There was some discussion about PAR vs Lumens and other lighting questions that should be answered by reading the following. I pulled this from a few sources: How is Light Measured? The "color" of light sources comes from a complicated relationship derived from a number of different measurements, including correlated color temperature (CCT) or Kelvin temperature (K), color rendering index (CRI), and spectral distribution (PAR Watts). However, color is most accurately described by a combination of Kelvin temperature and CRI. Color Rendering Index - CRI CRI is a numeric indication of a lamp's ability to render individual colors accurately. The CRI value comes from a comparison of the lamp's spectral distribution to the standard (e.g. a black body or the daytime sky) at the same color temperature. The higher the CRI the more natural and vibrant the colors will look. A bulb with a CRI of 85 or higher is excellent being that the sun has a CRI of 100. Eye Hortilux makes 90 -92 CRI bulbs that are used in aquarium, horticulture and other applications such as the 400W Eye Hortilux Blue 90CRI and 1000W Eye Hortilux Blue 92CRI. Standard Metal Halide bulbs have a CRI of about 70, so only 70% of colors will be rendered correctly. HPS bulbs have a CRI of 22. What is the Color Temperature or "K" - Kelvin Rating? The K rating is a generalized form of addressing the color output of a Light Bulb. Color Temperature is not how hot the lamp is. Color temperature is the relative whiteness of a piece of tungsten steel heated to that temperature in degrees Kelvin. HPS has a warm (red) color temperature of around 2700K as compared to MH at 4200K, which has a cool (blue) color temperature. The higher the kelvin temperature gets, the bluer. 10k lamps seem to be a nice crisp white, while higher kelvin can go from a blue/white to very blue and lower kelvin seem more like that of sunlight (6500k). Metal Halide bulbs go up to 20,000K (commonly used in aquariums) providing the bluest light. What is Spectral Energy Distribution & PAR Watts? The total visible spectrum is perceived by us humans as white light, but the "white light" is actually separated into a spectrum of colors from violet to blue, to green, yellow, orange and red made up of different wavelengths. Plants use the blue to red part of the spectrum as their energy source for photosynthesis. The different combinations and the relative intensity of various wavelengths of light determines the CRI of a light source. Only part of solar radiation is used by plants for photosynthesis. This active radiation Photo synthetically Active Radiation (PAR) contains the wavelengths between 400 and 700 nanometers and falls just within the visible spectrum (380 - 770nm). The light in this region is called PAR watts when measuring the total amount of energy emitted per second. PAR watts directly indicates how much light energy is available for plants to use in photosynthesis. EDIT: What are Lumens and how do Measure Them? “Lumen” is the unit of total light output from a light source. The problem is that the various units - candela, candlepower, lumens, lux, foot-candle - get very confusing. Lumens is a measure of light flux, so is independent of the area. It is essentially the amount of light available. So think of a bright light. It has a fixed amount of lumens, regardless of how near or far you are from it. Lux is the light level at a surface. If you think of a light bulb, the lux will vary with the distance from the light source because the light spreads out. 1 lux = 1 lumen per sq meter. Traditionally, lumens have been the benchmark of a lamps ability to grow plants; meaning the brighter the lamp the better the plant. However, studies have shown that a broader color spectrum lamp will perform much better than a lamp with high lumen output, especially when it comes to plant growth. Light meters: Most light meters read out in lux, which is a measure of the brightness at a surface. You will measure more luxes when you are closer to the bulb, than when farther away. Suppose you are measuring a compact flourescent lightbulb (CFL), which is roughly cylindrical in shape. You use your light meter and read 1000 foot-candles at about 1/2" from the lightbulb. How do you compute the lumens of the lightbulb? (1) Convert the foot-candles to lux: 1000 foot-candles = 10764 lux PIC NO. 1: SPECTRAL PAR Lighting Spectrum and Photosythesis The most common mistake people make with plants is to not understand photosynthesis and the visible spectrum of lighting that affects plant growth. Most people choose lighting solely based on the Kelvin temperature of a bulb. This tells you very little about what type of light within the spectrum is being emitted and at what strength. Visible light is on a scale in nanometers (radiated wavelength) from 