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Introduction
The Daily Light Integral (DLI) is is the amount of Photosynthetically Active Radiation (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 per square meter (m -2 ) per day (d -1 ), or: mol·m -2 ·d -1 (moles per day).
PAR = Photosynthetically Active Radiation
DLI = Daily Light Integral
The DLI measures the total amount of PAR received in a day. The growers and technicians can use a photo-radiometer to measure the number of light photons that accumulate in a square meter over a specific period (24h to calculate the DLI).
Quick explanations: example
A radiation of 1 micromole PAR light means that the plants receives 6,02 x 10^17 photons / m2.sec (1 micromole = 10^-6 mole)
PAR = Solar radiation x PAR % x 4,6 [micromoles/m2.s]
Where:
– Solar radiation = W/m2
– PAR % = it depends on the outside environment: clear sky = 45% ; fully cloudy sky = 60%
– 4,6 = number of micromoles PAR per Joule (for solar radiation)
For example,
A solar radiation of 300 W/m2 under a fully clouded sky
PAR = 300 x 0,6 x 4,6 = 828 micromoles/m2.s
The DLI is an important parameter to monitor and control. Indeed, it influences crops growth, development, yield, and quality. More particularly, in the Northern hemisphere where outdoor DLI values (lower) are between 5 to 50 mol·m -2 ·d -1 but also in some tropical countries with DLI variation during monsoon period and cloudy days / haze (Malaysia, Singapore, Indonesia… for exemple).
How to increase the DLI inside a greenhouse ?
- Select a proper covering type (glass/plastic) with high light diffusion proprieties
- Increase the light reflection inside your greenhouse using the right greenhouse design and option such as posts lacquering (white). A white ground cover can also help
- Make sure your roof covering (glass/plastic) is properly cleaned and not too old (diffuse effect can decreased over a certain time)
- Minimize overhead obstructions such as hanging gantry, heavy greenhouse design, equipment such as circulations fans, high-pressure fogging, irrigation booms…
- Choose permanent or complementary artificial lighting from High Pressure Sodium Lamps (HPS), Light Emitting Diodes (LEDs)…
DLI requirement for different plants
Sources:
Greenhouse Lighting 101 :
https://www.glexpo.com/summaries/2016summaries/GreenhouseVegetable.pdf
Light Management in Greenhouses I. Daily Light Integral: A useful tool for the U.S. Floriculture industry. James E. Faust.
Measuring Daily Light Integral in a Greenhouse ( Ariana P. and al.,):
https://www.extension.purdue.edu/extmedia/ho/ho-238-w.pdf
