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ISSUE-1 - 4.4.1 define - general flash and red flash thresholds

From: Simon Harper <simon.harper@manchester.ac.uk>
Date: Thu, 14 May 2009 19:35:19 +0100
Message-Id: <2BCBEE0C-E378-4F81-883C-FAC09A11955B@manchester.ac.uk>
To: UAWG list <w3c-wai-ua@w3.org>
So this is what I've dug up on this...


A potentially harmful flash occurs when there is a pair of opposing  
changes
in luminance (i.e., an increase in luminance followed by a decrease,  
or a
decrease followed by an increase) of 20 candelas per square metre  
(cd.m-2) or
more (see notes 1 and 2). This applies only when the screen luminance  
of the
darker image is below 160 cd.m-2. Irrespective of luminance, a  
transition to or
from a saturated red is also potentially harmful.

Isolated single, double, or triple flashes are acceptable, but a  
sequence of
flashes is not permitted when both the following occur:
i. the combined area of flashes occurring concurrently occupies more
than one quarter of the displayed (see note 3) screen area; and
ii. there are more than three flashes within any one-second period.
For clarification, successive flashes for which the leading edges are
separated by 9 frames or more are acceptable, irrespective of their
brightness or screen area.


Notes:
1. A luminance voltage of 234 mV results in light output of 20.1  
cd.m-2. If the brighter image of a
flash or pattern is above this level, then it is potentially harmful  
if the light output between the
darker and brighter images differs by 20 cd.m-2 or more.
2. A luminance voltage of 631 mV results in light output of 160  
cd.m-2. If the darker image of a
flash or pattern is below this level, then it is potentially harmful  
if the light output between the
darker and brighter images differs by 20 cd.m-2 or more.


Cheers
Si.

=======================

Simon Harper
University of Manchester (UK)

Human Centred Web Lab: http://hcw.cs.manchester.ac.uk

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Received on Thursday, 14 May 2009 18:35:57 GMT

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