Intensity times time
The light plants use for photosynthesis is called photosynthetically active radiation, PAR. Its intensity is given in micromoles per square meter per second (µmol/m²/s), often listed as PPFD, photosynthetic photon flux density. That is a rate, like miles per hour. Purdue Extension describes the daily light integral as the amount of PAR received each day as a function of light intensity and duration, measured in moles per square meter per day (mol/m²/day). It is the distance traveled, not the speed.
The 3,600 turns hours into seconds and the 1,000,000 turns micromoles into moles. For a measurement averaged over the full 24 hours, Purdue shortens it to PPFD × 0.0864, since a day has 86,400 seconds.
DLI by light level and hours
Each cell is the DLI in mol/m²/day for a constant PPFD at plant height running for the given hours:
| PPFD | 12 h | 14 h | 16 h | 18 h |
|---|---|---|---|---|
| 100 | 4.3 | 5.0 | 5.8 | 6.5 |
| 200 | 8.6 | 10.1 | 11.5 | 13.0 |
| 300 | 13.0 | 15.1 | 17.3 | 19.4 |
| 400 | 17.3 | 20.2 | 23.0 | 25.9 |
| 600 | 25.9 | 30.2 | 34.6 | 38.9 |
Two things follow. Doubling the hours doubles the DLI just as doubling the intensity does, so a modest light on for longer can match a strong light on for less time. And the PPFD that counts is the one at leaf height, so measure there with a PAR meter rather than relying on a single figure from the box.
Light groups and crop targets
Purdue Extension sorts crops by their DLI requirement into four groups, in mol/m²/day:
| Group | DLI |
|---|---|
| Low light | 3 to 6 |
| Medium light | 6 to 12 |
| High light | 12 to 18 |
| Very high light | Above 18 |
Specific targets from the two sources, in mol/m²/day:
| Crop | Minimum | Good | High quality | Source |
|---|---|---|---|---|
| Tomato (greenhouse) | 10 to 12 | 14 to 20 | 22 and up | Purdue |
| Petunia | 8 to 10 | 12 to 16 | 18 and up | Purdue |
| Shade-intolerant flowering crops, finish stage | 10 to 12 | – | – | Purdue |
| Boston bibb lettuce (one cultivar) | – | 17 | – | Cornell |
Purdue recommends a minimum of 10 to 12 mol/m²/day during the finish stage for many shade-intolerant flowering crops. The Cornell lettuce figure comes with a caveat in the handbook itself: 17 worked best for the one cultivar they grew, some cultivars develop tipburn at about 15, and the 17 target needed fans blowing air down onto the plants. Without that airflow they could not go above 12.
Working backwards to the light you need
Rearranged, the formula tells you the PPFD a light must deliver for a target DLI over a chosen photoperiod:
| Target DLI | 12 h | 14 h | 16 h | 18 h |
|---|---|---|---|---|
| 6 | 139 | 119 | 104 | 93 |
| 12 | 278 | 238 | 208 | 185 |
| 17 | 394 | 337 | 295 | 262 |
| 20 | 463 | 397 | 347 | 309 |
For lettuce at 17 mol/m²/day with 16 hours of light, the light must average 295 µmol/m²/s across the canopy. Stretching to 18 hours lowers that to 262, which a smaller fixture may manage. Check the crop's own growing guidance on day length before you run lights for 18 hours. In Cornell's greenhouse system the lamps only topped up daylight, adding 100 to 200 µmol/m²/s until the daily total reached 17.
How much light the sky gives
Purdue Extension puts outdoor DLI in the contiguous United States at about 5 to 60 mol/m²/day, depending on the month and the place. In northern latitudes, where most propagation happens from December to March, outdoor values run between 5 and 30. A greenhouse cuts that further: Purdue says greenhouse values seldom exceed 25, and shading from glazing, structures and hanging baskets can make them 40 to 70 percent lower, down to an average of 1 to 5 mol/m²/day in some spots.
To make up the gap, greenhouses add lamps. Purdue gives typical supplemental high-pressure sodium or metal halide lighting as 33 to 98 µmol/m²/s. Run for 16 hours, that adds 1.9 to 5.6 mol/m²/day, enough to lift a dull winter greenhouse day from 5 into the 10 to 12 range Purdue recommends for finishing flowering crops.
