In doing research on in-camera histograms, I have found repeated assertions that the luminance histogram is the same as the green channel histogram, color science to the contrary (the only RGB color space in which colorimetric luminance is the same as the green channel would have to have three identical primaries, all of them the white point, with the designation of one of them as “green” being therefore arbitrary). I thought that it was possible that camera manufacturers were taking computational shortcuts, so I constructed a test target with not much green. It’s R=255, G=0, B=255 in Adobe 1998 RGB; give it a try yourself, but read the next post before you do.
Here’s the result for the D4 (in-camera white balance, contrast, brightness, and saturation adjusted to approximate the real raw histogram):
And here’s what you get with the D800E (in-camera white balance, contrast, brightness, and saturation adjusted to approximate the real raw histogram):
Other cameras that I have produce similar results. Of the four or five cameras that I have tested, I have found none for which luminance is simply the green channel.
Notice that the spread of the histogram is different in the two pictures. Hold that thought…