What Does "Opaque" Mean in Metrology?

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Discussion Overview

The discussion centers around the definition of "opaque" in the context of metrology, particularly regarding its implications for focus detection instruments. Participants explore the relationship between opacity, reflectance, and the ability to measure surfaces using these instruments.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Technical explanation

Main Points Raised

  • One participant expresses confusion over the definition of "opaque" as it relates to metrology, noting that focus detection instruments require reflected light to function.
  • Another participant suggests that the common definition of opaqueness does not exclude reflection, citing that mirrors, which are opaque, still reflect light.
  • A participant agrees with the notion that the metrology text may have a misprint, as several references define "opaque" based on a lack of reflectance.
  • Some participants clarify that when light hits a surface, the sum of absorption, reflection, and transmission equals 100%, and that "opaque" specifically means transmission equals 0%.
  • One participant introduces the concept of opacity as related to the mass attenuation coefficient, indicating that opacity can involve both scattered and absorbed light, thus linking it to reflectance.
  • A later reply notes that the author of the metrology text clarified the original statement, emphasizing that the distinction was about the measurability of opaque surfaces with varying reflectances or levels of absorption and scattering.

Areas of Agreement / Disagreement

Participants express differing views on the definition of "opaque," particularly regarding the role of reflectance. There is no consensus on a definitive interpretation, and the discussion remains unresolved.

Contextual Notes

Some definitions of "opaque" may depend on specific contexts or interpretations, and the discussion highlights the complexity of measuring opaque surfaces with varying properties.

WaltLankor
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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.

Thanks
 
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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.
 
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.
 
When light is incident on a surface, in percentages :

Absorption + Reflection + Transmission = 100%

Opaque means that Transmission = 0%
 
Dr Lots-o'watts said:
When light is incident on a surface, in percentages :

Absorption + Reflection + Transmission = 100%

Opaque means that Transmission = 0%

Thanks
That is my understanding of the situation too.
 
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 )

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.
 
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