Thread Subject: Definition Consensus Decision: Relative Luminance

Note

This archival content is maintained by WebAIM and NCDAE on behalf of TEITAC and the U.S. Access Board . Additional details on the updates to section 508 and section 255 can be found at the Access Board web site.

From: Mike Paciello
Date: Sun, Mar 09 2008 4:40 PM

Following is the definition for Relative Luminance. If you do not agree that
this definition is acceptable, please reply and state your reason.

Relative Luminance

The relative perceived brightness of any point, normalized to 0 for black
and 1 for maximum white.

Notes:
The relative luminance of an sRGB color is defined as L = 0.2126 * R +
0.7152 * G + 0.0722 * B where R, G and B are defined as:

* if RsRGB <= 0.03928 then R = RsRGB/12.92 else R =
((RsRGB+0.055)/1.055) ^ 2.4
* if GsRGB <= 0.03928 then G = GsRGB/12.92 else G =
((GsRGB+0.055)/1.055) ^ 2.4
* if BsRGB <= 0.03928 then B = BsRGB/12.92 else B =
((BsRGB+0.055)/1.055) ^ 2.4 and RsRGB, GsRGB, and BsRGB are defined as:


* RsRGB = R8bit/255
* GsRGB = G8bit/255
* BsRGB = B8bit/255

Notes on the formulas

1. The "^" character is the exponentiation operator. (Formula taken from
[sRGB] and [IEC-4WD]).
2. Almost all systems used today to view Web content assume sRGB encoding.
Unless it is known that another color space will be used to process and
display the content, authors should evaluate using sRGB colorspace.
3. For dithered colors, use average values of the colors used (average R,
average G, and average B).
4. Tools are available that automatically do the calculations when testing
contrast and flash.
5. A MathML version of the relative luminance definition is available.




<http://www.paciellogroup.com/>

WebAIM is an initiative of:
Center for Persons with Disabilities (CPD) Utah State University