Photosynthetically active radiation, often abbreviated PAR, designates the spectral range (wave band) of solar radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. This spectral region corresponds more or less with the range of light visible to the human eye. 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. Photons at longer wavelengths do not carry enough energy to allow photosynthesis to take place.

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. Because of their pigments, they form colorful mats of green, red and purple.

Typical PAR action spectrum, shown beside absorption spectra for chlorophyll-A, chlorophyll-B, and carotenoids

Chlorophyll, the most abundant plant pigment, is most efficient in capturing red and blue light. Accessory pigments such as carotenes and xanthophylls harvest some green light and pass it on to the photosynthetic process, but enough of the green wavelengths are reflected to give leaves their characteristic color. An exception to the predominance of chlorophyll is autumn, when chlorophyll is degraded (because it contains N and Mg) but the accessory pigments are not (because they only contain C, H and O) and remain in the leaf producing red, yellow and orange leaves.

PAR measurement is used in agriculture, forestry and oceanography. One of the requirements for productive farmland is adequate PAR, so PAR is used to evaluate agricultural investment potential. PAR sensors stationed at various levels of the forest canopy measure the pattern of PAR availability and utilization. Photosynthetic rate and related parameters can be measured non-destructively using a photosynthesis system, and these instruments measure PAR and sometimes control PAR at set intensities. PAR measurements are also used to calculate the euphotic depth in the ocean.

## Units

The irradiance of PAR can be measured in energy units (W/m2), which is relevant in energy-balance considerations for photosynthetic organisms.[1]

However, photosynthesis is a quantum process and the chemical reactions of photosynthesis are more dependent on the number of photons than the energy contained in the photons. Therefore plant biologists often quantify PAR using the number of photons in the 400-700 nm range received by a surface for a specified amount of time, or the Photosynthetic Photon Flux Density (PPFD).[1] This is normally measured using mol m−2s−1. PPFD used to sometimes be expressed as einstein units, i.e., µE m−2s−1, although this usage is nonstandard and no longer used.[2]

The conversion between energy-based PAR and photon-based PAR depends on the spectrum of the light source (see Photosynthetic efficiency). 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.

T
(K)
η_v
(lm/W*)
η_photon
(µmol/J* or µmol s−1W*−1)
η_photon
(mol day−1 W*−1)
η_PAR
(W*/W)
3000 (warm white) 269 4.98 0.43 0.0809
4000 277 4.78 0.413 0.208
5800 (daylight) 265 4.56 0.394 0.368
Note: W* and J* indicates PAR watts and PAR joules (400–700 nm).

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 a black-body light source at 5800 K, such as the sun is approximately, a fraction 0.368 of its total emitted radiation is emitted as PAR. For artificial light sources, that usually do not have a black-body spectrum, these conversion factors are only approximate.

The quantities in the table are calculated as

${\displaystyle \eta _{v}(T)={\frac {\int _{\lambda _{1}}^{\lambda _{2}}B(\lambda ,T)\,683\mathrm {~[lm/W]} \,y(\lambda )\,d\lambda }{\int _{\lambda _{1}}^{\lambda _{2}}B(\lambda ,T)\,d\lambda }},}$
${\displaystyle \eta _{\mathrm {photon} }(T)={\frac {\int _{\lambda _{1}}^{\lambda _{2}}B(\lambda ,T)\,{\frac {\lambda }{hcN_{A}}}\,d\lambda }{\int _{\lambda _{1}}^{\lambda _{2}}B(\lambda ,T)\,d\lambda }},}$
${\displaystyle \eta _{\mathrm {PAR} }(T)={\frac {\int _{\lambda _{1}}^{\lambda _{2}}B(\lambda ,T)\,d\lambda }{\int _{0}^{\infty }B(\lambda ,T)\,d\lambda }},}$

where ${\displaystyle B(\lambda ,T)}$ is the black-body spectrum according to Planck's law, ${\displaystyle y}$ is the standard luminosity function, ${\displaystyle \lambda _{1},\lambda _{2}}$ represent the wavelength range (400 700 nm) of PAR, and ${\displaystyle N_{A}}$ is the Avogadro constant.

## Yield photon flux

Weighting factor for photosynthesis. The photon-weighted curve is for converting PPFD to YPF; the energy-weighted curve is for weighting PAR expressed in watts or joules.

PAR as described above does not distinguish between different wavelengths between 400 and 700 nm, and assumes that wavelengths outside this range have zero photosynthetic action. If the exact spectrum of the light is known, the photosynthetic photon flux density (PPFD) values in μmol/s can be modified by applying different weighting factor to different wavelengths. This results in a quantity called the yield photon flux (YPF).[3] The red curve in the graph shows that photons around 610 nm (orange-red) have the highest amount of photosynthesis per photon. However, because short-wavelength photons carry more energy per photon, the maximum amount of photosynthesis per incident unit of energy is at a longer wavelength, around 650 nm (deep red).

It has been noted that there is considerable misunderstanding over the effect of light quality on plant growth and many manufacturers claim significantly increased plant growth due to light quality (spectral distribution or the ratio of the colors).[4] A widely used estimate of the effect of light quality on photosynthesis comes from the Yield Photon Flux (YPF) curve, which indicates that orange and red photons between 600 to 630 nm can result in 20 to 30% more photosynthesis than blue or cyan photons between 400 and 540 nm.[5]

The YPF curve was developed from short-term measurements made on single leaves in low light. Some longer-term studies with whole plants in higher light indicate that light quality may have a smaller effect on plant growth rate than light quantity.[6]