Re: Colorimetric Accuracy in the Field
Re: Colorimetric Accuracy in the Field
- Subject: Re: Colorimetric Accuracy in the Field
- From: Thomas Lianza <email@hidden>
- Date: Fri, 07 Jun 2013 15:13:44 +0000
- Thread-topic: Colorimetric Accuracy in the Field
Hi to all.
There is a fundamental difference between colorimetry and appearance.
There is also a common problem with colorimetry itself. The standard
observer functions are based on averages, there are 10 degree and 2 degree
models that are different. Naturally there are different
illumination/collection geometries. We often see displays with different
backlight technologies measure exactly the same, but when viewed by a
single observer they appear quite different. We don't have strong science
in the area of adaptation in emissive/reflective environments. Colorimetry
and Spectrophotometry are very good for measuring color, but that doesn't
mean that the result of the measurement tracks the appearance of the
objects.
The argument of colorimetric accuracy in the field has very little to do
with the quality of the output with respect to appearance. On the other
hand, you do need some notion of the colorimetry to apply appearance
principles, but those principles modify the "colorimetry" to the point
that the notion of "accuracy" is really not much of a factor.
At the ICC, we are looking at some of these issues to improve the total
color appearance transfer of data from input to output.
http://www.color.org/icclabs.xalter
This effort is aimed at multispectral imaging chains and advanced imaging
work chains that carry multidimensional reflectance models. This work is
going to be an open source effort and we will be building a reference
implementation as well.
Regards,
Tom Lianza
Co Chair ICC .
On 6/7/13 10:06 AM, "Henry Davis" <email@hidden> wrote:
>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.
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