How is Luminosity histogram calculated from RGB values?

[C***@adobeforums.com]

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.

[C***@adobeforums.com]

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.

[L***@adobeforums.com]

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?

[C***@adobeforums.com]

white = 100% = 255 for 8 bit values