The ‘apparent’ size of an object is directly proportional to the distance it is away. Successfully visual perception depends primarily upon the brightness contrast, colour contrast and ‘apparent’ size of the detail to be perceived, the ability of the eye to successfully focus given the viewing distance and the ambient illumination. The environmental factors are now considered in further detail

The level of ambient illumination, together with the spatial relations between the light source, the product, and the user, impact on the ability to perceive visual detail. Examples highlighting this include reading the text on the back of a television, or reading a road sign whilst facing the sun.

Glare can cause significant difficulties when reflective surfaces such as a screen are viewed from a certain angle related to the sun or light source. Consideration of reflectivity, distance and viewing angle is particularly significant when the position of the product or user is fixed or constrained, such as when viewing a road-sign.