Thread Subject: FW: FLASH provisions.

From: Gregg Vanderheiden
Date: Mon, Mar 17 2008 5:05 PM

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Notes to TEITAC



Here is the revised text for the FLASH provisions and their definitions.
ALL aspects are included here for completeness.



1) this separates the hardware and software threshold definitions
because hardware must be based on luminance while software/content is on
'relative luminance' (software and web pages control light given off but do
not actually give off light.

2) This uses a similar function for hardware flashing as for software
except that it has a factor for very high brightness LEDs

3) The hardware provision continues the simple "less than 3 hz" but
also provides a way to meet it with flashing LEDs that do flash in
provocative range.

4) Covers very bright LEDs but lets ordinary LEDs used for indicators
etc pass.

5) removed 50 HZ from the SOFTWARE version since there is no line
frequency reason to have content changing from frame to frame in a flashing
manner. And 50% of people who are sensitive would be sensitive at 50 hz.

6) Keeps but restricts the hardware 50 and 60 hz exemption to products
that need it due to use of line frequency but does not allow other
frequencies in this range. It does allow flashing above 75hz which is the
recommendation of Graham Harding.

7) Includes a provision #6 for products that auto -adjust for ambient
light.

8) A number of other small fixes pointed out by people



The "NOTES TO ACCESS BOARD" are added to the Hardware Provision (and the
same note to the Hardware thresholds) because we are unlikely to be able to
reach consensus on the final equation for part #5 of the hardware provision
(even though we want to include it since it provides considerable
flexibility to industry in a way that preserves the protection level of the
rest of our provision) in the time we have left. We want the information
and provisions -but need more time to review them. Followup comments will
need to be submitted by by interested parties.





The hardware provision was designed to

a) provide easy solutions that are more restrictive

* any flash less than 3/sec

* any flash that is dim

* over 75hz

* 50 or 60 hz if tied to line freq.

b) then to provide more complicated but more flexible provision



Included is the following NOTE that gives example of what would pass
provision 5 - so that would make it easy to check these with your engineers
and see if it would cause any problems.



NOTE: Each of the following examples would meet option 5 (when viewed from
at least 12 inches) even when all of the sources are flashing together at
more than 3 Hz:

* 1 round LED with a diameter of 5 mm or less, and no more than 32,000
cd/m2.
* 1 rectangular indicator with a size of 10 by 20 mm or less, and no
more than 2,100 cd/m2.
* 3 square indicators (within a 2.1 inch circle) each 6 mm square or
less, and no more than 3,500 cd/m2
* 4 round indicators (within a 2.1 inch circle) each 8 mm in diameter
or less, and no more than 1,400 cd/m2.
* 5 rectangular indicators (within a 2.1 inch circle) each 6 by 2 mm,
and no more than 6,100 cd/m2.

(For light sources that are Red flashes (that have a 590 nm wavelength or
longer) the values for luminance in this note should be divided by 8.)

(For comparison, the maximum brightness of a white screen on an LCD computer
monitor is about 200 to 400 cd/m2)



REMEMBER - manufacturers can use LEDs brighter than these as well (as long
as they don't flash more than 3/sec. ) So you can have flashlights and
strobe lights... Just not flashing more than 3 per second.



Gregg



BELOW IS THE PROPOSED LANGUAGE FOR THE DIFFERENT PROVISIONS AND
DEFINITIONS.



{NOTE TO TEITAC REVIEWERS. This mirrors software but uses a different
threshold definition (and a qualifying "if" phrase to facilitate
elimination where it does not apply). Uses "either - or" language to make
it clearer that there are two choices. May want to consider that in
software provision}


2.1-B - Flashing (Hardware)


If products emit light in [flashes] at any time then either there must be no
more than 3 flashes in any 1 second period or the flashing must be below the
general flash and red flash thresholds for hardware.

