that the photosynthetic reaction takes place when a plant absorbs the photon, regardless of the photon's wavelength (provided it lies in the range between 400 and 700 nm). That is, if a plant absorbs a given number of blue photons, the amount of photosynthesis that takes place is exactly the same as when the same number of red photons is absorbed. Note, however, that the plant or coral may have an absorption response that preferentially absorbs more photons of certain wavelengths (more on this later).

Recall from Part II, PAR is measured as PPFD, which are Einstein/m2/s or µmoles/m2/s. One Einstein = 1 mole of photons = 6.022×1023 photons, hence, 1 µEinstein = 6.022×1017 photons.

Conversion from Radiometric Units to PPFD

If we know the spectral irradiance at any given wavelength (we can get this from the spectral power distribution), then we can determine the PPFD for the given wavelength by multiplying the spectral irradiance (watts/m2) by the watts to Einstein conversion factor for each wavelength (recall from Part I how to convert energy at a given wavelength into the number of photons). To compute the total PPFD over the range of 400-700 nm, compute the PPFD for each wavelength and sum over the range of 400-700 nm.

Summary:



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There are three basic forms of light measurement - radiometric, photometric and photosynthetic - and these can be measured at the source or at the object onto which the light falls. Photometric measurements are derived from the radiometric measurements by factoring in the human eye's response, and do not treat all radiation equally. Photosynthetic measures, on the other hand, treat all radiation equally. The starting point for all these measures is the spectral power distribution, from which all other entities can be derived. Conversion from one set of units to the others is simply not possible unless the spectral power distribution is known.

In addition to these lighting measures, additional measures such as Color Rendition Index (CRI) and Correlated Color Temperature (CCT) are used to describe light. These will be covered in the next part of this series.

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Part IV: Color Temperature


One the most abused and misunderstood terms in reef aquarium lighting is color temperature. Lamps, both fluorescent and metal halide, are being sold in the hobby based on color temperature ratings ranging from 5000K to 50000K, with a wide range of values in between. It is not uncommon to find lamps rated as 6500K, 10000K, 11000K, 12000K, 12500K, 13000K, 14000K, 15000K, 18000K, 20000K and 50000K being sold in the hobby. As interpreted by reef aquarists, these numbers tend to convey the apparent "blueness" of the light emitted by the lamps. The aquarium lighting industry has used this color temperature interpretation as a way to label their lamps, and use it to signify how their lamps would appear in comparison to other lamps and as a selling point for their lamps. It has been my experience, however, that these numbers often seem to be rather arbitrarily created and often there is very little correlation between the scientifically defined term of color temperature and the label on the lamp, thereby making it more difficult for the aquarist to make choices based on color temperature ratings. In this article I will explain the concept of color temperature, its relationship to spectral power distribution, and the color temperature nuances of light sources.

Understanding color temperature starts with understanding black body radiation and the Kelvin temperature scale. A theoretical black body is an object that has no color and is "black" because it absorbs all radiation incident on its surface and emits no radiation at 0° Kelvin. In the Kelvin temperature scale, 0° Kelvin (abbreviated by K) corresponds to -273.16° C and is the temperature where all molecular motion has ceased. This is called absolute zero. Recall that for radiation to be generated, the electrons have to be jumping to higher energy levels and releasing the energy as photons. At absolute zero all motion ceases and there is no energy being emitted. Hence, at 0K the black body emits no radiation.

As the black body is heated above 0°K it starts to emit radiation that lies within the electromagnetic spectrum. The radiation's spectral distribution depends on the black body's temperature. At low temperatures (e.g. room temperature) the black body is emitting radiation, but it is not in the range that is part of the visible spectrum. For visible radiation the back body must be quite hot. At about 1000K it looks red, yellow at about 1500K, white at 5500K, bluish-white at 6500K and more bluish at 10000K. The spectral irradiance of the radiation and color changes with temperatures have been well studied by physicists, and the relationships are given by the well-known black body radiation laws. Plank's law gives the spectral irradiance at different wavelengths, Wien's law provides the wavelength at which the peak irradiance occurs, and Stephan Boltzman's law relates the total amount of energy to the temperature of the black body. Details of these can be found in any physics textbook and will not be covered here.

Figures 1-3 below show the radiation of the black body at different temperatures and the peak of the radiation. It also shows the visible portion of the radiation as colored bands. This is how a perfect black body radiator behaves, and the radiation is a function of the temperature to which it is heated.


Figures 1-3: Black body radiation at various color temperatures. Source:

How does this relate to the light sources we use - fluorescent and metal halide lamps? Does this mean that a lamp being sold as a color temperature of 20000K is a black body radiator and has an actual physical temperature of 20000K? No, since the lamps are not black body radiators! To be able to assign a color temperature to a light source there must be a color match as well as a spectral match to a black body radiator. The spectral output of fluorescent lamps and metal halide lamps does not match with the black body spectral irradiance. Hence, the term color temperature, in fact, does not apply directly to these light sources. What it really means is that if we were to compare the lamp's color to a black body at 20000K, it would appear the same to a human observer. The technically correct term for this is Correlated Color Temperature (CCT) which is defined as the value of the temperature of the black body radiator when the radiator color matches that of the light source. CCT implies a color match to a black body at the specified temperature, but there is no spectral match. The table below shows CCT of various light sources:

1500 K Candlelight
2680 K 40-watt incandescent lamp
3200 K Sunrise/sunset
3400 K One hour from dusk/dawn
5000-4500 K Xenon lamp/light arc
5500 K Sunny daylight around noon
5500-5600 K Electronic photo flash
6500-7500 K Overcast sky
9000-12000 K Blue sky

This now brings up the issues of matching lamp color to color temperatures of the black body. Once we start talking about color, we have to remember that color is not a physical property but a physiological response created in the brain by the visible light seen by the eye. As someone adequately surmised, "Color is only a pigment of your imagination."

To be able to work with color mathematically, scientists have developed a mathematical means to specify color - where color is specified by numerical values called color coordinates or chromaticity. Correlated Color Temperature (CCT) can be determined by mathematical formula to find the chromaticity coordinates of the black body's color temperatures that are closest to the light source's chromaticity. (More on chromaticity and how it developed later.)

Since it is a single number, CCT is simpler to communicate than chromaticity or SPD, and is used as a shorthand for reporting the color appearance of light emitted from electric light sources. Correlated Color Temperature values are being used by the reef lighting industry to give a general indication of the apparent "blueness" of the light emitted by the source. According to aquarium lighting industry convention, lamps with higher CCT values provide light that appears "more blue."

