Daily light integral

Units

The daily light integral (DLI) is a function of photosynthetic light intensity and duration (day) and is usually expressed as moles of light (mol) per square meter (m⁻²) per day (d⁻¹), or: mol·m⁻²·d⁻¹. In other words, DLI measures the number of photosynthetically active photons (photons in the PAR range) accumulated in a square meter over the course of a day.

DLI is usually calculated by measuring the Photosynthetic Photon Flux Density (PPFD) in μmol·m⁻²·s⁻¹ (number of photons in the PAR range received in a square meter per second) as it changes throughout the day, and then using that to calculate total estimated number of photons in the PAR range received over a 24-hour period for a specific area. In other words, DLI describes the sum of the per second PPFD measurements during a 24-hour period.

If PPF stays the same for the entire 24-hour period, DLI in mol m⁻² d⁻¹ can be estimated from the instantaneous PPF from the following equation: μmol m⁻² s⁻¹ multiplied by 86,400 (number of seconds in a day) and divided by 10⁶ (number of μmol in a mol).

For example, 1 μmol m⁻² s⁻¹ = 0.0864 mol m⁻² d⁻¹ if light intensity stays the same for the entire 24 hour period.

Optimum daily light integral

Outdoors, DLI values vary depending on latitude, time of year, and cloud cover, and ranges from 5-60 mol·m⁻²·d⁻¹ in the United States. In greenhouses or growth chambers, DLI values are typically much lower, sometimes estimated at about half the outdoor DLI. Therefore DLI measurements in greenhouses rarely exceed 30 mol·m⁻²·d⁻¹.

A lower DLI can affect the shoot/root ratio, morphology, and the timing of flowering. Each type of plant has a different DLI range for optimal growth. DLI is directly correlated with plant quality, and a minimum amount of DLI is required for marketable plants. Measuring DLI over a growing season and comparing it to results can help determine which varieties of plants will thrive in a specific location.