How is Luminosity histogram calculated from RGB values?

Can someone from Adobe please help with this? I am working with a scanner manufacturer to debug their software, and I need to know how Photoshop calculates an L value displayed in the Luminosity histogram (or info palette) from the individual RGB values in a pixel.

I originally thought it was simply the average of R, G, and B, but it's not.

For instance, an image completely filled with the color R=125 G=156 B=223 gives L=154 in the histogram dialog box (but in Lab color in the Info palette, L=64.)

I'll appreciate a clear explanation, inquiring minds want to know!

Thanks,

-Chris.

The info palette doesn't display luminosity, only RGB, CMYK, and LAB. And the L in LAB is not Luminosity.

The RGB histogram in the Levels dialog is the total of the individual channel histograms.

The Luminosity histogram in the histogram palette/dialog is calculated as the approximate gray value of the image (Gray = 0.3 * Red + 0.59 * Green + 0.11 * Blue).

If you really want CIE L*, you'll need to consult a book on color science and define which RGB colorspace you're using.

Interesting, Chris, as those numbers add up to 1.0, which sets full white value in NTSC(100 IRE units). What gray value does it represent in PS?

white = 100% = 255 for 8 bit values