Accessibility’s hidden dimension

Challenges of delivering accessibility across multiple devices

Typography has to be accessible and consistent regardless of what platform or device it is viewed on. Office printers, smart home displays and smart watches are viewed within different contexts and from different distances. Furthermore, the rendering of fonts on these devices in point size is not consistent and has varying results.

For example, physical size of the rendered font depends on the font metrics. This means that Roboto will be different in size to Avenir even when both set to the same size in points. To complicate matters, the same font will slightly differ in size depending on the rendering engine; whether its Chrome, Safari, iOS, Android etc.

We believe that accessibility needs to go beyond guidelines and legislation and consider how the user actually ends up seeing something. We struggled to determine what font size was best for readability. So we turned to scientific research published in 2011, by G.Legge and C.Bigelow. The study found a broad but restricted range of font sizes for optimal and maximum readability. It argues that to be accurate in measuring font size you have to consider the angular size in degrees (projected image size at the eye retina), and how it is contingent on the physical size of type and viewing distance. An approximate conversion is as follows: Angular Size in degrees = 57.3 × Physical Size ∕ Viewing Distance.

Armed with this research and taking into consideration the context and the distance from which users are interacting with a device (A user is approximately 3 metres away from a TV, 65cm away from a monitor, 45cm away from a laptop, 30cm away from a mobile phone and 18cm away from a watch) our approach to defining a font size in the UI is as follows: We identify a range in the viewing distance by the user and then calculate the desired physical dimensions of the font based on viewing distance and the angular size for optimal reading speed. According to the formula above, the physical size can be calculated as: Physical Size = Viewing distance ×Angular size / 57.3. Once we have the desired physical size range, we measure the rendered font used on the particular device to find the corresponding point sizes to be used in the UI software code.

By drawing from scientific research we can confidently choose the right font size and avoid the pitfall of designing for accessibility in one dimension.

Are you interested in creating a digital product that works on multiple platforms? Contact us