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What I want to know is what does the Colormunki think 128,128,128 should look like? It must have its own definition of middle gray preprogrammed into it by the manufacturer. And this definition of middle gray must be in units of L*a*b because that's how the Colormunki spectrophotometer see things--in L*a*b.
It is the nature of this internal preprogrammed definition of middle gray as it exists in the eye of the Colormunki that lies at the heart of my inquiry.
Or is the rabbit hole getting too deep?
The Colormunki has no opinion about how 128, 128, 128 should look. It is a measuring device. The output of the system you use to print a target with no color management (the representation of the printer's native output) is measured by the Colormunki and compared to the known target values to form the basis for the ICC profile. The profiling software is the thing that will compare the target values for each patch (stored in a reference file, usually with a ".cie" file extension) to the measured values you acquired with the Colormunki.
Maybe it is not apparent to you - RGB color spaces are artificial representations of color - you need to know their primaries (chromaticities) and white point to understand how an RGB triplet represents a specific color that we can see. Because each RGB color space has its own primaries and white point, the color "Cherry Red" might be 235, 15, 35 in one color space and 220, 13, 40 in another - they both are the exact same color, but the RGB triplets are different because they originate in different RGB color models. This is what makes them DEVICE-DEPENDENT. In contrast, Profile Connection Spaces (PCS) are based in the physiology of how a "Standard" human observer perceives light of a specific "tristimulus" value - that is, how our visual systems interpret that light. These physically-based models of color form the basis for our ability to relate various artificial, device-dependent RGB models to each other - as such, they are DEVICE-INDEPENDENT. Hence the term "profile connection spaces" - the physically-referred PCS can connect one artificial RGB space to another, as long as the relationship between the RGB space and the PCS is known. It is sort of like Google Translate for color. Again, I would defer to the color experts here, as I am sure I am oversimplifying.
The printer internals are designed to lay down ink. How much gets laid down in response to a native RGB input is beyond me, but I assume that printer manufacturers have it figured out pretty well for their ink formulations and media types. It sounds like you are trying to adopt the Zone System for digital photography and you want to understand how Zone V in your digital image file in Photoshop in a particular RGB color space gets printed to a density of Zone V on a specific printer-paper combination. Take a deep breath and trust that color management is the way to go about characterizing this relationship.
Your first question should really be, what is Zone V in a device-(in?)dependent definition? Is it L*50? Is it the black density that your printer and ink can produce that is halfway between maximum black and paper white? At least these definitions have a physical meaning, in terms of your destination, which I hope like hell is printed output.
You may want to go to the Piezography.com website and download the free Piezography profiles - you will have no use for them, but in that download there is a user manual that you might want to read, particularly the Grayscale Management section.
Have fun!
kirk