To develop a mathematical and more unambiguous definition of color and color perception, the International Lighting Commission (Commission Internationale de l'Eclariage, referred to as CIE) established a colorimetry system for color matching that has, with minor changes, remained in use for the last 75 years. To understand the proper definition and meaning of CCT we need to understand color vision, how the chromaticity diagram was established, and how it is used to determine CCT of light sources.

Color Vision



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Before understanding the CIE color diagram, it is important to understand how the human eye sees color. The human eye contains two different kinds of receptors - rods and cones. The rods are more sensitive and outnumber the cones, but the rods are not sensitive to color. Color vision is provided by the cones. There are basically three types of color sensitive cones in the human eye, corresponding roughly to red, green and blue. The response curves of these different cones have been mapped by researchers. The perception of color depends on the neural response of the three types of cones. Hence, it follows that visible color can be mapped in terms of the response functions of these three types of cones. It was shown that color samples could be matched by combinations of monochromatic colors: red (700 nm), green (546.1 nm) and blue (435.8 nm). These matching functions were determined by experiments. By simply adding various amounts of these primary colors a large range of colors could be matched, but there were still some outside this range that could not be matched by pure addition. It was found, however, that by allowing negative values of red, all colors could be matched. Allowing negative values of red is the same as adding red to the color sample being matched.

CIE Chromaticity Diagram



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The CIE matching functions were derived from these Red, Green and Blue matching functions such that the matching functions are all positive, and any color can be considered to be a mixture of the three CIE primaries. These "primary colors" can be represented as mathematical functions of their wavelength, and are shown in Figure 4 below. The most commonly used CIE primaries were developed in 1931 using a two-degree field of view; since then, others have been defined using a 10-degree field of view and the functions were updated in 1964.


Figure 4: 1931 CIE Color Matching Functions.

The CIE color coordinates are derived by weighting the spectral power distribution (obtained by using a spectroradiometer) by these three functions. This gives three values, called the tristimulus values (X, Y, Z), from which the chromaticity coordinates are calculated. Without going into the mathematics of computing these values (we can let a program compute them), the Y value is a measure of luminosity, or how bright the light appears to an observer. These Y values are, in fact, defined to be the same as the photopic response of the human eye. Because perceived color depends on the relative magnitudes of X, Y and Z, the chromaticity coordinates are usually given by normalized coordinates x and y, where x = X/(X+Y+Z), y = Y/(X+Y+Z) and x+y+z = 1. The (x, y) coordinates are called the chromaticity coordinates. In the computation of the chromaticity coordinates the Y value is normalized to 100.

The figure below shows the 1931 CIE chromaticity diagram. The color temperature of a true black body is also shown on this chart. The path taken by the black body color temperature is called the black body locus. The pure spectrum colors appear on the outside along the curve, and points representing non-spectral colors are inside. A straight line connects the ends and represents colors that are combinations of wavelengths of 400 nm and 700 nm (blue and red).


Figure 5: The 1931 CIE chromaticity diagram.
Source: .

Mathematically, the Correlated Color Temperature of a light source is computed by determining the (x,y) color coordinates of the light source, and by finding the color temperature closest to the lamp (x,y) that lies on this black body locus. Details of this approach are beyond the scope of this article, but interested readers are referred to Reference 1.

What is important to note is that using such an approach, two points on either side of the black body locus can have the same CCT but different color coordinates. To prevent this from creating large differences in the perceived color of light represented by the same color temperature, a small tolerance zone is typically specified near the black body locus, and if the two points are outside this tolerance, then larger color differences will be perceived.

One drawback of the 1931 chromaticity diagram is the fact that equal distances on the chart do not represent equally perceived color differences because of the non linear nature of the human eye. The 1976 uniform chromaticity CIE chart (Figure 6) was developed to provide a perceptually more uniform color spacing for colors at approximately the same luminance. The coordinates used here are denoted (u',v') and can be computed from the 1931 x,y coordinates by the following transformation:

u'= 4x / (-2x + 12y + 3)

v'= 9y / (-2x + 12y + 3)

This excellent website provides the mathematical equations and a calculator to convert between the various color coordinates developed. In spite of its drawbacks, the 1931 color chart is still one of the most popular in use.


Figure 6: The 1976 CIE chromaticity diagram.

Another artifact of using the CCT arises from the fact that a single number is once again being used to characterize the SPD of the lamp. It is very possible that two very different spectral power density functions can have the same CCT, as shown in the Figure 7 below taken from . Light sources with different spectral distributions but with the same CCT are called metameric light sources.

Spectral power distribution of two metameric light sources:


Figure 7: The SPD on the left is that of an incandescent lamp with a CCT of 2856 K. The SPD on the right is of a red, green and blue LED mixed spectrum that is metameric with the incandescent lamp.

While it is too complex to represent the color appearance of a light source precisely by the color coordinates, it does provide a useful approximate representation of the appearance of the light source. The color theory can mathematically represent color and provide a mathematical specification of color, yet there is still a difference between color specification and humans' color experience. For example, brown and orange can have the same color coordinates on a CIE chart, but both produce a very different color experience in the human eyes. This artifact of color appearance is very difficult to represent in the CIE color chart and its mathematical representation of color. This situation arises due to the normalization of the luminosity function.

Color Coordinates of Metal Halide Lamps



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As seen from the above discussion, the main input required to calculate the CIE chromaticity coordinates is the spectral distribution and the functions for the CIE primary colors. The spectral data is obtained using the Licor 1800 Spectroradiometer. Software for the spectroradiometer has built-in functions to compute the 1931 CIE color coordinates. Using this, the color coordinates of a sampling of "blue" 250-watt mogul metal halide lamps sold as 13000K, 14000K and 20000K are computed and shown in the table below.

Color coordinates of some common "blue" 250-watt mogul metal halide lamps on different ballasts:
Lamp Ballast Lamp Wattage x y
Hamilton 14KK Icecap 250 0.22089 0.18143
Hamilton 14KK M58 250 0.21012 0.15708
Hamilton 14KK M80 250 0.23958 0.21082
PFO 13KK Icecap 250 0.2711 0.24933
PFO 13KK M58 250 0.25726 0.24004
PFO 13KK M80 250 0.2601 0.25546
Aquacon 14KK Icecap 250 0.26161 0.22229
Aquacon 14KK M58 250 0.23984 0.19343
Aquacon 14KK M80 250 0.28639 0.26015
Aquacon 14KK ReefFan 250 0.27209 0.23451
Radium - 20KK Icecap 250 0.19626 0.14491
Radium - 20KK M80 250 0.20159 0.15539
XM – 20KK Icecap 250 0.19235 0.12632
XM – 20KK M58 250 0.198 0.13989
XM – 20KK M80 250 0.20299 0.14727


These chromaticity coordinates are plotted on the CIE diagram, as shown in Figure 8 below. The background color for the plot is obtained by superimposing the color diagram from Figure 1. The plot is scaled to show only the relevant piece of the chart.

