Exploring the distribution of light in an aquarium gives us insight into how the different lighting solutions spread light throughout our tanks. From top to bottom and from left to right, how do the various light fixtures effectively spread the usable PAR from corner to corner so that our corals are receiving the light they need? 

Why Light Distribution Matters

Inconsistencies of light intensity throughout your tank can not only create problems for your corals but will also result in a splotchy visual effect with heavy shadowing in most circumstances. Areas of higher and lower PAR throughout the tank make it difficult to place the corals correctly based on their lighting needs and pose a challenge when tuning your lights using a PAR meter. You wind up with inconsistent growth patterns, poor coloration, and generally poor overall health. 

A light with poor distribution will create a severe hotspot in the center of your tank and as you move toward the edges of the tank, the PAR values drastically drop off. Turning down the light output to achieve target PAR levels in the middle will then create areas of proportionally lower PAR around the edges. A light that can more evenly distribute the PAR over a larger area simply makes the most efficient use of the light. You also wind up using fewer fixtures to achieve the even coverage you need. 

As it pertains to LEDs, distribution is all that more important because of the way an LED diode emits light. LEDs emit very focused beams of light which then requires special engineering and careful use of optics to effectively achieve an acceptable light spread for use over an aquarium.  

Light distribution visual grid

The Experiment

The goal is to collect data in the way of PAR measurements to see exactly how well each of the various lighting options distributes light inside an aquarium. 

Each of the 50+ lighting configurations was mounted over the top of a 24" cube aquarium at the BRS Recommended mounting height. We then recorded 36 PAR measurements in a grid pattern at 6", 12", and 18" depths for a total of 108 data points for every single light we tested. We then use software to visually map the light distribution and see exactly how each light performs.  

  • Which lights produce the most even horizontal spread?
  • Which lights penetrate the depths more effectively?

By testing such a large swath of different lighting options, we are able to compare and make great insight into the various light designs and engineering techniques. Which designs and optics make the most significant impact on distribution?

  • Large panel LEDs
  • Small & medium pendants with focused lenses
  • Small & medium pendants with wide-angle lenses
  • 3 Point Hybrids 

To see how your lighting performed, scan through the marked increments using the video player bar!

Radion XR30
Hydra 64

1. Penetrating the Depths

One of the most interesting observations was how the light penetrates down toward the bottom of the tank. Contrary to popular belief, most of the popular light options produced less than a 15% drop in average PAR from 6" to 18" below the water's surface. 

While the more focused LEDs sacrifice distribution to achieve that extra punch that reaches the depths, lights that use wide-angle lenses can achieve similar results by reflecting light off the interior tank walls. The wider angle pushes more light directly onto the inside glass walls where the light then reflects back down toward the bottom center of the tank, resulting in a more even distribution from top to bottom. 

Wide vs narrow angle lenses hotspot comparison

2. Focused Lenses Create Hotspots

Regardless of mounting height, a focused lens will create a hotspot at the top of the tank. While the total average PAR will be slightly higher, the increase is negligible in most cases and the hotspot does more harm than good. Wide angles will result in a slightly lower average PAR at the surface but will not create such a drastic hot spot.  

3. The Shape & Configuration of the Light Matters

Square and circular lights cover a square area more evenly while rectangular-shaped light fixtures cover rectangular-shaped areas more evenly. When using multiple lights, the individual cones of light emitted from each fixture will overlap creating zones of higher PAR. Exactly how you mount and configure multiple fixtures next to each other will affect the overall shape of the light spread and the distribution of light over that area. 

4. Even with Large Panel LEDs, Lens Design Makes a Difference

The Philips Coralcare LED has a diffusion-style lens that is optimized for mounting close to the water's surface versus the Orphek which engages individual, more focused lenses that perform best at a much higher mounting height.  Even though both of these light fixtures would be considered large, panel-style LEDs, the distribution is quite different when mounted at the same height. 

Hybrid vs T5 Light Spread

5. 3-point Hybrids Outperform T5 Fluorescent

A 3-point hybrid that contains a high-output pendant with LED strip lights or T5 lamps down the front and back side can produce an even better distribution than a bank of T5 lamps.  In the visual grid above you can see how the overlapping light created with the 3-point hybrid comes up with better distribution compared to the overlapping layers of light from each T5 lamp.

6. Which Fill Light Should I Use?

When choosing a fill light, look for something that is optimized for mounting at a similar height to your primary light. Since a hybrid setup is mounted with all lights at one level, different optimized mounting heights will result in at least one of the lights not performing their very best. 

Furthermore, choose a fill light that is roughly 50% - 70% of the wattage/PAR of your primary light. Generally speaking, the overlapping light spreads at this ratio produce the most consistently even distribution. Using adjustable LED strip lights as your fill light gives you some room to adjust this output intensity. 

The following chart suggests optimal 3-point hybrid pairings based on optimized mounting height. Find Your Optimal Mounting Height

Fill Light Primary Light
AI Blade, Reef Bright XHO, T5 Fluorescent GHL Mitras, Kessil A360, EcoTech Radion, Neptune Sky, and ReefLED 
Orphek OR3 Lights that require mounting heights beyond 12" from the water's surface.