PAR stands for photosynthetically active radiation, the specific range of light wavelengths that plants use to power photosynthesis. It spans from 400 to 700 nanometers, covering roughly the same visible spectrum you see as violet through deep red. Light outside this range can’t be absorbed and converted into energy for growth, which is why PAR is the standard measure for evaluating any light source used to grow plants.
Why PAR Matters More Than Brightness
The most common mistake people make when choosing grow lights is relying on lumens or lux, which measure how bright a light appears to the human eye. Your eyes are most sensitive to green, yellow, and orange wavelengths, so a lamp heavy in those colors looks very bright and scores high in lumens. But plants barely use green light. They rely heavily on red and blue wavelengths, which look dimmer to you but drive photosynthesis.
A light with impressive lumen ratings might illuminate a room beautifully while doing almost nothing for your plants. PAR flips the perspective: instead of measuring light the way human eyes perceive it, it measures the photons plants actually absorb and use. That’s why any serious discussion of indoor growing starts with PAR, not wattage or brightness.
How PAR Is Measured
PAR itself describes a range of wavelengths, not a specific number you can read off a meter. To quantify how much useful light your plants are actually receiving, growers use two related metrics: PPFD and DLI.
PPFD: Light Intensity at the Canopy
PPFD (photosynthetic photon flux density) measures how many PAR photons land on a given area each second, expressed in micromoles per square meter per second (μmol/m²/s). Think of it as a speedometer for light: it tells you the rate of useful photons hitting your plants right now. A reading of 200 μmol/m²/s means your canopy is receiving a moderate stream of usable light, while 800 μmol/m²/s is an intense flood suitable for high-light crops in full production.
DLI: Total Light Over a Full Day
Daily light integral (DLI) takes PPFD and multiplies it by the number of hours your lights run, giving you the total dose of PAR photons your plants receive in 24 hours. It’s measured in moles per square meter per day. If PPFD is the speedometer, DLI is the odometer. You calculate it by multiplying your PPFD by the number of seconds your lights are on (hours × 3,600), then converting to moles.
DLI is especially useful because two very different setups can hit the same target. A lower-intensity light running 16 hours can deliver the same DLI as a high-intensity light running 10 hours. Matching DLI to your crop’s needs gives you flexibility in how you schedule your lighting.
How Much PAR Different Plants Need
Light requirements vary dramatically by species and growth stage. Seedlings in their first week need only 25 to 50 μmol/m²/s, and pushing beyond that can cause stress. Over the following weeks, you gradually increase intensity: around 100 μmol/m²/s in week two, 150 in week three, and 200 to 250 by the time you’re ready to transplant.
Once plants are established, their needs diverge:
- Leafy greens (lettuce, spinach): 75 to 150 μmol/m²/s, with a DLI target of 12 to 20 mol/m²/day depending on the species
- Herbs (basil, cilantro, parsley): 100 to 175 μmol/m²/s, with DLI targets of 10 to 25 mol/m²/day
- Fruiting crops (tomatoes, cucumbers, zucchini): 100 to 200 μmol/m²/s at the seedling stage, scaling up significantly, with DLI targets of 20 to 30 mol/m²/day at maturity
Too little light produces leggy, pale plants that stretch toward the source. Too much causes bleaching, leaf curl, or heat stress. Starting low and increasing gradually over several weeks lets seedlings acclimate without damage.
Not All Colors in PAR Work Equally
The classic assumption has been that red and blue light are the most important wavelengths for photosynthesis. Most LED grow lights are designed around peaks at roughly 450 nm (blue) and 660 nm (red) for exactly this reason. But recent research complicates that picture.
Updated studies on the original McCree curve, which mapped how efficiently each wavelength drives photosynthesis, found some surprising results. In tomato plants, the lowest quantum yields were actually at 450 nm and 660 nm, the two wavelengths most grow lights emphasize. Green and amber wavelengths in the middle of the spectrum produced higher quantum yields than expected. Blue light was also found to be more efficient at driving photosynthesis than red light, contradicting the long-held assumption that red photons are the primary workhorses.
This doesn’t mean red and blue LEDs are useless. It means a broader spectrum that includes green and amber wavelengths may produce better results than a narrow red-blue combination. Plants evolved under full-spectrum sunlight, and their photosynthetic machinery reflects that.
Far-Red Light and Expanded PAR
The traditional 400 to 700 nm boundary of PAR was established in the early 1970s, but scientists have been pushing to expand it. Far-red light (700 to 750 nm), which sits just beyond the visible red end of the spectrum, makes up about 16% of photons in the expanded 400 to 750 nm range. Research from Utah State University showed that far-red photons can drive photosynthesis with efficiency similar to traditional PAR wavelengths, but only when delivered alongside shorter-wavelength light. On their own, far-red photons aren’t very productive, but combined with blue, green, or red light, they become nearly as effective.
Far-red light also influences plant shape and growth speed. It can accelerate crop growth through both direct photosynthetic contribution and morphological changes like increased leaf expansion, which helps the plant capture more light overall. Some growers and researchers now use the term ePAR (extended PAR) to describe the 400 to 750 nm range, though the traditional 400 to 700 nm definition remains the industry standard for now.
Measuring PAR Accurately
Standard light meters and smartphone apps measure lux or lumens, which are weighted toward human vision and unreliable for plant lighting. To measure PPFD, you need a quantum sensor, a device designed to respond equally to all photons across the 400 to 700 nm range regardless of color.
Not all quantum sensors are equal. Cheaper sensors built around gallium arsenide phosphide chips significantly undercount blue photons below 500 nm and have poor sensitivity to red photons above 650 nm. This means they can give misleadingly low readings under LED grow lights that peak in blue and red. Research-grade sensors like those from LI-COR maintain a flat response across the entire PAR spectrum with sharp cutoffs at 400 and 700 nm, giving accurate readings regardless of the light source’s color mix.
For hobby growers, mid-range quantum meters in the $100 to $300 range offer reasonable accuracy. If you’re using a lux meter because it’s what you have, understand that the readings will overvalue green-heavy light and undervalue the red and blue photons your plants actually use. The numbers won’t translate meaningfully to PPFD without knowing the exact spectrum of your light source.