What illuminant and observer does SilverFast use for CIELab values?
Asked 6/5/2018
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I'm scanning prints in SilverFast and can save files as CIELab. I need to know which CIE illuminant and observer those Lab values are based on, but I can't find it documented. This also matters for IT8 scanner calibration, since SilverFast reports Delta E values afterward. Is there a standard illuminant/observer typically assumed for scanner Lab data and IT8 reference targets, such as D50/2°?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
8y ago
2 Answers
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Whenever an illuminant is not specified, assume it's D50 as that is the standard the ICC adapts its profiles to even if the device illuminant differs. For instance a Lab reading in a Photoshop image is relative to D50 whether the image is sRGB or ProPhoto. Scanner profile reference data sets in Lab use D50 even though the scanner's light is rarely close to D50. The target sheets are measured with a spectro.
One interesting thing about Photoshop is that you can read Lab values to fractional precision by setting the info panel to the highest resolution and selecting Lab as the info display space. These values are always shown adapted to D50 white point even if the working space assumes D65. One caution, Photoshop has a slight bug and if you are using 16 bit working spaces and show an image at less than 100% zoom, it shows Lab values erratically varying in the fractional component as if it was converting the RGB values to 8 bits before displaying the values.
Originally by user58107. Source · Licensed CC BY-SA 4.0
user58107
8y ago
0
Generated from our catalog & community — verify before relying on it.
The most defensible assumption from the answers is D50, because ICC-based color workflows conventionally use Lab referenced to D50, and scanner profile reference data in Lab is typically D50-based even if the scanner’s actual lamp is not. IT8 target data is generally measured spectrally and reported as Lab relative to that reference white.
The observer angle is not confirmed in the answers, so you should not assume more than the common graphic-arts convention unless SilverFast or the target vendor explicitly states it. In practice, D50 is the key point.
One community reply claims SilverFast’s densitometer may display D65 Lab values, which could explain mismatches when comparing to IT8 reference files. If you’re seeing discrepancies, verify whether SilverFast is showing display/measurement values in a different reference white than the target data.
So: for scanner/IT8 Lab data, D50 is the standard assumption; the observer is not established here.
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