Re: RelCol Mapping of Middle Gray
Re: RelCol Mapping of Middle Gray
- Subject: Re: RelCol Mapping of Middle Gray
- From: Roger Breton <email@hidden>
- Date: Tue, 08 Mar 2005 13:04:39 -0500
> I know that reflective Lab readings are absolute measurements. However,
> what is the reference when making emissive readings?
>
> Ray Maxwell
Depends. My own best take on this is some chosen light source, ideally some
calibrated monitor with know spectral characteristics. This is left to the
instument manufacturer's discretion. What's critical is that the instrument
be factory-calibrated on sources typical of what the intent of the
measurement for. It's possible for an instrument to carry multiple
calibration. Hence, one for LCD typical spectrums and one typical of CRT
spectrums.
The whole accuracy of emissive measurements revolves around the factory
calibration. That's why I say, it's worth recertifying a colorimeter every
year when doing serious color work.
One way to measure emissive sources is by normalizing the measured XYZ to
the monitor white, thus making RGB = 255 map out to Lab = 100,0,0. This way,
every measurements taken is 'relative' to the monitor white point.
Another way is to measure in absolute terms, using either a
spectroradiometer or a colorimeter. Depending on the instrument, the
returned measurements can either be watts/sr/m2 (spectral data) or xyY
(integrated tristimulus).
And don't forget that in ICC monitor profiling there's also the business of
chromatic adaptation.
Regards,
Roger Breton | Laval, Canada | email@hidden
http://pages.infinit.net/graxx
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