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Because of the recent Web accessibility Law in my region, I need to ensure I'm designing with an appropriate contrast ratio.

WCAG says the value of this ratio is 4.5:1

How do I figure this out in in Photoshop and Illustrator?

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Non-related to the question, but could you explain/link to an article that explains this law? I've never heard of a law dictating web design before, so I'm quite curious. – Saaru Lindestøkke Apr 3 '14 at 16:45
It just came into effect in Ontario Canada. Here's the info: – Pdxd Apr 3 '14 at 18:28
Link to the actual regulation. It applies to any organization with more than 50 employees in Ontario and requires them to meet WCAG 2.0 Level A on all new websites starting this year, and to convert old websites by 2021. – AmeliaBR Apr 4 '14 at 14:39
Also: A reference on the WCAG site, which has links to the luminance formula, and calculators. As far as I can tell, both the W3 and Adobe use the sRGB colour space for calculating luminance values, so you should be able to just use the L values from your colour inspector to determine if the ratio is at least 4.5:1. – AmeliaBR Apr 4 '14 at 14:46

From that page, there is a link to a contrast calculator

And looking at the source, we can generalize:

RsRGB = Red Component  / 255
GsRGB = Green Component / 255
BsRGB = Blue Component  / 255

Calculate luminance
R = is (RsRGB <= 0.03928) then RsRGB/12.92 otherwise ((RsRGB + 0.055)/1.055)^2.4
G = is (GsRGB <= 0.03928) then GsRGB/12.92 otherwise ((GsRGB + 0.055)/1.055)^2.4
B = is (BsRGB <= 0.03928) then BsRGB/12.92 otherwise ((BsRGB + 0.055)/1.055)^2.4

Luminance is (0.2126 * R + 0.7152 * G + 0.0722 * B)

getContrastRatio {
L1 = Luminance of color1;
L2 = Luminance of color2;
round((max(L1, L2) + 0.05)/(min(L1, L2) + 0.05)*10)/10;

The code is embedded in that page, you can bookmark it or you can save it locally in case the page goes down.

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