[argyllcms] Re: fwa compensation unexpected results

Gerhard Fuernkranz wrote:

I think you've mistakenly mixed up "-i" and "-I". In the spec2cie utility I
had contributed, the capital "I" option permits overriding the instrument's
light source spectrum, i.e. the light source used by the instrument when it
takes the measurements. The default (if -I... is not specified on the
command line) is the light source assoiciated with the instrument type given
in the input .ti3 file. I had added this option 1. for experimental
purposes, and 2. because it looks like some instruments can use not only a
single, but different, selectable light sources (e.g. with or w/o UV filter
in the illumination path, or e.g. Spectrolino with D65 filter).

The discussion wasn't very specific about the tools being used, but I was assuming profile was being discussed - but looking back at the original post, you're right, Roberto was using spec2cie -f (why, I don't know.)

I have made a note to myself in the source code, that I went about
figuring out the instrument illuminant the wrong way - I should
have recorded the name of the instrument illuminant in the .ti3
file, rather than relying on the instrument type. That way the
SpectroLino could be used with it's D65 filter. I'll fix this
at some later stage, and the spec2cie -I flag should then not be needed.

So if "-I" is specified, then it should refer to the actual light source
spectrum used by the instrument when it takes the measurements. For many
instruments with a tungsten lamp, I guess A may be a good approximation,
but if an UV filter is in the play, then specifying an UV filtered ill.A
spectrum instead would IMO be more appropriate.

'A' is as close as one is going to figure it out, short for measuring it with another instrument.

(I have some doubts though, whether the FWA compensation will still work as
well for measuments that have been taken with an UV filterered instrument,
even if the actual (filtered) instrument light source spectrum is used for
computing the FWA compensation. I'm afraid that the prediction accuracy of
the FWA compensation will rather suffer in this case. What do you think?)

It has always been the case that a non-UV filtered instrument must be used if FWA compensation is to be applied. An instrument with a software selectable filter could actually give a better result, but alas, no such device exists.

Graeme Gill.

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