Reef keepers have long known that light is an integral part of the equation of keeping and propagating healthy invertebrates. Zooxanthellae use light for photosynthesis to feed themselves. The byproducts of the chemical reactions involved provide much of the energy and nutrients to the animals we appreciate. Light, however, is a tricky beast. What we refer to as light is merely a small section of the electromagnetic range that we can perceive. Even over that narrow spread, our eyes use a smaller subset, filling in the patches with mental gymnastics that, while effective for us, do not really provide the information to measure characteristics that dictate whether it is suitable or even usable, in our aquariums.
The various metrics that are used to describe light output fall short, primarily due to our use of light and how we have created it. In a world of incandescent bulbs, watts was a useful metric – higher watts, brighter light bulb. Lighting has evolved a great deal since then, but people have been slow to adopt better metrics when describing light sources. Here are several that are frequently used, and why they aren’t actually all that helpful. We propose to replace them when measuring light source efficacy in photosynthesis.

Lumens – Lumens depend on the human ability to perceive light. Lumens are a measure of candelas over time. Candelas themselves are measured by weighting those wavelengths the human eye can see. An easy example is an LED light with heavy UV/high energy blue. These aren’t wavelengths humans see well, though they are heavily used by Zooxanthallae during photosynthesis. To us, it may not appear very bright, but it will certainly feel that way to your Acropora.

CRI – The Color Rendition Index’s primary function is describing how a white light source renders colors when shone on other objects. It can be helpful when judging how two lights sources will appear over a tank, but that’s about it. Reef aquarium livestock frequently have the ability to absorb, or shift and then emit, specific wavelengths through pigments and carotenoids. This skill is one of the reasons corals and clams look as cool as they do. That means they don’t reflect what is shone upon them, robbing the CRI of a good bit of its value.

CCT – The Coordinated Color Temperature is the wavelength emitted by a theoretical black-body radiator. Good luck finding one. CCT is useful when describing white LEDs in terms of their shift from “cool” to “warm” white. When considering LED systems using many different emitters and combining direct emission and phosphor coated devices, CCT doesn’t measure up.

PAR/PUR – Photosynthetically Active/Useable Radiation is the current leading metric which lets users throw out the subjectivity of the other methods listed above, and actually count pure photons of light. There are still drawbacks to PAR/PUR. The biggest is that neither PAR nor PUR considers the relative importance/efficacy of specific wavelengths for photosynthesis. A PAR meter will read every photon from 400-700nm and count them the same way. The lack of a weighting function by wavelength means that even PAR/PUR doesn’t tell the whole story. For example, green wavelengths are of little use for chlorophyll-based photosynthesis, but it will show the same PAR value as blue if the radiant flux is the same. PUR might ignore these wavelengths, but it would not differentiate among those it does keep. This is an improvement, but still not the whole story.

AcroOptics, in our pursuit of a better LED light for reef aquariums, has started researching what we call the Chlorophyll Relative Absorption Value Estimate (CRAVE) index. Developed by Michael Hurowitz, E.E., we believe this would be a good first step in creating a more relevant metric for comparing the photosynthetic potential of a given light source. The proposed metric takes one step beyond PAR by measuring pure Spectral Power Density (W/nm) from 360-800 nm, and comparing that to the absorption profile for each photosynthetically/photobiologically active process. The metric explicitly assigns relative weights to each contributor to overall species health. The goal is to develop a true set of equations defining how light interacts with photosynthetic organisms. The equation model for calculating the CRAVE index would be as follows:
Simple two term CRAVE estimation formula
Wght(x) = weighting coefficient for given photoactive compound
Fλ=spectral power density at λ
xλ = Absorption efficiency at λ for photoactive compound (x)

In the version above, two photoactive compounds (e.g. chlorophylls a and c2) would have a weighting indicating that compound’s overall influence. The weighting would represent the contribution of the given compound to the photosynthetic process. Other pigments would be added as additional terms. Further knowledge could be embedded in this metric: good candidates would include
• Photo saturation points and photo-inhibition;
• Treatment of the spectral bands that trigger processes like spawning and flowering;
Moving to this level of detail, the term Fλ would become Faλ, to pick up on characteristics such as saturation points, or even Faλt, to differentiate how the SPD shifts over time.

As a metric, it sounds awfully appealing. Much work remains to be done to understand how the different compounds should be weighted, which will vary by species. We can already see the large differences in chlorophyll type concentrations between terrestrial and marine plants, and work is being done to identify similar patterns between types of Zooxanthellae found in coral and other marine invertebrates. Many of you will be familiar with the chlorophyll a/b chart below (I grabbed this one from Wikipedia):
Classic Chlorophyll a-b plot

Terrestrial plants have a higher concentrations of a/b, while coral and Tridacnid clams, with their symbiotic Zooxanthallae, primarily use a/c2, as well as other pigments. Thus, using the CRAVE metric, the weights assigned to chlorophyll a would be higher, and the absorption values for the blue and green wavelengths would be higher for corals than for terrestrial plants. In their excellent article about light and reef tanks (here), Dmitry Karpenko and Vahe Ganapetyan plotted a similar curve for Zooxanthellae:

Light absorption by zooxanthellae, Karpenko and Ganapetyan, 10/2012

Light absorption by zooxanthellae

A brief view of both charts shows that the green wavelengths, essentially unused in terrestrial plants, are a big part of the energy used by corals. Conversely, the red spectrum favored by terrestrial plants plays a smaller part of coral metabolism. New information is constantly being acquired about photobiologically-active compounds such as anthocyanins and peridinin. Chlorophyll f, present in cyanobacteria and key to the formation of stromatolites, were only identified in 2010.
“Stromatolites in Shark Bay”, Photographed by Paul Harrison (Reading, UK)
Chlorophyll f, present in cyanobacteria and key to the formation of stromatolites, was only identified in 2010.

As this information is acquired, it can be used to improve the metric, by adding additional terms and modifying the weighting parameters for the compounds and wavelengths. In the end, what is needed, and what the CRAVE index is designed to quantify, is a more holistic view of the interaction between photosynthetic organisms and their food source, the sun. I’m looking forward to being involved in CRAVE’s development.