Horizontally the DIN standard recommends horizontal viewing angles (i.e. viewing positions) to be within ±45o and within ±30o vertically. Again this depends on the actual application. For example the height of the image will depend on the seating layout!
Use the word ‘brightness’ carefully! Brightness refers to how we perceive luminance; it is not a scientific term. It’s hard to use the English language without using ‘brightness’ – even in this document! – but nevertheless be aware of the subjective associations of using this word.
Image ‘brightness’ and obtainable contrast ratios are the fundamentals for image quality.
As can be seen from the way the eye works, the image luminance (‘brightness’) and contrast ratio must be selected based on the average adaptation level in the specific application.
The higher the adaptation level, the ‘brighter’ the image that is required.
Adaptation level is a result of a combination of different factors including: ambient light level; reflection factors on the surfaces in the field of vision; and luminance of different light sources in the field of vision etc.
As a rule of thumb, the image luminance should be at least at the level of the average luminance level in the field of vision, provided that a sufficient contrast ratio can be obtained.
For simple displays (i.e. comprising simple graphic information such as PowerPoint slides) images become viewable from approximately 5:1 contrast ratio. More detailed content (e.g. spreadsheets and photos are viewable from approximately 10:1 contrast ratio.
An international panel of consultants and engineers agreed the following minimum contrast ratios:
- Image contrast ratio should be at least 10:1.
- Image contrast is acceptable/good for most content from 15:1
- Image contrast is good for all content from 20:1