Photosynthetic Active Radiation (PAR) designates the spectrum or range of colors of light from 400 to 700 nanometers that plants are able to use for photosynthesis. PAR measurements are usually expressed as photosynthetic photon flux density (PPFD) in units of μmol m^{-2s-1 -- how many micromoles of photons (602,214,150,000,000,000 photons) within the PAR wavelengths of 400nm-700nm that go through 1-square-meter each second, though most devices that measure PAR do it only at a single point, rather than over a whole square meter.}

PAR can also be expressed as a total photon flux (PPF) which counts all photons in the PAR range that leave a light fixture or bulb. This can be misleading depending on how it is actually measured.

There are important caveats, but in general, the higher the PPFD measurement a light has over its entire growing footprint, the better it will grow plants. Too much PAR (too much light) is wasteful and can even damage plants, although this is almost never a problem with artificial grow lighting.

More fundamentally, PAR lighting measurements do not account for the relative usefulness of particular wavelengths to the plant-- leaves' preferential absorption of different spectra (colors) mean some photons are more useful to the plant than others, even within the PAR range. For example, plant leaves reflect much of the green light that hits them-- much of it is not used by the plant, but this is right in the middle of the PAR spectrum at 495nm-570nm. In addition, plants require many spectra to perform well, although a light with a single color can have the same PAR measurement as a multi-spectrum light. Just having a high PAR value doesn't indicate that a light will grow plants well; the spectrum must also be considered.

PAR also assumes that all photons outside of the 400nm-700nm range have no use in photosynthesis or for overall plant health. However, plants do use light outside of PAR, such as far-red light above 700nm which increases photosynthetic efficiency (due to the Emerson effect) and for hormone regulation. UV light (below 400nm) increases canopy penetration and secondary metabolites such as vitamins, terpenes, THC and CBD.

PAR measurements can vary significantly within the lighting footprint of a light, so any single PPFD measurement is uninformative as to how the light will grow plants. For example, a laser can focus all of its light onto a PPFD-measuring instrument and have an incredible PAR value, but this doesn't mean it will grow plants well. Many LED grow lights are sold with secondary lenses which focus the light into a narrow cone. These lenses allow the light to achieve a very high PAR measurement at the center of its footprint, but just off to the sides the PAR falls to a point it can no longer sustain a plant. Companies using secondary lenses often manipulate PPFD measurements to make their light seem powerful even though it doesn't actually grow plants over the entire advertised footprint!

PAR measurements are also highly dependent on the distance they are taken from the light source. The inverse-square law of light means that the photon flux density (what PPFD is measuring) will decrease by the square of the distance from the source-- so if a PAR/PPFD measurement was 100 at 1 inch from the fixture, it will be 25 at 2 inches, and 11.1 at 3 inches. It's easy to claim a high PAR reading for a light fixture if the measurement is taken close to and directly below it. It is therefore critical to know how far from the light PAR measurements were taken, in addition to where in the footprint the measurement was taken.

For these reasons, single PAR measurements and PAR alone should not be used as a measure of how good a grow light is, and can be very misleading when comparing lights. Even multiple PAR measurements over the entire footprint at the recommended height will not indicate how well a light can grow, as the right spectral distribution of light is critical and is not considered by PAR. Only by measuring PAR over the entire footprint of the light, at the recommended hanging distance above the plants, and considering the entire spectrum (including outside of PAR!) can useful comparisons be made.