Colour Temperature

Theoretical emission spectrum of a blackbody at 6,500K

Introduction

The correlated colour temperature of gas discharge lamps is measured in Kelvin, which is in fact a temperature. It is based on a comparison of the spectrum produced by the lamp to that emitted by a true blackbody (a perfect emitter and absorber of energy, Planckian radiator) at a particular temperature, measured in Kelvin (K).

There is a common misconception that that the higher the colour temperature, the bluer the lamp. This is incorrect. This diagram shows the plot of a blackbody on a chromacity chart. As the temperature gets above 10,000K, the change in colour is quite subtle. It eventually gets to a point where no matter how high the temperature, the colour will not change. Unfortunately, this misconception has lead to some misleading colour temperatures given to lamps by resellers.

Appearance

In general, lower Kelvin lamps in the 6,500K range appear more yellow and the higher Kelvin lamps appear bluer. The lamp used on the tank select will effect the overall colour and brightness. The human eye is more receptive to the yellow spectrum than the blue. So 6,500K lamp will appear brighter to the naked eye than a 20,000K, even though the total energy output into the visible spectrum is similar.

There is plenty of speculation and conjecture that particular colour temperatures will effect the colour of corals. However, a lot of what has been said in this regard has not been sufficiently studied.

Ultimately deciding on the right colour temperature lamp for an aquarium comes down to personal preference, and how you want your tank to look. It can be quite helpful to see an aquarium using the lamp you intend to purchase.

Theoretical Spectrum Comparison

The relative amounts of light produced in the visible spectrum at the different blackbody temperatures can be seen in the below graphs. As the temperature increases, the amount of blue end of the spectrum produced relative to the red end increases. Hence the apparent colour change observed with lamps as the colour temperature increases.