Hi Edmund Sony brought out a camera with 4 color sensitivity to closely match the CMF's in 2003. It did not go over well. http://www.dpreview.com/news/2003/7/15/sonyrgbeccd Your point about "observer white point adoption" is very important and most folks just don¹t understand the real issue. We really only "see" white over a very small range of viewing conditions. That is why the chromatic adaptation matrices often fail to yield an image that even remotely represents the appearance of the scene. As the illuminant departs from a range of about 4800K to 6700K, the hue of the "white" starts to become noticeable. We don't adapt. If you hold up a white card at sunset, it looks orange. You can stare at the card for minutes and it won't appear white. The same is true at the other end of the CT range. There is a similar corollary with exposure. As it gets dark, a diffuse white no longer represents a diffuse highlight. This effect happens because we naturally maintain a large headroom to accommodate rapid changes from ambient dark to light. This means that we have a notion of "virtual" white when viewing in dim ambient. A "properly" exposed image of the dim scene will push the white in the scene into the reproduced highlight and the overall scene will appear much lighter than the scene as observed. In fact if the exposure is about 3EV or less, you should actually underexpose the scene by at least 1.5 stops to begin to simulate what you are seeing. If you keep the camera WB at 5K or slightly below, the relative white balance of the scene will track with appearance much better than adjusting white balance to the actual illuminant. This is especially true at lower light levels. Regards, Tom L. On 6/8/13 7:14 AM, "edmund ronald" <edmundronald@gmail.com> wrote:
It would seem that creating a camera with 3 accurate cone sensitivities, or even 4, is a solvable technical issue.
But how can one recreate the observer's white point adoption, especially in the presence of mixed light, if one does not register the 360 degree scene at high DR?
Edmund
On Sat, Jun 8, 2013 at 2:45 AM, Graeme Gill <graeme2@argyllcms.com> wrote:
Jeffrey Stevensen wrote:
If you meter the sunlight, what happens to your accuracy as more or less of these lights mix in different parts of the scene? It's my experience that color temperature will vary over as little as a couple feet or even the angle of the metering instrument to the light sources, or the angle of the target to the main light (the sun) and the mix of other reflections, or the color of a wall or pavement or greenery and trees.
Right, but this doesn't really matter - in the end it's what the human observer would use as their white point that counts. The illuminant color temperature is just a starting point to guess/estimate what that is.
It seems to me that the colorimetric accuracy would come down to only a very specific white balance in one tiny part of a scene.
Colorimetric accuracy is independent of white point - ie. XYZ is absolute, not white point relative. XYZ is the light levels integrated with certain spectral sensitivities - the ones typical of a human observer. The way these three levels are balanced (gain adjusted) in the eye and nervous system is what sets the observer white point.
White point is something of interest after you've captured the colorimetery, when you want to re-interpret a colorimetric image for a media/on a device which will cause the human observer to be adapted to a different white point than they would be in the original scene.
One would have to completely control and dominate the lighting with controlled lighting in order to have any predicted knowledge of the accuracy of a rendered color.
Not so if you have a colorimetric capture device.
And as Andrew has pointed out you would have to measure every color everywhere so as to have two "patches" to actually compare, an impossibility.
Not so, if you use a camera which is colorimetrically accurate. Using spot measurements of real world objects under real world illumination is just a way of confirming that your colorimetric camera is operating accurately.
There are two complementary ways in which a colorimetrically accurate camera can be approached: 1) change it's spectral sensitivities to better match the human observer 2) Come up with ways of compensating (ie. calibrating) for the interaction of illuminant, object reflectance spectra and the non-colorimetric camera sensitivities. The latter can never be a perfect way of repairing the first defect, and has various degrees of practical difficulty.
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