Light transmittance range of glass for visual comfort in an office environment

ABSTRACT ‘Nothing is as variable as the weather’ is a well-known Dutch expression demonstrating the instability of the Dutch climate and as a consequence the daylight availability. In rapidly changing climate conditions, a non-adjustable, structural shading device is unsuitable. Therefore a system that blocks the direct sunlight when necessary without obstructing the diffuse daylight next to keeping view intact would be the best solution, both for user comfort as for energy consumption. A glazing system with a rapidly adjustable transmittance might be a good solution for such climate. Smart Energy Glass (SEG), a glazing system being developed by Peer+ can quickly switch between two light transmittance values (max. range of 30%). The research question of this project was to determine, based on users comfort, these two values. An experimental set-up was made in the laboratory of Building Physics and Systems group with five identical rooms. All rooms were equipped with an equal sized daylight opening; four rooms had a different light transmittance (0.08, 0.25, 0.52, and 0.70). The fifth was the reference room and had normal glass with a transmittance of 0.70. During summer, autumn, and winter, the same 24 healthy test-persons (age 23±4 years) participated in the tests. By filling out a questionnaire regarding comfort, preferred transmittances and lighting situation under different daylight conditions was determined. The test people considered their working condition as comfortable with horizontal illuminance on the workplace of at least 267 lx. Regarding visual comfort, window luminance values of 6500 cd/m² were still accepted by the subjects, and glare was barely experienced or indicated. It could be concluded that during measurements with sunlight on the façade, the test space with a transmittance of 0.25 was mostly evaluated for its good lighting by daylight. During measurements without sun, the test spaces with a transmittance of 0.52 and 0.70 were mostly positively evaluated for their good lighting by daylight. In the summer period, transmittances of 0.25 and 0.52 were most preferred by the subjects. This also applies for sunny days in the autumn. During overcast days in the autumn and during winter, there was a desire for higher light transmission. Taken into account the maximum range of 30%, a transmittance range between 0.40 - 0.70 most users are satisfied with the lighting by daylight namely at least 71% of the time, up to 89% of the time.