CIE (1931 2deg) Chromaticity Coordinates of 250-watt Mogul "Blue" Lamps:

Figure 8: Color coordinates of some "blue" 250-watt mogul metal halide lamps.

Summary



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This article presented an overview of how color temperature is correctly defined and what it means. For lamps used in the aquarium hobby - both fluorescent and metal halide - the correct term to describe the color temperature is the Correlated Color Temperature (CCT). Correlated color temperature is derived from the chromaticity coordinates of the lamp, which, in turn, are determined by the spectral power distribution and the CIE color matching functions. Based on the mathematically accepted definitions, we should expect the CCT of metal halide lamps sold in the hobby to be close to black body locus, in the vicinity of their specified color temperature. Unfortunately, this is not the case with most metal halide lamps, especially the ones that have significantly "blue" appearance.

pictures
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Part V: Everything You Need to Know About
Metal Halide Lamps and Ballasts


Metal halide (MH) lighting is often an intimidating choice for beginning reef aquarists and DIY enthusiasts alike. This article will provide basic background information on MH lighting systems, as well as an understanding of the basic vocabulary used when talking about metal halide lights, and the hardware components that make up a complete lighting system.

There are four basic components in any MH lighting system (not including the mounting hardware and wiring):

the lamp;
the ballast;
the socket and;
the reflector.
Metal Halide Lamps



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Metal halide lamps are a type of HID (High Intensity Discharge) lamp; mercury vapor and high-pressure sodium lamps are also HID lamps. However, mercury vapor and sodium lamps are not typically used in the reef hobby but are widely used in the horticulture industry. The metal halide lamps used in the aquarium hobby are typically characterized and sold based on different attributes, such as:

the manufacturer’s/lamp’s name;
the lamp’s nominal wattage;
the type of method used to mount the lamp, along with the number of ends in the sockets used to mount the lamps and;
the lamp’s color temperature.
Metal halide lamps have two basic configurations; those with an outer envelope and those without. In the former, the lamp’s basic construction (see Figure 1) is an inner envelope (called the arc tube), which contains the arc, and an outer envelope (called the bulb) which filters out ultraviolet radiation (UVR) and shields the inner arc tube. These lamps are typically single-ended (SE) and use a threaded mount to screw into a socket. The second lamp configuration lacks the outer envelope and typically has two ends (double-ended, DE) that need to be inserted into a socket, as we shall discuss shortly.

The inner arc tube contains the electrodes and various metal halides, along with mercury and inert gases that make up the mix. The typical halides used are some combination of sodium, thallium, indium, scandium and dysprosium iodides. These iodides control the lamp’s spectral power distribution and provide color balance by combining the spectra of the various iodides used.


Figure 1. Anatomy of a typical mogul-based metal halide lamp (source Venture Lighting™).

Light is generated by creating an arc between the two electrodes located inside the inner arc tube. The inner arc tube is typically made of quartz, and this is a very harsh environment, with high temperatures approaching 1000°C and pressures of 3 or 4 atmospheres. To start a metal halide lamp, a high starting voltage is applied to the lamp’s electrodes to ionize the gas before current can flow and start the lamp. The outer jacket is usually made of borosilicate glass to reduce the amount of UV radiation emitted from the lamp. It also provides a stable thermal environment for the arc tube and contains an inert atmosphere that keeps the arc tube’s components from oxidizing at high temperatures.

Recently, some manufacturer’s catalogs have begun listing ceramic metal halide lamps. These refer to the fact that their inner arc tube is made of a ceramic material instead of quartz. Ceramic lamps can withstand higher arc temperatures and are supposedly better at holding the color temperature over the lamp’s life. To the best of my knowledge, no aquarium lamps currently are being sold with a ceramic envelope.

A characteristic of HID lamps is that they take several (from three to five) minutes to warm up, and during this warm-up period the light’s output varies in terms of intensity and color temperature. A lamp’s color temperature could take as much as 15-20 minutes after startup to stabilize. After any power interruption (1/20th of a second or more), a lamp that is hot will not restart immediately and must cool sufficiently before restarting. This time delay is called the restriking time and may take anywhere from 10-20 minutes for MH lamps.

As aquarists we are concerned with the characteristics that impact the selection of the lamp and its associated hardware. Metal halide lamps come in a wide variety of configurations differentiated by a number of factors: their wattage, their color temperature, their mounting base, their bulb’s shape, their electrical characteristics, their operating position, and their manufacturer. When working with a MH lighting system it is best to start by selecting the lamp first, and then selecting the other components based on the particular lamp chosen. In addition, some thought should be put into the types of lamps you may be using in the future. This has become more important recently, with many manufacturers of lighting systems selling specific ballasts for specific lamps.

The first choice to make is the wattage of the MH lamp to be used. Typical MH lamps are available in 70-watt, 100-watt, 150-watt, 175-watt, 250-watt, 400-watt and 1000-watt versions. The ones most commonly used for home aquaria are the 150-watt, 175-watt, 250-watt and 400-watt configurations, with the 70-watt lamps becoming more popular for nano reefs. The first determining factor in selecting the lamp’s wattage is dictated by the type of corals being kept, their light requirements and the aquarium’s depth. Generally speaking, for lamps with similar color temperatures, the higher the lamp’s wattage, the more light it produces.

Once the appropriate wattage has been determined, the next steps are to determine the type of lamp based on how it is to be mounted in the fixtures, and its starting requirements. The classifications that are commonly encountered in the reef hobby are single-ended (SE) and double-ended (DE). Single-ended lamps typically have a screw at one end and are designed to be mounted in a single socket. Double-ended lamps, on the other hand, are designed to be mounted in a pair of sockets, one at each end. Also, DE lamps do not have the large outer envelope typically found on single-ended lamps that is used to limit the lamp’s UV radiation. Double-ended lamps require an additional safety glass to be installed in the lamp’s fixture in order for them to be safe for your tank’s inhabitants.