Cuttings and seedlings
Young plants need less. Purdue cites Michigan State research in which 4 to 11 mol/m²/day during callusing and root development sped up propagation of petunia and New Guinea impatiens cuttings, and studies in which transplant quality of several bedding plant seedlings improved as DLI rose up to 12 mol/m²/day. In the Cornell lettuce system, seedlings were shaded from full sun on the first day after germination and then given the full 17.
Measuring instead of guessing
A single PPFD reading at noon says little about the day. Purdue recommends a quantum sensor that records instantaneous light once every 15 to 60 seconds, kept level and clean, with the readings summed over the day. Under a grow light with no daylight the job is easier: measure PPFD at several points across the canopy at leaf height, average them, and use the formula with the hours on the timer.
If your meter reads foot-candles
Converting foot-candles to plant light depends on the light source. Purdue's factors are 0.20 µmol/m²/s per foot-candle for sunlight and 0.13 for high-pressure sodium lamps. Its worked example: a 24-hour average of 184 foot-candles gives 184 × 0.2 × 0.0864 = 3.2 mol/m²/day under sunlight, and 184 × 0.13 × 0.0864 = 2.1 under HPS lamps. Purdue gives no factor for LED grow lights, so for those use a PAR meter or the manufacturer's PPFD map instead.
Light, cost and humidity
Every extra hour of light adds to the DLI and to the power bill in the same proportion. The grow light cost calculator turns a fixture's watts and daily hours into a monthly cost, so you can compare a stronger light for 12 hours with a weaker one for 16 hours at the same DLI. Light is not the only lever indoors: the plant VPD calculator and the VPD chart cover the balance of temperature and humidity that goes with it.
Frequently asked questions
What is daily light integral?
The total amount of photosynthetically active light a plant receives in one day, in moles of light per square meter per day (mol/m²/day). Purdue Extension describes it as a function of both light intensity and duration, so a dim light on for many hours can deliver the same DLI as a bright light for a few.
How do I calculate DLI from PPFD?
Multiply PPFD in µmol/m²/s by the hours the light is on, then by 3,600 seconds per hour, and divide by 1,000,000. A grow light giving 250 µmol/m²/s for 16 hours delivers 250 × 16 × 3,600 ÷ 1,000,000 = 14.4 mol/m²/day.
What DLI do tomatoes need?
Purdue Extension lists 10 to 12 mol/m²/day as the minimum acceptable for greenhouse tomatoes, 14 to 20 as good and 22 and up for high quality. That puts tomatoes in its very high light group for top results.
What DLI does lettuce need?
The Cornell controlled environment lettuce handbook recommends 17 mol/m²/day for its hydroponic lettuce system, noting that this worked best for the one Boston bibb cultivar it used. It also says the 17 target needs downward airflow to prevent tipburn, and without the airflow they could not go above 12.
How much light does a window give?
Outdoors, Purdue Extension says DLI in the contiguous United States ranges from about 5 to 60 mol/m²/day depending on the month and location. For a spot indoors, measure: Purdue notes that even greenhouse values seldom exceed 25 and that shading can make them 40 to 70 percent lower, and a windowsill varies just as much. A PAR meter gives the real number.
Sources
- Purdue Extension HO-238, Measuring Daily Light Integral in a Greenhouse (definition and units; PAR × 0.0864 for a daily integral; outdoor DLI 5 to 60 in the contiguous US; greenhouse values seldom above 25 and 40 to 70 percent lower with shading; low, medium, high and very high light groups; table of crop DLI requirements including tomato and petunia; foot-candle factors 0.20 for sunlight and 0.13 for HPS; minimum 10 to 12 for finishing shade-intolerant crops; 33 to 98 µmol/m²/s from supplemental HID lamps; propagation and seedling research; sensor readings every 15 to 60 seconds)
- Cornell University CEA Hydroponic Lettuce Handbook (seedlings shaded on the first day after germination; 17 mol/m²/d recommended for its Boston bibb cultivar; supplemental light of 100 to 200 µmol/m²/s; tipburn and downward airflow; 12 mol/m²/d without airflow)