Lighting quality goes beyond task performance to address other needs related to aesthetic judgment, mood and atmosphere and visual comfort, all of which indirectly affect user satisfaction and task performance.
The first two needs, often left out of the retrofit process, become primary considerations in a relighting or new construction project, covering patterns of light, room surface brightness and uniformity. These topics are covered in detail in Chapter 3.
Visual comfort generally means avoiding excessive luminance or luminance contrasts that can cause either visual fatigue or glare.
Tired eyes and uniformity
Visual fatigue is the result of long exposure to excessive non-uniform contrast in a space, particularly when performing critical tasks, caused by transient adaptation, or the eye continually adapting to one level of luminance to another. For this reason, relatively uniform light levels are recommended for office, classroom, industrial and similar environments, where users perform sustained and/or critical visual tasks. Harsh patterns should be avoided. Another lighting problem that can contribute to visual fatigue is flicker.
Uniformity entails a relatively consistent distribution of light intensity across the task plane, an imaginary plane placed on the level of primary tasks in the space. In an open office, this may be the desktop. In a hallway, the task plane may be the floor. Emotionally, the presence of good uniformity of light intensity in the visual environment, combined with higher light levels and less intensity at the room perimeter, can promote a public “work” atmosphere. Uniform distribution with high light levels and bright walls and ceilings can create a further sense of spaciousness.
Task luminance should be higher than its surround to focus attention but not so high that uniformity is lost and visual fatigue may result. As a guide, IES provides recommended max-to-min luminance ratios for various applications. Generally, up to three times more light on a workplane task than its immediate surround will direct attention to the task but not cause visual fatigue.
