In doing my measurements, I marked an area of a piece of paper. I made sure that I could see that mark both on the paper directly, and reflected in the utensil. I took my ratio of reflection colors by using the same spot, reflected color divided by direct color.

But even this was too casual and has a flaw. The utensil is reflecting light onto the paper, changing its color. Measuring these two this way, I'd be pulling in measurements from a feedback loop. This would give incorrect results.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)