Re: measurement device trivia question
Re: measurement device trivia question
- Subject: Re: measurement device trivia question
- From: Graeme Gill <email@hidden>
- Date: Fri, 05 Dec 2008 10:19:33 +1100
email@hidden wrote:
The old Spectrolino devices can be fitted with a D65 filter.
When using it with Measure Tool, I can specify D65 illumination.
I presume this is an appropriate combination.
These two aspects are not directly related. The underlying measurement
is reflectance - that means that the instrument is measuring the percentage
of light returned in each wavelength band. The spectral shape of the illuminant
it is using to do this measurement is divided out, so (to a first order
approximation) the spectral reflectance is independent of the illuminant
it uses.
The illuminant here is a real illuminant, determined by the
nature of the instrument. Fitting a filter changes the effective
illuminant being used, but in most situations the reflectance
readings will be unaffected by this, since the instrument illuminant
is divided out of the measurement.
When you ask for a CIE XYZ value (or equivalent), a model is applied to
compute how a material with that spectral reflectance will look to
a human observer. The model involves an illuminant, the materials
spectral reflectance, and a model of how our eyes detect light at
different wavelengths.
The illuminant here is a hypothetical illuminant, and
typically is chosen to be the same as an industry standard (ie. D50),
so that the CIE values can be compared with other CIE values.
Q. Why is the filter available then, if it doesn't change the readings ?
A. It does change the readings in some circumstances, particularly
when the material being measured has fluorescence, because with
fluorescence the nature of the measurement illuminant spectrum
cannot be easily divided out. To do that would require a very expensive
bi-spectral instrument. By making the instrument illuminant closer to
the actual viewing illuminant, the effects of things like fluorescence
are better accounted for.
Graeme Gill.
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