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Subject: Re: ACR Calibration


Chris was refuting a statement from Bruce Fraser. Bruceís comment was similar to mine. Chris referred to some colorimeter filters. Colorimeters can and do use XYZ metrics. But to imply they use optical filters that match XYZ is a bit of a stretch. I donít think we need to go into the fundamental differences between a colorimeter and a camera, so I will still simply agree to disagree.

BTW, a spectrally calibrated and high quality camera could be used as a reflected light colorimeter. You would need an ACR option to select Lab mode as your color space instead of RGB. The Lab values could be easily converted to XYZ or the PS Info panel could offer an XYZ display option.

I do find Thomas Knollís comments to be on target. If I understood him, the math and matrices he refers to would be the CIE color matching functions that depend on spectral data about the sensor. Mapping the sensor data to XYZ coordinates (chromacity) is the fundamental objective of color matching. The data can be vendor provided, measured by another vendor (or the consumer), or simply a WAG. This is what could be in the ISO TIFF/EP tag or simply published.

As Tom states, the data is not a linear combination of the cone responses. Thatís what color matching algorithms and tables address.

I assume, because Adobe tells me so, that some kind of calibration is done for each ACR supported model. I donít know what the steps are or how thorough it is because Adobe doesnít tell me. I am disappointed when I experience the effort required on my part to calibrate something that logically should already be close at least.

Cheers, Rags :-)


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