Toolshop/Guides/Light

Light

PPFD, DLI, and the light spectrum

PPFD is how much usable light lands on a leaf at one moment. DLI is how much lands over a whole day. Plants grow on the daily total, so most lighting decisions come down to a trade between brighter light and longer days. The labs below use the same formula as the PPFD and DLI Calculator.

Updated October 2026 5 min read By Connor Schmitz

PAR and PPFD

Plants use light between 400 and 700 nanometers for photosynthesis. That band is called PAR, photosynthetically active radiation. What drives photosynthesis is the number of photons in it, so plant light is measured by counting them.

PPFD (photosynthetic photon flux density) is the count landing on a square meter each second, in micromoles per square meter per second (µmol/m²/s). Midday summer sun is about 2,000. A bright windowsill is often 100 to 300, and most of a room is under 50.

PPF µmol/s Total photons a fixture puts out each second. A spec-sheet number for the light, regardless of where the light lands.
PPFD µmol/m²/s Photons arriving on one square meter of canopy each second. Depends on the fixture, the hanging height and the walls. This is what a quantum sensor measures.
Efficacy µmol/J Photons per watt-second of electricity. Current LED fixtures run about 2.5 to 3; older HPS fixtures about 1.7.
Lux, foot-candles for eyes Weighted to human vision, which peaks in green. The conversion to PPFD changes with the spectrum: about 54 lux per µmol/m²/s in sunlight, and different for every lamp.

DLI: the daily total

DLI (daily light integral) is every photon that landed during the day, in moles per square meter per day. For a light that holds steady, it is PPFD multiplied by the seconds it is on:

DLI = PPFD × hours × 3,600 ÷ 1,000,000

Try it

Set the light and the day length

DLI, mol/m²/day23.0

Inside the band for vegetative growth.

Low-light houseplants 2 to 6
Most houseplants 6 to 12
Seedlings and cuttings 8 to 15
Lettuce and leafy greens 12 to 17
Vegetative growth 18 to 30
Flowering and fruiting 30 to 45
Scale 01020304050 mol/m²/d
Indoor target bands, the same ones the PPFD and DLI Calculator grades against. Lettuce stops at 17 because more light than that brings on tipburn.

Outdoors, a clear summer day delivers 40 to 60 mol/m²/day across most of the US. A northern winter day can be under 10. A greenhouse loses a third to half of that to glazing, structure and dust, which is why winter greenhouse crops so often need supplemental light.

Quick check

Which delivers more light in a day: 600 µmol/m²/s for 12 hours, or 400 for 18 hours?

Answer: The same. 600 × 12 and 400 × 18 both come to 7,200 µmol-hours, a DLI of 25.9. The plant gets the same daily total either way. Try both in the lab above.

Intensity or hours

Since DLI is intensity times time, the same total can come from a strong light for a short day or a weaker light for a long one. The longer, dimmer day is usually the better deal:

  • Leaves use dim light more efficiently. Photosynthesis climbs steeply at low PPFD and levels off as light gets stronger. Each extra photon at 1,200 µmol/m²/s adds less growth than the same photon at 300.
  • Fewer fixtures. Stretching from 12 to 18 hours reaches the same DLI with two-thirds the PPFD, and so two-thirds the fixtures.

Day length has limits of its own, though:

  • Flowering triggers. Short-day plants such as chrysanthemum and poinsettia flower only when nights are long enough, and long-day plants such as spinach bolt as days lengthen. Day length on these crops is a flowering decision first and a DLI decision second.
  • Continuous light. Tomatoes, peppers and some other crops develop yellowed, mottled leaves under 24-hour light. Most crops are lit 16 to 18 hours at most.
  • Heat at close range. Hanging a light lower raises PPFD fast and leaf temperature with it. Leaf temperature affects transpiration and VPD as well as growth.

Quick check

A grower wants poinsettias to bloom for December, and runs the lights 18 hours a day at lower PPFD to save on fixtures. What happens?

Answer: Poinsettias are short-day plants. They color up only after weeks of long, unbroken nights, so an 18-hour day keeps them leafy and green.

PPFD and DLI Calculator PPFD and DLI from fixture output and grow area, graded against the target band for the crop and growth stage.
Open the calculator

Spectrum

PAR, 400 to 700 nm Far-red
Wavelength in nanometers. PPFD counts every photon from 400 to 700 equally. Far-red, just past 700, is left out of PAR but still drives photosynthesis alongside shorter wavelengths.
Blue 400 to 500 Keeps plants compact, with thicker leaves and shorter stems. Opens stomata. A little goes a long way: most crops look normal with 10 to 20% blue.
Green 500 to 600 Less of it is absorbed by the top leaf, and more passes through to the leaves below, so it works deeper in a dense canopy. Also what makes a plant look normal to the eye.
Red 600 to 700 Drives photosynthesis most efficiently per photon, and red LEDs are the most efficient to make, so most grow lights are heavy in red.
Far-red 700 to 750 Signals shade to the plant, so stems and leaves stretch. Adds to photosynthesis when mixed with PAR. Some references count 400 to 750 as ePAR to include it.

For most crops, the number of photons decides how much the plant grows, and the spectrum mostly decides its shape, such as how compact it grows, how thick the leaves are and what color they turn. A white LED and a purple one at the same PPFD grow about the same amount of plant. The white one is easier to work under, and makes pests, deficiencies and disease easier to spot.

One measuring note: older quantum sensors under-read some LED spectra, especially deep red. Newer full-spectrum sensors fix that. Phone light meter apps measure lux, and their conversion to PPFD is only as good as their guess at the lamp's spectrum.

Quick check

Two fixtures each deliver 500 µmol/m²/s at the canopy. One looks purple, one looks white. Which grows more lettuce?

Answer: About the same. Plants use green light too, and growth follows the photon count. The white fixture looks brighter only because human eyes are most sensitive to green.

Summary

  • PPFD is light at one moment. DLI is the day's total, and plants grow on the total.
  • DLI = PPFD × hours × 0.0036. Halving the hours takes double the PPFD for the same DLI.
  • Longer days at lower PPFD use light more efficiently, up to 16 to 18 hours, unless day length controls flowering.
  • Lux is for eyes. Measure plant light with a quantum sensor.
  • Photon count sets growth; spectrum mostly sets shape.

Keep reading

Nutrition

EC, ppm, and the 500 and 700 scales

The same bucket reads 1,000, 1,280 or 1,400 ppm depending on the meter. Which scale a meter uses, how to convert, and what temperature does to a reading.

5 min read Lawn

Fertilizing a lawn

Pounds of nitrogen per 1,000 sq ft is the number that matters. What the bag says, how much to spread, when, and how to check the spreader.

5 min read Index

All Bizon guides

Everything published so far, and what is being written next.

Browse the guides