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Re: [diy_3d_printing_and_fabrication] Re: Projector Calibration/ Lens Distortion

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  • Jon Elson
    ... Because of automatic contrast/exposure adjustment of webcams and digital cameras, it may be very hard to get repeatable measurements with them. Also,
    Message 1 of 39 , May 21, 2013
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      Graham Stabler wrote:
      >
      >
      > you can buy a photodiode for a few dollars from any electronics
      > supplier but then you need to build some electronics, not hard though,
      > the webcam avoids that though it was just a thought.
      >
      Because of automatic contrast/exposure adjustment of webcams and digital
      cameras,
      it may be very hard to get repeatable measurements with them. Also,
      projectors
      with color wheels will be rapidly flashing cyan/magenta/yellow frames which
      may give the camera a banded image. A photodiode (or even an LED) a
      9 V radio battery and a DVM set to current mode may do much better in
      giving a repeatable numeric result. You want to set up the diode in
      photoconductive mode to get the most linear result, so the + of the
      diode should go to the minus of the battery (providing reverse bias).

      Jon
    • Graham Stabler
      ... The reason I suggested a diffuser is because I was talking about converting a camera into a sensor so the image you get is actually an average of the
      Message 39 of 39 , May 23, 2013
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        On Wed, May 22, 2013 at 11:32 PM, Bhatt Himanshu <bhatthimanshu@...> wrote:

        @Graham
        I read about ND filters and i think i can get it from local camera shop. Do i need diffuser if i use ND filter/filters properly. 

        The reason I suggested a diffuser is because I was talking about converting a camera into a sensor so the "image" you get is actually an average of the light hitting the sensor. The idea of an ND filter is just to reduce the light levels though as I mention on another thread you can do this by projecting darker images anyway I think. 

        I want to make it clear that I am not suggesting trying to measure the output of a specific pixel on the projector but rather to get the average intensity over a fairly small area (because crappy webcam sensors are tiny) and then use a grid of these measurements to produce a correction which should be fairly smooth if the aberration is only caused by the optics.

        Cheers,

        Graham
         
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