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How to Measure and Calculate Supplemental Light in a Greenhouse or Indoors Using LUX and PAR Meters


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How to Measure and Calculate Supplemental Light in a Greenhouse or Indoors Using LUX and PAR Meters

Tyler Baras


How to Measure and Calculate Supplemental Light in a Greenhouse or Indoors

Growing is a science. Many growers know their EC/ppm, pH, air temperatures, relative humidity… but a surprising number of growers do not know how much light they are giving their crop. To start, let's look at four key terms.

Photosynthetically Active Radiation (PAR)

This is light that plants use to power photosynthesis.

Photosynthetic Photon Flux Density (PPFD)

This is how much PAR reaches the crop. PPFD measures how many photosynthetically active photons (measured in μmol) are landing in a square meter (m2) each second (s)... the unit used is μmol/m2/s.


Lux is another way to measure light intensity per square meter, but instead of using PAR it uses a measurement of light intensity based on brightness perceived by the human eye.

Daily Light Integral (DLI)

A PPFD measurement shows light intensity per square meter per second. A DLI measurement shows the light intensity delivered per square meter per day. That means adding up the PPFD for each second throughout the day. The unit used is mol/m2/d. DLI does not use μmol because the number would be HUGE. A mol is 1,000,000 μmol.


The following process can be used to calculate how long to run lights in a greenhouse or indoors to meet the required DLI for your crop. If you are growing indoors, start at Step 4.

STEP 1: Determine Outside DLI

There are a few ways to determine outside DLI for your location and season:

  • Measure with a PAR meter. Some PAR meters have an option to take PPFD measurements throughout the day and add these instantaneous measurements into a daily total, or a Daily Light Integral (DLI - mol/m2/s)
  • Use the Clear Sky Calculator
  • Check out the DLI Maps (image below) developed by Dr. Jim Faust

STEP 2: Determine Greenhouse Light Transmission

The best way to determine transmission is by measuring outside PPFD and comparing it to an average inside PPFD. If you do not have a meter, below are some reference transmission levels for common greenhouse coverings. The transmission levels below do not factor in other obstructions in the greenhouse that might reduce transmission. Dirt and dust on the coverings can also reduce transmission.

  • Single Polyethylene Film - 85%
  • Double Polyethylene Film - 77%
  • Corrugated Polycarbonate - 91%
  • Double Wall Polycarbonate - 78%-82%
  • Glass - 78%-90%

STEP 3: Calculate DLI Inside Greenhouse

Multiply the outside DLI by the transmission to get your inside DLI.

Example: If the outside DLI is 25 mol/m2/d and transmission is 80%. 80% of 25 is 20. So the inside DLI is 20 mol/m2/d.

STEP 4: Find Crop's Target DLI

Purdue Extension has an awesome pdf with target DLIs for various crops. These DLI recommendations should be used as a reference, not as a required minimum or maximum.

Below are some DLIs that I see growers use in greenhouses and indoors. The target DLI will depend on many factors including crop, crop stage, air temperature, humidity, CO2 levels... but these are some general targets to get you started.


Vegetative Growth (Leafy Greens/Herbs): Minimum 17 mol/m2/d

Flowering Crops (Peppers/Tomatoes): 25-45 mol/m2/d


Microgreens: 6-12 mol/m2/

Vegetative Growth (Leafy Greens/Herbs): 12-17 mol/m2/d

Flowering Crops: 20-40 mol/m2/d

STEP 5: Determine Required Supplemental DLI

To figure out how much light you will need from your supplemental source (HPS, MH, LED, T5...), you need to figure out the difference between your inside DLI and your target DLI. If you are indoors, your starting inside DLI is 0. For greenhouse growers, use the inside DLI calculated in STEP 3.

Example: If the target DLI is 30 mol/m2/d and the inside DLI is 20 mol/m2/d, then your required supplemental DLI is 10 mol/m2/d.

STEP 6: Calculate Output From Lights

The best way to do this is with a PAR meter. Take PPFD readings from multiple locations to create an average PPFD. Measure output at night when there is no interference with sunlight. If you have a Lux meter, use these conversion tables to convert your Lux to PPFD. If you do not have a PAR or Lux meter, some manufactures can give you an estimated output for their lights at various heights above the crop. 

At this point you could go to this DLI calculator and enter in your measurements to determine how long to run your lights to reach your target supplemental DLI OR you can continue with the process below to learn the math yourself!

STEP 7: Convert Output From Lights Into mols/m2/h

Multiply the PPFD (μmol/m2/second) value determined in STEP 5 by 60 to get μmol/m2/minute. Multiply that by 60 again to get μmol/m2/hour. Then divide by 1,000,000 to convert from μmol to mol, giving you the output of your lights in mols/m2/hour.

Example: PPFD of 400 μmol/m2/s * 60 seconds/minute gives 24,000 μmol/m2/minute THEN 24,000 μmol/m2/minute * 60 minutes/hour gives 1,440,000 μmol/m2/hour THEN 1,440,000 μmol/m2/s divided by 1,000,000 μmol/mol gives 1.44 mol/m2/hour!

STEP 8: I See The Light!

Last step! Divide your Supplemental DLI from STEP 5 by your mols/m2/h from STEP 7 to calculate how long to run your lights to reach the target DLI.

Example: Supplemental DLI 10 mol/m2/day divided by output of lights at 1.44 mol/m2/hour gives us 6.94 hours... meaning if we ran these lights for 6.94 hours each day we would give the crop an additional 10 mol/m2/day of light.

Well, that was a fairly math intensive post... if you are feeling like you need to give your brain a break, check out this video! It has spider mites dying and cute kids...!

The lights used in the "How to Measure and Calculate Supplemental Light in a Greenhouse Using Lux and PAR Meters" were provided by Hydrofarm. They've been very supportive and I really appreciate them helping me create educational content. Thanks!