Follow-up comments on LED lumens/Watt efficacy: More than meets the eye

Take a look at a couple of comments on the previous post regarding the efficacy of production LEDs, Real-world numbers on production HB LEDs: 32% efficiency . A reader points out that using 683lm/W (from Wikipedia) as the 100% luminous efficacy figure, (theoretical maximum lm/W), then Cree’s 114lm/W works out to a luminous efficacy of 114/683=17%, which is the original figure from the first post.

Doug Leeper responds: 

(I’ve posted the comment below to save clicking back and forth.)

 “On the max 100% luminous efficiency numbers, one has to consider the spectrum…which greatly alters that number.

The luminous flux is the part of the power that is perceived as light by the human eye, and the figure 683 lumens/watt is based upon the sensitivity of the eye at 555 nm, the peak efficiency of the photopic (daylight) vision curve. The luminous efficacy is at that single wavelength or frequency-just for 555nm only. However the visible light spectrum needs to be considered.

Wise and thoughtful comment but the basis is utterly flawed.

At some single wavelengths the maximum efficiency is only 10 lumen/watt.

When calculating the efficiency of a YAG White LED, you must add up the eye’s efficiency for the entire visible spectrum. For Ce:YAG White, there are sight variations in the emission spectrum, as well as the color temperature of the White, and the maximum efficiency can vary from 230 l/W all the way up to a maximum of 330 lm/W for each LEDs visible emission spectrum.

The curve for the human eye also changes from the Photopic, to mezotopic, to Scotopic- depending on the ambient lighting conditions. This is as in the different conditions you are utilizing rods or cones, or a combination of both.

A little more on these can be found here:

4colorvision.com/pdf/18abnormalities.pdf   

If you visit here, you can see the lumens per watt graphed at different frequencies, and the daylight vs. nightime vision response:

hyperphysics.phy-astr.gsu.edu/hbase/vision/bright.html#c2

More detailed lm/W graph for the human eye photopic:

www.molalla.net/~leeper/human_~1.jpg

Both: www.molalla.net/~leeper/humaney.jpg

So, for you, look up the lm/W of the human eye at 400nm, and you should start to grasp the concept of having to sum or integrate the spectral output vs. the human eye and find that the 683 lm/W idea is fundamentally flawed.

Sharp thinking though, Kudos!”

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