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Re: Clarification of duration of flashing or flickering (Success Criteria 2.3.1)

From: Gregg Vanderheiden <gv@trace.wisc.edu>
Date: Wed, 28 Oct 2009 08:06:33 -0400
To: Phill Jenkins <pjenkins@us.ibm.com>
Cc: public-comments-wcag20@w3.org, w3c-wai-ig@w3.org
Message-id: <AFF03BBA-5298-4416-B4FA-A7AE1E9103FA@trace.wisc.edu>
Hi Phill

    For hardware guidelines - an upper limit is made.    For software  
guidelines it was omitted because there is no reason in software to  
blink something above 30 hz because no one can see it.     The only  
thing would be subliminal messages and that would require a frame rate  
of 160 frames per second and it would be clearly visible -- so it  
wouldn’t be subliminal.    So there would be no reason to flash  
something intentionally in software at that frequency.     Frame  
flicker would not meet the intensity provision.   The frames would  
have to differ significantly back and forth over a large enough area  
with enough luminosity difference.

For hardware you can get flicker.   And we are forced to set the upper  
limit at 50 hz because of line frequencies in Europe -- but that is  
actually a bad frequency (anything above 3 and below about 75hz is not  
good).    So no upper limit is set for software because there is not a  
need for one, and  it would be set different than hardware which could  
be very confusing.

Your suggestion for  "a clearer discussion between flicker frequency  
that the developer has no control over verses the flash rate during  
that one second period ".      Sounds like a good idea.  If you can  
send more about what you are thinking of here it would be helpful.   
You can do on-list or off.

Thanks

NOTE:  These are quick feedback notes from me personally to advance  
the discussion on the IG.   Responses from the working group itself  
will come more slowly as the topics sent to public comment are logged  
and addressed in turn by the group.

Thanks Phill

Gregg
-----------------------
Gregg Vanderheiden Ph.D.
Director Trace R&D Center
Professor Industrial & Systems Engineering
and Biomedical Engineering
University of Wisconsin-Madison










On Oct 28, 2009, at 6:06 AM, Phill Jenkins wrote:

>
> Thanks Gregg for clarification - it was the key word "any" that I  
> missed - as in: "any one second".
>
> But I think there is still a need for an upper limit during that any  
> one second period.  For example, if it flashes 80 times in that one  
> second period I do not believe it is a problem - is it?
>
> Maybe a clearer discussion between flicker frequency that the  
> developer has no control over verses the flash rate during that one  
> second period would help?
>
> Regards,
> Phill Jenkins,
Received on Wednesday, 28 October 2009 12:07:09 GMT

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