Re: 75 lm/Watt

Actually, I have yet to hear of monochrome LEDs of such overall luminous efficacy being available. The high conversion efficiencies apear to me to be in blue and red, which are not the best for high lumens/watt.

It appears to me that maximum possible overall luminous efficacy would be for a blue LED chip to have its output entirely fluoresced into green or yellow, but there is little effort there since the big money is after white. So we have blue LED chips combined with a broadband yellow-glowing phosphor producing red through green, and the phosphor coating is thin enough to let some of the blue light through.

These phosphors are not perfect and have some losses. For one thing, it appears to me that a layer of blue-utilizing phosphor thick enough to give a "warm white" color loses enough light to result in lower overall luminous efficacy than is available with a "cool white" achieved with a thinner phosphor layer. The latest high lumen/watt figures for laboratory prototypes tend to have color temperatures in the 4,000's to about 5,000 Kelvin. Most white LEDs already in production have even higher color temperature - in the 5,000's and 6,000's and I have seen some even higher (a noticeably bluish shade of white). I suspect yellow would fare worse than warm white. Perhaps the most efficient yellow LED one can get today is a white LED with a yellow filter over it.

- Don Klipstein ( snipped-for-privacy@misty.com)