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I want to profile my scanner using an IT8.7/2 chart. Analyzing the ICC profile I generated it shows that the Media White Point is close to D65 illuminant (6501K). What does this mean? Does it mean that whenever I apply this profile to an image it will have a D65 white point?

Thanks.

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  • How are you analyzing the "Media White Point?" Examining 2 different scanner profiles, one made with Argyll and one made with XRite's software, the "Media White Point" is exactly D50 though the luminance is reduced for the XRite profile. That is the XYZ are D50 but scaled to the "Y" of the whitest IT8 patch but it remains D50 (xy chromaticity coordinates) for both. – doug Feb 8 at 21:50
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I want to profile my scanner using an IT8.7/2 chart. Analyzing the ICC profile I generated it shows that the Media White Point is close to D65 illuminant (6501K). What does this mean?

It means that it has analyzed the results of your scan and has determined that the illuminant for your scanner is centered on 6501K.

Does it mean that whenever I apply this profile to an image it will have a D65 white point?

It means that when you apply this profile to images that were created using the scanner in question the color will be accurate (to within the limits of your color chart and software).

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  • Thanks. In this particular case I want to use my scanner as a "poor man's" colorimeter (e.g. to measure the color of sand), although I'm aware of the limitations. In scientific literature D65 white point is usually preferred for sample color characterization rather than D50. How does this relate to ICC targets? Aren't they all based on D50 measurements? If a scanner is calibrated using a IT8.7/2 target which are based on D50, does this necessarily mean that the color of my samples will be based on D50? – Lee1010 Jan 8 at 23:08
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    Targets are the color they are. They're not based on D-anything. The light illuminating them is D50 or D65. D50 is the preferred environment when comparing output from an emissive screen to output from a subtractive printer (which can use a wide variety of materials in the inks it uses). I guess it's possible your target might demonstrate metameric failure when illuminated by D65 light instead of D50 light, but I'd highly doubt it. It's almost certainly designed to be useful over a wide range of lighting conditions (and probably not produced by the same process that a photographic print is). – Michael C Jan 9 at 7:14
  • But your scanner is not a scientific instrument and isn't intended to be used in that way. Your target may or may not be suitable, but your imaging device certainly is not. – Michael C Jan 9 at 7:16
  • Agree mostly except that the scanner illuminant is irrelevant. Also, a scanner has no idea what the scanned media's white point is. Thus is limited to absolute colorimetric intent. Even there scanners tend to suffer from metameric failure and something called large area crosstalk. Both can produce large deltaE errors. While some scanner profiles provide relative and perceptual capabilities these are, at best, distortions since a scanner has no way to identify what "white" is on any given media under scan. – doug Feb 8 at 5:49
  • @doug Scanned media has no white point. Only the light illuminating it does. – Michael C Feb 11 at 7:36
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Ignore the Media White Point in Scanner Profiles

I want to profile my scanner using an IT8.7/2 chart. Analyzing the ICC profile I generated it shows that the Media White Point is close to D65 illuminant (6501K). What does this mean?

For reflection scanners, nothing. And the Media White Point in an ICC scanner profile is not that of the scanner's illuminant.

Reflection scanner profiles convert "rgb" values from a scanner to D50 Lab values. Always. These are then typically converted to a standard profile such as sRGB or ProPhoto RGB by the application using the scanner profile. D50 is the common reference for ICC profiles. While "Media White Point" has a critical meaning for operations such as printing, this is because the printer paper is never a perfect white and the white point of unprinted paper or unexposed photo paper must be measured and taken into account for calorimetrically accurate printing.

Scanner profiles, on the other hand, not only don't use this information, they have to no way to determine it. Scanner profiles are made from the scanner RGB data together with the D50 Lab data that is supplied for the IT8 scan target. Nothing else is needed or available. For instance here is the spectrum of the illuminant measured from my Epson V850 scanner. It's not anywhere close to D50 or D65 yet the Media White Point in an XRite IT8 profile is D50.

enter image description here

For accurate reflection scanning, it simply doesn't matter what the scanner's illuminant, alone, is. What does matter is the illuminant, multiplied by the RGB filters' and sensor response at each wavelength. Ideally, they should match, in a linear, combinatorial sense, the CIE D50 color matching functions. This allows high quality profile creation.

Does it mean that whenever I apply this profile to an image it will have a D65 white point?

No. Applying a profile to a scanner image results in Lab values based on D50 regardless of what the Media White Point is set to.

Other Problems with Scanners Profiles Can't Fix

Two significant sources of error are intrinsic to scanners. Metameric Failure Error, and Large Areas Spacial Crosstalk.

Metameric Failure Errors occur when two colors that are visually the same but significantly differ in spectra, have different colors when scanned. IK have run across this when comparing scans of a ColorChecker card and the same colors printed. Even though the measured colors are within 1 deltaE of each other with a spectrophotometer, the colors of the scans differ with the two images varying by as much as 9 deltaE. That said, scanners can produce quite accurate scans if they are profiled from charts that use similar media. For instance scanning chemical color prints will produce the best scans when profiles are made from IT8 charts using the same chemical color process. Similarly good quality can be achieved with inkjet printed IT8 charts and these profiles are excellent for CYM printed materials. I've found remarkable consistency across multiple printers with the same inkjet chart profile. However, large errors still occur scanning natural colors or pigments used, for instance for artwork.

Large Area Spacial Crosstalk occurs when nearby light from a scanned object bounces off near structures on the scanner and adds light to the points being scanned. Since this process is linear it can be estimated mathematically. I have seen as much as a delta E of 8 occur with an Epson V850. A detailed description (and program to correct it) is here.

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