BRStv Investigates PUR & the Quality of Light - Lighting Showdown

We know corals need light to survive but the key to maintaining thriving coral in captivity goes far behind simply illuminating the tank. The color of the light (spectrum), amount of light (PAR), and quality of the light (PUR) combine to create suitable conditions for our corals to grow and if any one of those aspects of lighting is out of balance, the corals will not thrive. 

In this BRStv Investigates, the team examines PUR - photosynthetically useable radiation to find out which lights do the best job of providing the ideal quality of light for our corals.

PUR vs PAR

The difference between PAR and PUR is important and both of these are used in different ways to ensure we provide the most ideal lighting conditions. While PAR is a measure of the radiant energy within the spectral range of 350 - 750nm, PUR refers to only the radiant energy that is usable for photosynthesis. With any measurable level of PAR, you can illuminate the corals and make them visible to the human eye but it requires PUR to promote efficient photosynthesis.

So what wavelengths of light make up PUR? 

PUR is the wavelengths of light that are optimized for the coral's primary light collection pigments - Chloraphyll-A, Chlorophyll-C2, and Peridinin. These pigments are what use the light energy to carry out photosynthesis and provide energy to the coral. Each of these pigments has a slightly different preference for light meaning they are most efficient at collecting light within certain wavelengths. 

410-430 nm

450 nm

480nm

Coincidently enough these preferences overlap and result in a "biological band" or range within the spectrum that promotes photosynthesis best. This band exists at 410 - 484nm and the ultimate goal is to provide light peaks across this entire range.  

When gauging PUR, you're evaluating the peaks of light within this particular nanometer range of the spectrum. A "high quality" light source will produce light peaks within this biological band and the wider the peak, the better the PUR.

Keep in mind, a light source may produce high PAR values but that doesn't mean it will also produce high PUR values. In order to achieve high PUR, a light must produce wavelengths that peak within the biological band (410nm - 484nm). If you do not provide the light that peaks within this particular nanometer range, the coral will suffer and not be able to obtain sufficient energy from photosynthesis, regardless of PAR levels. 

The Experiment

The goal is to graph the spectrum produced by 50 of the most popular reef tank lighting options to find out which ones produce the best "quality of light".  Which lights provide us with the widest blue range of light? 

Each of the 50 different reef lights was measured at 100% output across all available color channels using a spectrometer. The resulting spectral plot gives us a visual representation of the wavelength peaks measured in nanometers. We use nanometers instead of color because it's a defined measurement that is not open for interpretation.

Which Lights Produce the Best PUR?

It's best to scrub the video and view the particular lighting options you are interested in to see exactly how well they produce these usable spectrums of light. Ultimately, a light that can produce wavelengths across the entire 410nm - 484nm range is ideal. 

In many cases, the lower (violet) part of the blue band (where Chlorophyll-A is most efficient) will be the lacking part of the blue band. This is because almost all reef tank lights will prioritize royal blue and cool white which covers the upper half of the blue bland. The use of supplemental lights like the AquaIllumination Blade Glow or Reef Brite Actinic XHO strips can help to fill in the lower 410-430nm range. 

It is also important to remember your LEDs are adjustable and that most modern LEDs offer color channel control. Pay attention to the bottom half of the spectral graph within the blue band; it is the base of that spectral peak that will tell you the range of blue your LED is capable of covering. 

So long as you have an LED light that is not dependent upon running all channels at 100% to achieve target PAR values, you can manipulate the color channels to achieve a more ideal spectrum ratio. In the case of the Radion Blue pictured above, reducing the royal blue color channel by 50% creates one of the widest spectrums we tested with the best color peaks. Something to keep in mind when purchasing an LED and adjusting the color spectrum.  

An LED light that has dedicated color channels to cover the full range from 410nm to 484nm means you ultimately have the ability to control the valuable light peaks.  Also, note that this is putting aesthetics aside and a tank that ONLY contains this blue band, won't be able to produce the level of coloration most reef tank owners desire. 

The long and short of it - now that we have these spectral graphs for reference, we are all equipped to make educated decisions about the spectrum in our aquariums. Deciding whether or not you need to supplement your existing lighting or how you should be using the color channel controls need not be a guess.

What about the orange-red spectrum?

While both Chlorophyll-A and C2 are able to absorb light within this nanometer range, the value of this wavelength for coral is still up for debate. Warm-colored light is absorbed first as light travels through the ocean's depths so the light that ultimately hits the corals doesn't contain these warm peaks. Furthermore, the most proven spectrums of light for coral husbandry (ATI Blue Plus) are all but completely void of orange-red colored light. There are additional theories and only future science can tell us definitively whether or not these warmer spectrums play even the slightest role in a coral's biological processes.  

Biological Process is the key component here and for this discussion, we are primarily focused on photosynthesis. On the topic of coloration and creating a visually appealing appearance, this warm-colored light plays a big role in the colors your eyes perceive which is why it's not a total loss and is still usable over our tanks.

What about the UV (ultra-violet) spectrum?

True UVA exists at 315nm - 400nm and it is still debated in terms of how useful this spectrum is. Classically, this true UV spectrum really isn't prioritized and the theories around the use of 380nm are exactly that, theories. We have seen success without using 380nm light for many years which suggests it's just not a critical component to our success.