One of the go-to lighting metrics of greenhouses is DLI, or Daily Light Integral, and you may see this acronym when you’re trying to figure out how much light a specific type of plant needs. Put simply, DLI is a measurement of how much usable light hits a certain area (one square meter, to be exact) per day.

How is DLI Measured for LEDs?

DLI is measured in moles of light in the PAR (Photosynthetically Active Radiation) range per square meter per day. For a quick brush-up on the best way to measure plant lighting, check out this post. If you want to figure out what your lighting system’s DLI is, the best place to start is to determine what your PPFD is at plant canopy height. PPFD refers to Photosynthetic Photon Flux Density and refers to the number of photosynthetically active photons that hit a square meter of area per second. The units for PPFD are micromoles per meter squared per second (written as μmol m-2 s-1).

To measure your PPFD, your best bet is a quantum sensor like an Apogee SQ-500 or MQ-500, or a Li-Cor 190R, however you can also get a close estimate by simply entering values into a PPFD calculator like this one: (note – this links to the rollitup forum which focuses only on growing weed, and is NSFW. Link is here, link to download is on post #243). It’s worth noting that you can also get an estimate of DLI by measuring your light source with a lux or foot-candle meter and applying a conversion factor to get PPFD, though these conversion factors are typically only available for metal-halide or halogen lamps, and not LED lighting.

How to Calculate DLI

Once you have your PPFD, you can calculate your DLI:

  1. PPFD measures micromoles per second, so take your PPFD and multiply it by 60 to get micromoles per minute
  2. Multiply your figure above by 60 again to get micromoles per hour (you can combine steps 1 and 2 by multiplying your PPFD by 3,600)
  3. Multiply the above figure by the number of hours your lights will be on to get micromoles per “day” (photoperiod)
  4. Divide the above figure by 10^6 to convert it from micromoles per day to moles per day (using a calculator, enter 10, then press the xbutton, then enter 6)

This will be your DLI.

For example, let’s say we have a PPFD of 400μmol m-2 s-1 and we leave our lights on 14 hours a day.

  1. 400 * 60 = 24,000
  2. 24,000 * 60 = 1,440,000
  3. 1,440,000 * 14 = 20,160,000
  4. 20,160,000 / 10^6 = 20.16

DLI = 20.16

If you’re lazy, I’ve got you covered. Just use this little calculator:

How to Use DLI to Calculate Required PPFD

You can also work backwards and figure out how high your PPFD would need to be, in order to hit a certain DLI:

  1. Multiply DLI by 10^6 to convert to micromoles
  2. Divide this figure by the number of hours per day your lights are on
  3. Divide this figure by 60 to convert to minutes
  4. Divide again by 60 to convert to seconds

For example, let’s say you do some research and discover that your tomatoes require a DLI of 22 moles per day. If we only want to run our lights for 12 hours a day, what will our PPFD need to be at canopy level?

Using the steps above:

  1. 22 * 10^6 = 22,000,000
  2. 22,000,000 / 12 = 1,833,333
  3. 1,833,333 / 60 = 30,556
  4. 30,556 / 60 = 509

PPFD = 509μmol m-2 s-1

And the calculator for this:

What Are Typical Natural Outdoor DLI Values?

DLI varies depending on time of year and geographical location. Below is a map of the United States showing the different DLI values throughout the year:

Different Plants Require Different DLIs

From plant to plant, there’s quite a difference in how much light is required per day. For example, lettuce may only require a minimum DLI of 15, while peppers and tomatoes require 22+. For a look at the needs of some different plants, check out this document on Measuring Daily Light Integral in a Greenhouse, created by the Purdue Department of Horticulture and Landscape Architecture.