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KELVIN, K OR CORRELATED COLOUR TEMPERATURE (CCT)
Traditionally a kelvin or K rating is used by aquarist as a guide to help choose a light system or lamp that is spectral most suitable for their biotope. Different biotopes and organisms have different spectral requirements and this should be viewed as a separate issue to the quantity of light (or light power). We should look at spectra as a measurement of quality or fit for purpose for that specific biotope. As a rule fresh water lamps have a lower K (redder) and marine lamps have a higher K (bluer).
So the where does the Kelvin scale for the light sources on our aquariums come from? If we imagine a black object being heated it would glow red first then white hot and eventually blue. The chart below shows this heating process and attaches the value K against stages of light colour radiated from the object.
A chart like this is used for characterising terrestrial light as K and usually through the day a transition of K occurs from sunrise (2500K) to daylight (6500k) and occasionally overcast days can rise to 10,000K or higher. Why does K increase when the sky is overcast? The answer is simple water droplets filter light and it filters red out first. If we apply this knowledge to the aquatic environment and use the term K we can imagine that a plant in shallow rivers will receive a light close to daylight whereas coral on reefs meters down will receive a very different spectra even though it's from the same light source (the sun) this is because the red content is filtered out and continues to with depth as shown below.
Therefore fresh water plants and coral have evolved different survival strategies with types of chlorophyll that are tuned to the spectra they would naturally receive; supplying the wrong spectra can either starve organisms or encourage other nuisance species such as algae's. The seneye device has a small database of plants and coral with their light requirements that it hopes to build over time, the purpose of this is to allow the user to pick the best place to locate corals or to determine if they have enough light for that organism.
In basic terms K is dictated by the biggest (tallest and widest) spectral peaks from a light source and how close the overall colour produced falls next to the Planckian locus line. The Planckian locus (or black body locus) line is shown as the black line on the CIE colour space diagram below.
The seneye device works out the K of the lamps by passing zyx colour coordinates of the light it receives through a set of known equations. The device can measure very far along the plank line when compared to traditional devices and this is important as the growth of LED's has meant that light sources can in fact have high levels of blue and violet which will very steeply raise K.
K is a useful tool but as many aquarium owners use mixed light sources or tubes with different phosphor in the combined light given off is often difficult to understand and it will change throughout the aquarium due to shadow and depth. This is why the seneye is submersible and allows the user to move the device to specific parts of the aquarium, log and displaying the light reading. More importantly and less understood is how aquarium light output can change over time as from the second they are turned on no matter what type of device they are they will change spectral output as they age. Often with metal halide blue output will degrade first and K will shift down toward red, regularly logging K along with power will help you understand the state of your lighting over time.