Photosynthetically active radiation, or PAR, is the slice of the light spectrum between 400 and 700 nanometers that plants use to power photosynthesis. This range roughly corresponds to visible light, stretching from violet-blue at 400 nm through green and yellow to deep red at 700 nm. Within this band, plants absorb light energy and use it to convert water and carbon dioxide into sugars and oxygen.
Why 400 to 700 Nanometers?
Sunlight contains a broad spectrum of radiation, from ultraviolet through infrared. Plants evolved to harvest the portion that is both abundant at Earth’s surface and energetic enough to drive the chemical reactions of photosynthesis. Light below 400 nm (ultraviolet) carries so much energy per photon that it can damage cellular structures. Light above 700 nm (far-red and infrared) doesn’t carry enough energy to reliably split water molecules, the key step that kicks off the photosynthetic chain.
The 400 to 700 nm definition was formalized through the work of plant scientist Keith McCree in the early 1970s. McCree measured the photosynthetic response of over 20 crop species across different wavelengths and found that virtually all useful photosynthetic activity fell within this window. His data became the basis for what researchers still call the McCree curve, a graph showing how efficiently plants use each color of light.
Not All Colors Drive Photosynthesis Equally
Although the entire 400 to 700 nm range counts as PAR, plants don’t use every color with the same efficiency. Red light (600 to 700 nm) consistently produces the highest quantum yield, meaning plants fix the most carbon dioxide per photon absorbed in that range. Blue light (400 to 500 nm) is absorbed very strongly by leaves, even more than red, but the absorbed blue photons are used less efficiently in the actual chemistry of photosynthesis. Green light (500 to 600 nm) is the least absorbed, which is why leaves look green to our eyes.
There’s a surprising twist at high light intensities. When light is abundant, the high absorptance of red and blue wavelengths becomes a disadvantage: most of those photons get captured near the upper surface of the leaf, saturating those cells while deeper tissue stays underlit. Green light, because it’s poorly absorbed, penetrates further into the leaf and distributes more evenly. At high intensities, green light can actually match or exceed red light in its net photosynthetic contribution per photon hitting the leaf. This matters in dense canopies and in greenhouses where light levels are pushed high with artificial sources.
Why Lumens and Lux Don’t Work for Plants
A common mistake is using lumens or lux to judge how much useful light a grow lamp provides. Lumens and lux are weighted to human vision, which peaks in sensitivity around 555 nm (yellow-green). A light source can produce a very high lumen rating by emitting mostly yellow-green light, which our eyes perceive as bright but which plants absorb poorly. Meanwhile, deep red and blue LEDs that are highly effective for photosynthesis may score low on a lux meter simply because human eyes aren’t very sensitive to those wavelengths.
PAR measurements solve this by treating every photon between 400 and 700 nm equally, regardless of how bright it looks to us. The relevant metric is photon count, not perceived brightness.
How PAR Is Measured
The standard unit for PAR intensity is photosynthetic photon flux density, or PPFD. It’s expressed in micromoles of photons per square meter per second (µmol/m²/s). One micromole equals roughly 602 trillion photons, so even modest sunlight delivers an enormous number of photons. Full midday sun typically produces a PPFD around 1,800 to 2,000 µmol/m²/s, while an overcast day might drop to 200 or less.
PPFD tells you how much photosynthetic light is hitting a specific point at a specific moment. That’s useful, but plants don’t just care about intensity. They care about total light received over a full day. That’s where the daily light integral, or DLI, comes in. DLI adds up all the photons that land on a square meter over 24 hours, expressed in moles per square meter per day (mol/m²/d).
If you’re using grow lights on a fixed schedule, the math is straightforward: multiply PPFD by 3,600 (seconds in an hour), multiply that by the number of hours the lights run, then divide by 1,000,000 to convert micromoles to moles. A grow light delivering 200 µmol/m²/s for 16 hours produces a DLI of about 11.5 mol/m²/d. Most greenhouse crops thrive somewhere between 10 and 25 mol/m²/d, depending on the species.
PAR in Practice
Growers use PAR data to make real decisions. If a greenhouse crop is receiving a DLI of only 6 mol/m²/d during winter, a grower can calculate exactly how many hours of supplemental lighting at a known PPFD will bring the total up to the target range. Conversely, in summer, PAR sensors can trigger shade curtains to prevent light stress when PPFD climbs too high.
For indoor gardeners choosing LED panels, PAR maps (grids showing PPFD at different points under a fixture) are far more useful than wattage or lumen ratings. Two lights may draw the same power but deliver very different PPFD at canopy level depending on their spectrum, optics, and mounting height. A PAR map shows you exactly where the light is strongest and where the edges fall off, so you can position plants accordingly.
The Case for Extending PAR
The 400 to 700 nm definition has held for over 50 years, but recent research is pushing the boundaries. Far-red light (700 to 750 nm) was traditionally considered outside PAR because it’s too low in energy to drive photosynthesis on its own. However, scientists at Michigan State University and other institutions have shown that far-red photons work synergistically with shorter-wavelength light, boosting the overall rate of photosynthesis beyond what the shorter wavelengths achieve alone. This has led to the concept of extended PAR, or ePAR, which spans 400 to 750 nm.
The practical implication is that grow lights supplemented with far-red LEDs can increase plant growth without proportionally increasing energy costs. Fixture manufacturers have started incorporating far-red diodes, and some PAR sensors now offer an ePAR measurement mode. Whether the formal definition of PAR will officially shift remains an open question, but the science supporting the extension is strong enough that many commercial growers already factor far-red into their lighting strategies.