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Underwater PAR Measurements for Reef Aquariums

Underwater PAR Measurements for Reef Aquariums

4 min read

The entire reef ecosystem depends on light for photosynthesis, which is measured as photosynthetically active radiation (PAR). But without correction, PAR meters calibrated in air provide only relative values underwater. This article explains how to take accurate PAR measurements underwater as well.

As we all know, lighting is extremely important when it comes to living organisms such as plants, animals, and even humans. Measuring photosynthetically active radiation (PAR) is crucial in assessing the quantity and quality of light for plants. PAR is not only the most important measure of light in terrestrial photobiology research for plants and crops, but also for underwater plants such as corals and other reef plants.

Reef Lighting

The entire reef ecosystem depends on light for photosynthesis, which is the process where corals derive 80-85% of their energy from. Furthermore, reef lighting stimulates the corals' chromoproteins that are responsible for creating beatifully vivid and vibrant colors within your reef aquarium.

Healthy corals showing vibrant and vivid colors

Using Our App For Underwater PAR Measurements

There is absolutely no issue in using our Grow Light Meter app under water – it just needs a bit of preparation and some adjustments.

Without correction, PAR meters calibrated in air provide only relative values underwater as more radiation is backscattered out of the diffuser due to the refractive index of water (1.33) being greater than for air (1.00) resulting in a phenomenom called the immersion effect. Fortunately, our app offers a handy calibration feature that allows to take care of this.

Immersion Effect Correction Factor

As the immersion effect is obfuscating PAR measurements, a correction factor must be used to approximate the correct PAR measurement. Luckily, there already is some great research on the topic provided by Apogee Instruments.

As our Grow Light Meter is calibrated using an Apogee SQ-520 Full Spectrum Quantum Sensor, we can use its immersion effect correction factor of 1.32 as recommended by Apogee.


One obvious step is to prepare the smartphone and diffuser for its submerged usage. Even though most modern smartphones are water-proof to some extent, we recommend putting the smartphone including the attached diffuser in a clear plastic bag.

Putting the smartphone in a clear bag ensures that our whole setup is waterproof and the paper won't get soaked

Having some kind of translucent plastic in front of your camera will inevitably block some light and therefore require calibration to counteract the loss in transmission.


Calibration is not only required to to compensate the water-proof setup but also to factor in the physical changes in light transmission that is different in air and water. Therefore, we apply the immersion effect correction factor for accurate PAR measurements underwater with our air-calibrated Grow Light Meter.

As a first step, we need to calibrate for our waterproof setup. This requires us to take a reference measurement without the plastic bag that we'll then use within the calibration flow.

Taking the initial measurement under stable conditions
The same conditions but now with the clear plastic bag over the camera

As for the calibration, we'll use the "Light- / PAR-Meter" option and input our initial measurement as the reference meter's measurement:

The three broad steps to counteract the plastic bag in front of the camera

So far we've now successfully compensated for the plastic bag's transmission loss and should measure exactly the same with the bag as without the bag before.

As the second and final step, we need to apply our immersion effect correction factor of 1.32 to the measurement. We can again use the regular "Light- / PAR-Meter" calibration option to do so:

A second calibration is required to apply the immersion effect correction factor

A little hack is required: At the third step, input a pseudo measurement to get to the "Detected Peak Light" at the fourth step that is otherwise locked. After taking this measurement, you need to multiply it by 1.32 and input this result as the reference in step three that you can access by swiping back.

The broad three steps to factor in the immersion effect correction factor

Then press "Calibrate" and you're done: You can now use our Grow Light Meter as an underwater PAR meter.

New To Our App?

In case you aren't already using our app, we can fully and confidently recommend to use our grow light meter for any light measurement desires. We even wrote a white paper covering the topic of how to achieve the highest accuracy as possible. Additionally, our support team is always happy to help to achieve the most accurate results possible and you can reach out to us any time. Thank you for reading!

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