Re: Illuminant change
Re: Illuminant change
- Subject: Re: Illuminant change
- From: Dan Reid <email@hidden>
- Date: Fri, 06 Apr 2001 09:23:39 -0600
On Wed, 04 Apr 2001 08:52:16 -0700, Robin Myers <email@hidden> wrote:
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Dan,
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If you change the illuminant, the XYZ and L*a*b* values will change. The XYZ
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values are calculated from a product of the reflectance, the illumination and
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the observer. The L*a*b* values are then calculated from the XYZ values.
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Therefore, as you change XYZ you can expect the L*a*b* values to change. It is
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best to use spectral data to calculate the L*a*b* results of illuminant
changes.
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If you change the illuminant and the XYZ values change, but the L*a*b* values
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do not change, then something is wrong. I suspect you had the rendering intent
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set to perceptual or relative colorimetric, which will try to keep the L*a*b*
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values constant. This is something some color engineers have done to make life
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easy for them but this is wrong.
That's what I was I seeing in ColorShop v2.6 as I flipped illuminants and
rendering intents
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The most recent work on an appearance model, which will predict the change in
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perceived color with illuminant changes, is the CIECAM 97 work. It is still in
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its infancy, changes often, and has not been put through extremely rigorous
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testing, yet. So stay tuned on this issue.
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Creating color matches tuned for the display illumination is optimal, but
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often impractical. It is very difficult to get users to understand that once a
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print is tuned to a particular lighting, they must adhere to this requirement.
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The user may ask for a match made for tungsten lighting which may be displayed
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under cool white fluorescence. When the image does not look true to the
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original, who gets the blame? Look at all the problems Epson is having with
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the 2000P and its metamerism issues.
Yeah but if you know the client will be a displaying the images under a
particular illuminant, wouldn't it makes sense to soft-proof simulating the
viewing conditions?
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This is what the standards for matching and viewing were supposed to address.
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In the U.S. the standard is D50, equivalent to direct sunlight viewing. In
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Europe, the standard is D65, equivalent to indirect sunlight viewing. Which is
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best? You must decide. But remember, the exact match is only valid for the
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standard viewing conditions.
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Can prints be made that do appear similar under different lighting, yes,
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but... It is mainly a function of the colorants used, the substrate materials,
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the display environment (e.g. surrounding colors) and the illumination (there
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are other factors but they tend to be lower order effects). As the
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illumination changes, the goals are to have prints that maintain their basic
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color balance (e.g. do not acquire a color cast), memory colors remain
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somewhat similar (e.g. sky stays blue, grass green, flesh tones maintain
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healthy appearance) and the tonality of the image is maintained. Most printing
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systems achieve this. The printing industry has worked on this issue for a
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long time, specifically to address the metamerism issue and has selected
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colorants and materials that minimize problems with illumination changes. As
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an example, take a magazine page into different lighting. The images and
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colors maintain their basic relationships even though they may be viewed under
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tungsten, fluorescent, natural light, or combinations of these. The exact
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color values do change, but the relationship to the whole image is maintained
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within the tolerance of our acceptability.
But wouldn't the spectral energy from the different illuminants emphasize
certain colors and *break* that relationship?
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Let me know if this helps or hinders and I will attempt to clarify these
issues.
Always enlightening. Thanks for your input.
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Robin Myers
--
Dan B. Reid
RENAISSANCE PHOTOGRAPHIC IMAGING
Color Imaging Solutions Provider
http://www.rpimaging.com | email@hidden
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