Metal halide lamps are sensitive to the manner in which they are mounted, due to the sensitivity of the arc’s shape, in the arc tube. Lamps are designed to operate best only in a certain orientation. However lamps marked as “Universal lamps” can be operated in any position, but the lamp’s life and its light output are reduced when it’s used in an off-vertical position. For best performance, if the lamp’s operating position is known in advance, the position-oriented bulbs are best. Various codes are used to designate a lamp’s recommended burning position (e.g., U = universal, BH = base horizontal, BUD = Base up/down (vertical), etc.). Most modern aquarium lamps tend to be universal lamps and are usually used in horizontal burning positions. When using lamps in the horizontal position, it may be best to orient the lamp so that its exhaust tip (affectionately called the nipple) on the inner arc tube points up.

Starting Metal Halide Lamps



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A metal halide lamp’s starting requirements are important because they impact the type of ballast that the lamp requires. Two methods are used to start MH lamps: probe start (standard start) and pulse start. Probe start refers to the method used to ignite the arc in the arc tube. A traditional or probe start metal halide lamp has three electrodes – two for maintaining the arc and a third internal starting electrode, or probe. A high open circuit voltage from the ballast initiates an arc between the starting electrode and the operating electrode at one end of the arc tube. Once the lamp reaches full output, a bi-metallic switch closes to short out the probe, thereby discontinuing the starting arc.

Pulse-start MH lamps do not have a starting probe electrode. An igniter in the pulse start system delivers a high voltage pulse (typically 3 to 5 kilovolts) directly across the lamp’s operating electrodes to start the lamp, eliminating the probe and bi-metallic switch needed in probe start lamps, as shown in Figure 2. Without the probe electrode, the amount of pinch (or seal) area at the end of the arc tube is reduced, which allows for increased fill pressure and reduced heat loss. Furthermore, using an ignitor with a lamp reduces tungsten sputtering by heating up the electrodes faster during starting, reducing the lamp’s warm-up time.


Figure 2. Two diagrams showing the difference between a probe start and a pulse start lamp design (source Venture Lighting™).

ANSI Lamp Designation



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To provide a common system for identifying lamps, and to allow lamps to be cross-referenced with different manufacturers to select the proper ballast, the ANSI (American National Standards Institute) system is used to designate lamps. Designation of MH lamps that follow the ANSI system starts with “M” followed by a number (an “H” designation stands for mercury vapor lamps, and “S” for sodium lamps), which identifies the lamp’s electrical characteristics and, consequently, the appropriate ballast. After the lamp’s numbers are two letters that identify the bulb’s size, shape, finish, etc., excluding color. After this section, the individual manufacturer may, at its discretion, add any additional numbers or letters to indicate information not covered by the designation’s standard section, such as the lamp’s wattage or color. For the purpose of ballast selection, only the letter “M” and the number that follows are important. For example, a lamp with the ANSI designation M59-PJ-400 will operate with any ballast designated for M59 lamps. A lot of European lamps are used in the hobby, and these do not exactly follow the ANSI standard but, instead, use the European standard, which, in some cases, may be slightly different from the ANSI standard.

Another term typically encountered in the aquarium lighting industry is HQI. HQI is a trademark of OSRAM, and stands for a specific brand of lamps made by OSRAM. The aquarium industry has been quite loose with this term and has applied it to any European metal halide lamp and now, even more loosely, to any DE lamp. European MH lamps do not directly conform to the ANSI standard and have different operating current and voltage requirements. In most cases, a direct match may not exist with U.S. ballasts, so the aquarium industry has tried to find ballasts that work with those lamps, and have labeled these as HQI ballasts. For example, the M80 and M81 ballasts are called “HQI ballasts” in the aquarium industry, for 150-watt and 250-watt lamp applications, respectively.

Table 1 shows a list of the available lamps frequently used in the hobby, along with their ANSI designations.

Table 1 Watts
Single or
Double-Ended
Base/ Mounting
Code
Starting Method
ANSI
Designation

150W
SE
Medium
Pulse
M102/M142

DE
Rc2
Pulse
M81

175W
SE
Mogul
Probe
M57

Pulse
M137/M152

250W
SE
Mogul
Probe
M58

Pulse
M138/M153

DE
R7
Pulse
M80

400W
SE
Mogul
Probe
M59

Pulse
M135/M155




Choosing the Color Temperature



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The wattage, type of lamp (SE or DE) and starting requirements determine the ANSI code associated with the lamp (and the ballast). The aquarist next faces the choice of selecting the lamp’s color temperature. Lamps below 5000 Kelivn (K) are not usually recommended for use in reef aquaria. Desirable lamps in the hobby usually have color temperatures designated as 5500K, 6500K, 10,000K, 11,000K, 12,000K, 12,500K, 13,000K, 14,000K, 15,000K, 18,000K, 20,000K or 50,000K, with the generally loose understanding that the higher the lamp’s color temperature, the more blue its light output, however, this often is not the case.

Each lamp has its own spectral signature (how much light it produces at different wavelengths), and it’s important to remember that the ratings and color temperature supplied by aquarium vendors often do not correspond to the lamp’s actual output. Additionally, the lamp’s output is affected by the ballast used to drive it. It is the spectral characteristics and the intensity at different wavelengths that should be of greatest interest to reef aquarists. It is worthwhile to check the spectral characteristics of the lamps and ballast that you select by reviewing the data on the following website: www.reeflightinginfo.arvixe.com.

Lamp Bases



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After the specific lamp has been selected, the next step is to check the lamp’s mounting base. The lamps most commonly used in the hobby are single-ended, with a screw-type mounting base. The size of the base and the threads is also described by a code, although names are more commonly used. For example, Base - E39 is commonly called a mogul base. European lamps also have a mogul base, but it is slightly different from the E-39 and is called E40. The differences are small enough that the E40 base lamps will work fine in the typical E39 mogul base used in the U.S. Double-ended 150-watt lamps use the RSC (RX7s) base, while the 250-watt double-ended lamps use the Fc2 base. Figure 3 shows the various shapes and configurations of lamps and bases, along with the restrictions on their various operating positions.

Figure 3. Designations of various bulbs and bases, and operating positions
(source Venture Lighting™).



Figure 4. Deterioration of the arc tube and contents over time.

Metal halides lamps have a finite life and deteriorate with use. While the lamp may be rated for several tens of thousands of hours of use, in typical reef applications the lamps’ output may drop by 30% or more in a year, necessitating a change of lamp. Several effects take place in these arc tubes that ultimately affect the lamp’s light output: deposits of electrode material build up on the arc tube’s wall, changes occur in the arc stream’s chemical composition, the quartz deteriorates to a more crystalline form that is opaque to light, etc. Each time a MH lamp is turned on; tungsten sputters from its electrodes and, over time, blackens the arc tube’s wall. Figure 4 compares a new lamp (left) to a lamp used for over a year (right). The deterioration in the lamp’s arc tube manifests itself as a change in its spectrum, shifting the lamp’s color temperature toward lower Kelvin values. This is often referred to in the hobby as a spectrum shift, and results from decreased output at different wavelengths, with larger reductions at smaller wavelengths i.e. towards the blue end of the spectrum.

