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using camera flasher for CMOS sensor illumination.

Discussion in 'Electronic Design' started by [email protected], Apr 13, 2007.

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  1. Guest

    hi,

    im working on a project which involves image aqusition from a gray
    scale image sensor. (KAC - 9630). the aqured image is at a distance of
    5 inches from the sensor and is flat. for best results i need a way to
    make the illumination level on the surface constant, uniform and very
    high. i've tried white LED illumination, but could not get a uniform
    field. therefore im planning to use a flasher of a normal camera for
    this. Problems im having are:

    is it possible to continuously power a flasher. if yes, what might be
    the powrer consumption?
    how do i power a flasher. do i need specialy circuits for that? if
    yes, where can i get some sample circuits?
    if continuous powereing is not possible, will it be possible to turn
    it on just before the image is taken. the image rate of the sensor is
    580 frames/sec.

    are there any alternatives to this scheam?

    thank you.
    CMOS
     
  2. As the name implies, a flasher isn't for continuous operation. It puts
    out a high intensity for a short time. Trying to flash it at 580 /
    sec might make it explode if the intensity was high. Maybe you could
    use a ring of high intensity white LEDs around and just behind the
    sensor, maybe with a diffuser, to get even illumination of the object.
     
  3. A flash that operates at 580 flashes per second is a stoboscope
    (and a fast one).
    However the CMOS sensor has an exposure time, and the stroboscope would
    need to be synchronized with it.
    Also a 'flash' does _not_ guarantee uniform lighting.
    To sync the stroboscope wit hteh sensor would requrie electronics and likely
    some programming (embedded), if at all possible, things like
    flash duration, light output curve, CCD shutter time etc need to be looked at.
    So perhaps it would be much simpler to add some extra normal lightsources
    to light out your object.
     
  4. default

    default Guest

    Flash tubes aren't designed to stay on - they burn out if you try.

    A short arc xenon lamp or short arc mercury lamp would work - but you
    may have to filter out the UV or risk bleaching the gray scale target.

    A circular fluorescent lamp at close proximity to the target and
    around the sensor might be another cheaper alternative. - I used a
    fluorescent lamp for a "dark field light box" I built, along with a
    cmos camera and ran into a problem with "aliasing," 60 hertz flicker
    of the light was in and out of sync with the camera's vertical
    sampling.

    The company with the software was not willing to modify their design
    to average the images over time (one solution) for a reasonable cost,
    so I ended up using four tubular aquarium bulbs to provide
    illumination and had to add a timer so the thing wouldn't cook itself
    to destruction if someone forgot to turn it off (160 watts)

    I should have stuck with the fluorescent and made a high frequency
    ballast for it, but the light box was wanted in the lab and they
    wouldn't let me tinker with it.
     
  5. Paul Mathews

    Paul Mathews Guest

    To make a more uniform field with LEDs, choose wide-angle or unlensed
    types, use many of them, extending over and beyond the desired
    coverage area. If results are not uniform enough, add some sort of
    scattering element in between. The prismatic plastic sheet material
    used with fluorescent lighting fixtures (troffers) has lower losses
    than milky diffusers. Finally, consider using processing to deal with
    the non-uniformities. Some imager chips do this for you.
    Paul Mathews
     
  6. colin

    colin Guest

    A block of LEDs the size of your target,
    or as big as needed to make a uniform light on it,
    with the camera looking through a hole in the middle of it.

    Colin =^.^=
     
  7. Look at Xenon discharge lights. Small units are available for bicycles,
    while slightly larger units, are used as car headlights. Basically, a
    'flasher', is a Xenon light, giving high intensity for a short time.
    However the tubes are not designed for continuous operation, and will
    overheat if driven continuously. Units designed for continuous repeat
    operation, are those used in stroboscopes, while the units designed for
    continuous operation, are the Xenon discharge lights.
    You are going to have to design carefully, to get even illumination
    though. This is why professional photographers, will have multiple lamps,
    and diffusers. What about a EL panel?.

    Best Wishes
     
  8. Hawker

    Hawker Guest

    I had to do this very same thing for a client not to long ago, only I
    used the Micron sensor instead. We only needed the IR band.

    I ended up using a large grid of IR LEDs. The trick was spacing them for
    even light. Our ID dude helped, and armed with the dispersion angle of
    the LEDS and the distance (luckily fixed) the image he suggested the
    spacing to use, which was close enough. There was some 512 - 768 (I
    don't remember anymore it was almost 2 years ago) LEDs to the array. The
    distance was about 18" or so.

    This replaced a photo flash system that used 4 flashes and gave more
    even light than it did.

    Good luck.
    Hawker
     
  9. jasen

    jasen Guest

    no, it'd probably melt or explode.
    580/second would be much like having it on full time.
    Some automotive lamps are labeled as xenon lamps, you could try them,
    (I only mention it because xenon discharge tubes are the camera flash)

    or just a regular spotlamp, halogen, incandescent, stage lighting,
    or whatever.

    The output of a projector (eg slide projector) is a very uniform
    field too. (especially if you replace the AC supply to the lamp with
    regulated DC)

    Or the sun if you can get suitable weather and work in the sun
    or arrange a heliostat.

    Bye.
    Jasen
     
  10. Lionel

    Lionel Guest

    No. It'll overheat.
    You're going to find it extremely difficult to flash a Xenon tube at
    that rate. LEDs will handle it easily, & can be pulsed at much higher
    currents than can be used for continuous illumination.
    The reason you can get a fairly flat field from a camera flash is that
    the reflector has been designed very carefully. The easiest solution I
    can think of for your problem is to do the same for a housing for a
    bunch of LEDs.
     
  11. Lionel

    Lionel Guest

    Indeed. And for small subjects, up close (which is what the OP seems
    to be talking about), you'd normally use a ring-flash, which is
    typically a pair of 1/2 circle xenon tubes or a ring of LEDs in the
    shape of a doughnut, that fits around the lens itself.
     
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