Irradiance refers to the light falling on a given surface. It measures the light density at a given point. The unit used is uE/m2 (per second). Its often found as PPFD (Photosynthetic Photon Flux Density).
Its the radiometric equivalence to the more known iluminance (
photometric, for humans, as lm) wich is measured on lm/m2=lux. Light concepts preceded by a "i" are refered to the lighted object or surface, and not to the light's source.
The term light's "intensity" is too used for irradiance, but i prefer to avoid using it, due there is other light "intensity" concept wich is refered to the light source, wich is measured in Cd (candles) or W/sr (optic watt per steroradian). It could lead to confusion, thus ill only use the irradiance term, wich not only is clearly refered to the lighted surface, but its too a radiometric concept: radiometric units refers to physical entities (watts, photons) while photometric units refers to how human sense the light (lm, cd), thus they are very misleading when used for plants lighting.
There is a relatively simple way of measuring irradiance with a standard luxometer (light meter) knowing the spectrum of the light: its explained on the
"Bulb Comparision" thread wich is absolutely complementary to this thread.
Irradiance depends strongly on the point where its measured. It drops sharply with the increased distance and its very affected by how the reflector distribute the light. A concept very similar is when we calculate the average light avalaible, by dividing lm by sq meter (or sq ft). If instead of using the photometric lm, we use number of photons and divide it by sq meters where it's distributed, we get an average uE/m2 figure. Irradiance is too given on uE/m2, but it relates to a point with a given position and distance to the lamp while the calculated uE/m2 is just an average of the light throwed to a given space, without taking into account position or distance.
Sorry for the long introduction to the term irradiance, but its a very bad understood concept wich is very important to know how light affect plants, thus i want it very clear. Please ask later any doubt about it: optimizing the lighting of a grow is a task mainly of improving irradiance distribution along the grow room so the better you understand it, the better you will can improve your lighting.
(Im trying to condense on a few post what is often studied along a year. If you dont have previous knowledge about this topic, is very probable you dont understand all on a first read, neither on a second one. But trying to understand it worth, and its not as complex as it seems, just take your time and ask for explanations
Photosynthesis behaves on a typical way depending of the irradiance level al leaves, as this graph shows:
(From
The photosynthesis 'light response curve')
(Read too for more indeep graphs and explanations on
Eutrophication - light and growth)
There is a first part of the curve wich is near linear, meaning that equal increase in irradiance level lead to a equal increase of photosynthesis. The slope of this line determine the maximun photosynthetic rate of the plant. This is called the "light limited" part of the P-E curve (photosynthesis(P) vs irradiance(E)), because what limit the photosynthesis is the amount of light. Along levels of irradiances of this part of the curve (the lowest), more light produces more photosynthesis.
But there is a point where the curve goes flattening, called max Photosynthesis rate point, often noted as A. This is the part of the curve called "CO2 limited", because is internal CO2 concentration in leaves what limit P. Plants is using more CO2 than its able to absorb from the air, thus part of the light cant be used for photosynthesis. The higher the irradiation from this point, the more light is wasted. For cannabis, this point is about 300 uE/m2 at ambient CO2 concentrations: at higher CO2 levels, this point happen at higher irradiances, aswell as the flattening of the curve is less pronounced, because plant is able to keep internal CO2 higher.
Finally, there is a point when further increase of irradiance dont get any increase on P, wich is called the "saturation" point. If we still increase irradiation, plant finally protect itself from damage due to excess light and P decreases. There are differents ways used by plants to do it, but at really high irradiances plants desactivate photosynthetic systems and chorophill is retired from the leaves, producing the effect known as "light bleaching" (because leaves becomes white), wich is irreversible (permanent damage).
There are many practical consecuences of this pattern of the P-E relationship:
-Light use is higher as closer to the max P rate point (A) the irradiance we use, thus higher productivity of light (g/uE).
-The most efficient use of light is when we can distribute the light on a way all leaves works on similar irradiances, close to A. This mean zenital lighting (from top) isnt desiderable ideally, as it produces high irradiances at the top of the plant (thus, wasting light) and low irradiances at the bottom (wasting the ability of those leaves to produce more photosynthesis if they have more light avalaible). Although is desiderable a slighty higher irradiances at top than the bottom, because old leaves are less efficient doing P than new ones, in general we must try as even lighting along all the grow volume (in 3 dimensions) as we can. This is one of the main advantages of LEDs over other ways of lighting: small sources of light may be distributed along the grow, without the requeriment of use light from top, as with HIDs, due to heat. NASA has achieved up to 35% higher yields using same light just by moving part of the LEDs arrays from top to side lighting. With our plant, doing it not only increases light productivity, but allows to harvest fat buds from the bottom part of the plant instead of small ones.
-CO2 enrichment is little useful at low irradiance levels, but very useful as higher the irradiances used.
This article models C3 plants P-E behavior on each part of the curve based on the limitating factors very deep. I only recomend reading it with strong botanic backgroud, its intended as a model for predicting P-E after genetic engieenering.
PS: P-E curve of C4 plants is different due the different way of keeping internal CO2 concentration, but i wont enter on that as Cannabis is a C3 plant