400nm (violet) to 700nm (red). Simple matter of photosynthesis: plants can only utilize light that is absorbed. Bright light is essential yet only a portion of this white light is used for photosynthesis. The blue and red zones of the visible spectrum are the most beneficial to plants. Green plants appear green because it is reflected light. How "bright" a light appears has more to do with how much light is output in a given area visible to the human eye, with "brightness" being at a maximum in the green spectrum (middle of visible spectrum, or around 550nm). PIC 2: VISUAL SPECTRUM Lighting for a growing plants should not be chosen on color temp alone. It is true that 'full spectrum' bulbs are referred to as bulbs between 5000 Kelvin (K) and 6500 K and are considered to be best for plants. Yet this does not indicate what wavelength in nanometers the bulb is actually emitting. If you want to optimize plant leaf development (blue light) and stem elongation and color (red light) you need light in both the blue and red spectra for photosynthesis. You need a mix of blue and red for your plants, and green for you (brightness as perceived by humans). If your lighting looks extremely bright and your plants seem ultra-green, it means that you have lighting that outputs strongly in the green spectrum. Do not equate this with good lighting for your plants, because plants don't use light in the green spectrum for photosynthesis. Sunlight peaks in the blue spectrum at 475 nanometers (nm). This is a shorter wavelength than red light and is used by both plants and algae. As light passes through water the intensity decreases. The shorter wavelength blue light penetrates water better and more quickly than red, which is slower and absorbed more quickly. Chlorophyll, the photosynthetic pigment used by plants traps blue and red light but is more efficient with red light at 650 – 675nm. Blue is used at the same rate as red because it is more available for reasons mentioned above. For green plants the lighting peaks that are most important: Chlorophyll-a: 430nm/662nm Chlorophyll-b: 453nm/642nm Carotenoids: 449nm/475nm Red pigmented plants use more light in the blue area of the spectrum. PIC 3: CHLOROPHYL CHART Beyond choosing lighting that is optimal for photosynthesis, as above, you should choose lighting with the color temperature that best suits the plants needs. From a color temperature standpoint, blue-colored light will enhance blues in your plants. Green-colored light will make the plants look bright to humans and enhance the green color of your plants. Red-colored light will enhance the reds in your plants. Lux is lumens/square meter, so they are similar. They are both defined in terms that are meaningful to human perception of light – not plants. They stress the amount of energy in the green band to which humans are most sensitive – not plants. Artificial light sources are usually evaluated based on their lumen output. Lumen is a measure of flux, or how much light energy a light source emits (per unit time). The lumen measure does not include all the energy the source emits, but just the energy with wavelengths capable of affecting the human eye. Thus the lumen measure is defined in such a way as to be weighted by the (bright-adapted) human eye spectral sensitivity. PIC 4: Human Eye Sensitivity Lumen ratings are usually available, but when you use them you have to keep in mind what they mean. Lamp A can have a higher lumen rating than lamp B and appear brighter to you, while lamp B provides more useful light for plants. Compare the lumen ratings for cool white and GroLux bulbs of the same wattage and you will see what I mean. A 40-watt cool white bulb is rated at 3050 lumen; a 40-watt GroLux bulb (not the wide spectrum) is way lower at 1200 lumens. The big difference is because GroLux lamps provide very little green light and cool whites provide a lot of green light. SOME SAY: The GroLux bulb is perhaps the best plant bulb available because it has very little green light. Yet if you add some other lighting such as a Philips 6500K the effect is more pleasing to the eye and still beneficial to the plants. I find that the GroLux along with a GroLux wide spectrum (89 Color Rendering Index) has a great effect for use as dawn/dusk lighting. (A Sylvania rep. told me it was best to use both together.) PIC 5: GROLUX Kelvin rating and lumens does not equate for plants. The Kelvin scale is more of how your plants will look to you/us and is totally subjective. It is true that the lower Kelvin ratings like 3000K will have more red light and a 10,000K will have more blue light. Lumens are meaningless for plants, as green plants do not utilize green light for photosynthesis. A higher lumen rating at the same wattage often means greener light. Lumen is a rating