PPFD, short for photosynthetic photon flux density, is the amount of plant-usable light landing on a surface each second. It is measured in micromoles of photons per square meter per second (µmol/m²/s), and it is the most useful way to describe how intense a grow light is at the height of your plants.
PAR, PPF and PPFD: three terms, three jobs
Grow light specifications use three similar-looking abbreviations. Each answers a different question.
- PAR stands for photosynthetically active radiation. It names the range of light plants use for photosynthesis, roughly 400 to 700 nanometers. It is a range, not an amount.
- PPF is photosynthetic photon flux. It is the total amount of PAR a light fixture gives off every second, in micromoles per second (µmol/s). It describes the light, not where the light goes.
- PPFD is the density of that light: how many photons actually land on each square meter every second (µmol/m²/s). It describes what your plants receive.
A simple picture: PPF is the water coming out of a shower head, and PPFD is how much water lands on each square inch of your skin. The same shower can feel very different depending on how far away you stand.
Why PPFD matters more than watts or lumens
Watts measure electricity, not light
Two lights with the same wattage can produce very different amounts of plant light, depending on the efficiency of their diodes and drivers. See LED vs HID grow lights for how efficiency varies.
Lumens and lux measure brightness to people
Lumens and lux are weighted to the human eye, which is most sensitive to green and yellow light. They undercount the red and blue light that plants use heavily, and they overcount green. A light can look bright to you and still deliver little useful light to a plant. Quantum sensors, which count photons in the 400 to 700 nanometer range, avoid that problem.
What do PPFD numbers look like?
To give a sense of scale, bright full summer sun delivers around 2,000 µmol/m²/s, while a typical indoor windowsill in winter gets a small fraction of that. Indoor crops are grown at levels far below full sun. Commonly used ranges, which vary between sources, are:
| Plant stage or crop | Typical PPFD at the canopy (µmol/m²/s) |
|---|---|
| Seedlings and cuttings | About 200 to 400, often less at first |
| Lettuce and leafy greens | About 150 to 300 |
| Herbs | About 200 to 400 |
| Vegetative growth of fruiting plants | About 400 to 600 |
| Flowering and fruiting plants | About 600 to 900 |
These are starting points. A review of lettuce research found 150 to 300 µmol/m²/s optimal under artificial light, and one study found fresh weight peaked near 200 µmol/m²/s with 16 hours of light, then fell when the daily dose was increased too far. More is not always better. Excess light can stress plants and waste electricity.
PPFD changes with distance and position
PPFD is not one number for a light. It changes with the height of the fixture and with the position under it:
- Height. The closer the light, the higher the PPFD. Raising a light spreads the same photons over a wider area. See how far to hang LED grow lights from plants.
- Position. Light is strongest under the center of a fixture and weaker at the edges, so a PPFD map shows a grid of readings, not a single figure.
- Dimming. Turning a light down lowers PPFD at every point.
When a maker publishes a PPFD map, check the stated height, the area it covers and the average and lowest readings. A light that reads 900 in the middle and 300 at the edges is much less even than one reading 600 across the whole area.
How to measure PPFD
- A quantum sensor or PAR meter. The accurate way. Place the sensor at leaf height and read the value in several spots.
- A phone app. Apps use the phone's camera or light sensor and estimate PPFD from lux. They are only rough guides and can be far off with colored or narrow-spectrum lights.
- The maker's map. Use it as a starting point, then check your plants.
From PPFD to DLI
PPFD is a snapshot. To know how much light a plant gets in a day, combine it with the hours the light is on. That total is called the daily light integral. The formula is simple: DLI = PPFD × hours of light × 3,600 ÷ 1,000,000. At 200 µmol/m²/s for 16 hours the DLI is 11.5 mol/m²/day. Read daily light integral explained.
PPFD, lumens and lux compared
| Unit | What it measures | Useful for plants? |
|---|---|---|
| Watts (W) | Electrical power the light draws | Only to work out running cost |
| Lumens and lux | Brightness as the human eye sees it | Poorly; they overweight green and underweight red and blue |
| PPF (µmol/s) | Total plant light a fixture produces | Yes, for comparing fixtures |
| PPFD (µmol/m²/s) | Plant light landing on each square meter per second | Yes, the best measure at the plants |
| DLI (mol/m²/day) | Plant light received over a whole day | Yes, the best measure for a crop |
Reading a PPFD map
A PPFD map is a grid of readings taken at one height under the light. Here is an invented example, for illustration only, of a map for a square growing area at 12 inches:
| Left | Center | Right | |
|---|---|---|---|
| Back | 420 | 560 | 430 |
| Middle | 540 | 720 | 550 |
| Front | 410 | 540 | 420 |
The center reads 720 and the corners about 420, so this light is strongest in the middle. The average is about 510, and the lowest is 410. For a crop that wants 400 to 600, plants anywhere in this area are fine. For a crop that wants 600 to 900, only the center is good, and the corners would be too dim. Use the average and the lowest reading, not the peak, to judge a light.
Questions about PPFD
Is a higher PPFD always better?
No. Each crop can use only so much light. Beyond that point extra intensity adds heat and stress and wastes electricity. Match PPFD to the crop and the stage.
How does a window compare?
A sunny summer window can deliver many hundreds of µmol/m²/s near the glass, but the figure drops steeply a few feet back and in winter. That is why a grow light gives steadier results than a window.
Can I tell PPFD by looking at the light?
No. Our eyes are poor at judging plant light. A light that looks dazzling may have low PPFD for plants, and a light that looks pink may have high PPFD. Use a sensor or the maker's map.
Key takeaways
- PPFD is the plant-usable light landing on a surface per second, in µmol/m²/s.
- PAR is the 400 to 700 nanometer range, PPF is the fixture's total output and PPFD is what your plants receive.
- Lumens and lux describe brightness to people, not light for plants.
- PPFD changes with height, position under the light and the dimmer setting.
- Use the map's average and lowest readings, and watch the plants.
A quick way to use PPFD
- Choose a target PPFD for your crop and stage from the table above.
- Check the light's PPFD map at your planned height.
- Hang the light, then adjust the height or the dimmer until the plants sit in the target range.
- Watch the plants. Stretching means too little light, and bleached or curling leaves close to the light mean too much.
My LED Grow Lights have a dimmer switch, so you can set the intensity to the target instead of moving the fixture each time. See all of our LED grow lights, or go back to how plants use light.
Related reading
- Daily Light Integral (DLI) Explained
- How Many Hours of Light Do Plants Need?
- Back to How Plants Use Light