Ballasts



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The ballast provides the proper starting voltage, operating voltage and current to the lamp to initiate and sustain its arc. High intensity discharge (HID) lamps have negative resistance, which causes them to draw an increasing amount of current; hence, they require a current-limiting device. The ballast provides the following functions:

It provides starting voltage and, in some cases, ignition pulses. All ballasts must provide some specific minimum voltage to ignite the lamp. In the case of pulse start lamps, an additional high voltage pulse is needed to ionize the gases within the lamp. These pulses are superimposed near the peak starting voltage waveform;
It regulates the lamp’s current and power. The ballast limits the current through the lamp once it has started. The ballast’s current is set to a level that delivers the proper power to the lamp. In addition, the ballast regulates the lamp’s current through the range of typical line voltage variations, thereby keeping the lamp’s power fairly stable to maximize the lamp’s life and performance and;
It provides appropriate sustaining voltage and current wave shape to achieve the lamp’s rated life. The ballast provides sufficient voltage to sustain the lamp as it ages.
Choosing the Ballast



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Once the lamp is selected, the next step is to select the ballast that will be used to drive it. When putting together a MH lighting system, it is very important to match the ballast to the lamp(s). An easy way to do this is by using the ANSI designation. For example, when using a lamp designated M-57, look for a ballast with that same designation. For lamps with no ANSI designation, it is best to call the lamp’s manufacturer for their recommendation on the correct ballast to use. Table 2 shows a complete list of aquarium metal halide lamps and their ballast requirements as specified by their respective manufacturers. There is a trend in the reef aquarium hobby to use ballasts that do not match the lamp’s design specifications, often to “overdrive” a lamp, and hence, to generate more light output from a given lamp. This can lead to premature failure and shorter lamp life and, in some cases, explosive failure due to rupturing of the lamp’s inner and outer arc tube.

Table 2. Aquarium Metal Halide Lamps and Their Recommended Ballasts as Specified by Manufacturers (Compiled by Paul Erik Hirvonen). 70/75-watt MH DE Lamps Lamp Standard
Lamp Type
Recommended
Ballast

Aqualine Buschke
HQI/M85
PULSE
M85

BLV
HQI/M85
PULSE
M85

Ushio
HQI/M85
PULSE
M85

Venture
HQI/M85
PULSE
M85/M98


150-watt MH SE Lamps
Lamp Standard
Lamp Type
Recommended
Ballast

Iwasaki 6500K Med
M102
PULSE
M102

Iwasaki 50,000K Med
M102
PULSE
M102


150-watt MH DE Lamps
Lamp Standard
Lamp Type
Recommended
Ballast

Aquaconnect
HQI/M81
PULSE
M81

Aqualine Buschke
HQI/M81
PULSE
M81

CoralVue
HQI/M81
PULSE
M81

Giesemann Megachrome
HQI/M81
PULSE
M81

BLV
HQI/M81
PULSE
M81

Iwasaki
HQI/M81
PULSE
M81

Radium
HQI/M81
PULSE
M81*

Sylvania
HQI/M81
PULSE
M81

Ushio
HQI/M81
PULSE
M81

Venture
HQI/M81
PULSE
M81/M102

XM
HQI/M81
PULSE
M81

* Note Radium Blue/20,000K (HRI-TS 150W/230/B/RX7S) is rated at 160W and may not operate on electronic ballasts with safety shutoff.


175-watt MH SE Lamps
Lamp Standard
Lamp Type
Recommended
Ballast

Aquaconnect 14,000K
Euro PS
PULSE
M137/M152

Aqualine Buschke 10/13,000K
Euro PS
PROBE*
M137/M152*

BLV Nepturion 10,000K
Euro PS
PULSE
M137/M152

CoralVue 10,000K, 14,000K, 20,000K
?
PROBE
?

CoralVue ReefLux 10,000K, 12,000K
?
PROBE
?

EVC 10,000K, 14,000K, 20,000K
M57
PROBE
M57

Hamilton 14,000K
M57
PROBE
M57

Helios 12,500K, 20,000K
M57
PROBE
M57

Iwasaki AQUA 2 15,000K
M57
PROBE
M57

PFO Lighting 11,000K, 18,000K
M57
PROBE
M57

Ushio Aqualite 10,000K, 14,000K, 20,000K
M137/M152
PULSE
M137/M152

Venture 5,000K,10,000K
M57
PROBE
M57

XM 10,000K, 15,000K, 20,000K
M57
PROBE
M57

* Note Aqualine Buschke 10/13,000K (175W SE) is a probe start lamp but is recommended for use on a pulse start ballast. A probe start ballast may not provide adequate sustaining voltage during warm-up and might cause cycling.
* Note Euro PS refers to low lamp current Pulse Start European specification.


250-watt MH SE Lamps
Lamp Standard
Lamp Type
Recommended
Ballast

Aquaconnect 14,000K
M80/HQI
PULSE
M80/HQI*

Aqualine Buschke 10/13,000K
?
PROBE*
M138/M153*

Blueline 10,000K+, 10,000K Superwhite
M58
PROBE
M58

BLV Topflood 5200K
M80/HQI
PULSE
M80/HQI*

BLV Nepturion 10,000K, 14,000K, 20,000K
M80/HQI
PULSE
M80/HQI*

BLV Colorlite Blue (20,000K)
M80/HQI
PULSE
M80/HQI*

CoralVue 10,000K, 14,000K, 20,000K
?
PROBE
?

CoralVue ReefLux 10,000K, 12,000K
?
PROBE
?

