What is the Daily Light Integral?

The Daily Light Integral (DLI) is the total amount of PAR received by a plant during a single photoperiod. DLI is expressed in units of moles per meter squared per day (mol m-2 day-1). DLI is simply taking PPFD measurements every second during a 24-hour day and adding them up. The DLI concept is like a rain gauge. Just as a rain gauge collects the total rain in a particular location over a period of time, so DLI measures the total amount of PAR received in a day. Since light intensities change throughout the day in a greenhouse, growers can purchase PPFD loggers that automate the process.

DLI = (PPFD) * (Photoperiod)

Calculating indoor DLI is easy. It’s as simple as taking an average PPFD reading and doing some math. PPFD measurements only need to be done once a day since the light intensity does not typically change over a day. Additionally, there are quite a few DLI calculators available online if math is not your strong point. All you need is your PPFD readings and photoperiod length to use one.

DLI is an important variable to measure because it influences plant growth, development, yield, and quality. 

Use the equation below to calculate the DLI generated by your grow light. This example assumes a grow light that provides a PPFD of 1,400 µmol m-2 sec-1 during a 12-hour photoperiod:

(1,400 µmol/m²·sec)* (60 sec/min)*(60 min/hour)*(12 hour/day)*(1 mol)/(1,000,000  µmol)  =  60.5 mol /m²·day

For reference, the maximum DLI that can be achieved outside in full summer sun is about 65 moles/day. For most crops this level should not be exceeded. Many commercially grown plants have published DLI ranges.