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DLI is the amount of light that the plant receives in a 24 hour period. The unit of measurement is mol/m2/day or 'moles per square meter per day'. This usually only counts the top light, and intracanopy or side lighting can greatly increase these numbers. DLI does not take in to account that as the intensity of the light increases in PPFD that the photosynthetic efficiency of the plant decreases.

  
Daily Lighting Integral representation (source)
 

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. Growers can use light meters to measure the number of light photons that accumulate in a square meter over a 24-hour period. DLI is an important variable to measure because it influences plant growth, development, yield, and quality. For example, DLI can influence the root and shoot growth of seedlings and cuttings, plant quality (such as branching, flower number and stem thickness), and timing. Torres, Ariana and Lopez, Roberto. Measuring Daily Light Integral in a Greenhouse. Purdue University

How to calculate DLI

This formula will give the DLI for a 24 hour photoperiod: (PPFD/100) * 8.6. Following this logic you can multiply the result with the percentage of light on time per day: the formula will be ((PPFD/100) * 8.6) * (% hours on per 24 hours). As a reference, there is a Standford study that determined that lettuce does best at around a DLI of 17 or 17 moles of photons per square meter per day.

• Example: 200 µmol/m2/sec on 18 hours per day. (200/100=2) (2 * 8.6=17.2) (17.2 * 0.75=12.9 mol/m2/day)
• Example: 1200 µmol/m2/sec on 12 hours per day. (1200/100=12) (12 * 8.6=103.2) (103.2 * 0.50=51.6 mol/m2/day)