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An Opaque question.

  1. Aug 31, 2010 #1
    I'm puzzled.
    In an authoritative metrology text I read.
    "Focus detection instruments cannot be used to measure 'opaque' surfaces"
    as they require a finite level of reflected light to function.
    Non transmittance being (to my understanding) the key definition of "opaque", I looked up the meaning of opaque in various places. A significant number of references give non transmittance and reflectance as the definition of an "opaque" surface.

    Any definitive comments on this would be appreciated.

  2. jcsd
  3. Aug 31, 2010 #2
    I could be wrong, but I don't think that the "normal" definition of opaqueness excludes reflection at all.
    For example, a good mirror is opaque, yet certainly reflects.
  4. Sep 2, 2010 #3
    Indeed that was my understanding and I would have put the reference from the metrology test down to a misprint if I had not found several references quoting definitions for Opaque as being dependent on lack of reflectance.

    Thanks for the reply.
  5. Sep 2, 2010 #4
    When light is incident on a surface, in percentages :

    Absorption + Reflection + Transmission = 100%

    Opaque means that Transmission = 0%
  6. Sep 3, 2010 #5
    That is my understanding of the situation too.
  7. Sep 3, 2010 #6
    Apparently Opacity can be defined as 'mass attenuation coefficient', which is the sum of scattered and absorbed light, so opacity can be a function of reflectance in effect.
    http://en.wikipedia.org/wiki/Opacity_(optics [Broken])

    Anyway the author of the text in question has clarified the sentence quoted in my original post.
    "Focus detection instruments cannot be used to measure 'opaque' surfaces"
    Taking into account a contextual comment regarding focus detection instruments requiring a finite amount of light to be reflected into their detectors from the sample.
    The intention of the queried comment was to distinguish between the measurability of opaque surfaces with differing reflectances, or perhaps differing combined levels of absorption and scattering.
    So there you go.

    Thanks all.
    Last edited by a moderator: May 4, 2017
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