Photosynthetically active radiation
Photons at shorter wavelengths tend to be so energetic that they can be damaging to cells and tissues, but are mostly filtered out by the ozone layer in the stratosphere.
Other living organisms, such as cyanobacteria, purple bacteria, and heliobacteria, can exploit solar light in slightly extended spectral regions, such as the near-infrared.
These bacteria live in environments such as the bottom of stagnant ponds, sediment and ocean depths.
Chlorophyll, the most abundant plant pigment, is most efficient in capturing red and blue light.
[1][2] Because green and yellow wavelengths can transmit through chlorophyll and the entire leaf itself, they play a crucial role in growth beneath the plant canopy.
In these contexts, the reason PAR is preferred over other lighting metrics such as luminous flux and illuminance is that these measures are based on human perception of brightness, which is strongly green biased and does not accurately describe the quantity of light usable for photosynthesis.
When measuring the irradiance of PAR, values are expressed using units of energy (W/m2), which is relevant in energy-balance considerations for photosynthetic organisms.
In relation to plant growth and morphology, it is better to characterise the light availability for plants by means of the Daily Light Integral (DLI), which is the daily flux of photons per ground area, and includes both diurnal variation as well as variation in day length.
It has been noted that there is considerable misunderstanding over the effect of light quality on plant growth.
Many manufacturers claim significantly increased plant growth due to light quality (high YPF).
[9][10] But the YPF curve was developed from short-term measurements made on single leaves in low light.
Blue light, while not delivering as many photons per joule, encourages leaf growth and affects other outcomes.
The following table shows the conversion factors from watts for black-body spectra that are truncated to the range 400–700 nm.
It also shows the luminous efficacy for these light sources and the fraction of a real black-body radiator that is emitted as PAR.
For example, a light source of 1000 lm at a color temperature of 5800 K would emit approximately 1000/265 = 3.8 W of PAR, which is equivalent to 3.8 × 4.56 = 17.3 μmol/s.
For artificial light sources, that usually do not have a black-body spectrum, these conversion factors are only approximate.
Radiation reaching a plant contains entropy as well as energy, and combining those two concepts the exergy can be determined.
Thus, as a consequence of the entropy content, not all the radiation reaching the Earth's surface is "useful" to produce work.
Therefore, the conversion factor of the organism will be different depending on its temperature, and the exergy concept is more suitable than the energy one.
Researchers at Utah State University compared measurements for PPF and YPF using different types of equipment.
Both YPF and PPF sensors were very inaccurate (>18% error) when used to measure light from red-light-emitting diodes.
[8] Photobiologically Active Radiation (PBAR) is a range of light energy beyond and including PAR.
