Luminous Landscape Forum

Raw & Post Processing, Printing => Digital Image Processing => Topic started by: Jonathan Wienke on July 25, 2005, 02:35:02 am

Title: Lab vs aRGB midpoint question
Post by: Jonathan Wienke on July 25, 2005, 02:35:02 am

LAB and Adobe RGB do not have the same luminance gamma.

Title: Lab vs aRGB midpoint question
Post by: Hermie on July 25, 2005, 02:14:59 am

Can someone help me out on this one?

A Lab luminance value of 50% in Adobe RBG space equals RGB 118,118,118 and a brightness of 46% in HSB.
Now aRGB is a perceptual uniform color space with gamma 2.2, so why isn't a luminance of 50% situated at RGB 128,128,128? Does this mean that aRGB is not as uniform as Lab?

When I do the math on 18% reflectance I also arrive at 46%: 0.18^0.4545=46% (0.4545 = function of lightness sensation, the inverse of a gamma of 2.2). Or am I mixing up things?

Thanks

Herman