weighted entirely towards human perception. It has little to do with the value of a light for either growing or viewing plants. The Kelvin rating is an indication of color temperature. The higher the temperature, the more blue the light. Here's a rough scale: - Reddish/Yellowish Endpoint - Incandescent Light: 2700K Daylight: 5500K Blue Sky: 10,000K - Blue Endpoint – PIC 6: Kelvin Color Map Don't be fooled by color temperature as an indication of what wavelength of light may or may not be present. The emitted wavelengths of light for two bulbs with the same color temperature could be wildly different. Therefore, color temperature is not what you should use to determine useful light for growing plants. It will, however, give you an idea of how things in your grow will look. For example, the sky has a color temperature of 10,000K and looks blue. Lighting that has a higher color temperature, indicating that it is bluish, does point to the fact that blue wavelengths are dominant. This, in turn, just means that it will activate green plants in the blue range, which is a good thing. Red photosynthetic pigment is less efficient at utilizing light and requires stronger light as a result. The less efficient red carotenoid pigment must rely on blue and some green light as well as more intense lighting. There are some plants that that are able to change the pigment they use for photosynthesis depending on available lighting. We see this in red-leaved plants that turn green if the lighting is too low, not enough blue and/or green light. Alternatively, some green leafed plants produce red foliage when closer to the light source or with overly bright lighting. The Kelvin color designation of a particular bulb is not always true to the black body locus line on a CIE Chromaticity map. This is why some 5000K bulbs look yellow and others white, especially when trying to compare a linear fluorescent with a CF or MH. This is where Kelvin ratings of bulbs can fall prey to marketing schemes/hype. PIC 7: CIE Map The standard measure that quantifies the energy available for photosynthesis is "Photosynthetic Active Radiation" (aka "Photosynthetic Available Radiation") or PAR. It accounts with equal weight for all the output a light source emits in the wavelength range between 400 and 700 nm. PAR also differs from the lumen in the fact that it is not a direct measure of energy. It is expressed in "number of photons per second". The reason for expressing PAR in number of photons instead of energy units is that the photosynthesis reaction takes place when a photon is absorbed by the plant; no matter what the photon's wavelength is (provided it lies in the range between 400 and 700 nm). In other words if a given number of blue photons is absorbed by a plant, the amount of photosynthesis that takes place is exactly the same as when the same number of red photons is absorbed. This is why it is so important to get the spectral output of a bulb before deciding if is a 'good plant light'. You may need to add/mix bulbs to get a lighting that has good visual effects for the human eye and proper light for plants because 'plant bulbs' tend to be purplish. There is an additional term called "Photosynthetic Usable Radiation" or PUR which takes in to account blue and red light only. I don't understand why people insist on distinguishing between lamps on the basis of their color temperature. No lamp renders color correctly or looks natural unless its Color Rendering Index (CRI) rating is very high. When CRI is over 90 the color temperature shouldn't make much difference; colors rendered accurately will always look about the same regardless of the Kelvin rating. Many bulbs render red and orange colors poorly and give you a look with very flat color contrasts. Other bulbs produce a lot of green light and don't render either blue or red very well at all. CRI or Color Rendering Index is an indication of how close the light is to daylight (full spectrum) on a scale from 0 to 100 with respect to how it makes objects appear. In the case of the Philips PL-L 950, the CRI is 92, so it has pretty good color rendering properties. Two bulbs with the same Kelvin temperature but different CRI ratings can produce very different appearances. Compare a 5000K that has an 80-something CRI with a 5000K that has a 90-something CRI. The 80 CRI bulb is very bright, but it renders greens with a distinct yellow cast. The 90 CRI bulb is dim, but it renders rich colors across the whole spectrum. Whether or not a bulb looks "natural" to you is totally subjective. It depends in part on what you're used to. If you only see the world under cool white fluorescents then that is probably what looks natural to you. If you live somewhere with frequently hazy or overcast skies then you may be accustomed to "natural" light