NOTE ACCESS BOARD: the working group came up with the definitions for
"general flash and red flash thresholds for hardware" after much effort but
there was NOT sufficient time to explore them as much as necessary to come
to consensus on them. The "general flash and red flash thresholds for
hardware" therefore DOES NOT HAVE CONSENSUS. What the group did consense
on were the following

1. very bright point sources or very bright small sources can be a
problem (per Graham Harding)

2. Very bright point sources or very bright small sources should be
allowed if they flash less than 3 per second

3. Flashing above 3 per second should be allowed if is as equivalently
safe as with the Software flash thresholds.

4. Some metrics for identifying 'equivalently safe' were created but
more time is needed by everyone to study them and their derivation before
they are used by anyone including the Access Board.

5. The numbers were included in this report in order to facilitate
review by different parties.

There IS consensus on the flash provision for software and content and the
general flash and red flash thresholds for software and content.




3-J - Flashing (Software/Content)


Content or user interfaces must not contain anything that flashes more than
3 times in any one second period or the flashing must be below the general
flash and red flash thresholds for content and user interfaces.

{no change except to add "for content and user interfaces" at the end.}




flash


a pair of opposing changes in luminance (relative luminance for software and
content) that can cause seizures in some people if it is large enough and in
the right frequency range

Note 1: See general flash
<http://www.w3.org/WAI/GL/WCAG20/#general-thresholddef> threshold and red
flash threshold definitions for information about types of flash that are
not allowed.

Note 2: See also blinking <http://www.w3.org/WAI/GL/WCAG20/#blinksdef> .



{New - from WCAG. To clarify FLASH and BLINKING. Also needed for
definitions of thresholds}







blinking


switch back and forth between two visual states in a way that is meant to
draw attention

Note: See also flash <http://www.w3.org/WAI/GL/WCAG20/#flash-def> (It is
possible for something to be large enough and blink brightly enough at the
right frequency to be also classified as a flash).





{New - from WCAG. To clarify FLASH and BLINKING.}





{New - based off of software flash thresholds but considerably different to
handle hardware issues.


general flash and red flash thresholds for hardware


a hardware flash is below the threshold (i.e. software or content passes) if
any of the following is true:



1. there are no more than three General Flashes and / or no more than
three Red Flashes within any one-second period; OR



2. the flash frequency is 50 or 60 Hz and is due to a refresh that is
intrinsically tied to the local line frequency OR



3. the flash frequency is 75 hz or greater OR



4. general flashes (that have less than a 590 nm wavelength) are no
more than 20 cd/m2
AND red flashes (that have a 590 nm wavelength or longer) are no more than
2.5 cd/m2 OR



5. general flashes are no more than (1200/N0.3)(0.006/AREA - 1) cd/m2
AND red flashes (that have a 590 nm wavelength or longer) are no more than
(150/N0.3)(0.006/AREA - 1) cd/m2 OR



6. there is an adaptive brightness feature that always keeps the
changes in luminance of flashes from all sources (that might flash > 3 hz)
below the maximum of (1, ambient(in lux)/700 lux) times the threshold values
in 4 or 5 above.



- where N is the number of sources that are flashing together more
than 3 times per second within a 0.024 steradian circle

- where AREA is the summed area of the N sources, measured in
steradians (at the minimum typical/expected viewing distance)

- and where sources that are separated by less than .4 degrees (
~2mm at 12 inch viewing distance) are treated as a single source with AREA
being the area from edge to edge of the group.



NOTE: Most products can safely use a minimum typical / expected viewing
distance of 12 inches where 0.006 steradians would be a circle of 1.05 inch
diameter.



NOTE: Red sources with wavelengths of 590 or more nm are especially
provocative since, due to the way the eye and brain process light with
longer wavelengths.



NOTE: Each of the following examples would meet option 5 (when viewed from
at least 12 inches) even when all of the sources are flashing together at
more than 3 Hz:

* 1 round LED with a diameter of 5 mm or less, and no more than 32,000
cd/m2.
* 1 rectangular indicator with a size of 10 by 20 mm or less, and no
more than 2,100 cd/m2.
* 3 square indicators (within a 2.1 inch circle) each 6 mm square or
less, and no more than 3,500 cd/m2
* 4 round indicators (within a 2.1 inch circle) each 8 mm in diameter
or less, and no more than 1,400 cd/m2.
* 5 rectangular indicators (within a 2.1 inch circle) each 6 by 2 mm,
and no more than 6,100 cd/m2.

