I'll consider giving more details on my project if it gets documented well enough to be published. I'm definate my project takes quite a while to get finished though.
Basics for my project are like this: Adjustable current sources for rgb channels, a lot similar to previous schematics on this thread. Couple op amps and detector diodes for measurements. CPU has analog outputs for current sources, analog inputs for measurements. Everything else is done by software.
My basic idea is to keep electronics very simple. Once you have a couple measurement channels (for baseboard and mixing chamber light level), know the heat sink temperature and can control the light level you're done. With those components you can create almost everything with software.
I started with an idea of Heiland Splitgrade type controllable G/B light source including the timer. Unfortunately that can give very diffent pictures for different paper types with different responses. So, I'll include a measured table for each paper to get actual contrast on a paper independent scale. Most measuring systems seem to have reference points at just visible white and just visible black. The difference between these is the ISO R-speed which correlates well with contrast grades. B&W zones are just some steps between these two points.
Paper manufacturers document ISO P-speed as well. However, I'm considering using the white reference point as a speed reference (to avoid adding any extra measurements). That's what one would basically do for split grade printing as well.
The real pain is in the calibration. Software must include methods for easy calibration or the head is not that usable really. My goal is at 1% absolute repeatability which means reasonably accurate measurements and the need to do runtime correction during a long exposure as well. (Heat changes => light level changes)
Basically I'll probably measure a base line to get a correction table depending on power level and temperature. That get's me close and the tiny correction is left for the runtime during exposure.
I'm adding features one by one. I have a complete goal known pretty much so I can design user interface and the input/outputs for accessory harware coming later on. I'll start by using a simple guessed/measured G/B table for contrast and probably a density based RGB as well for color. That allow me to print. Then I'll add more features one at a time over a longer period.
If anyone has good insite or references how to make a very usable color metering system I'm interested on all ideas. I'm not very familiar with color metering systems at this point.