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Optical filter for better viewing of blacklight effects

  1. Oct 30, 2012 #1
    Hi guys :)
    I'm designing an automatic soap dispenser, in which the soap particles contain phosphors. These will emit visible light in the presence of UV light. I intend to have a small UV light source underneath the dispenser.

    Thing is, its really bright in bathrooms, and its tricky to see the UV/phosphor glow from the soap unless one is in the dark

    Would there be any way to block out the ambient bathroom light via an optical filter, which only lets in the light from the phosphor particles?

    The optical filter would be installed on the end of the dispenser, such that you could see the soap particles glowing if you looked through the filter, but everything else would be dimmed.

    Thanks, just not too sure what sort of wavelengths the emitted light is, or whether it would be possible to isolate this.
  2. jcsd
  3. Oct 30, 2012 #2


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    Staff Emeritus
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    Of course. There are plenty of filters out there. You just need one that lets the specific wavelength of the emitted light through.
  4. Nov 1, 2012 #3
    Since most of the ambient light is reflected from the outside of the soap container,
    you might try a sheet of polarizing film.
    Just like polarized sunglasses cut down of reflected glare, the film should limit the
    reflected ambient light.
    The light from the phosphor is "new light" and random so much of it will pass the
    polarizing film.
  5. Nov 1, 2012 #4
    Why don't you use a UV LED for the UV source. If you modulate the UV LED with a specific frequency (not to include 60 or 120 Hz) your photosensor can be designed to amplify just that frequency.

    What color is the fluorescent radiation of the soap? An optical filter for that color might help.
  6. Nov 2, 2012 #5


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    Gold Member

    I'm thinking you'd be better off with a brighter blacklight.
  7. Nov 2, 2012 #6
    How will your dispenser use the phosphor glow, to determine how much soap to dispense. Since the light levels in restrooms varies tremendously and even varies as people move around, I think it will be very difficult to accurately measure the amount of soap dispensed. Even the skin color of the hand may cause a different amount of soap to be dispensed.

    The more I think about this it seems to me it would not be a good idea to use an optical filter for the phosphor glow. What would happen if you decide to change vendors for your phosphor and the color changes?

    I'm even more convinced that the only way you can get an accurate measurement of the glow is to modulate the source and have a receiver that can measure the amplitude of just the modulated reflection.
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