YielixDaily Light Integral (DLI) Calculator: Supplemental Lighting DLI and Lamp Hours
Use this when you are planning greenhouse supplemental lighting and want to know how much light your crop gets per day, or how long the lamps must run to reach your own DLI target. Enter the PPFD your lamps deliver at the canopy and the lamp hours to get supplemental and total DLI in mol/m²/day, or switch to Lighting hours needed to solve the runtime. Natural light means daylight measured inside the greenhouse at the crop, not outdoor DLI.
The Constant-Light Approximation
PPFD is an instantaneous rate: micromoles of photosynthetically active photons landing on one square metre each second. DLI adds that rate up over a day, in moles per square metre. Each hour has 3600 seconds and each mole has 1,000,000 micromoles, so one hour at 1 μmol/m²/s delivers 0.0036 mol/m². The University of New Hampshire Extension run time worksheet uses the same 0.0036 factor.
The calculator treats lamp PPFD as constant for every lamp hour. That suits fixed-output LED or HPS lamps measured at a steady mounting height. Dimmed, staged or daylight-controlled lighting changes PPFD through the day; enter the average PPFD over the lamp hours, or add up each period separately.
Measured Canopy PPFD, Not Fixture Ratings
A lamp datasheet gives PPF in μmol/s, the total photon output of the fixture. The crop receives PPFD at its own height, which depends on fixture spacing, mounting height, reflectors and wall losses, and it is usually lower at bench edges than in the middle. Measure PPFD with a quantum sensor at canopy height, at night or with daylight excluded, at several points, and use the average. Re-measure after raising lamps or as the crop grows toward them.
Natural Light Inside the Greenhouse
Outdoor DLI maps and weather stations describe light above the greenhouse. Glazing, framing, shade or energy screens, dirt and overhead equipment all absorb or block part of it, so the crop gets noticeably less. The calculator only uses natural DLI that you measured inside, at the canopy, and never substitutes an outdoor value. If you leave the field blank, natural light counts as zero and the result covers lamps only.
Lighting is one part of the climate. Running lamps adds heat and changes leaf temperature, so check transpiration demand with the VPD calculator and the night heating load with the greenhouse heating cost calculator. Extra light raises the crop's capacity to use CO₂, which you can cost with the greenhouse CO₂ enrichment calculator, and the greenhouse ROI calculator helps judge whether a lighting investment pays back.
Inputs Explained
- Calculation mode
- Calculate DLI turns lamp PPFD and hours into supplemental and total DLI. Lighting hours needed solves the lamp runtime that closes the gap between your measured natural light and your target.
- Supplemental PPFD at canopy
- Photosynthetic photon flux density from the lamps at plant height, 0 to 3000 μmol/m²/s, measured with a quantum sensor when daylight is excluded (for example at night). Use the average over several points on the bench.
- Lamp hours per day
- How long the lamps run each day, 0 to 24 h. Used in Calculate DLI mode only.
- Natural in-greenhouse DLI (optional)
- Daylight DLI measured at the canopy inside the greenhouse, 0 to 100 mol/m²/day. Outdoor DLI is not used, because glazing, shade screens and structures reduce the light reaching the crop. Leave blank to count lamps only.
- Target DLI
- The daily light goal for your crop and growth stage, from your own source, 0 to 100 mol/m²/day. Optional in Calculate DLI mode and required in Lighting hours needed mode. No default target is applied.
- Maximum lighting window (optional)
- The longest daily lighting period you allow, 0 to 24 h, for example to respect a photoperiod. The calculator warns when the target needs more lamp hours than this window.
How This Calculator Works
Formula
DLI_lamps = PPFD × h × 3600 ÷ 1,000,000 = 0.0036 × PPFD × h- Supplemental DLI in mol/m²/day from canopy PPFD in μmol/m²/s and lamp hours h per day.
DLI_total = DLI_lamps + DLI_natural- Daily total at the canopy. DLI_natural is measured inside the greenhouse and is 0 when left blank.
