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
http://www.w3.org/TR/UNDERSTANDING-WCAG20/visual-audio-contrast-contrast.html
How do I figure this out in in Photoshop and Illustrator?
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.
Personally, I test UI's using a contrast checker app, aptly named "Contrast":
You can check not just your files but anything on screen, including other websites — anything on screen!
It's not very practical to measure pure contrast ratios because it's taxing cognitively, numbers have to be compared in the mind like 5.2 > 4.5 ? so apps invented an abstraction — scores based on letters, for example, Contrast Mac app would give "AA" for 4.5:1 at least.
It's a paid Mac app but it doubles as a screen colour picker too:
