Originally Posted by TropTrea
........ Very little light is utilized in the green spectrum for photosynthesis. Most of the green light is not absorbed by the plants but is reflected back at us which is why many plants look green to us. .....
I know this is a pretty old post, but I do think it should be corrected:
If you look at the illustration in the first post here, that shows the spectral range for lumens, which are intended to be close to normal human eye spectral response, you will see that our eyes are most sensitive, by far, to green light. And, much less sensitive to red and blue light. So, if we were to look at light with a perfectly flat spectrum from 400 to 700 nm, we would most likely see green light. So, the fact that we see plants as green is mostly because that's the color we see most intensely.
It is true that plants use more red and blue light than than green light, but they use more than a trivial amount of green light. Complicating this is the fact that we see a variety of spectral mixes of light as green, not just light of a wavelength associated with green.
To standardize our data on light, I think it is best to just use photosynthetically active radiation intensity as our measure of light (PAR - miicromoles of photons per second per square meter). No method within reach of hobbyists allows us to measure that with great accuracy, but even very cheap PAR meters are useful for judging how intense our aquarium lighting is, and the standard Apogee meters are even better, but still no where near perfect.
The light intensity in an aquarium varies widely, as much as from 20 PAR at the center, at the substrate to 100 PAR at the water surface to 10 PAR at each end of the tank at the substrate. So, to be useful I like to specify light intensity by the PAR at the center at the substrate level, and just accept the variation as the way it is. That lets us use a simple and useful set of PAR readings as a measure of low light, medium light, high light. And, we can define what low, medium and high mean several ways.