[argyllcms] Re: Using an i1 to measure FWA content?
- From: Gerhard Fuernkranz <nospam456@xxxxxx>
- To: argyllcms@xxxxxxxxxxxxx
- Date: Tue, 09 Feb 2010 19:55:37 +0100
Graeme Gill wrote:
> Gerhard Fuernkranz wrote:
>> Well, if we'd assume that the filtered light does not contain any
>> energy below 400nm or 420nm, then basically we would have to discard
>> the readings for these wavelengths completely, as they would be the
>> result of a zero by zero division (no light multiplied by the
>> reflectance of the sample (+ noise), divided by no light multiplied
>> by the reflectance of the reference tile (+ noise)).
>
> This could well be the case, especially as it's an incandescent source
> and has less energy in the blue to start with. But in theory there is
> no need to be zero, it just needs to be a lot less than the
> illumination level used at the emission wavelengths, so in theory the
> short wavelengths could be measured in spite of the UV filter.
I also doubt that the filter cuts off completely, but it's still the
question what SNR can be eventually obtained at the short wavelenghts.
Without any opportunity to look "inside" (i.e. at the raw readings from
the sensor) it will be certainly hard to assess.
For comparison, I'm wondering what's actually the situation with the
Color Munki? Isn't its light source UV-poor too? Is the SNR still large
enough at the very short wavelengths, so that the driver can still
safely divide the (offset-adjusted) raw readings obtained from samples
by the raw readings from the reference tile, w/o yielding "wild"
results? Or does the Munki simply not report this wavelength range at all?
Btw, I rather don't think that the reflectance of the white reference
tile has actually a drop-off at about 400nm as suggested by the UV-cut
measurements, but I find it more likely that this apparent drop-off is
caused by the instrument (If manufacturer supplied reference numbers are
available for the white reference tile, then this could be easily verified).
Regards,
Gerhard
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