Thread Subject: Re: Definition Consensus Decision: Relative Luminance

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From: Sean Hayes
Date: Sat, Mar 15 2008 11:05 AM

I don't disagree with this definition per se. But I would like to point out it is only defined under the viewing conditions referenced in the sRGB standard. Thus it should not be applied outside of that limited context (that is roughly CRT displays in an office environment)

Sean Hayes
Incubation Lab
Accessibility Business Unit
Microsoft

Office: +44 118 909 5867,
Mobile: +44 7875 091385

From: = EMAIL ADDRESS REMOVED = [mailto: = EMAIL ADDRESS REMOVED = ] On Behalf Of Mike Paciello
Sent: 09 March 2008 22:37
To: 'TEITAC Committee'
Subject: [teitac-committee] Definition Consensus Decision: Relative Luminance

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.

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