Luminous Landscape Forum
Raw & Post Processing, Printing => Colour Management => Topic started by: nirpat89 on August 31, 2019, 11:31:10 am
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I am trying to convert some spot-measured Lab's to RGB using Bruce Lindbloom website facility. It requires specification of the illuminant. Can't find from X-rite literature which one it would be. I am guessing D50 but not sure. Anyone to confirm for me?
Thanks.
:Niranjan.
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I am trying to convert some spot-measured Lab's to RGB using Bruce Lindbloom website facility. It requires specification of the illuminant - either D65 or D50. Can't find from X-rite literature which one it would be. I am guessing D50 but not sure. Anyone to confirm for me?
Thanks.
:Niranjan.
For spot measures of prints, it's D50. Gets more complicates with displays. For accuracy, best to measure them as xyY. ICC profiles adapt different color spaces like sRGB to D50 even though the illuminant is speced at D65. The three RGB points are adapted from their true D65 xy to D50 xy and placed in the ICC profile as coordinates.
L*a*b* is usually referenced to D50 especially when used w/o a white point reference. When a WP is included L*a*b* is referenced to that such that L*=100 yields the white point.
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For spot measures of prints, it's D50. Gets more complicates with displays. For accuracy, best to measure them as xyY. ICC profiles adapt different color spaces like sRGB to D50 even though the illuminant is speced at D65. The three RGB points are adapted from their true D65 xy to D50 xy and placed in the ICC profile as coordinates.
L*a*b* is usually referenced to D50 especially when used w/o a white point reference. When a WP is included L*a*b* is referenced to that such that L*=100 yields the white point.
Thanks Doug. Yes, I am measuring printed output. I realized I used the wrong terminology here. The CIE Color Calculator on Lindbloom website asks for Reference White and not Illuminant as I said. I am not sure what the difference is. May be they are interchangeable? Does the D50 illuminant make the profiles created with ColorMonki to be M1 profiles?
Anyway, when I use D50 along with gamma of 2.2, Bradford Adaptation, and AdobeRGB (which is the Color Space I am using) the resulting values come out to be the pretty close to what I see in Photoshop. So that must be the way to go. I am trying to compare how far are the printed/measured patches from the image RGB values and see if they can be corrected with a correction curve in making of a toned black and white print or a digital negative for alternative processes.
:Niranjan.
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Thanks Doug. Yes, I am measuring printed output. I realized I used the wrong terminology here. The CIE Color Calculator on Lindbloom website asks for Reference White and not Illuminant as I said. I am not sure what the difference is. May be they are interchangeable? Does the D50 illuminant make the profiles created with ColorMonki to be M1 profiles?
No, it doesn't. The ColorMonki only makes M2 profiles since it uses a white LED w/o uV. The profile is exactly the same if the paper has no OBAs.
Anyway, when I use D50 along with gamma of 2.2, Bradford Adaptation, and AdobeRGB (which is the Color Space I am using) the resulting values come out to be the pretty close to what I see in Photoshop. So that must be the way to go. I am trying to compare how far are the printed/measured patches from the image RGB values and see if they can be corrected with a correction curve in making of a digital negative.
:Niranjan.
When you print using Relative Colorimetric the measured print Lab value L* will be lower than what you read in Photoshop and the a* and b* shifted in the direction of the paper white.
When you print using Absolute Colorimetric the printer will attempt to print exactly the Lab values you see in Photoshop. Since most papers can't print above about L* 94 to 97 as they aren't "perfect" whites, the whitest areas will clip. This is why Abs. Col. is rarely used for printing except when you are trying to replicate exact colors. But to test printer accuracy, printing with Abs. Col. is the way to go. Just make sure the colors are within the printer's gamut. Then most colors measured with the CM should match what's shown in Photoshop within 1 dE.
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What about the non-zero Black point. That would make the L* values on the shadows portion higher than the image values? How does BPC takes affect that?
:Niranjan.
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What about the non-zero Black point. That would make the L* values on the shadows portion higher than the image values? How does BPC takes affect that?
:Niranjan.
Yes, for instance if the actual (measured) BP is Lab=(5,0,0), and you print the same using Rel. Col., it will measure a bit higher, about L*=6. Rel. Col. with BPC expands the lower end so if you print, say, L*=3, and measure it you will get about L*=8. There is a specification from Adobe included in ICC documents at color.org that details how BPC is implemented.
It's common to enable BPC to prevent dark shadow clipping since Rel. Col. alone only expands the upper end (white point).
Perceptual intent is up to the profile maker's software but common practice is to incorporate BPC as well though some will clip at L*=3. Newer printers often have a very low L* black on glossy media. My Pro1000 comes in at just under 2.0 for blacks.
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Yes, for instance if the actual (measured) BP is Lab=(5,0,0), and you print the same using Rel. Col., it will measure a bit higher, about L*=6. Rel. Col. with BPC expands the lower end so if you print, say, L*=3, and measure it you will get about L*=8. There is a specification from Adobe included in ICC documents at color.org that details how BPC is implemented.
It's common to enable BPC to prevent dark shadow clipping since Rel. Col. alone only expands the upper end (white point).
Perceptual intent is up to the profile maker's software but common practice is to incorporate BPC as well though some will clip at L*=3. Newer printers often have a very low L* black on glossy media. My Pro1000 comes in at just under 2.0 for blacks.
I checked out the icc paper:
http://www.color.org/WP40-Black_Point_Compensation_2010-07-27.pdf
Nice write-up. A good comparison of how the various RIs function. So it looks like what BPC does is extend the linearization to the whole range from L*min to L*max whereas Relative Colorimetric by itself takes care of the upper limit only. It is actually quite similar to what is done in making of digital negatives, i.e. creating a "Correction Curve" to map the image dynamic range to the process dynamic range.
Thanks again for clearing up some of these concepts.
:Niranjan.