To calculate contrast ratio the following formula is used (source):
(L1 + 0.05) / (L2 + 0.05)
where:
L1 is the relative luminance of the lighter of the colors, and
L2 is the relative luminance of the darker of the colors.
My question is, what is the number 0.05 in this formula?
The Gamma correction and RGB constants are derived from the Standard Default Color Space for the Internet (sRGB), and the 0.05 offset is included to compensate for contrast ratios that occur when a value is at or near zero, and for ambient light effects. - http://juicystudio.com/article/luminositycontrastratioalgorithm.php
Well, without it, if L2 is darkest black, which has a relative luminance of 0, the ratio would be undefined since you can't divide by zero.
So they decided to add a small factor to make sure the ratio is always defined. The consequence of using 0.05 is that the maximum ratio is 1.05 : 0.05 = 21 : 1.
Note 1: Contrast ratios can range from 1 to 21 (commonly written 1:1 to 21:1).
Why they chose exactly this number, I have no idea.
The 0.05 addition was intended to represent the flare on the face of the monitor from ambient illumination that was typical of CRT type displays, this flare value is recited in Annex D of the sRGB standard for Typical Viewing Conditions
I discuss the history in this GitHub thread
Unfortunately, adding the flare value to both sides of the formula doesn't actually correct the problem with using a simple contrast ratio in terms of its lack of perceptual uniformity, so that algorithm really only works when the background is very light or white.
Ratios are not the best way to model human perception of text contrast at suprathreshold levels on a self illuminated display. For an example of a contrast method that uses a more comprehensive model of contrast perception, see apcacontrast.com (disclosure, I am the creator of the APCA algorithm).
