Glare Metrics Research Articles

Predicting visual comfort in side-lit open-plan core zones: Results of a field study pairing high dynamic range images with subjective responses

Abstract Transmitting sufficient daylight to core zones while maintaining a visually comfortable work environment is critical for the effective use of daylight to reduce lighting energy and enhance indoor environmental quality. Although a range of indicators exists to predict visual comfort from windows, data comparing indicators with occupant subjective data collected from the core zones of daylit buildings are limited. This paper presents results from a study conducted in the core zones of a side-lit office building located in San Francisco, California. Subjective measurements of visual comfort were collected using a repeated-measures study design involving (N = 14) participants over 2 weeks under clear sky conditions. Desktop polling devices were used to pair subjective data with concurrent luminance measurements acquired from high dynamic range (HDR) imaging cameras, resulting in a total of 523 observations. Single-variable logistic regression models generated from paired physical and subjective data were used to examine and rank 15 indicators of visual discomfort. Discomfort indicators based on luminance contrast ratios and absolute measures were found to be more effective than glare metrics or the more basic measures of vertical or horizontal illuminance. Results are compared and discussed in context with existing guidance for measuring and assessing discomfort glare.

Daylighting in an atrium-type high performance house

Different lighting strategies for an atrium-type high performance house that was developed for the 2013 Solar Decathlon competition in Datong, China are discussed in this paper. Although daylighting may be abundant in an atrium, with large exposed window surfaces, such design is typically avoided in high-performance houses due to the poorer thermal performance of glazing compared to opaque building envelopes. However, advantages of the atrium design exist in different weather conditions and are investigated in this paper. Insights from a Solar Decathlon 2013 project with a central atrium and a high amount of glasses in the external envelope (80 square meters of glasses) are given. The house was designed considering weather of Datong and Worcester, Massachusetts (US). Simulation results obtained with Radiance and Daysim allow assessing different parameters such as the daylight autonomy index and the useful daylight index. The challenge of visual comfort for a house with such a great quantity of windows is realized considering different parameters related to the visual glare, including the instant and cumulative daylight glare probability indexes. These glare metrics are often inconsistent as the probability of glare was sometimes depending on the selected index. Different chances to have glare issues resulted using different indices. Finally, the influencing factors for blind adjustments in residential buildings are discussed, and according to these factors and to simulation results with several software, the design of the shading systems and their schedules are evaluated.

Post occupancy evaluations relating to discomfort glare: A study of green buildings in Brisbane

Glare indices have yet to be extensively tested in daylit open plan offices, as such there is no effective method to predict discomfort glare within these spaces. This study into discomfort glare in open plan green buildings targeted full-time employees, working under their everyday lighting conditions. Three green buildings in Brisbane were used for data collection, two were Green Star accredited and the other contained innovative daylighting strategies. Data were collected on full-time employees, mostly aged between 30 and 50 years, who broadly reflect the demographics of the wider working population in Australia. It was discovered 36 of the 64 respondents experienced discomfort from both electric and daylight sources at their workspace.The study used a specially tailored post-occupancy evaluation (POE) survey to help assess discomfort glare. Luminance maps extracted from High Dynamic Range (HDR) images were used to capture the luminous environment of the occupants. These were analysed using participant data and the program Evalglare.The physical results indicated no correlation with other developed glare metrics for daylight within these open plan green buildings, including the recently developed Daylight Glare Probability (DGP) Index. The strong influence of vertical illuminance, Ev in the DGP precludes the mostly contrast-based glare from windows observed in this investigation from forming a significant part of this index. Furthermore, critical assessment of the survey techniques used are considered. These will provide insight for further research into discomfort glare in the endeavour to fully develop a suitable glare metric.

The ‘adaptive zone’ – A concept for assessing discomfort glare throughout daylit spaces

Discomfort glare is an underutilized parameter in contemporary architectural design due to uncertainties about the meaning of existing metrics, how they should be applied and what the benefits of such analysis are. Glare is position and view direction-dependent within a space, rendering it difficult to assess compared to conventional illuminance-based metrics. This paper compares simulation results for five glare metrics under 144 clear sky conditions in three spaces in order to investigate the ability of these metrics to predict the occurrence of discomfort glare and to hence support the design of comfortable spaces. The metrics analyzed areDaylight Glare Index, CIE Glare Index, Visual Comfort Probability, Unified Glare Rating and Daylight Glare Probability. It is found that Daylight Glare Probability yields the most plausible results. In an attempt to deal with multiple positions and view directions simultaneously, the concept of an ‘adaptive zone’ is introduced within which building occupants may freely adjust their position and view in order to minimise the effect of glare. The spatial and directional extents of the adaptive zone depend on furniture layout and the freedom of occupants’ tasks. It is found that applying the adaptive zone concept to a sidelit office with manually operated venetian blinds reduces the predicted hours of intolerable discomfort glare from 735 to 18 occupied hours per year and increases the annual mean daylight availability from 40% to 72%.

A daylighting knowledge base for performance-driven facade design exploration

Heuristics and simple design guides are important tools for designers who wish to incorporate daylighting into the early design process. This paper describes a simulation-based method for generating simple rules for facade design based on specific climate data. This approach uses a design of experiments methodology to populate a detailed daylighting knowledge base. To enable consideration of both visual performance and visual comfort, the knowledge base contains information about whole-year, climate-based illuminance and glare metrics. The design parameters considered include window and shading device geometry and glazing materials. The approach illustrates how various design elements affect daylighting performance and provides a simple way to approach daylighting early in the design process. It can be employed for design and in educational settings.

An interactive expert system for daylighting design exploration

Architects increasingly use digital tools during the design process, particularly as they approach such complex problems as designing for successful daylighting performance. However, while simulation tools may provide the designer with valuable information, they do not necessarily guide the user toward design changes which will improve performance. This paper proposes an interactive, goal-based expert system for daylighting design, intended for use during the early design phases. The expert system consists of two major components: a daylighting knowledge-base which contains information regarding the effects of a variety of design conditions on resultant daylighting performance, and a fuzzy rule-based decision-making logic which is used to determine those design changes most likely to improve performance for a given design. The system gives the user the ability to input an initial model and a set of daylighting performance goals in the form of illuminance and daylighting-specific glare metrics. The system acts as a “virtual daylighting consultant,” guiding the user toward improved performance while maintaining the integrity of the original design and of the design process itself. Two sets of case studies are presented: first, a comparison of the expert system results to high performing benchmark designs generated with a genetic algorithm; and second, an evaluation of the expert system performance based on varying levels of esthetic constraints. The results of these case studies indicate that the expert system is successful at finding designs with improved performance for a variety of initial geometries and daylighting performance goals.