(For light sources that are Red flashes (that have a 590 nm wavelength or
longer) the values for luminance in this note should be divided by 8.)

(For comparison, the maximum brightness of a white screen on an LCD computer
monitor is about 200 to 400 cd/m2)

RATIONALE: The hardware flash values are based on the CIE Small Angle
Disability Glare Equation in Johannes J Vos "On the cause of disability
glare and its dependence on glare angle and ocular pigmentation" in Clinical
and Experimental Optometry 86.6, November 2003 and "Commission
Internationale de l'Eclairage CIE. CIE equations for disability Glare. CIE
report #146. Vienna: CIE 2002. This equation also includes an age factor.
62.5 was used as the age in generating the constants in equation in #5
above.

NOTE TO ACCESS BOARD: the working group came up with the definitions for
"general flash and red flash thresholds for hardware" (above) after much
effort but there was NOT sufficient time to explore them as much as
necessary to come to consensus on them. The "general flash and red flash
thresholds for hardware" therefore DOES NOT HAVE CONSENSUS. What the group
did consense on were the following

1. very bright point sources or very bright small sources can be a
problem (per Graham Harding)

2. Very bright point sources or very bright small sources should be
allowed if they flash less than 3 per second

3. Flashing above 3 per second should be allowed if is as equivalently
safe as with the Software flash thresholds.

4. Some metrics for identifying 'equivalently safe' were created but
more time is needed by everyone to study them and their derivation before
they are used by anyone including the Access Board.

5. The numbers were included in this report in order to facilitate
review by different parties.




general flash and red flash thresholds for software/content


a flash or rapidly changing image sequence is below the threshold (i.e.
software or content passes) if any of the following is true:



1. there are no more than three General Flashes
and / or no more than three Red Flashes within any one-second period; or



2. the combined area of flashes occurring
concurrently occupies no more than a total of .006 steradians within any 10
degree visual field on the screen (25% of any 10 degree visual field on the
screen) at typical viewing distance

where:

* A General Flash is defined as a pair of opposing changes in
relative luminance of 10% or more of the maximum relative luminance where
the relative luminance of the darker image is below 0.80; and where "a pair
of opposing changes" is an increase followed by a decrease, or a decrease
followed by an increase, and



* A Red Flash is defined as any pair of opposing transitions
involving a saturated red.



Exception: Flashing that is a fine, balanced, pattern such as white noise or
an alternating checkerboard pattern with "squares" smaller than 0.1 degree
(of visual field at typical viewing distance) on a side does not violate the
thresholds.



Note 1: For general software or Web content, using a 341 x 256 pixel
rectangle anywhere on the displayed screen area when the content is viewed
at 1024 x 768 pixels will provide a good estimate of a 10 degree visual
field for standard screen sizes and viewing distances (e.g. 15-17 inch
screen at 22-26 inches). (Higher resolutions displays showing the same
rendering of the content yield smaller and safer images so it is lower
resolutions that are used to define the thresholds.)



Note 2: A transition is the change in relative luminance (or relative
luminance/color for red flashing) between adjacent peaks and valleys in a
plot of relative luminance (or relative luminance/color for red flashing)
measurement against time. A flash consists of two opposing transitions.



Note 3: The current working definition in the field for "pair of opposing
transitions involving a saturated red" is where, for either or both states
involved in each transition, R/(R+ G + B) >= 0.8, and the change in the
value of (R-G-B)x320 is > 20 (negative values of (R-G-B)x320 are set to
zero) for both transitions. R, G, B values range from 0-1 as specified in
"relative luminance" definition. (Harding and Binnie 2002)



Note 4: Tools are available that will carry out analysis from video screen
capture. However, no tool is necessary if flashing is less than or equal to
3 flashes in any one second period (content automatically passes (see #1 and
#2 above)).

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