EVC 10,000K, 14,000K, 20,000K
M58
PROBE
M58

Giesemann Megachrome Marine 12,500K
M80/HQI
PULSE
M80/HQI*

Giesemann Megachrome Coral 14,500K
M80/HQI
PULSE
M80/HQI*

Hamilton 14,000K
M58
PROBE
M58

Iwasaki 6500K Clean Arc
M58
PROBE
M58

Iwasaki 6500K Clean Ace
H37
PROBE
H37

Osram Daylight (HQI-T 250W/D)
M80/HQI
PULSE
M80/HQI*

PFO Lighting Krystal Star 11,000K, 18,000K
M58
PROBE
M58

Radium Blue/20,000K
M80/HQI
PULSE
M80/HQI*

Sunburst 12,000K
M58
PROBE
M58

Ushio Aqualite 10,000K (UHI-S250AQ/10/CWA)
M58
PROBE
M58

Ushio Colorlite Blue/20,000K (UHI-S250/E39/BLUE)
M80/HQI
PULSE
M80/HQI*

Venture 5,000K,10,000K
M58
PROBE
M58

XM 10,000K, 15,000K, 20,000K
M58
PROBE
M58

* Note Aqualine Buschke 10/13,000K (250W SE) is a probe start lamp but recommended for use on a pulse start ballast. A probe start ballast may not provide adequate sustaining voltage during warm-up and might cause cycling.
* Note HQI refers to high lamp current European specification. Operating it on a standard American pulse start (ANSI M138/M153) ballast will reduce its output and may cause color shift.


250-watt MH DE Lamps
Lamp Standard
Lamp Type
Recommended
Ballast

Aquaconnect 14,000K
M80/HQI
PULSE
M80/HQI

Aqualine Buschke 10,000K, 20,000K
M80/HQI
PULSE
M80/HQI

BLV Nepturion 10,000K, 14,000K, 20,000K
M80/HQI
PULSE
M80/HQI

CoralVue 10,000K, 14,000K, 20,000K
M80/HQI
PULSE
M80/HQI

CoralVue ReefLux 10,000K, 12,000K
M80/HQI
PULSE
M80/HQI

EVC 10,000K, 14,000K, 20,000K
M80/HQI
PULSE
M80/HQI

Giesemann Megachrome Marine 12,500K
M80/HQI
PULSE
M80/HQI

Giesemann Megachrome Coral 14,500K
M80/HQI
PULSE
M80/HQI

Giesemann Megachrome Blue 22,000K
M80/HQI
PULSE
M80/HQI

Hamilton 14,000K
M80/HQI
PULSE
M80/HQI

Helios 12,500K, 20,000K
M80/HQI
PULSE
M80/HQI

PFO Lighting Krystal Star
M80/HQI
PULSE
M80/HQI

Phoenix Electric HexArc 14,000K
M80/HQI
PULSE
M80/HQI

Ushio Aqualite 10,000K, 14,000K, 20,000K
M80/HQI
PULSE
M80/HQI

XM 10,000K, 15,000K, 20,000K
M80/HQI
PULSE
M80/HQI


400-watt MH SE Lamps
Lamp Standard
Lamp Type
Recommended
Ballast

Aquaconnect 14,000K
HQI
PULSE
HQI*

Aqualine Buschke 10,000K
?
PROBE
M135/M155

BLV Nepturion 10,000K, 14,000K, 20,000K
HQI
PULSE
HQI*

BLV Colorlite Blue (20,000K)
HQI
PULSE
HQI*

CoralVue 10,000K, 14,000K, 20,000K
?
PROBE
?

CoralVue ReefLux 10,000K, 12,000K
?
PROBE
?

EVC 10,000K, 14,000K, 20,000K
M59
PROBE
M59

Giesemann Megachrome Marine 12,500K
HQI
PULSE
M135/M155/HQI*

Giesemann Megachrome Coral 14,500K
HQI
PULSE
M135/M155/HQI*

Hamilton 14,000K
M59
PROBE
M59

Helios 12,500K, 20,000K
M59
PROBE
M59

Osram Daylight (HQI-BT 400W/D)
HQI
PULSE
M135/M155/HQI*

Osram Daylight (HQI-T 400W BLUE)
Euro PS
PULSE
M135/M155

PFO Lighting Krystal Star
M59
PROBE
M59

Radium Blue (20,000K)
Euro PS
PULSE
M135/M155

Sylvania Aqua Arc 10,000K
M59
PROBE
M59

Ushio Aqualite 10,000K (UHI-S400AQ/10)
HQI
PULSE
M135/M155/HQI*

Ushio Aqualite 10,000K (UHI-S400AQ/10/CWA)
M59
PROBE
M59

Ushio Aqualite 14,000K, 20,000K
HQI
PULSE
M135/M155/HQI*

Ushio Colorlite Blue (20,000K) (UHI-S400BL)
M59
PROBE
M59

Venture 5,000K,10,000K
M59
PROBE
M59

XM 10,000K, 15,000K, 20,000K
M59
PROBE
M59

* Note Aqualine Buschke 10/13,000K (400W SE) is a probe start lamp but is recommended for use on a pulse start ballast. A probe start ballast may not provide an adequate sustaining voltage during warm-up and might cause cycling.
* Note HQI refers to high lamp current European specification. Operating it on a standard American pulse start (ANSI M135/M155) ballast will reduce its output and may cause color shift.
* Note Euro PS refers to low lamp current Pulse Start European specification. Operation on an HQI ballast is not advised by the manufacturer and will overdrive the lamp.


400-watt MH DE Lamps
Lamp Standard
Lamp Type
Recommended
Ballast

CoralVue 10,000K, 14,000K, 20,000K
HQI
PULSE
M135/M155/M108/HQI*

Hamilton 10,000K, 20,000K
HQI
PULSE
M135/M155/M108/HQI*

IceCap 10,000K, 20,000K
M108
PULSE
M135/M155/M108

Osram Daylight (HQI-TS 400W/D)
HQI
PULSE
M135/M155/M108/HQI*

PFO Lighting Krystal Star 11,000K, 18,000K
HQI
PULSE
M135/M155/M108/HQI*

Radium 5,200K (HRI-TS 400W/D/230/FC2)
HQI
PULSE
M135/M155/M108/HQI*

* Note HQI refers to high lamp current European specification. Operating it on a standard American pulse start (ANSI M135/M155) ballast will reduce its output and may cause color shift.



Ballast Circuit Types



--------------------------------------------------------------------------------

Several different types of circuits are used for HID ballasts. Probe start and pulse start lamps need different ballasts. The different types of circuits used with standard MH lamps are Reactor (R), High Reactance Auto Transformer (HX-HPF), Constant Wattage Autotransformer (CWA), Super Constant Wattage Auto Transformer (SCWA), Constant Wattage (CW), etc. There are specific advantages and disadvantages to these different circuit types, and lamps may be designed to work with a specific type of circuit. It is not the intent here to discuss the various circuits and their utility, but rather to make the reader aware that there are differences in the circuit that pertains to how the ballast behaves with respect to variations in input voltage and also in their output to the lamps.

