Checkpoint 03: Flickering

PDFs must not contain content that flashes or flickers more than three times per second. This requirement exists to protect people with photosensitive epilepsy from potentially life-threatening seizures.

What This Means

Flickering or flashing content refers to any visual element that rapidly alternates between different states, colors, or brightness levels. In PDFs, this can occur through:

• Animated GIF images
• Embedded video content
• JavaScript-triggered animations
• Flash or multimedia objects (in older PDFs)
• Rapid color transitions in embedded media

When content flashes more than three times per second, particularly with high contrast or large areas of the screen, it can trigger photosensitive seizures in susceptible individuals. This is not merely an inconvenience but a serious health and safety concern.

The "three flashes or below threshold" rule comes from medical research on photosensitive epilepsy. Content is considered safe if it either flashes no more than three times in any one-second period, or the flashing area is sufficiently small and the contrast is below specific thresholds defined in the WCAG guidelines.

Why It Matters

Photosensitive epilepsy affects approximately 1 in 4,000 people, but the consequences of triggering a seizure can be severe:

• Immediate health risk: Seizures can cause loss of consciousness, falls, injuries, or in rare cases, can be fatal
• Real-world incidents: The 1997 Pokemon episode "Electric Soldier Porygon" caused seizures in nearly 700 Japanese children due to rapid flashing sequences
• Legal requirements: WCAG 2.3.1 (Level A) makes this a baseline accessibility requirement that applies to all conformance levels
• Universal benefit: Many people experience discomfort, nausea, or headaches from flickering content even without having epilepsy

This is one of the few accessibility requirements where failure can cause immediate, physical harm to users. For this reason, it is categorized as critical priority regardless of how likely a PDF is to contain such content.

Common Violations

The Matterhorn Protocol defines three failure conditions for flickering content. All three require human testing because automated tools cannot reliably analyze the temporal characteristics of embedded content.

03-001: An Action Causes Flickering

What's Wrong: An interactive element in the PDF triggers flickering when activated. This could be a button that launches an animation, a form action that causes visual feedback, or a link that activates multimedia content.

How to Identify:

• Manually test all interactive elements in the PDF
• Click buttons, submit forms, and activate links while watching for rapid visual changes
• Pay particular attention to any feedback animations or transitional effects
• Note any content that rapidly alternates colors or brightness when activated

Risk Scenarios:

• "Click here" buttons that trigger animated confirmations
• Form validation that rapidly flashes error indicators
• Navigation elements with pulsing or blinking hover effects
• Success/error states that use flashing to draw attention

03-002: A Multimedia Object Contains Flickering

What's Wrong: An embedded video, animation, or multimedia element contains sequences that flash more than three times per second. This is the most common source of flickering violations in PDFs.

How to Identify:

• Review all embedded videos and animations frame by frame
• Use video analysis tools to check flash rates
• Look for strobe effects, rapid scene transitions, or pulsing elements
• Check animated GIFs for rapid frame changes
• Pay attention to any content with alternating high-contrast colors

Risk Scenarios:

• Training videos with transition effects between slides
• Product demonstrations with attention-grabbing animations
• Embedded advertisements or promotional content
• Animated diagrams with pulsing or blinking elements
• Video content with strobe lighting or rapid cuts

03-003: JavaScript Causes Flickering

What's Wrong: JavaScript code in the PDF creates visual effects that flash more than three times per second. This includes scripts that manipulate element visibility, colors, or positions rapidly.

How to Identify:

• Test the PDF with JavaScript enabled and disabled
• Look for any animated elements that rely on scripting
• Check for countdown timers, progress indicators, or attention-grabbing effects
• Review JavaScript code if you have access to the source
• Test interactive calculators, forms, or dynamic content for visual flickering

Risk Scenarios:

• Countdown timers with flashing digits
• Progress bars with pulsing animations
• Alert messages that blink to draw attention
• Dynamic form validation with rapid visual feedback
• Custom cursor effects or highlight animations

How to Fix in Adobe Acrobat

Adobe Acrobat provides tools to identify and remove problematic multimedia and JavaScript content.

Identifying Multimedia Content

1. Open your PDF in Adobe Acrobat Pro
2. Go to Tools > Rich Media
3. Review any embedded video or audio content
4. Click on each multimedia element to preview it
5. Note any content that appears to flash rapidly

Removing Problematic Multimedia

If embedded media contains flickering that cannot be edited:

1. In the Rich Media tool, select the problematic element
2. Press Delete to remove it
3. Consider replacing it with:
   • A static image capturing the key information
   • A link to an external video with appropriate warnings
   • A text description of the content

Checking for JavaScript

1. Go to Tools > JavaScript
2. Select Document JavaScripts to see document-level scripts
3. Review the code for any timing functions (setInterval, setTimeout) that could cause rapid visual changes
4. Select Edit All JavaScripts to view and modify scripts if needed

