Thank you, that's interesting and prompts me to ask: do colorimeters/ spectrophotometers attempt to mimic the human observer? I suppose there can only be a claim for mimicking - for how could it ever be verified? But we use them for measuring and determining a colorimetric match so we have to rely on the claim because besides that there is only subjective observers. Or we rely on both. My question about a measured match between two samples that don't match visually is still important to me and I think the answer puts a lot of stress on how colorimetric accuracy is defined. Henry Graeme wrote: Henry Davis wrote:
Nobody is saying that the illuminant isn't a hugely important factor.
The main reason that the illuminant is a factor is that many/most cameras aren't colorimetric. By that I mean that their spectral sensitivities don't match a human observer. If they did, and their (light level) linearity was known, then (by definition) they'd be a high spatial resolution colorimeter. The practical situation is analogous to the one with cheap display colorimeters: They can be very accurate with a correction matrix if the spectral nature of the samples they are reading is known. With a display device that has colors that are (largely) a liner combination of the three primary spectra, a matrix does nicely. For a camera, it can't be that easy, because real world object colors are composed of many metamers, and the spectrum hitting the camera sensor is that times the illuminant spectrum. So a "calibration" matrix can fudge it for a range of expected object spectra x expected illuminant, but ultimately it's going to work the best over the widest range of conditions if the camera spectral sensitivity is closer to a human observer. It would be interesting to know which current cameras best fulfil this ideal :-) Such a camera would capture "what you see" with much less need for manual post capture tweaking. Graeme Gill.