What Photoshop fill for 18% grey card?

Started Dec 16, 2005 | Discussions thread
technoid Senior Member • Posts: 2,276
Just Use RGB 117,117,117 in Adobe RGB (1998)

Iliah Borg wrote:

Petruska wrote:

What is Y100?

Y100 is Y coordinate of XYZ normalized to 100%,

see http://brucelindbloom.com/ColorCalculator.html

enter Lab values (for 18%: 49.4961, 0, 0)

check "scale XYZ"

press "Lab" button to calculate XYZ


Also XYZ values are linear, aka gamma=1.0

To get the RGB values for a given reflectance use this when the colorspace has a pure gamma.

255 * .18^(1/gamma)

Turns out Adobe RGB gives the closest results when rounded.

So just enter this in google and it will calculate the result for you

255 * .18^(1/2.2)


Which rounds nicely to 117.

Now enter RGB 117,117,117 and fill a region in Adobe RGB (1998). Then use the info tool to examine the L*a*b* value. You can set the info tool to 32 bits and Photoshop will display 2 places of precision. The L*a*b* value is 49.53. This is 18.01% reflectance.

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