Most ballasts (except electronic ballasts) used for metal halide lighting are the CWA (constant wattage autotransformer) type. This is a lead circuit ballast and consists of a high reactance autotransformer (core-coil) with a capacitor in series with the lamp. However, ballasts for 150-watt and 250-watt DE lamps tend to be the HX-HPF circuit type and require an igniter along with the capacitor and core-coil. Ballasts for pulse start lamps also have an additional igniter to start the arc, and have their own set of ballast circuit types.

Capacitors are needed to improve a ballast’s (input) power factor, and are integral components of CWA and regulated lag circuits; they will not operate without capacitors. Both oil-filled (wet) and dry-film capacitor technologies are commonly used with ballasts. Oil-filled capacitors come in metal cases and are filled with a dielectric fluid; dry-film capacitors do not use a dielectric fluid. High intensity discharge lamp igniters provide a brief, high voltage pulse or pulse train to break down the gas between the electrodes of an arc lamp. Pulses can range from several hundred volts to 5kV. Typical durations are in the µsec range. They are usually timed to coincide with the peak of the open circuit voltage.

It is important to realize that a particular ballast specification corresponds to specific operational characteristics and, for proper functioning of lamps, it is important that the right ballast be used. To see the differences in the way ballasts operate lamps, compare the ANSI and European specifications (Table 3), which specify the arc operating voltage, current and required starting voltage (ignition voltage) / current for each standard/specification.

Table 3: ANSI and European Lamp/Ballast Requirements
(Compiled by Paul Hirvonen)

ANSI Code
Wattage Rating
Arc Operating Voltage
Arc Operating Current
Starting Method
Minimum Starting OCV For Lag (Reactor) Ballast
Minimum Starting OCV For Peak Lead (CWA) Ballast
Lamp Starting Voltage Pulse Height
Lamp Starting Current

Nominal Ratings
Vrms
Vpeak
Vrms
Vpeak
Min.
Max.
Min.
Max.

M58
250W
133V
2.1A
Probe Start
350
495
270
1.8 CF
N/A
N/A
2.1A
3.5A

M153
250W
133V
2.1A
Pulse Start
254
345
254
483
3.0kV
4.0kV
2.1A
3.2A

M80
250W
100V
3.0A
Pulse Start
230
325
TBD
TBD
4.0kV
6.0kV
3.0A
5.2A

250 HQI*
250W
100V
3.0A
Pulse Start
198
TBD
198
TBD
4.0kV
5.0kV
3.0A
4.5A

M59
400W
135V
3.25A
Probe Start
350
495
280
1.8 CF
N/A
N/A
3.2A
5.0A

M155
400W
135V
3.25A
Pulse Start
254
345
254
483
3.0kV
4.0kV
3.2A
5.0A

400 HQI*
400W
120V
4.0A
Pulse Start
198
TBD
198
TBD
4.0kV
5.0kV
4.6A
7.5A

*The 250 HQI and 400 HQI are European metal halide specifications which most single-ended and double-ended European lamps are built to.



The ANSI information in the table above was compiled from the following references:

ANSI M58 Document: ANSI C78.1378-1997
ANSI M153 Document: ANSI C78.01650-2003
ANSI M80 Document: ANSI C78.1387-2001
ANSI M59 Document: ANSI C78.1375-1997
ANSI M155 Document: ANSI C78.01650-2003
The ANSI specifications for universal operating position SE metal halide lamps are tested and rated with the lamp’s operating base up. These operating values vary with the lamp’s operating position. Operation of metal halide lamps below the limits of the ANSI standards will reduce the lamps’ efficacy and may result in color shift, arc instability, flicker and reduced lamp life. The lamp’s manufacturer should be consulted before operating metal halide lamps at reduced wattage or in applications using dimming circuits.

Ballast Configurations



--------------------------------------------------------------------------------

Any ballast rated for the lamp it is designed for will function properly, but may come in different configurations, each with its pros and cons, as well as differences in prices and the amount of DIY work required. Several manufacturers’ ballasts will operate the same lamps, and often the exact brand is not important. The ballasts can be cross-referenced with another company's ballasts if a specific brand is desired. Ballasts for MH lamps are often available in a multi-tap configuration. This means that the same ballast can be used with different input voltages (e.g., 120/208/240/277) by selecting the right combination of wires leading into the transformer’s coil. The correct combination of wires to use for a specified voltage is indicated in the wiring diagram and is often labeled on the wires, too.



Figure 5. Components of a core-coil ballast.
(1) Core & Coil Ballasts
The most commonly used ballast is the core and coil type, which basically consists of a transformer (core and coil) and a capacitor. Core-coil ballasts are sold as kits, which include the transformer, capacitor (and ignitor, if needed) and mounting rails. These ballasts are the cheapest kind because they are mass-produced for commercial MH lighting purposes. Unfortunately, they are designed to fit into the standard housing of commercial lighting fixtures, meaning that a box will have to be found or fabricated to house the ballast. This is the ballast configuration typically found inside the commercially sold ballast boxes in the aquarium industry. Running these ballasts exposed is extremely dangerous and is therefore not a good idea.

(2) F-can Ballasts
The F-can ballast is very similar in appearance to the fluorescent ballast is potted in fluorescent type cans and utilizes asphalt based insulating materials. F-can ballasts may also have thermal protectors, which cut off power to the ballast if it overheats. These ballasts are also called tar ballasts due to the black asphalt based material used in their construction. Because these ballasts are already enclosed in a fluorescent type case, they are much easier to work with than core and coil ballasts, and can easily be mounted anywhere. Because these ballasts are designed for indoor use, they tend to have less "ballast hum." They are more expensive than core-coil ballasts, but their additional cost may be offset by the cost of the box and additional DIY work needed to install core-coil ballasts.

(3) Electronic Ballasts
Recently, electronic ballasts based on digital electronics have become available for metal halide lamps. Manufacturers claim that these ballasts provide better performance in a smaller package, have a high power factor, save energy, generate less heat, have less change in output power and have lower maintenance costs than F-can and core/coil ballasts. Manufacturers also claim that high frequency ignition reduces blackening on the arc tube’s wall, which gives better color stability and longer lamp life. In addition, electronic ballasts can dim the lamp up to 33% of its full light output. Two concerns with electronic ballasts operating at high frequency are acoustic resonance and electromagnetic interference. Several users have reported interference with other electrical signals, such as TV, and interference with home automation signals, such as those used for X10 devices.

Reflectors/Reflecting Surfaces



--------------------------------------------------------------------------------

To maximize utilization of a lamp’s light output, the use of reflectors or reflecting surfaces is highly recommended. One approach is to use commercially available reflectors that are designed to provide a good spread and intensity of the light. Several such reflectors are available commercially. Before using such a reflector, it would be wise to check to see if it will fit in the hood/enclosure you are planning to build. Also, make sure that it will leave enough room for any additional fluorescent lamps you intend to add in the future. It is my experience that any reflector is better than no reflector, and that reflector designs do make a significant difference in how light is distributed over the tank. For more information on the differences between various reflectors, see the following list of articles.

