Basically the testing suite uses an image of known brightness profile (if you scroll down to the IE7 section you can see an example of a test image). It looks l

Basically the testing suite uses an image of known brightness profile (if you scroll down to the IE7 section you can see an example of a test image). It looks like what its doing is grabbing the interpolated image from the browser and plotting the brightness against x-axis, which it can then compare to the actual brightness function.