Influence of wall luminance and uniformity on preferred task illuminance

Literature suggests an influence of the luminance from non-horizontal surfaces in our visual field on our visual and psychological assessments of an office space. These assessments are believed to directly relate to our expressed preferred task illuminances. This paper describes an evaluation in a mock-up office, wherein wall conditions with a non-uniform and a more uniform light distribution of 3 different average luminance levels have been evaluated regarding their effect on users' preferred task illuminance. Each condition is evaluated starting from three different initial desk illuminances. For all test conditions, a wall with a non-uniformly distributed average luminance of 200 cd/m2 lead to significantly lower selected desk illuminances than a uniformly lit wall with the same average luminance level. In all cases, preferred task illuminances set were significantly lower when offering the lowest starting level for dimming of 300 lx. The range of preferred illuminance levels between subjects was also found to be smaller for dimming with the starting level of 300 lx at desk level. The study suggests that when providing users with personal control they will control the total perceived brightness in their visual field, even though they are only directly affecting their task illuminance level. Triggering the selection of lower preferred illuminance levels due to a personal control starting level of 300 lx, will positively influence the energy used for lighting. The smaller range of preferred illuminance levels between subjects at the starting of 300 lx could reduce the risk of lighting preference related conflict between people. However, more research is needed to confirm that these smaller differences are also perceivable by users