Is there any liklihood that a self-luminous colour checker would work? Initially, I was thinking of a lightbox + dyed gel filter array affair, but a tablet with suitably calibrated screen then sprang to mind. Using the in-built brightness sensor to adjust to ambient light. Advantages - freedom from ambient lighting variations. Disadvantages - doesn't take account of ambient lighting variations. Only works in fairly dim light and fraught with all sorts of other potential issues..... but..... maybe? Non-starter or worth pursuing? Thoughts please.