Measuring Light - Day Light Integral (DLI) & Photosynthetically Active Radiation (PAR) Meters

An excellent way to measure the power of light is by using a PAR meter. A PAR meter measures the photons per square meter per second. It measures how intense the light is in a given location.


This is an image of the PAR meter.

To get a reading, you plug the meter into your computer. And set the par meter down in the location you want the reading. It will then give you a reading onto a software. This particular meter uses Aplgee Connect.


This is the PAR meter in a GBE unit.

The readings look like this:


This is a reading for par in Aplgee Connect.

We 3D printed these special PAR meter holders to ensure that the PAR meter is steady to get accurate readings.


This is the par meter in the 3D printed holder

I tested various locations with the par meter:

  • Sun Outside - 1692.2
  • Outside in Shade - 238.6
  • Under a Lamp - 16.4
  • MARSfarm Office - 11.1
  • In a GBE Unit - 178.7
  • In an MV.1 - 338.6

The Daily light integral (DLI) is the amount of PAR received each day. To convert PAR to DLI:

    1. add all PAR readings; 2) multiply by the time interval in seconds between measurements; 3) divide by 1,000,000 to complete the unit conversion.

OR multiply your PAR value by (based on how long the light time interval):

  • 24-hours: 0.0864
  • 16-hours: 0.0576
  • 12-hours: 0.0432
  • 10-hours: 0.036
  • 6-hours: 0.0216

DLI and PAR values can give you insight into plant growth. DLI can influence the root growth of seedlings and can affect the plant’s final quality.

You can read more about PAR and DLI values at MARSfarm here:

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A new method to evaluate PAR values is with the Photone app for a phone. It uses an existing sensor within a phone to get PAR readings without using an expensive PAR sensor, and it is available on both Android and iOS.

After hearing about this, I decided to compare it to our PAR sensor, which we know to be very accurate. This was done with both inside a GBE FG.V3 box, using a setup posted below:


Setup of a phone running Photone and a PAR sensor plugged into a laptop

Here are my results:

Photone Mode Light Settings PAR Meter Reading (PAR) Photone Reading (PAR)
LED Full Spectrum White 255 640 684
LED Full Spectrum White 175 354 379
LED Full Spectrum White 125 190 205
LED Full Spectrum Red 255 504 318
LED Full Spectrum Green 255 112 128
LED Blue/White Blue 255 371 32
LED Blue/White White 255 638 711
LED Blue/White Blue 175 White 175 483 327
LED Blue/White Blue 125 White 125 282 198
LED Red/Blue Red 255 504 550
LED Red/Blue Blue 255 372 53
LED Red/Blue White 255 639 1175
LED Red/Blue Red 150 Blue 150 416 299

The main point to be taken is that the app does a fairly decent job with white LEDs in the full spectrum mode, overshooting by only around 5-10%. It does slightly worse with reds and greens, at about a 10% error, but it simply cannot be trusted with blues, which it seems to undershoot by a factor of 11 – only measuring about 10% of the true value. This may be an artifact of my phone’s sensor, but it may be inherent to the app.

In summary, this app can provide some value for base-level checks, but for better accuracy, a professional sensor would be recommended.

I hope this brief overview of a new sensor technology was helpful for everyone in their quests learn more about agriculture.

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Check out this new video of MARSfarm CEO Peter Webb explaining what a par meter is, and walking through different par values inside our units and around HQ.

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DLI Maps for the United States. These are a great way to learn what the DLI is where you live at any time of year - good talking point and poster to print out for a classroom to discuss light as an environmental variable.

citation: https://www.researchgate.net/figure/Maps-of-monthly-outdoor-DLI-throughout-the-United-States-Source-Mapping-monthly_fig1_242551992

How this map can be used to help students create their own custom recipes for the MV1:

  1. Start by deciding what you want to grow.
  2. Using the known growing seasons, calculate the ‘average’ DLI that the plant would receive during its phases of growth.
    • Use the map in the first link to calculate the DLI for that location and time.
    • For shorter duration plants (leafy greens) this is shorter - 2-3 months. If it’s longer, worry more about the period when the fruit would be ripening - whenever it gets the most light.
  3. Convert that DLI into a ‘recipe’ which can be run on the MV1.
    • This table shows the settings for the three pre-made DLI ‘recipes’ (DLI: 30, DLI: 21, DLI: 17) that you can pick from to run in your MV1: What does a MARSfarm recipe do? What is JSON? (ChatGPT-4 Response) - #4 by Peter
      • Honestly, those three are great place to start for growing any type of leafy green.
      • DLI 30 = too much light for most leafy greens.
    • Keep in mind, the conversion we make assumes plants that are ~6" tall.
      • If you’re growing a tomato or pepper - the DLI will be even higher closer to the lights.
    • Other posts earlier in this thread show what a PAR meter is and how we use it to calculate DLI settings for each recipe.