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Luminance of small normal LED green bulb

Discussion in 'Electronic Repair' started by Maria Mustafa, Jul 26, 2013.

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  1. Hello every one

    I want to know the luminance of simple LED green bulb of normal small size in cd/m2 at 2.5V

    Can anyone help

    Regards

    Maria
     
  2. Mike S.

    Mike S. Guest

    If you have the manufacturer's part number, you usually can find a PDF of
    their data sheet. "Simple LED green bulb" can mean jsust about anything.
     
  3. Hello Arfa

    Actually, the cd/m2 unit is not so meaningless - it's the measurement unit
    of
    luminance (http://en.wikipedia.org/wiki/Luminance). You may have heared the
    unit "nit" being referred to (apparent brightness of CRT, LCD screens, EL-
    foils, or any other flat and rather regular "area light sources"), it's the
    same unit. 1 nt = 1 cd/m2. The integration over the visible wavelengths is
    weighted with the human eye perception sensitivity, so it measures how
    bright
    a luminous area appears to be (independent of the radiating area, distance,
    and viewing angle, for that part see the cos(beta) term in the definition).

    In principle this measurement applies to any area that can emit light, the
    active die area of the LED chip including. I've got no real experience with
    actual values, but the German wiki page on luminance has some examples.
    (http://de.wikipedia.org/wiki/Leuchtdichte#Beispiele). A white LED is listed
    as 50Mnt, a fluorescent tube as 11knt. The website for a Philips "Altilon"
    (http://www.philipslumileds.com/products/automotive-lighting/luxeon-altilon)
    high-power LED mentions 60Mnt when driven at its rated current of 1000mA,
    but this looks like a thermal challenge. This LED integrates a copper heat
    spreader that has to be mounted with M3 screws to a sizeable heatsink. A
    typical "bulb of normal small size" is rather unlikely to drive the chip at
    a current density comparable to the above, so the perceived brightness per
    unit area should be less. Green LEDs however will have a noticeably higher
    nits rating than white LEDs of the same chip current density because of the
    peak in the human eye response to green wavelengths (and because the white
    LED prosphor is never 100% efficient).

    It seems like typical LED datasheets mostly skip this parameter, and if
    any specify it, those tend to be LEDs that are intended to be mounted in
    secondary optics designed for a very high luminous intensity (headlights,
    searchlights and the like). There's some (partial) sense in it, since the
    luminance is a figure-of-merit parameter when it comes to integrating the
    LED into high intensity narrow beam optics, but is a little less useful
    otherwise. The other use case where this parameter gets important is the
    "sunlight visibility" - the LED visibility under bright sunlight. To be
    visible the LED chip needs to have a considerably higher luminance than
    the LED and its immediate surroundings would have by way of reflection
    when under direct sunlight (this is easier said than done as most LEDs
    don't achieve such high luminance values). Green LEDs help in this regard
    because of the eye sensitivity and because of the colour contrast, the
    sunlight not being green after all.

    To the OP: There's a regular on the SE* newsgroups who knows a lot about
    light sources - Don Klipstein. He's got a page http://donklipstein.com/
    and since he knows a lot more than I do about all sorts of light emitting
    things, he might know some more reliable typical values from experience.

    Regards,
    Dimitrij
     
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