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Discussion in 'Lighting' started by Andrew Gabriel, Nov 7, 2005.

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  1. I recently bought a small CCD camera, which falls back to IR mode
    in low light level and has 12 IR LEDS which come on in the dark.
    They seem to project quite a bright beam which you can see in
    the resulting image (the beam isn't as wide an angle as the
    camera, but that doesn't much matter in this case). However,
    much to my surprise, you can see the LEDs glowing a dim red
    when they're on. Do all IR LEDs do this at high power?
    I had imagined they would be invisible.
  2. I am guessing that these LEDs are GaAlAs type with peak wavelength
    probably around 880 nm. In my experience, these are slightly visible.
    And I don't find them so dim as to require a really dark room to see them
    glowing - just substantially dimmer than dimmish-normal room light.
    I have in the past tried using a reflective diffraction grating to see
    the spectrum, and I saw it. It appears to me that what is visible is the
    shorter wavelength end of the main emission band, with wavelengths mostly
    in the low 800's nm. I remember seeing spectral power distribution curves
    for GaAlAs LEDs before, and they do not show spurious emissions nor
    multiple peaks.

    I have found such wavelengths to be slightly visible. I have seen,
    with the above reflectrive diffraction grating and filtering to block
    wavelengths near and below 700 nm, the 766.5/769.9 nm potassium lines and
    the 818.3/819.5 nm sodium lines in the spectra of high pressure sodium
    lamps. Also the "Z" line in the solar spectrum, at 823 nm (atmospheric
    oxygen? - more visible when the sun is lower).

    There is another chemistry for IR LEDs - GaAs. Although this is older
    than GaAlAs, I have found it easy recently to find IR LEDs with this
    chemistry. The peak wavelength is usually 940-950 nm. In my experience,
    these usually do not visibly glow. Sometimes I can barely see one
    glowing, with full power, full dark adaptation and a very dark room - but
    normally I find these to not visibly glow at all.

    - Don Klipstein ()
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