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Optics Help

Discussion in 'General Electronics Discussion' started by Arouse1973, Jan 14, 2014.

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  1. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    I have an interesting problem I need help with. I have an Opto Reflector that I need to detect the presence of a metal surface 50mm away. Circuit works fine, but if I put certain transparent objects in its path as a cover to protect it from dust it reflects of it.

    Now you might think this is obvious but one of them is optical glass and the other is an IR acrylic lens. The IR material is rated to pass the 950nm IR.The glass should start attenuating at around 3 microns, the IR device transmits at 0.95 microns. The surface of the glass is very smooth and the surface of the IR lens has a roughish texture. Now this is the weird thing.

    If I take an A4 protective wallet, you know the ones you put paperwork in to protect it which have an almost frosted look. I don't get a problem. If I take a component draw plastic insert, shiny on one side frosty on the other I again don't get a problem on either side shiny or frosted.

    What do you think is happening? Total scattering (Lambertian diffuser) or the thickness of the material causing in phase reflection at that wavelength of light. This can happen if the material thickness is a multiple of the wavelength of the light. Or do you think it might just be the surface finishes of the material itself?

    Thanks
    Adam
     
  2. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    Is it *either* the optical glass *or* the acrylic lens on their own which affect the circuit? (i.e. have you tried one at a time)

    I think you probably have, but just wanting to be sure.

    Is it the same piece of glass/acrylic covering the LED and the sensor, or 2 different pieces? (or are the LED/sensor *very* close together and must be covered by a single cover?)

    Does the sensor see a continuous signal, or nothing at all with the cover(s) in place?
     
  3. duke37

    duke37

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    Jan 9, 2011
    Hve you tried the glass/acrylic at an angle so that the reflection does not get back to the detector?
     
  4. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    If I use either the glass or the IR lens both reflect too much. The lens will be one entire lens made of a suitable material. Both lenses still let most of the IR through but the reflection is too great coming back and I can't reduce the TX or RX.
     
  5. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    We did try this but it was not that controlled. It might be worth looking at this again as you have mentioned it.
     
  6. jpanhalt

    jpanhalt

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    Nov 12, 2013
    Reading post #4, I get the impression that you want to place both the TX and RX on the same side of the same protective sheet. If that is correct, what you see can be explained by lack of scatter by the glass and acrylic The frosted material scatters a lot, so the reflected beam is not intense enough to affect appreciably your receiver.

    My solution to that problem was to place the emitter outside the protective cover (Actually, I mounted it in the cover). A second option would be to use separate covers or a cover with an opaque block/baffle between the TX and RX.

    John.
     
  7. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    High John

    It's in the same package. the reflection is bouncing back of the surface. Yes agree with that but why is the optic glass reflecting this light when it should pass it. Also the shiny surface of the other side of the frosted plastic didn't reflect.
    thanks

    Adam
     
  8. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Didn't mean you were tall. Meant to say Hi :)
     
  9. jpanhalt

    jpanhalt

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    Nov 12, 2013
    Any interface between media of different RI's will reflect. Sure, a lot will get through the clear plastic or whatever, but at close range, it doesn't take much reflection to affect the receiver.

    We used to use a number of like 5 to 10% reflection for an optical quartz-air interface. Try sticking a baffle between the TX and RX to see if it cures the problem.

    Remember, the TX LED probably has a fairly narrow cone of emission. So a smooth reflecting surface with less random scatter is going to affect the receiver more. You can do a lot with geometry, but the baffle often works.

    John
     
  10. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Yeah valid point about the cone, hadn't thought of that. The cone is where you are guaranteed half power so this makes sense if the cone is narrow you have a larger irradiance at a point so 1% of that coming back is worse that 1% of say double the cone angle. How do you make the baffle.
    Thanks
    Adam
     
  11. jpanhalt

    jpanhalt

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    Nov 12, 2013
    I would use black vinyl electrical tape. Stick a doubled piece to your lens or filter and bend it down so it is between the TX and RX. It does not have to be light tight.

    I forgot to ask, are the TX and RX separable? If so, call the RX the lowest surface, moving the TX so it is a bit higher toward the protective elements may help too.

    John
     
  12. Arouse1973

    Arouse1973 Adam

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    Dec 18, 2013
    Ah yes we call this a separator, yes we have used this many times on other jobs. I can't separate the unit it's all build in one. But I will try what you have mentioned.
    Thanks
    Adam
     
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