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I have one question still; these targets are all 8bit and so 256 neutral grey patches are the most it can measure for BW.
For Bw it does not make a difference if you have the 918 chart with 256 gray patches or the 3315 chart with 256 grey patches .
Or am i wrong?
A number of technical considerations are packed into this question, perhaps too many to totally unpack, but I'll try to answer the main considerations. First, an 8 bit color target doesn't negate a 16bit CMM workflow. It simply limits the choice of what RGB triplet values are used in the design of the color target to be printed. 8 bits is enough to offer 16 million discreet RGB triplet combinations which in turn are more than humans can observe as discreet colors or would ever use to design a color target, hence more than enough precision. Yet, you are right, out of those 16 million RGB choices, only 256 combinations are theoretically "pure neutral triplets" in the RGB source image color space. But these pure neutral triplets in the source image typically do not print as dead neutrals in the output destination color space, if for no other reason than the media white properties are often not neutral to begin with. This is where adding more near neutral triplets in the color target (or resorting to a two step color profiling approach) is also important because some of those near neutral combinations will ultimately be required to output a uniformly neutral tone scale or even a slightly tinted B&W ramp (e.g. for making warm tone, sepia, cool, or split tone B&W prints) with the chosen printer/ink/media output color space. This is one aspect (but not the only one) where the 3315 Aardenburg chart can improve monochrome printing performance on a color printer if the printer is stable and repeatable enough in its printed output.
I should also note that the Z series HP printers are unique in that HP uses essentially 100% gray component replacement. RGB neutral triplets printed in the no color adjust mode use only the gray inks when printing the color profiling target. Canon and Epson printers don't do that. Thus, HP's approach has an inherent advantage when trying to print accurate neutral and near neutral tones in a full color printing pipeline. It helps a smaller patch count target such as a TC918 color chart to deliver a pretty good B&W print without going to additional heroics, but a larger patch count target like the 3315 Aardenburg chart does refine HP's approach even more. It's subtle, but in my testing, I see it. Epson, for example, takes a different approach than HP with regard to accurate B&W printing by resorting to an entirely different "advanced Black and white" (ABW) mode for monochrome printing. Nevertheless, HP's 100% GCR approach and its inherently neutral photo gray inks do deliver superior performance when printing B&W images in a full color output mode, and this is why the Z's have a well deserved reputation for making great B&W prints without having to resort to other methods like ABW, Quad Tone RIP, piezography ink sets, etc., to print excellent B&W output.
cheers,
Mark
http://www.aardenburg-imaging.com