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detecting laser beam in sunlight

Discussion in 'Electronic Design' started by pave, May 25, 2004.

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  1. pave

    pave Guest

    I'm using Siemens BPW34 photodiodes to detect 635nm and 670nm rotating
    laser beams. It is working somehow but I have one Major problem.
    Sunlight is disturbing my detector because the diode I'm using is more
    sensitive at 850nm. I have used red perspex to filter those infrared
    wave-lengths but it is not enough when there is a very sunny day. The
    detector receives hits from sunlight.

    Have you guys solved this kind of problem and how? Should I use
    different kind of photodiode or perspex? I haven't found very good
    alternatives.
    Or could it be solved by electronics?

    Thanks!
     
  2. You can buy narrow-bandwidth optical filters to remove the sunlight
    but let the laser through. But I think these are expensive.
    The "electronic" solution would be to modulate the lasers. You then
    put the photodiode signal through an electronic filter that only
    accepts the modulation frequency.
     
  3. The usual way is to modulate the laser beam in some way, and build a
    receiver that is (mostly) only sensitive to that modulation. The
    sunlight isn't modulated, and will be (partially) rejected.

    A suitable modulation would be 100% AM at several kHz. (Or perhaps
    much faster, depending on the speed of your rotating laser beams.)

    Please read this (long) sci.electronics.design thread about an IR link
    with a double conversion superhet receiver:
    http://groups.google.com/groups?threadm=JyhW7.91717$1.on.home.com

    Regards,
    Allan.
     
  4. On closer inspection, Win's circuit (post #9 in the thread) actually
    modulated the light with a 2.5kHz carrier with 250Hz BPSK.

    Regards,
    Allan.
     
  5. Al

    Al Guest

    Try a differential receiver. One photodiode is exposed to the laser and
    the sunlight, the other is exposed to just the sunlight. Detect the
    diffference between the two. This won't work if the sunlight saturates
    the photodiodes.

    Al
     
  6. Chris Carlen

    Chris Carlen Guest

    Interference filters are tricky in that they are angle sensitive, which
    will let more sunlight in than expected from their transmission graph.
    And since diode lasers don't have very tight wavelength tolerances, the
    filter may need some angle tuning to maximize transmission of the
    lasers' wavelength.

    If the two wavelengths are going to the same detector, than an
    interference filter can't be used.

    In either case some good long wave pass and short wave pass filters can
    be stacked together to make as narrow a bandpass as possible. Such
    Colored glass filters don't have angle tuning issues. But they will
    attenuate the signal quite a bit along with the sunlight.

    A filter stack combined with the differential method suggested below by
    Al might do a good job if fast response is needed without needing to
    resort to modulation.
     
  7. Paul Mathews

    Paul Mathews Guest

    Red acrylic 'perspex' is absolutely transparent to near IR. It blocks
    only blue and green wavelengths, so helps only a little with sunlight.
    However, it can be used in conjunction with a 'hot mirror', which is
    a kind of short wavelength pass dielectric filter. There is a piece
    of hot mirror filter glass over most video camera chips, or you can
    buy some from scientific suppliers. Alternatively, use a red filter
    with 'heat absorbing glass'. This is a slightly greenish appearing
    material, usually about 5 mm thick. You can salvage a piece of the
    latter out of any old slide projector.

    Paul Mathews
     
  8. Dave VanHorn

    Dave VanHorn Guest

    Make a densitometer.

    Take the appropriate IRleds, and a phototransistor, and set up the circuit
    so that an op-amp drives the LED to get a specific current out of the
    phototransistor.
    Set up the mechanicals to allow samples of the material to be placed between
    the emitter and detector.
    Measure the LED drive current relative to the current with just air, and you
    have the density of the material.

    Plastic dyes that have a lot of carbon, absorb IR.
    Exposed photographic negatives make a pretty passable IR pass filter.

    I used this to investigate a plastics substitution problem, where the stuff
    they used turned out to be "frosted glass" for IR. The stuff I specified
    was IR Opaque, and of course the substitution caused a problem.
     
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