How to Measure Accurately

Learn what affects the accuracy of the Photone Grow Light Meter

How to Measure Accurately

Although we try to design our app as simple and easy-to-use as possible, there are still a few things that might affect the accuracy of your measurement. This guide explains how you can measure accurately and what you can do if you don't measure what you would expect.

How Accurate Is Photone?

When used correctly, Photone is accurate down to a few percent of deviation compared to best in class PAR meters and illuminance meters. You may read more about accuracy and check out some sample measurements in our white paper.

Diffuser Required

To get an accurate measurement you must use a diffuser. This will evenly disperse the light before it enters the camera. You can easily build a diffuser yourself in less than a minute by attaching a piece of printer paper in front of your camera.

Follow the instructions within the app or check out this video:

Prerequisites for an Accurate Measurement

Light Source Setting

Make sure you have selected the correct light source setting. This will allow the measurement algorithm to adapt to your light's spectrum. If you don’t know which light setting is correct for your light source, read our guide on which light to choose.

Clean Camera

Make sure your camera lens (i.e. the glass covering it) is clean as even a bit of dust or finger grease can impact the measurement accuracy.

Screen Protector

If you’re using a screen protector, make sure that it doesn't cover the front camera. If it does, it might affect the initial factory calibration and you'd have to recalibrate accordingly. Performing a custom calibration is not a big deal and there are many options available. Read our guide on how to calibrate to learn how to do it properly.


Use a diffuser as explained within the app. If a diffuser is detected, the orange (!)-sign in the background and the warning will disappear (reach out to our support if it doesn't). Also make sure to use 80g/m2 or 22 lb standard white matte multi-purpose printer paper. Make sure there's no gap between camera and diffuser. You may tape the diffuser directly onto your phone if other provided methods don't work for you. If you’re using any other type of material as a diffuser, you’ll probably need to calibrate Photone.

Taking an Accurate Measurement

To take a measurement, place your phone or tablet at the top of your plants' canopy and aim it at the light source at a 90° angle. Doing so allows Photone to measure the light that falls on your plants' leaves and thus, what actually makes them grow. Make sure that you don't interfere with the measurement, by placing your hand or body too close.

Place your phone or tablet at the top of your plants' canopy and aim it at the light source at a 90° angle

Additional Tips

Under bright lighting, it may be hard to read the measurement due to the display's reflections or lack of brightness. If that is the case, you can enable the HOLD functionality by touching and holding the lower third of your screen. This freezes the measurement until you release the touch.

As an additional help for your measurements, our Pro Settings allow you to display peak and average readings as well as providing a position indicator to indicate if your device is completely leveled.

Not Measuring What You Expect?

Please note that, even though Photone is factory calibrated for every iOS device, some minor differences between devices may occur. A 5-10% difference is to be expected and nothing unusual for almost any light meter. This minor difference won't be noticed by your plants at all. Please mind that even 5% can seem quite high when measuring really bright lights.

If you're comparing the reading of Photone to a cheap illuminance meter or PAR meter, Photone is most likely more accurate. We routinely test common light meters and most of them deviate a lot compared to our professional reference equipment.

Nevertheless, if you have higher trust in your meter or the specifications of your light manufacturer, you can use Photone's calibration option that can be found on the settings page. Read our guide on how to calibrate to learn how to perform your own calibration properly.

Additionally, if you're certain that you followed all the above steps and still aren't measuring what you would expect you can always contact our support team. Make sure to provide us with the required information to troubleshoot your issue, namely:

  • Expected measurement and actual measurement
  • Screenshots (or photos)
  • Type of device (include if you have a screen protector)
  • What kind of diffuser that you're using, if any
  • Type of grow light that you're measuring

How to Measure Accurately

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