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1. Re: Identifying and quantifying a color cast in a black and white print
normfb Nov 5, 2014 9:17 AM (in response to j4644)The Info Panel has the answer. In RGB mode, the three numbers should be identical for neutral gray.
The shift from identical would indicate the color cast.
Working in LAB the A and B channels should read zero and:
A as + number indicates magenta cast, and as - number indicates a green cast
B aa + number indicates yellow cast, and as - number as blue cast.
If you make the RGB numbers identical you will remove the cast
If you zero out the A and B in Lab you will remove the cast and not affect the tonal scale.
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2. Re: Identifying and quantifying a color cast in a black and white print
j4644 Nov 5, 2014 9:28 AM (in response to normfb)Hi normfb,
Thank you. I do know that within PS if you have the R,G, and B channels all the same values that it is a neutral color. Eg. 5,5, and 5 is pretty much a deep solid black. I wanted to know how to read the actual hard print. In PS, I have my B and W processed image pretty neutral but my Canon printer, with a well calibrated monitor and ICC profiles, wants to add some color to it and make it less neutral with some sort of a color cast.To correct the cast in the print, it would be easiest to know what the color cast is in the first place.
So unless I am missing something here, I do not think the above will help.
Thank you for your response,
Jim
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3. Re: Identifying and quantifying a color cast in a black and white print
G.Hoffmann Nov 5, 2014 9:44 AM (in response to normfb)I'm measuring regularly the Lab values in B/W prints by X-Rite's Digital Swatchbook.
This is since many years a highly reliable instrument, but unfortunately discontinued.
Please search for newer replacements:
Judging by scans of the print is in my opinion just wasted time.
Best regards --Gernot Hoffmann
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4. Re: Identifying and quantifying a color cast in a black and white print
normfb Nov 5, 2014 9:55 AM (in response to j4644)Reading a print requires a type of reflection densitometer fitted with R G and B filters and it delivers a density readout. Essentially, it feeds out a known quantity of R G or B light, reads the return reflection as a fraction, inverts the fraction to get density and then converts that number to its log. For example, a reflection of 50% is 1/2. Its inverse is 2/1 or 2. The log of 2 is 0.3 density. Separate RGB readings will reveal the precise cast. I believe your photo lab probably has a densitometer and can tell you the cast. Certainly, if you deal with color lithographer, a densitometer is available. I hope this information helps.
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5. Re: Identifying and quantifying a color cast in a black and white print
j4644 Nov 5, 2014 10:33 AM (in response to G.Hoffmann)G.Hoffmann,
Thank you. So I need a spectrophotometer.
Jim
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6. Re: Identifying and quantifying a color cast in a black and white print
j4644 Nov 5, 2014 10:38 AM (in response to normfb)normfb,
Now that is what I was looking for. So I need special equipment other than my i1 colorimeter and PS. I print my own images so I do not have a lab I work with. Let me ask you this, if I scanned the image, imported back into PS, would I be able to read the cast then?
Thanks,
Jim
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7. Re: Identifying and quantifying a color cast in a black and white print
normfb Nov 5, 2014 10:51 AM (in response to j4644)You would be introducing to the mix the variables inherent in the particular scanner. I assume you currently print a satisfactory, predictable gray. Please consider this alternative: If you are using a manufacturer's recommended ink and paper, ask that firm for the particulars.
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8. Re: Identifying and quantifying a color cast in a black and white print
j4644 Nov 5, 2014 3:14 PM (in response to normfb)Norm,
Thanks.
Jim

