Throught photosynthesis, plants gets the required energy to disociate water molecules (H2O) into its components (H and O) and build, together with the C from the air's CO2 (carbon dioxide), the organic matter used to build the plants. Some other elements are required in small amounts, and they are uptaked by roots (they are called plant's nutrients). So a plant need mainly air (CO2), water and light to thrive.
The process of photosynthesis requires light, water and CO2 and produces O2 (oxigen). So its possible to measure accurately photosyntesis by measuring CO2 uptaken or O2 produced. Photosynthesis is limited for the required factor (light,water,CO2) wich is depleted earlier. On indoor growing, it shouldnt be a lack of enough water, and the grower should provide enough CO2 by air renovation or directly CO2 supplementing if want to optimize the grow, so the main limiting factor is the light avalaible. Its a must on a well designed grow room.
Photosynthesis is tightly linked with total amount of photons absorbed. This concept is the base of all, and it should be clear for any grower. So im going to analyze it deeper:
-Amount of photons. Not of watts, or lm. Plants use photons, so the number of photons is the esential figure to consider. The more the photons wich reach the plant, the better (up to a limit).
Its important to note that same energy (for example 1 watt) of blue (450nm) have 33% less photons than of red ones (670nm) (450/670=0.67 : as noted before, energy carried by a photon is inversely proportional to its wl) if we take the amount of red photons as base. If we take the amount of blue photons as reference, then 1 watt of red ones carries 49%, near half, more photons (670/450=149). So very often, producing as more red photons possible is the most effective way of using artificial light for growing plants (if the efficiency of producing 1 watt of each are similar).
This effect is what does that plants have adapted their systems to use 670nm photons the more efficiently, as sunlight reaching Earth's surface has higher number of then than of any other wl, while the higher energy (watts) received is of green light.
-Absorbed. Plants absorb differentially photons of differents wl, and it depends too slighty of the light density they are receiving. One of the ways of improving plants lighting is by optimizing the light level and the spectrum in order to get the max absortion (and less reflection) of photons possible.
This way is how cannabis reflect different wl:
Note that although the higher reflection is of green (around 550nm), and thats why see the plant green, just a small fraction of green is reflected back, as 15%, and not all as you can read many times on the net.
That plants reflect back all the green light is a false statement.
This false statement is found very often linked to other false one, that plants dont use green photons for photosynthesis. Plants reflect back more green photons than of other wl, as they use them with lower efficacy, but as max its used at half the efficacy of red ones. Lower efficacy of green, yes, but its not wasted at all.
That green light is wasted is another false statement. Ill analyze this topic deeper later, as its qualitative and not quantitative analysis.