Appropriate to use for all general Web content including special cases like material that is specifically designed for a display in a foyer.
This technique relates to Success Criterion 2.3.1: Three Flashes or Below Threshold (Sufficient).
The purpose of this technique is to provide an easy way to pass the success criterion for things that flash, but are small.
If you have something that flashes more than 3 times in a one second period (so G19 can't be used), but the area that is flashing is less than 25% of 10 degrees of visual field (which represents the central area of vision in the eye), then it would automatically pass.
The 10 degree of visual field represents the central area of vision in the eye. This area is highly packed with visual sensors. Flashes in this area are transmitted to the visual cortex. For those with photosensitivity, this flashing of activity on the visual cortex can cause seizures. Flashing on other areas of the eye (which have far fewer sensors) has much less effect on the cortex. Hence, the focus on just the 10 degrees of central vision.
- If the content is for general Web use, you can use Formula 1: Small Safe Area for Web Content.
- If the content is for a known display (e.g., in a company foyer) then Formula 2: Small Safe Area for Known Displays should be used.
Formula 1: Small Safe Area for Web Content
Most Web authors do not know how to translate visual field to pixels, which is what they generally can deal with. This technique provides that translation.
At this point in time, the most prevalent display is 1024 x 768 and about 15-17 inches diagonally. When viewed at a typical viewing distance (22-26 inches) a 10 degree visual field will capture an area approximately 341 x 256 pixels. This is not circular, but neither is the central vision of most users, and the difference is so small (and at the edge of the central vision where sensors are fewer) that it is not important.
Since the criterion is 25% of any 10 degree visual field, any single flashing event on a screen (there is no other flashing on screen) that is smaller than a contiguous area of 21,824 sq pixels (any shape), would pass the General and Red Flash Thresholds.
1024 x 768 was chosen because it represents the most common screen size. It also works with higher resolution screens since the tighter pixel density would result in a smaller and safer image size.
Users with lower resolution displays or that enlarge or view their screens closely would have a higher risk depending on the viewing distance. To address the needs of this group, should be used since it is independent of screen resolution or viewing distance.
Formula 2: Small Safe Area for Known Displays
To calculate the small safe area (in pixels) on the screen when the screen size, resolution, and viewing distance is known, use the following procedure.
For a number of reasons (distribution of central vision sensors often non-circular, simplicity, computational convenience, historical), a 4:3 rectangular approximation of the central 10 degree of visual field is used that is 10 degrees wide and 7.5 degrees high. This has an area of 75 square degrees, vs the 78.5 square degree area of a true circle of 10 degrees.
- To convert viewing distance to rectangle size, multiply the viewing distance by 0.1745 (10 * Pi / 180) to get the width of the rectangle, and multiply the viewing distance by 0.1309 (7.5 * Pi / 180) to get the height of the rectangle. (This calculation can be done in inches, or millimeters, or any other unit of length.)
Determine size of 10 degree angle of view in pixels.
To do this, multiply the width and height of the rectangle from step 1 by the resolution of the screen, in pixels per unit length, to get the horizontal and vertical size of the rectangle in pixels.
- For a 1080p widescreen display (which is 1920 by 1080 pixels), the resolution of the screen in pixels per inch is 2203 divided by the diagonal screen size, in inches.
- For a 720p widescreen display (which is usually 1365 by 768 pixels), the resolution of the screen in pixels per inch is 1566 divided by the diagonal screen size, in inches.
- For an LCD computer monitor which specifies the pixel pitch in millimeters / pixel, the resolution of the screen in pixels per inch is 25.4 divided by the pixel pitch in millimeters.
For any display, if you know the actual diagonal screen size in inches, and the horizontal and vertical resolution of the display in pixels, then the resolution of the screen in pixels per inch is the square-root of ( (horizontal resolution in pixels) * (horizontal resolution in pixels) + (vertical resolution in pixels) * (vertical resolution in pixels) ).
- Multiply the width of the rectangle by the height and divide by 4.
- An author creates an animation that will be displayed on a screen in the entrance lounge at a company. Using the size and resolution of the display and the closest distance that a person can stand when viewing the display, they calculate the size of 25% of the 10 degree of central vision in pixels (using the formula above). This would be the small safe area. They then are careful to never flash any area larger than the small safe area.
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- The small safe area is calculated.
- Check that only one area of the screen is flashing at any time.
- Check that the flashing content would fit into a contiguous container whose area is less than the small safe area.
- Checks #2 and #3 are true.