having a color temperature near 7000K. If you live somewhere with clear skies and infrequent cloudy days then your natural light might have a color temperature closer to 5000K. If you are used to north skylight then maybe a color temperature close to 10,000K seems more natural. In any case of actual natural light the light will render colors pretty well. That is usually not the case for fluorescent lamps with a high Kelvin temperature rating. If you want a high K lamp that does render colors accurately then you might try finding the Philips C75. It has a 7500K color temp and a 90+ CRI. It could be hard to find and a bit pricey. Plants will grow with ordinary bulbs as they tend to have both some blue and red emissions. The problem is that they also have wavelengths between 500 and 600nm, which algae likes. Green algae and green plants use the same pigments for photosynthesis (chlorophyll a/b & carotenoids). So, light that helps one helps the other. The algae that are different are the blue-green algae (cyanobacteria), which contain Phycocyanin and absorb light heavily in the low 600nm (orange-red), which is unfortunately present in most standard fluorescents. PIC 8: Plant Pigment Bulbs sold as generic plant/aquarium bulbs usually have OK energy in blue and not much in red. A bulb sold as a generic "sunshine" bulb may or may not have some useful red, depending on the bulb. You can put any fluorescent lighting on your plants and do OK, but if you want to maximize plant growth, it's best to compare lighting options and, if possible, try to find the graphs/data for spectra output, rated life and output decay over time. Unfortunately, CF bulbs haven’t caught up with linear bulbs in the ability to offer light (tri-phosphor type) in the proper areas of the spectrum. Fluorescents lose efficiency over time. Some lose more than others - some bulbs may only suffer 10% drop in output, while others may drop 30% or more in the same time frame. The less the drop over time, the less you have to replace them, depending on your application. SOME SAY: Linear fluorescent tubes should be changed out every six months and compact fluorescents every year. I'm not too sure about that, as I thought CFL were built to last... Fluorescent bulbs marketed for aquaria are often more expensive and not necessarily better than generic versions. They are also not necessarily marketed correctly. Many bulbs offer spectral output graphs. However, many of these graphs are measured in relative power on the Y-axis rather than a known reference like watts per nanometer per 1000 lumens. All that 'relative power' lets you know is that 100% is the highest peak at a given nanometer and all other peaks are relative to this. So, don't be fooled by nomenclature and packaging (marketing hype). If you get a CRI in the 80s, you're doing fine. This is only a measure of how much something looks (to humans) under the bulb light as it would under "normal" light. Any fluorescent will work, but triposhphor (aka sunlight, full-spectrum) bulbs seemt to work a bit better, covering more parts of the spectrum. Plants aren't allthat fussy about the spectrum except that regular fluorescents have strong output in the green part of the spectrum and plants reflect much ofhte green light back. Lumens are the visible (to humans) light so if two bulbs have the same lumen ratings and one looks brighter, the "extra" light might be only what humans see, not what plants like. Unless there is a big diff in the green part of the spectrum between bulbs, it doesn't matter than much to plants. Color temp gives only a rough idea of how things will look under the light, whether there will be a strong blue aspect to the white light (higher temps) or yellowish or reddish. Actually, they only give a ersatz measurement of the overall spectral output, not how the light will look. They don't tell one much about spectral output, just the overall value (the sum of the peaks and dips in the spectral output.) Diff spectra can have the same color temp and even appear to be a somewhat diff hue. A high narrow peak in the blue region will pull up the color temp rating without making the light seem much bluer. A slightly depressed but wide slump in the red region will raise the color temp but so will a a deep narrow slump in the red and green. So high color temp doesn't always mean "bluer" or low color temp mean "redder". ***Note that the color temps are different shades of white, not say blue vs red bulbs. And note that plants don't seem to mind much about color temp ratings. Get what looks good to your eye-- otherwise don't worry about color temp. There is more red is some and more blue in others, so don't get that confused. You probably won't find standardised PAR ratings on enough diff bulbs to be able to make comparisons. But