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Red/green bi-color LED mcd ratings

Discussion in 'Electronic Basics' started by Ben Jackson, Nov 30, 2005.

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  1. Ben Jackson

    Ben Jackson Guest

    I was reading the specs on a bunch of red/green (diffuse white when off)
    bi-color LEDs (real party animal I am). I noticed that the standard
    was for about a 4x higher 'mcd' brightness rating for the green output
    vs the red. Eg 3mcd red, 12mcd green.

    Does this result in even brightness as perceived by the human eye? Does
    turning both on at full brightness produce a uniform yellow? Or do you have
    to take the ratio into account?
  2. Bob Myers

    Bob Myers Guest

    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

    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.
  3. 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

    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
    580's or close to 590 nm).

    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.

    2. 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

    - Don Klipstein ()
  4. 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 ()
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