On Jan 5, 2018, at 1:52 PM, forums@walkerblackwell.com wrote:
All other things can go where they go for now. If the print is near the sun I’m fine with it blinding me, if the print is in shadow I’m fine with it looking too dark. But linear printed L* values (percentage of reflected light) related to a TILE WHITE of a spectro is what I’m after.
First...it's really important to understand that L* values are _not_ the percentage of reflected light. L* values (ideally but not actually) represent 100 perceptually-equal step wedges between absolute black (*NOT* dMax, but an actual absence of light) and a perfect lambertian reflector (of which, again, PTFE thread tape is an astonishingly good approximation) under close-to-typical illumination. You're familiar, I hope, with the famous Kodak 18% gray card? It reflects (within manufacturing tolerances, which are pretty broad) 18% of incident light. Plop your spectrometer on it, and it'll give you a D50 Lab reading of L*=50, a*=0, b*=0...plus or minus at least five units (did I mention manufacturing tolerances?). That works out to a gamma power curve of roughly two (rounded to one significant digit) -- and for very good reason, because that's a mirror to the human visual system. The gray card might _look_ like it's midway between black and white, but it's only reflecting about 18% of the light -- much darker, in reality, than we perceive it to be. (But not really, of course...there's that whole inverse square thing of light going on such that the logarithmic nature of perception actually does make sense; it's just that we naively expect everything "simple" to be linearly additive, like counting apples.) If you truly want linear reflectivity values...profiling probably isn't for you. Instead...either grab a densitometer or figure out how to use your spectro in a densitometer mode. Print your step wedges in non-color-managed / null transform / profiling / whatever mode. Plug the measurements into a spreadsheet and calculate the offset needed to make the numbers come out the way you want. Create a tone curve layer in Photoshop using those numbers -- oh, and make sure that your working space in Photoshop is gamma 1.0, or else you'll have to bake in the inverse of the gamma into your spreadsheet. That will probably get you "close enough" on the first try, but you can certainly add as many iterations as you want until you're chasing diminishing returns. ...but, I gotta warn you. The ICC profiling infrastructure, complex and seemingly over-engineered as it may be, exists for a reason. The densitometer approach you're trying to re-create is the way things were done in the pre-digital age of the printing press and halftoning cameras. The existence of great works from that era demonstrates that great work is possible, no doubt...but, by any objective measure, modern systems technically run rings around their quarter-century-ago counterparts. That includes factors that you're so casually dismissive of, like neutrality, as well as ones (like dMax) that you seem concerned with. A modern black-and-white inkjet print from a latest-generation top-of-the-lne fine art printer on baryta paper by a skilled operator is going to be more neutral _and_ have a greater dMax _and_ have a whiter dMin _and_ significantly more detail and less grain than any large-format photographic print you'll find from the pre-digital age. If that's the standard you're chasing...densitometry is the worng path to follow. Cheers, b&