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: Sat, 08 Jun 2013 12:50:00 +0000
- Thread-topic: Colorimetric Accuracy In the Field
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 donĀ¹t 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white no longer represents a diffuse highlight.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" <email@hidden> 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 <email@hidden> 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.
>>
>> Graeme Gill.
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