Background knowledge
All texts or graphic objects in a PDF document should have sufficient contrast values in relation to the background or adjacent graphic objects. This is to ensure that even users with poor eyesight can perceive all texts and graphic objects well.
Sufficient contrast is provided when the foreground and background colors differ from one another sufficiently and the required contrast values are met.
Required contrast values
The required minimum values for the contrast of texts are defined in WCAG 2.1, check point 1.4.3, level AA:
- for (large) font with at least 18pt (not bold): minimum contrast 3: 1
- for (large) font with at least 14pt (bold): minimum contrast 3: 1
- for (normal) fonts smaller than 18pt (not bold): minimum contrast 4.5: 1
The minimum values required for the contrast of graphic objects are defined in WCAG 2.1, check point 1.4.11, level AA:
- Minimum contrast 3: 1
How can you measure the contrast value?
There are various free programs for this. For example, we work with the "Color Contrast Analyzer", which you can download from the following link: Colour Contrast Analyser (CCA).
Alternatively, you can also use web-based applications for this - you can find a contrast calculator, for example, at Constrast Checker by WebAIM.
Positive examples
- Sufficient text-background contrast for normal-sized 11pt font
- Sufficient contrast between two graphic objects (pie chart)
Further examples and explanations can be found in Understanding Success Criterion 1.4.11: Non-text Contrast | WAI | W3C.