How Many LED's to Use and What Colour
A few members are starting to put together their own DIY LED builds so I thought it was about time I finished off this 'theory' section of this LED series and got onto the 'good stuff' of actually putting it all together.
I have to say that this topic is very subjective and 100 people will have 100 different answers to the question of how many LED's to use and what colour they should be. There are a lot of variables which will affect the final result such as optics, the type of LED, the type of driver, how the LED's are arranged over the tank....the list goes on. I am going to jump in the deep-end and provide some guidelines but please remember that this is just a starting point for your own research and decision making. The great thing about building this yourself is the ability to change something if required - if you don't have enough light then add some more LED's; if the colour is not right then replace some LED's with different colours.
The recommendations are based on high quality 3w LED’s such as CREE – if you use other types then you may need to compensate accordingly. I have also assumed that the LED’s are driven at around 750ma – if you use a different current level then you may need more or less LED’s. In a future post I plan to discuss the relevant efficiency of the LED’s at different currents.
I think it is fair to say that the old reference to watts/gallon is no longer applicable when applied to LED’s for a number of reasons:
- The light supplied by the individual diodes is in a much narrower spectrum that that provided by fluorescents or metal halides. We are able to target the required wavelengths much better to meet the needs of our corals without wasting a lot of energy on unwanted wavelengths. This is why tanks with LED’s often do not look as bright as those with alternative light sources.
- LED’s are much more efficient than alternative lighting methods i.e.
Incandescent Bulbs ------- 5-35 lumens/watt
T8 Fluorescent ------------- 80-100 lumens/watt
T5 Fluorescent ------------- 70-104 lumens/watt
Metal Halide ---------------- 65-115 lumens/watt
CREE XPG R5 ------------ 112-132 lumens/watt (based on current from 350ma to 750ma)
One guideline currently being used is 1 led every 10 to 20 square inches (65 to 130 cm2) – 10 square inches for SPS dominated etc (400w MH equivalent), 15 square inches for a general reef tank (250w MH Equivalent), and 20 square inches for lower light inhabitants (175w MH equivalent).
This results in the following numbers for some common sized tanks:
Some adjustments to these numbers will be required to fit the capacity of your drivers but it is a starting point. I also feel that these are the maximum number that would be required - I have 60 on a 4x2x2 but have then located where my corals will be as I see no sense in lighting the weir, or the back of the tank. Careful placement can reduce the number of LED's used, and money.
Now that we have the numbers sorted, what colours are best?
For a few years now most LED lights have used a combination of Cool White and Royal Blue.
As a general guide:
- 1 Royal Blue to 1 Cool White => 10K – 12K
- 2 Royal Blue to 1 Cool White => 14K – 16K
- 3 Royal Blue to 1 Cool White => 17K – 20K
Relatively recently there has been a lot of discussion surrounding the spectrum produced by this combination and there is now a school of thought that using a combination of Cool White, Neutral White, Royal Blue and Blue provides a much better coverage of the required spectrum especially as the Neutral White provides more of the red spectrum.
We have previously discussed the required wavelengths utilised by corals as falling between 400-550nm and 620-740nm. As an example, this is the mix I am currently using (CREE LED’s)
XPE – Royal Blue D40 455-460 nm
XPE – Blue B30 465-470 nm
XPE – Blue B40 470-475 nm
XPG – Cool White 0D0 R4 Approximately 8,300K
XPG – Cool White 1C0 R5 Approximately 7,000K
I am using 50% white and 50% blue which appears to give a colour equivalent to 16K, when compared to my previous 14K MH bulbs. Whilst I am generally happy with the light produced, purple and red corals do not look as good as I would like and I am considering replacing some of the Cool White LED’s with some Neutral White and Purple.
Some people are also experimenting with the use of other colours such as red, green and UV LED’s.
Hopefully, this will give you a guide to start your own deliberations. There is no right or wrong answer and everyone’s requirements will be different depending on your tank and its inhabitants.
Please post any questions or comments in this thread.