PAR tells you how much light the bulb makes that some plants can use for photosynthesis -- so everything else being equal, higher PAR means more light for the plants. It's usually not hard to get enough light over plants, so PAR isn't terribly useful for making critical determinations between which bulb to buy, especially since it is such a uncommmonly available rating. IT'S GREAT TO KNOW THAT YOU WANT TO FIND A PAR LEVEL FOR THE LIGHTS YOU NEED. IT'S ANOTHER TO ACTUALLY FIND IT LISTED ON A PACKAGE OR IN AN ONLINE DESCRIPTION OF THE BULB. If you see bulbs you really like the look of, you can grow plants just find with those bulbs, even if they are cheap old shop lights. Triphosphor, full spectrum/sunlight bulbs generally will have a more "sunlight" appearance -- although some made especially for aquaria can be kind of purplish due to big spikes in the red and blue parts of the spectrum. Personally, I think purple and pink bulbs belong on Christmas trees or in festival parades, but it's a matter of personal choice. Watts is a measure of the amount of energy the lamp consumes, assuming you use a particular standard ballast under standard conditions. What are the standards? They are pretty much whatever the manufacturer used to rate the bulb and somnetimes you can look them up, but usually not. So watts ratings don't tell you the actual light output of a bulb or even the actual watts that it will consume, but it will be reasonably close on the energy consumption. So when your out shopping for bulbs, try to find bulbs in the 6500-5000k (w/ 5500k the best) aka "FULL SPECTRUM" or "DAYLIGHT" bulbs for vegging, that have a high CRI, the higher the better. I would recommend looking at these bulbs: Duro-Test Corporation's Vita-Lite (c) and Vita-Lite Supreme (c). The original Vita-Lite hit the market in 1967 (!) as the world's first patented, natural-daylight-stimulating fluorescent tube. For over twenty five years (until the advent of their Vita-Lite Supreme) Duro's Vita-Lite was the closest simulation of natural daylight ever created by anyone, anywhere. (No, I'm not being paid for this plug) Specifications: 5500 K, 91 CRI, 2180 Lumens. For folks looking for more luminosity Duro-Test offers another lamp, the Vita-Lite Plus; the only specification difference being the generation of 2,750 lumens. The Vita-Lite Supreme offers 5500K, a CRI of 96 at 2000 lumens; it is the best match yet to natural outdoor light. These are great (the best available) lamps for the marine aquarist, aquatic gardener, herptile keeper, photographer wanting to skip filters, and human work place. They grow aquatic organisms better than any other light system, without specialized fixturing at the lowest cost. What is more, your fishies and photosynthetic organisms look and live better under these lamps. Yes, these products are that good. BENIE BREEDING ANYONE? Also, in all fairness, I'd like to mention three other manufacturers of full-spectrum fluorescents. They are Philips with the Colortone 50, General Electric with their Chromaline 50 and Verilux with lamps of the same name. These companies also 'private label' full spectrum lamps for other labels. You will have to look for the CRI, Temperature in Kelvin, Luminosity in lumens, power curve, and average life ratings to make your own judgments. As far as flowering bulbs go, 2700-2100k, you probably wont find a bulb w/ a CRI over 82 in CFL'S or Flouro's, & even lower in HPS at around CRI of 22. FOR YOU CFL AND FLOURO USERS: Natural sunshine is 100 CRI & 5300K at peak Vegging CFL Bulbs BlueMax Full Spectrum HD CFL Bulbs: 5500k, CRI 93+ BlueMax Full Spectrum HD CFL Bulbs: 5900k, CRI 93+ Indoor Sunshine Full Spectrum CFL Bulbs: 5300k, CRI 95 Duro-Test Color Classer 75: 7500k, CRI 93 Duro-Test Daylight 65: 6500k, CRI 92 Duro-Test Vita-Lite: 5500k, CRI 91 Duro-Test Vita-Lite Plus: 5500k, CRI 91 (higher lumens) Duro-Test Optima 50: 5000k, CRI 91 Duro-Test Color-Matcher 50: 5000k, CRI 90 NaturesSunlite: 5000k, CRI 85 NaturesSunlite: 6500k, CRI 85 NaturesSunlite Full Spectrum: 5500k, CRI 93 Vegging Fluorescent Tubes Sylvania Gro-Lux GRO/AQ (these dont have specs, but are great for growing) Sylvania Gro-Lux Wide Spectrum: 3400k, CRI 89 Verilux Tru-Bloom Full Spectrum: 6280k, CRI 94.5 AgroSun Full Spectrum: 5850, CRI 93 BlueMax Full Spectrum HD: 5900k, CRI 93+ BlueMax Maxum: 5000k, CRI 91+ BlueMax Prolume: 6500k, CRI 91 BlueMax Spectra: 5500k, 5600k, 5900k, CRI 93 Duro-Test Vita-Lite: 5500k, CRI 91 NaturesSunlite: 5500k, CRI 93 NaturesSunlite: 5500k, CRI 96 Most Flowering CFL & Fluorescent bulbs are the same at 2700k, with CRI ranging around 80-85, though MaxLite & Sunblaster have the highest CRI of 84 & 85 That was a little extra to help out you flouro growers, as I see there are quite a few. thanks to jcj for the content pics.