# Blue vs White LED - efficiency



## blasterman (Oct 3, 2009)

After I found that cool-white LEDs are stellar for photosynthesis in my reef tank, I started pondering further at what type of applications this would be good for. 

As I understand it, all white LEDs start out as blue LEDs, but have additional phosphors added to produce the additional wavelengths. Correct?

Ok, does that mean a 3-watt blue LED produces the same about of blue light as a 3-watt cool-white (assuming both are in a similiar technology group), or does the 3-watt white emitter produce less blue energy?

Intuition would indicate that the white emitter would produce less blue energy because it's consuming the same amount of power to produce additional spectra, but I've been wrong before.

Relative spectral energy graphs would answer my question, but I can't find any.


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## jtr1962 (Oct 4, 2009)

All things being equal (same die, same drive current), a blue LED will emit a lot more blue light, and also more total light energy in mW, than a white LED. The phosphor in the white LED converts a substantial portion of the blue light to longer wavelengths. Also, some energy from the blue LED is lost as heat in the conversion process (roughly 20%).

For example, let's take Cree's new XP-G at 350 mA. Assume R5 bin, and also assume middle of the bin output (143.5 lumens). Total light energy the LED outputs is roughly 440 mW (going by the very rough estimate of 330 lm/W for the spectra of typical white LEDs). Only a relatively small percentage of this 440 mW falls in the blue area of the spectrum.

Now let's say you make an XP-G using this exact same blue LED, but with no phosphor. Obviously 100% of the output is blue. Moreover, the total light energy outputted is roughly 440/0.8, or 550 mW (Cree is using blues with 50+% efficiency in their best XP-Gs!). Roughly 20% of this energy would be lost in the conversion process to white light if you wanted to make a white LED with this blue LED. But in any case it's easy to see the blue LED produces several times the amount of blue output that the white LED produces.


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