Including Dynamic Viewing Behavior in Determining Vertical Eye-Level Light: Does it Matter for Office Design?

When considering non-image-forming (NIF) light effects on people, knowing the light vertically at eye-level is necessary. However, people are dynamic in their behavior and constantly change their posture and viewing direction. This means that light measured vertically toward a constant direction might differ from the actual light that reaches people’s eyes. If the difference is large, occupant behavior might need to be included in lighting design measurements and simulations predicting the potential of NIF light effects. This paper presents an experiment on the difference between the actual dynamic eye-level light of office workers while seated at a desk (dynamic condition) and light measured statically toward a computer screen (static condition). Eye-level light conditions in relation to NIF effects were quantified using the melanopic Equivalent Daylight Illuminance (mEDI) and the relative alerting response (r_D). Measured dynamic mEDI was 4% to 10% larger than the static one. Analysis of r_D was not possible due to saturation of the response. The dominant face orientation was toward the computer screen, with a deviation in the horizontal and vertical direction within 15 degrees from the center. The results show that measuring and simulating personal light conditions of office workers statically toward a computer screen might be a small underestimation of actual eye-level light, but the relevance of this underestimation in lighting design practice is not evident.