This is a very big topic.
To continue from Lindsey:
-Instantaneous light level
Full sunlight on a cloudless day at noon is about 2000 umol photons m-2 s-1. That level is higher than the level to saturate photosynthesis for most leaves. Most leaves, even leaves on plants in high sunlight, are light saturated by around 500 umol photons m-2 s-1.
A greenhouse would cut this by about 50%, so maximum level in a greenhouse is likely around 1000 umol photons m-2 s-1, maybe a bit more with careful cleaning, materials and design. 1000 umol photons m-2 s-1 is still more than enough to saturate leaf photosynthesis for most species.
But leaves do not lie flat on the ground, they have an orientation and are usually in a canopy of other leaves that lowers the instantaneous light.
(Also, humans have a logarithmic response to light; typical room light is ~40 umol photons m-2 s-1, 1/50 the level of full sunlight. So it is tricky for us to gauge light levels.)
Exposing a leaf to light above its saturation level often leads to photoinhibition; a decline in photosynthesis or growth rate caused by excess light, which has toxic effects. In most cases it is not the absolute level of light, it is whether light is above or below saturation. The saturation level varies with species, developmental history, temperature, nutrient status and prior light acclimation; most leaves have some capacity to acclimate over time to changing light.
And, saturation of leaf level photosynthesis is not the same as light saturation of growth, nor of production of the desired crop. Growth usually light saturates at levels below leave light saturation.
The length of the light exposure in the day. Plants have many photoreceptors aside from the photosynthetic system, and some are sensitive to very low levels of specific light. Some plants are able to exploit long photoperiods (tomatoes in Alaska etc.); others are not, and some respond to changing photoperiod to complete their developmental cycle.
Depending upon species light acclimation can flatten the response of growth to photoperiod somewhat; long low light photoperiods can sometimes drive growth at a rate similar to higher light shorter photoperiods.
The colour of the light has regulatory effects upon the photoreceptors, with different plants responding differently. Some wavelengths (UVB) are strongly inhibitory.
Total photons m-2 in a given period. This can be a factor; it is the product of instantaneous light and photoperiod (actually the integral of instantaneous light over photoperiod). Think of a laser flash that delivered a whole days light in a ms; the plant would be incinerated. Then think of a very dim light for a very long time; the plant is unable to grow at all because the light level is insufficient to drive net photosynthesis, so no matter how much total light is delivered slowly there is no growth. The useful photon dose also interacts with nutrient status, temperature, presence of mycorhizae, species, developmental history etc.
-UVB & UVA effects are complicated, dose and species specific.
sunnily yours, Doug Campbell (Canada Research Chair in Phytoplankton Ecophysiology, Mount Allison University)