Removing JavaScript That Causes Flickering

1. In the JavaScript panel, identify problematic scripts
2. Select the script and click Delete to remove it entirely
3. Alternatively, edit the script to:
   • Remove animation effects
   • Slow down any transitions to below three per second
   • Replace animated feedback with static alternatives

Checking Embedded Animations

1. Go to View > Show/Hide > Navigation Panes > Content
2. Expand the content tree to find XObjects (which may include images)
3. Look for animated GIF files or other animated formats
4. Right-click and select Delete to remove problematic animations
5. Replace with static images if the content is needed

Adding Warnings for External Content

If you must link to external content that may contain flickering:

1. Add a clear text warning before the link
2. Example: "Warning: The following video contains flashing lights that may affect photosensitive individuals."
3. Ensure the warning is properly tagged and in the reading order before the link

How to Fix in Microsoft Word

Preventing flickering issues starts with the source document. Word has limited support for problematic animations, but here are best practices.

Avoiding Animated GIFs

1. Do not insert animated GIF files that flash rapidly
2. If you need to reference an animation, use a static screenshot instead
3. Add descriptive text explaining what the animation shows
4. Link to the animation externally if users need to view it, with appropriate warnings

Using Static Images Instead

1. If you have an animated graphic, create a static version:
   • Open the GIF in an image editor
   • Export a single representative frame as PNG or JPEG
2. Insert the static image in Word
3. Add alt text describing both what the image shows and that it represents an animation

Checking Embedded Videos

1. If you embed video in Word, review it for flickering before insertion
2. Use video editing software to analyze flash rates
3. Consider linking to external videos with warnings instead of embedding
4. Ensure any embedded video has pause/stop controls visible

Export Settings

1. When saving as PDF, go to File > Save As > PDF
2. Click Options
3. Under "Include non-printing information," uncheck options that might preserve animations
4. Consider using PDF/A format which has stricter restrictions on dynamic content

Best Practices for Source Documents

• Use transitions and animations sparingly in presentations converted to PDF
• Test all interactive elements manually before distribution
• Include text warnings before any potentially problematic content
• Provide alternative versions of documents containing multimedia

Testing Your Fix

Because flickering violations require human testing, verification must be done manually and thoroughly.

Manual Visual Testing

1. Open the PDF and view it at full screen
2. Interact with every clickable element
3. Watch for any rapid visual changes
4. Time any animations to ensure they flash no more than 3 times per second
5. Pay special attention to:
   • Page transitions
   • Form interactions
   • Embedded media playback
   • JavaScript-driven content

Using Video Analysis Tools

For embedded video content:

1. Extract or screen-record the video content
2. Use tools like the Photosensitive Epilepsy Analysis Tool (PEAT)
3. Analyze the frame rate and luminance changes
4. Ensure no sequence exceeds three flashes per second

Testing with Different Viewers

1. Test in Adobe Acrobat Reader (most common PDF reader)
2. Test in web browsers (Chrome, Firefox, Edge PDF viewers)
3. Test in Preview on macOS
4. Note that different viewers may handle multimedia differently

Seizure Safety Checklist

• No content flashes more than 3 times per second
• No large areas (more than 25% of the screen) flash at any rate
• Red flashing is minimized or eliminated (red is particularly problematic)
• Interactive elements do not trigger rapid visual feedback
• Embedded videos have been analyzed for flash content
• JavaScript does not create flickering effects
• Warnings are provided before any potentially problematic external content

Document Your Testing

Because these tests cannot be automated:

1. Create a testing log documenting what was checked
2. Record the date and tester name
3. Note any borderline content and the decision made
4. Keep records for compliance purposes

Additional Resources

Official Standards and Guidelines

• W3C WCAG 2.1 Success Criterion 2.3.1: Three Flashes or Below Threshold
• W3C WCAG 2.1 Success Criterion 2.3.2: Three Flashes
• PDF Association Matterhorn Protocol 1.02

Medical and Technical Background

• Epilepsy Foundation: Photosensitivity and Seizures
• W3C Technical Paper on Flash Thresholds
• Understanding Photosensitive Epilepsy (WebAIM)

Testing Tools

• PEAT (Photosensitive Epilepsy Analysis Tool) - Free tool for analyzing video content
• Harding Test - Professional broadcast-standard flash testing
• PAC (PDF Accessibility Checker) - Note: Cannot detect flickering automatically

Related Guidelines

• BBC Guidelines on Flashing Images
• Ofcom Guidance on Flashing Images (UK Broadcasting)

This documentation is based on the Matterhorn Protocol 1.02, the definitive reference for PDF/UA validation. Flickering violations require human testing and cannot be detected by automated tools. For the most current information, consult the PDF Association and W3C WCAG guidelines.