Exploring the consistency of UGR calculations, measurements, and evaluations

Building simulation results are frequently expressed in terms of the computed values of various performance indicators. Specifically, simulations of visual conditions in buildings yield estimated values of performance indictors such as illuminance levels and daylight factors (at specific points, or averaged over a reference plane) or glare-related indices such as UGR (Unified Glare Rating).

Aside from visual performance simulation, such visual performance indicators may be also used in buildings’ energy simulation as virtual sensors that feed into control algorithms for operation of luminaires and shades. As such, it is important to examine the fitness and dependability of these kinds of performance indicators and their utility in the building performance simulation process on a regular basis. In this context, the present contribution presents a case study focused on UGR, an occupant-centric performance indicator, which is meant to express the level of visual discomfort experienced by people exposed to the presence of bright light sources in the field of view. This case study address issues of calculation, measurement, and occupants’ evaluation.

Embedded routines for the calculation of UGR in simulation applications require information (computed in a previous step) on the background luminance, the luminance of luminaires present in the observer’s field of view, the solid angles extended from the observer’s eye to the luminaire weighted according to their relative position to the line of sight. Actual UGR values in a given spatial setting can be obtained using luminance meters or specially calibrated cameras.

Moreover, empirical studies have established relationships between measured UGR values at a reference point of view and the human observers’ evaluation of glare discomfort using standardized Likert-type scales. The present case study entails the results of calculations and measurements of UGR as well as corresponding subjective evaluations of visual discomfort levels in an office space.

The results provide, for the specific setting examined, answers to the following questions: Do the UGR measurements agree with prior calculated values? Are observers’ judgment of the discomfort level caused by glare agree with the relevant reference information in literature and standards? Specifically, the findings regarding the difference a) between calculation and measurement results on the one side and b) between standard-based predictions of subjective evaluations on the other side provide insights into the dependability of simulation-based UGR estimations as the basis for the evaluation of visual comfort in indoor environments.