Light spectrum

The colour composition of the light emitted by the lamp. Each wavelength has different effects on plants, corals and colour perception.

Detailed explanation

The white light we see is made up of all visible wavelengths. But not all wavelengths are equally useful for photosynthesis or aquarium aesthetics.

Aquatic plants mainly absorb red (620-700nm) and blue (430-470nm) light for photosynthesis. Chlorophyll a has absorption peaks at 430nm and 662nm. Green light is largely reflected (this is why plants are green).

For the reef the ideal spectrum is shifted towards the blue. Blue LEDs (450-470nm) penetrate deeper and stimulate the fluorescence of corals. The typical spectrum of a reef ceiling light is composed of a mix of royal blue (450nm), blue (470nm), cool white (10000-20000K), UV (400-420nm) and sometimes red and green.

Colour temperature is measured in Kelvin (K). 6500K is neutral daylight, great for plants. 10000K is cool white, good for a marine effect. 14000-20000K is very blue, ideal for the reef. Below 5000K the light is yellowish and unpleasant for most aquarists.

UV LEDs (380-420nm) make the fluorescent pigments of corals "glow". Too much UV can damage coral tissue and human eyes. It is used in moderation.

In planted freshwater aquariums a full-spectrum spectrum with peaks in red and blue is ideal. Lamps with too much green make plants very green but do not stimulate growth as much. The colour rendering (CRI > 90) makes the colours of the fish appear more vivid and natural.

The old T5 and T8 fluorescent lamps are disappearing in favour of LEDs but they remain valid. The combinations of tubes (e.g. two "day" tubes + one "plant" + one "blue" for marine aquariums) allow you to customize the spectrum.

How it is measured

With a PAR metre the intensity useful for photosynthesis is measured. The spectral composition is analysed with a spectrometer. In practice most aquarists rely on the manufacturer's specifications.

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