OK I've uploaded a modified version of my light meter, called the BBGammaMeter.

The center will show diffuse light levels just like my BBLightMeter. But the rim will show render gamma level.

gamma = 1 will be black

gamma = 1.8 will be almost all white

gamma = 2 will be yellow

gamma = 2.2 or higher will be red

You can get it here:

https://sites.google.com/site/bagginsbill/file-cabinet/BBGammaMeter.zip?attredirects=0&d=1

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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)