We understand light quality for the distribution of wl that emits the light. The graph wich show it is called SPD (Spectral Power Distribution), many times abreviated as spectrum.
Reading other threads about lighting, it seems that spectrum is the only parameter wich determines the efficacy of light for growing. But its not true, what determines mainly the efficacy is the energy efficiency of the bulb, wich determine the amount of PAR watts delivered. But spectrum affect the efficacy by two ways: changing the amount of photons that carries each PAR watt and because there are wl wich are more efficients promoting photosynthesis than others.
Opposite to general belief, differences in efficiency promoting P are relatively small, and almost always is below a 20% for wide spectrum lights (white of differents tones). Using LEDs is possible to get somewhat higher enhancement, but its up to 50% in best cases (meaning an optimized spectrum may reach 50% more photosynthesis for the same amount of photons than for example, an HPS). But its very important to note than this enhacement may be applied to the number of photons, wich, again, is the base of all. If you use half photons with an average 50% more efficacy promoting P you get a final efficcacy of just 75%.
If you use half photons, dont mind how good is the spectrum, you never get as much photosynthesis (thus, plant growth). This is very often forgotten by LED grow lights designers, wich largely overstates the importance of spectrum alone. This mistake is due many people use "action spectrum" curves, wich are curves build from laboratory
absorbance of photosynthetic pigments, and not from measuring P promoted by each wl with live plants.
Main general studies about this topic were performed by McCree and Inada in the 70's of last century. Their results have been repeated many times for specific plant species, so their results are widely accepted as valid. Both performed the studies by irradiating the plants with narrow bandwiths of light (2nm) and measuring the photosynthesis (by O2/CO2 changes).
The most used today is the McCree curve, wich shows P for each mol of photons absorbed. You may find it many times plotted as background of horticultural lamp's SPDs:
The curve of Inada, however, shows the P promoted by watt of incident enrgy, and not of absorbed photons. Thus, it gives together the effect of photon's absorbance and its efficacy promoting P:
1 is the average of 27 herb plants and 2 the average of 7 trees.
Studing both arises some conclusions:
-Maximun photosyntetic efficacy is achieved by red photons. And differences between different red wl are small. The max is at 670nm, but all the range between 610 and 670nm gets very good efficacies. There is a sharp drop in efficacy at 685nm, so is important to try not use LEDs emiting part of the light there.
-Blue and green photons efficacy is similar.
-There is a drop at the end of the blue range, where is the minimun efficacy, and not in the green. 470nm blue leds are the worst, but Royal Blue leds emiting at 450nm od slighty less are very good.
-Use of photons almost all the PAR range is similar. The curves are pretty flat, with the minimun being near 65% of the max. Enhancement due to optimized spectrum maybe nice, but it may be up to an 25% against white sources in best cases (25% of the max: if we take the lower efficacy spectrum as baseline, as that of HPSs, it may be up to 50%. But in the practice is going to be very difficult get enhancements over 20%). Claims of LED sellers of 8x (800%) or higher enhancements are completely off base, as many people has checked, unfortunately.
These curves have a severe limitation: dont take into account possible
synergies between differents wl. a famous synergy is known as the "Emerson effect": if a plant is irradiated with 660 and 700nm light at same time, P produced is higher than if its irradiated with them separately. Its possible there are more synergies like that we still dont know. However, there have been studies that has calculated the P of white light by adding the P promoted by each wl showing than measured results are between a 7% error margin of calculated values using the McCree curve. Its a decent error margin wich confirms the validity of the method.
But things are more complex, due photosynthetic systems of plants are very adaptable, and they change to use the light quality they are receiving the best. Plants grown under HPS may have up to double chlorophill b than those grown using sunlight, in order to use better the yellow light of them. So we always may expect that after plants acclimatation to a given light quality (after about a week under it), P promoted is going to be higher than calculated. This effect still reduce further the improvement margin by spectrum optimization.
Another effect to keep in mind is that most plants have shown better results when exposed to wide spectrums than to a nearly monochromatic ones. There are exceptions, as wheat, so we must check it with cannabis, but my personal results point toward cannabis liking wl along all the PAR range, still being a little demanding plant specie in terms of light quality. The main task of LED grow lights researchers is to find the best wl distributions for cannabis, wich is still unknown.
In order to show the importance of this task, i strongly recomend to read
OPTIMIZATION OF LAMP SPECTRUM FOR VEGETABLE GROWTH. It shows how tomato, wich is known to be a very little demanding specie about spectrum (as cannabis), still produce more when is grown under decent amounts of blue and green light (for the same energy used), but it produces more when using higher red proportion than with cucumbers, wich need way more green to perform fine.
Other article of that page shows how plants adapted to lighting of differents colors perform different than those grown under sunlight. It shows too how different quality of lights get saturated at differents irradiances:
Wich point out that higher content of blue light may worth when using high irradiances. It aswell shows how perfomance under different light qualities is affected by leaves age:
All the articles on the same page worth the reading:
International Lighting in Controlled Environments Workshop. Some of the coments about leds are clearly obsolete (its from 1994), but most of the lighting concepts are completely valid and very accurate.