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AB+ Have you ever wondered why one of the most popular LED light profile is called AB+? Well I guess one of the reason is that the spectrum attempts to match the absorption peaks of chlorophyll a (Chl a) and chlorophyll b (Chl b), both of which are found in zooxanthelle. Consider this adsorption spectra (source: Wikipedia) The highest absorption peaks for Chl a and Chl b ranges from 420 nm to 490 nm range. These wavelengths correspond to violet and blue light. Therefore, most of the modern reef lights produce a large amount of blue light. These caters well to the need of Chl a and b. On the other side of the spectrum, the 650 nm to 680 nm range correspond to amber and red spectrum. These wavelength also contribute to photosynthesis. Many reef lights provide these wavelengths through red diodes, and to a lesser extend, white LEDs. It'a also worthwhile to note that the absorption is very low in the green region. On the other hand, human eyes are very sensitive to green light. Therefore some output from green diodes can make the reef aquarium looks much brighter, while not contributing much to coral growth. Caveats There are a few caveats concerning the above (simplified) information. 1. There are more pigments in zooxanthelle than Chl a and b. There are beta-carotene, xanthophyll, Chl f, etc. They do make use of other part of the spectrum. However, Chl a and b are the dominant pigments in coral system. 2. The absorption spectrum of Chl a and b extends well in to sub 400 nm wavelengths (UV-a), while Chl f absorption spectrum extended into the IR region. The reason most absorption spectrum shows 400 nm -700 nm is because this is the range defined for PAR. 3. This discussion of spectrum is with reference only to photosynthesis. This is not a discussion for coral color (yet).

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