Thread Subject: Re: 2.1.A - Contrast Ratio for Display

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From: Robert_Nerhood@Dell.com
Date: Fri, Oct 12 2007 6:45 AM

Michael, I agree the addition of the "at least" phrase in necessary to
help with this one. Does that address your concerns with this proposal?

The challenge here is with regards to the usage of the concept of
Contrast Ratio. Contrast ratio has different definitions dependent on
the discipline addressing it. For software/web content it appears to
have one involving the RGB capabilities.

For hardware, the simplest form of contrast ratio is what is defined
below as the ratio of screen brightness as high state to brightness at
low state. The high and low state values are measured via photometric
measurement methods. Not calculated as is done with the software
option.




-----Original Message-----
From: = EMAIL ADDRESS REMOVED =
[mailto: = EMAIL ADDRESS REMOVED = ] On Behalf Of Gregg
Vanderheiden
Sent: Thursday, October 11, 2007 11:56 PM
To: 'TEITAC desktop/portable (hardware) subcommittee'
Subject: Re: [teitac-hardware] 2.1.A - Contrast Ratio for Display

Good comments Michael,


We need to talk about "luminosity contrast ratio" for content etc so
that it
is clear we do not mean that here for the hardware. You are correct -
that
is a content & software measure not hardware.


For this we need to look at the measure used for displays. But we
don't
want it to be as high as CRTs. That is more than is needed but I don't
know
what the right value is.

And Michael is also correct that it should be "at least xxxx".




Gregg
-- ------------------------------
Gregg C Vanderheiden Ph.D.



> -----Original Message-----
> From: = EMAIL ADDRESS REMOVED =
> [mailto: = EMAIL ADDRESS REMOVED = ] On Behalf Of
> Takemura, Michael (HP Accessibility)
> Sent: Thursday, October 11, 2007 7:06 PM
> To: TEITAC desktop/portable (hardware) subcommittee
> Subject: [teitac-hardware] 2.1.A - Contrast Ratio for Display
>
> Rob...
>
> The standard uses a method for contrast ratio measurement
> that is not common in the industry. We use a direct
> measurement method such that CR of 800:1 or 1000:1 is quite
> common. This proposed 508 standard uses a calculation based
> on red, green and blue components in a ratio determined by
> the sRGB set point. The max possible CR is 21:1.
>
> We would need to conduct some lab experiments or calculations
> to see what 5:1 looks like.
>
> My interpretation of the rule is that the display must be at
> least 5:1.
> Is that the way you read it, or does it need to be exactly
> 5:1--that would be a problem.
>
> Michael
>
> Michael Takemura
> HP, Director, Accessibility Program
> www.hp.com/accessibility
> 281 518 0739
>
>

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