Measuring Light - Photosynthetically Active Radiation (PAR) Meters

steven · June 8, 2023, 9:24pm

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:

ben · July 6, 2023, 9:02pm

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.

steven · July 12, 2023, 9:45pm

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.