Joshi, S. and Marks, Timothy, 2003. Analyzing Reflectors: Part I - Mogul Reflectors. Advanced Aquarist, March 2003.
Joshi, S. and Marks, Timothy, 2003. Analyzing Reflectors: Part II - Double Ended Lamp Reflectors. Advanced Aquarist, July 2003.
Joshi, S. and Marks, Timothy, 2004. Analyzing Reflectors: Part III. Advanced Aquarist, March 2004.
Joshi, S. 2004. Analyzing Reflectors: 400W DE Reflectors. Advanced Aquarist, Dec. 2004

pictures
http://www.reefkeeping.com/issues/2007-03/sj/index.php

 

[Seamaiden]

I love it when I see Sanjay's stuff used like this.

 

[AlienBoyD7]

2 much for me. I just know cannabis is interested in all kinds of light. I got a case of Mercury Vapor 1,000 watts for nothing. I run them with HPS 1,000 's ...works real well...

 

[dextr0]

more light stuff

http://www.weedfarmer.com/cannabis/lightguide_guide.php
Grow Light Guide

Horticultural lighting systems allow you to extend the growing season by providing your plants with an indoor equivalent to sunlight. This is a great advantage for those of you who appreciate having a year-round supply of fresh flowers, veggies and herbs. Artificial lighting is also a great way to jump-start spring by starting your seedlings months ahead of the last frost. There are three main types of horticultural lighting systems.

HID (High Intensity Discharge) Lighting

HID lighting is the most efficient way to convert electricity into light that is available to the consumer. There are two types of HID grow lights used for horticultural lighting:


HID Light Efficiency


One 1000 watt HPS bulb
puts out as much light as
111 incandescent 100 watt bulbs. One 400 watt MH bulb
puts out as much light as
20 fluorescent 40 watt tubes.

Metal Halide - MH
Metal halide bulbs produce an abundance of light in the blue spectrum. This color of light promotes plant growth and is excellent for green leafy growth and keeping plants compact. It is the best type of light to be used as a primary light source (if no or little natural sunlight is available). The average lifespan is about 10,000 cumulative hours. The bulb will light up beyond this time but due to the gradual decline of light, it is not worth your while to wait for the bulb to finally burn out. If you compare their lumen (brightness) per unit of energy consumed, metal halides produce up to 125 lumens per watt compared to 39 lumens per watt with fluorescent lights and 18 lumens per watt for standard incandescent bulbs.


High Pressure Sodium - HPS
High pressure sodium bulbs emit an orange-red glow. This band of light triggers hormones in plants to increase flowering/budding in plants. They are the best lights available for secondary or supplemental lighting (used in conjunction with natural sunlight). This is ideal for greenhouse growing applications.

Not only is this a great flowering light, it has two features that make it a more economical choice. Their average lifespan is twice that of metal halides, but after 18,000 hours of use, they will start to draw more electricity than their rated watts while gradually producing less light. HPS bulbs are very efficient. They produce up to 140 lumens per watt. Their disadvantage is they are deficient in the blue spectrum. If a gardener were to start a young plant under a HPS bulb, she/he would see impressive vertical growth. In fact, probably too impressive. Most plants would grow up thin and lanky and in no time you will have to prune your plant back before it grows into the light fixture. The exception to this is using a HPS light in a greenhouse. Sunlight is high in the blue spectrum which would offset any stretching caused by HPS bulbs.

HID Light Output
Primary Growing Area
Supplemental Growing Area
100 watts
2' x 2'
3' x 3'
250 watts
3' x 3'
4' x 4'
400 watts
4' x 4'
6' x 6'
600 watts
6' x 6'
8' x 8'
1000 watts 8' x 8' 12' x 12'


HID Lighting Helpful Tips
.Hanging height: Due to the heat that is emitted from these types of fixtures, you should hang them according to size. Smaller wattage systems (100 and 250) should be hung about 2 to 3 feet from the top of the plants. Medium wattage systems (400 and 600) should be hung around 4 feet from the top of the plants. High wattage systems (1000 and up) should be placed at least 4 to 6 feet from the plant tops.
.How long should lights run? This depends on the type of plant. Most plants and vegetables need about 10 to 12 hours of light to promote growth. Plants that produce fruits or flowers will show improvement with up to 16 hours a day of supplemental light.

Fluorescent Lighting

This type of light is perfect for starts and seedlings. They are also popular for growing low-light plants like herbs and African violets. Fluorescent lights are low intensity and need to be placed within 8" (up to 15" for shade loving plants) of the plants to be effective. They are a poor light source for flowering and budding primarily because of their low lumen output.

Incandescent Lighting

These lights are also good for starts and seedlings and provide an inexpensive alternative to HID lights, because they do not require a ballast. These lights are only good for individual plants or small groups of plants because of their low lumen output and limited range.


The Cost to Run a Lighting System
To get the operating cost per hour for a light, take the lights combined wattage, and divide it by 1000 to get the kilowatts used. Then multiply that number by the amount your electric company charges per kilowatt hour. HID lights will use the number of watts it emits per hour, ie; 600w system will use 600 watts per hour (regardless of spectrum).
(light wattage output / 1000) x electricity cost per kilowatt hour = Operating cost per hour
operating cost per hour x hours used per month = Operating cost per month

How the Sunlight Effects Plant Growth

200 - 280 nm UVC ultraviolet range which is extremely harmful to plants because it is highly toxic.
280 - 315 nm Includes harmful UVB ultraviolet light which causes plants colors to fade.
315 - 380 nm Range of UVA ultraviolet light which is neither harmful nor beneficial to plant growth.
380 - 400 nm Start of visible light spectrum. Process of chlorophyll absorption begins. UV protected plastics ideally block out any light below this range.
400 - 520 nm This range includes violet, blue, and green bands. Peak absorption by chlorophyll occurs, and a strong influence on photosynthesis. (promotes vegetative growth)
520 - 610 nm This range includes the green, yellow, and orange bands and has less absorption by pigments.
610 - 720 nm This is the red band. Large amount of absorption by chlorophyll occurs, and most significant influence on photosynthesis. (promotes flowering and budding)
720 - 1000 nm There is little absorption by chlorophyll here. Flowering and germination is influenced. At the high end of the band is infrared, which is heat.
1000+ nm