Deficit = max(0, DLI_target − DLI_natural)- The daily light the lamps must supply, in mol/m²/day.
Hours needed = Deficit ÷ (0.0036 × PPFD)- Lamp runtime in h/day. Only defined when PPFD is above 0; it is 0 when natural light already meets the target.
Source
- Supplemental Lighting Run Time Worksheet (opens in a new tab), University of New Hampshire Extension — uses the 0.0036 conversion from μmol/m²/s × hours to mol/m²/day to work out supplemental lighting run time
Assumptions
- Lamp PPFD at the canopy is constant for every lamp hour; enter an average if the lamps are dimmed or staged.
- PPFD is the average measured at canopy height across the growing area, not a fixture PPF rating or a single centre point.
- Natural DLI is daylight measured inside the greenhouse at the canopy. A blank value is treated as 0 and labelled as lamps only.
- Lamp light and daylight add directly; the calculator does not model crop light-use efficiency or saturation.
- Inputs are limited to PPFD 0 to 3000 μmol/m²/s, 0 to 24 lamp hours and DLI values of 0 to 100 mol/m²/day. Results are rounded only for display.
Worked Example
LED lamps delivering 300 μmol/m²/s at the canopy, with 10 mol/m²/day of measured natural light inside the greenhouse
- 1. Lamp DLI for 14 hours
0.0036 × 300 × 14 = 15.12 mol/m²/day
- 2. Total DLI
15.12 + 10 = 25.12 mol/m²/day
- 3. Gap to a 22 mol/m²/day target
max(0, 22 − 10) = 12 mol/m²/day must come from the lamps
- 4. Lamp hours needed
12 ÷ (0.0036 × 300) = 11.1111 h/day, about 11 h 7 min
14 lamp hours give 15.12 mol/m²/day of supplemental light (25.12 total); reaching 22 mol/m²/day needs about 11.11 lamp hours per day
How to Interpret Your Results
| Condition | What It Means |
|---|---|
| Gap to target is negative | Total daily light is below your target by that amount. Add lamp hours, raise canopy PPFD or improve daylight transmission. |
| Lamp hours needed is 0 | Measured natural light already meets your target, so no supplemental runtime is needed for that day. |
| Target cannot fit the lighting window | The required runtime is longer than your maximum lighting window or 24 h. Running the lamps longer is not an option, so increase PPFD or lower the target. |
| Runtime not available | Supplemental PPFD is 0 while the target is above natural light, so no runtime can close the gap. Enter the PPFD your lamps actually deliver. |
Limitations and Cautions
- Natural DLI changes from day to day with weather and season. A single day's measurement does not describe a whole month, so plan runtime from logged averages and check it again as days lengthen or shorten.
- The result is a daily total. It does not show how light is spread over the day, and some crops respond to photoperiod as well as to DLI.
- No crop or growth-stage DLI targets are built in. Any target you enter comes from your own source.
- Lamp output falls as fixtures age and get dirty, so a PPFD reading taken at installation can overstate later DLI.
- Supplemental lighting also affects heating, humidity and electricity cost; this calculator covers light quantity only.
Common Mistakes to Avoid
Using outdoor DLI as natural light
Greenhouse covers and shade often transmit well under 100 % of outdoor light. Measure DLI inside, at the canopy, or leave natural light blank.
Using the lamp catalogue PPF instead of canopy PPFD
Fixture PPF (μmol/s) is total output; the crop receives PPFD (μmol/m²/s) at its height, which depends on mounting height, spacing and reflection. Measure it with a quantum sensor.
Treating one DLI target as right for every crop
Suitable DLI depends on crop, cultivar and growth stage. Enter a target from a source for your crop.
References
- Supplemental Lighting Run Time Worksheet (opens in a new tab), University of New Hampshire Extension — the 0.0036 factor converting PPFD and hours into DLI, and the supplemental lighting run time calculation
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