Putting an end to the ‘blue light means slow coral growth’ myth
It is time to end a misconception in this reefkeeping hobby, blue light does not slow down the growth of corals. Let me say that again, blue light doesn’t slow down the growth of corals.
Back in the days when metal halide was king of the lighting arena of our hobby (yes yes, I can hear some of you saying “What!? Halides are still best!”, but that is for another discussion), colour temperature, Kelvin rating, K, was a big deal. In general a 6500K lamp was said to put out light similar in colour to the sun at midday, while 10000K was slightly blue and 20000K being the bluest of the common ratings.
As a general rule of thumb, corals were found to grow fastest under halides with a K rating of 6500K but lacked colour due to their yellow output. The amount of PAR (Photosynthetically active radiation) these lamps produced made them great ‘grow’ lamps. For example a 250w Iwasaki lamp produces around 159 PPFD as per Sanjay Joshi’s testing. But because these 6500K lamps weren’t very visually appealing, reef keepers started to lean towards high K rated lamps such as 10000K (182 PPFD – XM 250w SE), 14000K (159 PPFD- IceCape 250w SE) and of course 20000K (106 PPFD – Happy Reefing 250w SE).
As a general rule of thumb, as the K rating increases, the PAR levels start to come down.
The high K rating lamps gave us the bluer looking aquariums we enjoy and started to excite certain pigments in corals. At this point you’re probably going to be saying, yes, so blue light does mean slower coral growth…
As the K rating trend took off, the use of the K value became more of a marketing gimmick, to explain, give you an idea of what colour you could expect from your lamp. I’ve seen 14000K lamps that are more yellow than 10000K lamps, and I’ve even seen some very white 20000K lamps. And I’m sure you’ve seen the same. Conventional reef lighting thinking would have you believe that the higher the K rating, the slower the growth, but we need to consider that this simply applies to halide lighting and to a certain extent T5’s but what about LEDs?
The beauty (and curse) of LED lighting is that the light generated is very spectrum specific unlike halide lamps or T5 tubes which have a greater blend of spectrums in a single lamp.
With LEDs we can fine tune our lighting to only supply the most essential of spectrums – but because we like to look at our corals, we throw in a nice variety of spectrums to balance out the overall picture.
Chlorophyll A does best in wavelengths from 380nm through to 440nm. There is a peak at 410nm and a large spike at 430nm. Now this is in the blue side of spectrum. Chlorophyll A goes onto to look for more red light at 662nm.
Chlorophyll B peaks 453nm and again at 642nm.
So just very quickly, based on the Chlorophyll absorption peaks, blue light is infact better for corals, for both growth and health.
Just remember, if it’s not a halide and K is being talked about, it’s the overall colour rendition to our eyes and hardly an indication of growth application.
Although this is overly simplified way of looking at it, don’t just assume blue lighting will mean slower growth for your corals, infact too much blue light could end up being a bad thing too.
(Photo by Menashri via Flickr)