Red/green bi-color LED mcd ratings

have

Turning both the red and green LEDs on produces a yellow glow; precisely WHAT "yellow" you will perceive depends on the ratio of red to green, but it will be uniform no matter what (you'd have to be pretty close to the emitters, and with no diffusing layer in place, for the eye to distinguish the separate red and green emitters.

The difference in the "brightness" figures (technically, the candela, and therefore the millicandela, is a measure of luminous intensity, which is the luminous flux per unit solid angle, corrected per a standardized model of the sensitivity of the human eye) is due to the differences in the efficacies of these LEDs - greens are typically better than reds by quite a bit - which in photometric units results in large part from the fact that the eye is most sensitive to green, less so to red and blue.

Bob M.

Green will look brighter than red, but human vision will "compress" range of brightness to make 12 mcd look only about 2.5-3 times as bright as 3 mcd.

As for making yellow: 3 mcd of red and 12 mcd of usual-yellowish-LED-green will add up to 15 mcd of yellow, rather than only a slightly more yellowish green.

My calculations:

Typical cheap red LED that gets only 3 mcd has dominant wavelength (color specification that is approximately but not always exactly "hue") close enough to 650 nm. CIE chromaticity coordinates would be about x=.725, y=.275, z=0.

Typical cheap green LED in a non-green-tinted package has dominant wavelength in the upper 560's to close to 570 nm, and CIE chromaticity would be about x=.438, y=.56, z=.002.

The 1931 CIE standard has "y" related to luminosity as a function of wavelength. (x is roughly reddishness, y is roughly greenishness, z is roughly bluishness.) The "photopic function" of wavelength had an update in 1988 making it differ from the "Y-bar function" in the deep blue and violet, but this usually only slightly invalidates the below calculations, and not at all when wavelengths shorter than greenish blue are not involved.

So, as for "X-impact", "Y-impact" and "Z-impact", I would do this:

First: Multiply chromaticity coordinates by ratio of photometric specification to the y chromaticity coordinate.

That results in:

Red: X becomes 7.91, Y becomes 3, Z remains zero

Green: X becomes 9.39, Y becomes 12, Z becomes .043

Add these up, and you get:

X totals 17.3, Y totals 15, Z totals .043

To get a chromaticity specification from these, divide each by the sum of all three. That yields:

x=.535, y=.464, z=.001

This is close enough to the color of about 583 nanometers, which is yellow and not greenish, although a little less orange than usual yellow/"amber" LEDs (which have dominant wavelength typically in the upper

If this 2-color LED has a red chip that is of an orangish shade of red as opposed to pure red, expect the yellow to be slightly less orangish still, but probably not greenish.

What to watch out for:

1. Photometric specifications for LEDs are often only loosely approximate.

1. Some 2-color red/green LEDs have a red chip that gets less efficient and more orangish as current increases above a milliamp or two, while the green chip has efficiency increasing with current until the current gets past 10-30 mA or more. Such an LED may have a lemon-yellow color at 20 mA per chip, but a more orangish color at lower currents, possibly even reddish orange at 1 mA per chip, and may be a slightly greenish shade of yellow at 30 or more mA per chip (if it does not overheat). This is the result if the red chip is a GaP red with peak wavelength around 690-700 nm, and red LEDs with such GaP chips are sometimes called "low current red".

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

I have often seen the diffusion in diffused bicolor LEDs to be weak enough to allow the LED color to vary significantly with what direction youlook at it from.

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