What Photoshop fill for 18% grey card?

Started Dec 16, 2005 | Discussions thread
technoid Senior Member • Posts: 2,276
Re: What Photoshop fill for 18% grey card?
2

MU84 wrote:

RGB: 124,124,124

#7C7C7C

No, it's not. First, no colorspace is given so I'll pick 3 common ones. RGB(124,124,124) is actually over 20% reflectance in both sRGB and Adobe RGB (1998) and way over that at 27% in ProPhoto RGB.

Using Adobe RGB the values should be (117,117,117). This is 18.01% reflectance when printed using Absolute Colorimetric. Absolute must be done because otherwise the card won't be neutral but shifted in the direction of paper white. Printing in any other intent other than Absolute will also shift the gray depending on how reflective the unprinted paper.  RGB (255,255,255) in Relative/Perceptual/Saturation Colorimetric Intent is unprinted paper white whether it is 85%, 90% or 95% reflective and smaller numbers are scaled to that paper white.

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