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What are lumens, lux, and all that?

What is Light?​

Light is a form of electromagnetic radiation that can be understood as either a wave or a particle, leading to the concept of wave-particle duality. Light exhibits phenomena that can be explained by its wavelike characteristics, such as interference, as well as some that can be best explained by its particle-like properties, like the photoelectric effect.

When considering light as a particle, it consists of many particles called photons emitted by a light source. These photons have a unique property: their energy. When treated as a wave, this property is referred to as wavelength and is perceived by humans as color. The key takeaway is that photons with the same energy (wavelength or color) from different sources are identical.

Understanding Light Intensity​

Light intensity refers to the flux of photons per unit area per unit time. In simpler terms, the more photons present, the higher the intensity.

Lumens, Lux, and Foot-candles: What do they mean?​


Lumens are a unit that defines "luminous flux," which is the energy emission adjusted for the sensitivity of the human eye. In the context of a light bulb, it measures the total number of emitted photons, adjusted based on the human eye's sensitivity to different photon energies. Different colors are weighted differently according to a "luminosity function" that describes our eyes' sensitivity.

Lux and Foot-candles​

Lux is defined as the number of lumens per second incident upon one square meter. It represents the number of photons striking a finite area per unit time, weighted by the luminosity function. Foot-candles are a derivative unit equal to 10.76 lux, based on the square foot for area conversion.

Implications of Multiple Light Sources​

Lux represents the number of photons striking a unit area per unit time, weighted by a luminosity function. We've also established that photons from the same light source are indistinguishable, as long as they have the same energy/wavelength/color.

This means if you place two lights at the same distance from a point, and each light provides N photons per unit area at that point, with two lights, you'll have 2N photons per unit area at the point. Since intensity is a measure of the number of photons per unit area, the light is twice as intense, regardless of the unit used. It will be twice the lumens, twice the lux, and twice the foot-candles.

However, there is a practical caveat when using multiple low-intensity light sources. It's important to note that the lights must be at the same distance. For instance, while you can get 27,000 lumens from 10 x 42W CFLs, it's challenging to place them close enough to be effective. If arranged in a line, each successive light is farther from the meter, and the effective increase will be reduced due to the 1/d^2 rule. On the other hand, this can be an effective way of distributing light, whereas with HID, you need to distribute the plants around a single point light source.
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