Re: Art Duplication
Re: Art Duplication
- Subject: Re: Art Duplication
- From: Robin Myers <email@hidden>
- Date: Fri, 18 Sep 2015 14:26:17 -0700
Dr. Miller:
> <snipped>
> I might just see where some of my issues come in. Could it be the cardboard x-rite CC? (I refuse to use the passport CC).
The ColorChecker Passport uses the same patches on its ColorChecker Classic page as the cardboard ColorChecker Classic so there is no reason not to use the ColorChecker Passport. There are added benefits on the facing Photo Enhancement page with finer spaced dark grays and light grays and the higher saturated spectrum colors. These extra patches can be used to evaluate your images and profiles. More information on the ColorChecker Passport may be found at http://rmimaging.com/information/ColorChecker_Passport_Technical_Report.pdf.
>
> Workflow is camera white balanced with what is supposed to be a white target on one side, and black, gray, and white on the other side. These three shades are for me to determine, exposure. Then, I use the white only side to determine white balance in camera.
This is one source of your color error. If you examine the spectra of the white patch on this chart you will find that it is not white but very pale yellow. The color is specified as Munsell N9.5 and an average of 12 ColorCheckers of various ages produces L*a*b* values of L* 96.4, a* -1.0, b* 3.1. We perceive the color as white because it is the whitest object in our field of view and the human vision system adapts to make it appear white. Because the white patch is not white, the white patch should be used for setting the exposure only.
Instead, you should set the neutral balance, white balance is a misnomer, with the gray patch on this chart. The gray patch is specified as Munsell N5 which I have measured and averaged from 12 ColorCheckers as L* 50.9, -0.4, 0.1. This is definitely more neutral than the white patch and a better choice for neutral balancing. It is also light enough to produce a good signal-to-noise ratio in the camera’s sensor.
The black patch is specified as Munsell N2 and should be used to check for veiling glare. Veiling glare can be controlled by using a lens hood and surrounding the subject with a black background.
<snipped>
Mr. Goren:
> <snipped>
>> Workflow is camera white balanced with what is supposed to be a white target on one side, and black, gray, and white on the other side.
>
> While some white balance targets are more spectrally flat than others, nothing, not even Spectralon, is perfectly flat. Click-to-white-balance relies on a nonexistent physical property.
While PTFE references such as Spectralon® and Fluorilon® do not exhibit perfectly flat spectral reflectances, they are the best materials presently available for white reflectance standards and are used as such for calibrating spectrometers by many companies and NIST. PTFE is the only white material I recommend for neutral balancing cameras. The majority of white objects, including the white patch on ColorChecker charts, use a titanium white pigment which is actually a pale yellow, as noted above. If these white objects are used for neutral balancing a camera the resulting images will have a slight blue color cast.
Robin Myers
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