Okay, so if you let the toaster cool down in between rounds, it is (more or less) linear. (Contra Dave's experiment.)
Here, also, is an array of toast:
- Equalize the white balance on the toast photos,
- Desaturate them,
- Blur the heck out of the images,
- Sample the center point of each slice, and then
- Check the brightness value.
You get, well, rather uninteresting results. Had to scratch that itch, though.