Have You Been Misled About Your Reef Tank Light’s PAR? Biggest Light Testing Mistake: Water vs Air

A majority of aquarium light manufacturers test and compare their own light's performance using PAR measurements.  These measurements are not taken inside an aquarium in 99% of the cases.  Instead, they measure the output in open air and don't take into account how light changes when it passes through water inside a glass or acrylic box.

Way back in 2017, BRStv investigated exactly why this was misleading based on reflection and refraction of the light underwater. The bottom line, you will get drastically higher PAR readings inside an aquarium, underwater where it matters. This is exactly why we always measure PAR underwater when investigating reef aquarium light fixtures. So why are manufacturers still giving us the PAR readings measured through the air? 

Ryan and the crew revisit this subject to further explore why using PAR measurements taken underwater, inside an aquarium is a far more accurate way to measure an aquarium light. We not only affirm our light testing methods are sound but also further our understanding of why some lights perform better than others over a reef aquarium.

Reflection and Refraction

Before diving into our experiments, it's important to understand how reflection and refraction are going to affect your aquarium light. It's pretty simple to understand by the most basic definition, as it pertains to lighting.

• Reflection: The return of light waves from a surface.
• Refraction: The change in direction of light waves passing through one medium to another based on the density of the medium.

In our aquariums, light is reflected off of the surface of the water and also reflects off the walls. Light refracts as it passes through water and glass/acrylic walls. Both of which will affect the PAR that is hitting your corals and reaching the depths of your tank.

The Laser

Our first experiment was a simple laser pointed at the tank.  We compared how the laser reacts going through an empty tank vs how it acts going through a tank full of water. This profoundly demonstrates why we measure PAR underwater and helps to better explain both light spill and the improved results we get from scatted, well-diffused light sources.  

The laser passes right through the wall of the tank without changing direction when the tank is empty (through the air).  When the tank is full of water, a majority of the laser is reflected inwards back into the water column. Exactly why get higher PAR numbers underwater, rather than losing light out the sides of the tank, it is reflected back inside. Reflection and refraction are both at work, changing how the light acts over our corals.

Upon closer examination of the laser, we see some reflection off the surface of the water which creates that light spill above our tanks.

We also see some refraction through the glass walls, changing the direction of the light, creating the light glow we see surrounding our tanks on the floor. 

Black Wall Effect

This was the last phenomenon we noted with the laser.  Many tanks have a black wall on the back of the tank. Black absorbs light so as suspected, a majority of the light doesn't reflect off of an aquarium wall colored black. The light is absorbed instead.  This results in lower PAR readings in the area closest to the back wall of the tank.  This shows up in our BRStv Investigates light testing experiments where we see a row of lower PAR measurements across the back because we are testing using a tank with a black background. 

PAR Measurements AIR vs WATER

Here, we used our PAR meter to set up identical scenarios to measure PAR using an Aqua Illumination Hydra, Red Sea ReefLED 90, and Neptune SKY. The setup is very similar to the PAR testing we do during our BRStv Investigates light testing and comparisons. We tested the PAR at the very same distance from the light, one with the PAR meter through the air and one with the PAR meter underwater.  

Needless to say, the results further drove this home. Testing underwater produced significantly higher PAR values when tested underwater. This is because the light is reflected and refracted back into the tank instead of escaping through the walls.

The most interesting lesson learned through this experiment was not the increase in PAR, rather how much of an increase we got based on the type of light. Let's start by examining the lens and output from each of the three LED lights we tested.

• Aqua Illumination Hydra: Narrowest angle lens
• Red Sea ReefLED 90: Wider angle lens
• Neptune Systems SKY: Diffused panel style LED

When we look at our results from the PAR tested underwater, we see much larger boosts in PAR from the ReefLED 90 and Neptune SKY. This is because of the wider spread. The more light that hits the walls, results in more PAR reflected back into the tank. Exactly why we see better performance in our tanks from widespread, diffused LED light sources.

It also helps light to wrap around the corals as it is reflected back into the tank from different angles and creates a boost in PAR toward the bottom. This is why T5 lights and diffused LEDs can help eliminate the shadows we get from larger, more mature corals and help us to achieve optimal PAR throughout as much of the tank as possible. This leads us to the third and final experiment.  

Boosting PAR Where It Matters

Our third and final test was where we examined the boost in PAR we got in the bottom of the tank during our PAR measurements. Above we have each of the three light fixtures tested, with PAR results from the BOTTOM grid only.  This shows how much of an increase we see across the bottom of the tank between testing in AIR vs testing inside a tank filled with water.

Notice the large increase in PAR (over 100%) we get from the wider angle ReefLED 90 and Neptune SKY.  Wider angle lights produce better PAR results across the bottom of the tank, helping hobbyists achieve those ideal PAR numbers throughout a larger portion of their tank. Wide-angle or diffused LEDs are simply a more efficient light source for illuminating a reef tank. 

What Did We Learn?

We are confident in our approach to testing the efficiency and performance of an aquarium LED light underwater.  This is the intended environment and the results will more accurately demonstrate how the light will perform. When it comes to PAR values for your reef aquarium lights, don't just take the manufacturer's word for it, measure the PAR yourself because more often than not the advertised PAR is not an accurate representation of the values you will get inside your tank.

Reflection and refraction have a profound effect on aquarium light as it passes through water.  Mostly to the benefit of the tank for it results in higher PAR values across the board and helps increase PAR across the bottom area of the tank where we need it most.  

When choosing an LED light, the output angles and spread really do matter. Not only for effectively covering the entire footprint of your aquarium but also ensuring your get sufficient PAR throughout the depths of your tank too. Furthermore, if your aquarium has a black or painted wall, this will produce a lower area of PAR directly in front of that painted wall no matter what kind of light you're using.