Daily light integral

Daily light integral (DLI) is the amount of PAR received each day as a function of light intensity (instantaneous light: μmol·m⁻²·s⁻¹) and duration (day). It is expressed as moles of light (mol) per square meter (m⁻²) per day (d⁻¹), or: mol·m⁻²·d⁻¹ (moles per day).

The daily light integral (DLI) is a measure of the amount of light received in a single day in a particular area. For example, the number of moles of Photosynthetically Active Radiation (PAR) photons received during a single day in a square meter, or: mol·m⁻²·d⁻¹. Another example is the total solar radiation received during a single day in a square meter, or W·m⁻². Depending on the application or research question the time integral of incident light may be a more useful tool than the instantaneous incident light. DLI varies depending on latitude, time of year, and cloud cover, and ranges from 5-60 mol·m⁻²·d⁻¹ in the United States for PAR. In greenhouses or growth chambers, DLI values are typically much lower, which can affect the shoot/root ratio, morphology, and the timing of flowering.

On a sunny winter day in the middle latitudes, a plant receives about 9 moles/day. If it is cloudy, the DLI drops to 3 moles/day. In the summer, the DLI for a sunny day is about 26 moles/day and 12 moles/day for a cloudy day. Each type of plant has a different DLI range for optimal growth. DLI is directly correlated with plant quality, and a minimum amount of light is required for marketable plants. Measuring DLI over a growing season and comparing it to results can help a grower decide which varieties work for his or her location.

Example calculation:

• DLI (mol/m²/day) = average light intensity (μmol/m²/s) · time (in seconds) / 1.000.000
• DLI (mol/m²/day) = PAR · h · 3600 / 1.000.000

References

• Korczynski et al., 2002
• Bunnelle et al., 2005 and Ilias et al. 2005