Daily Light Integral Calculation
Calculate DLI from constant or time-varying photon flux and identify errors caused by units, sampling, and wavelength-band mismatch.
Educational reference · evidence, sources, and limits shown below
Calculate DLI from constant or time-varying photon flux and identify errors caused by units, sampling, and wavelength-band mismatch.
Terms to know
- DLI
- Daily light integral: accumulated photons per square metre per day, expressed as mol m-2 d-1.
- Instantaneous PPFD
- Photon flux density measured at one moment.
- Integration interval
- Time represented by each logged value in a numerical sum.
- Temporal profile
- Change in photon flux through the light period, especially important under sunlight or dimming.
Core science
For a constant PPFD, DLI = PPFD x photoperiod hours x 0.0036. The factor converts micromoles per second and hours to moles per day. For example, 500 micromoles m-2 s-1 for 18 hours equals 32.4 mol m-2 d-1. The calculation must retain whether the input is PPFD or ePPFD.
When light changes with clouds, curtains, sunrise, dimming, or control events, DLI must be integrated from logged readings: sum(PFD_i x interval seconds) / 1,000,000. A midday spot reading multiplied by day length can substantially overstate or understate the true integral.
DLI compresses time into a daily total. Equal DLI can come from different intensities and photoperiods, but plants may respond differently because photosynthesis is nonlinear, signaling depends on timing, heat and water loads differ, and dark-period biology is not preserved by the arithmetic.
Why this matters in cultivation
- DLI is useful for comparing delivered photon dose across days and for coordinating greenhouse supplemental lighting with sunlight.
- A stage plan should record DLI together with photoperiod, PPFD distribution, spectrum, temperature, carbon dioxide, irrigation, and plant response rather than treating DLI as a standalone recipe.
Measure and record
Band and metric
PAR-DLI or ePAR-DLI, instrument model, and calibration status.
Time series
Logging interval, complete timestamps, missing data, daylight-saving handling, and control events.
Geometry
Sensor position, canopy height, representative area, shading, and whether the sensor moved.
Calculation
Formula, units, interval weighting, averaging method, and software version.
Crop context
Stage, cultivar, photoperiod, PPFD map, environment, irrigation, and observed response.
Common misconceptions
Correction: PPFD is a rate at a moment; DLI is the time-integrated daily dose.
Correction: Intensity, duration, spectrum, timing, dark period, and other resources can change the response.
Correction: Variable natural light requires time integration with an appropriately placed sensor.
Evidence limits
Published DLI responses depend on genotype, stage, environment, canopy, and measurement band. A DLI value alone cannot establish a universal cannabis optimum or safety limit.
Related encyclopedia topics
- THC-ENC-101-102, THC-ENC-104-106, THC-ENC-109-111, and THC-ENC-120.
Source notes
- Apogee Instruments (2021). The New 400-750 nm ePAR Range Explained.
- Collado C.E. et al. (2024). Supplemental Greenhouse Lighting Increased the Water Use Efficiency, Growth, and Cutting Production of Cannabis Stock Plants. Frontiers in Plant Science 15:1371702.
- Rodriguez-Morrison V., Llewellyn D., and Zheng Y. (2021). Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment. Frontiers in Plant Science 12:646020.
- THC Cannabis Plant Science Source Packet v1.1 (project source, May 2026).
This lesson summarizes the source material and its evidence limits for education. Use direct measurement, controlled comparison, and the cited sources when conditions differ or a decision carries meaningful risk.