It can’t be stressed enough: natural light is crucial to the health and well-being of building occupants. However, it can also be a source of glare and visual discomfort. Although this risk is well known in theory, it isn’t always dealt with in practice, and awareness of indicators and prediction tools is still limited.
The aim of this blog post is to provide an overview of the methods and tools available to characterize and quantify glare from natural light. Its purpose is to help developers better understand the issue of glare and help them assess the efficiency of a particular design strategy.
Due to its subjective nature and the many different parameters that affect it — lighting conditions, building morphology, orientation, size and position of windows, interior layout, properties of surfaces, occupation of space, activities or tasks to perform, to name a few — glare is difficult to characterize and quantify. This is particularly true of daylight glare1.
Evaluating glare has been the subject of numerous studies since the 1970s. They have resulted in the gradual development of many models and metrics, mostly based on lab experiments interviewing and examining subjects exposed to different lighting conditions. These models have regularly been called into question, compared, corrected and refined2.
Glare metrics today usually take one or more of the following parameters into account: luminance of glare sources, their size and position in the field of vision, and adaptation luminance3,4 (i.e. the luminance to which the eye has adjusted in a given moment). These metrics can be categorized into three groups5:
- Metrics based on saturation, which define the luminance or illuminance limits for the eyes. They mainly characterize the glare caused by a large amount of light entering the eyes. For instance, there is vertical illuminance at eye level, or simplified Daylight Glare Probability (DGPs).
- Metrics based on the effect of contrast between adaptation luminance and a very bright element in the field of vision: these include, for instance, the Daylight Glare Index (DGI), the Unified Glare Rating (UGR) and the CIE Glare Index (CGI).
- Hybrid metrics: based on both of the effects above, such as Daylight Glare Probability (DGP).
Other metrics based on the quantity of light on a horizontal plane, such as Useful Daylight Illuminance6 (UDI) or Annual Sunlight Exposure7(ASE), are also worth mentioning due to their use in certain standards8,9. However, they are less precise than the metrics based on luminance10 and are more commonly recommended as an estimate when performing an initial quick evaluation11,12.
These metrics are also associated with scales and/or limit values that indicate the probability of risk of glare.
The indicator most recognized by the scientific community for quantifying daylight-based glare is DGP. It ranges from 0 to 1 and indicates whether a glare situation will be imperceptible (DGP ≤ 0.35), perceptible (0.35 < DGP ≤ 0.40), disturbing (0.40 <DGP ≤0.45) or intolerable (DGP > 0.45) to a majority of occupants.
Example of DGP calculation in an office space (source: ESTIA)
There are certain situations, however, where this indicator is less appropriate, such as when the light comes from a window in the roof or in deep or dark spaces13. Furthermore, the applicability of DGP to all types of facades and solar protection has not yet been verified14.
Generally speaking, there is no scientific consensus on the glare metrics developed to date — and the associated limits15. In particular, the lack of consideration of individual, contextual and psychological factors (e.g., the presence of a view) is a source of ongoing debate. Future research will be required to refine knowledge in this area. However, it seems unlikely that this will result in a single metric that applies to every combination of spaces, lighting conditions and potential users4.
Calculating and measuring glare
Most building simulation software — TRNSYS, DesignBuilder, IESVE, DIAL+, IDA-ICE — now allows you to calculate levels of natural light in a room over the year in a given climate. But as mentioned above, these levels are not necessarily closely correlated with glare perception. To go further and calculate more sophisticated metrics such as DGP, special software such as Radiance, Diva-for-Rhino or LightStanza is required. It must be noted that these metrics are calculated for a specific time of year and viewpoint and that each calculation can take up to several hours to perform. Different times of year and viewpoints must be examined for a full analysis.
Measurement tools, such as luminance meters and HDR cameras, associated with specific protocols16,17 allow us to identify potential sources of glare in a given scene or measure certain metrics such as DGP. They are, however, usually reserved for use by experts. There is a recognized need to pursue research into measurement methods, which goes hand in hand with future studies on metrics.
Examples of (a) an HDR camera and (b) a luminance meter permitting high dynamic range photography (c) aimed at identifying sources of glare.
Standards and Labels for Glare
To date, only the EN 17037 standard specifically sets out how to assess glare associated with daylight. It is based on the DGP indicator. Recommendations on anti-glare protection are also given.
Building labels also include glare criteria. For the most part, however, these are anti-glare protection criteria that are primarily quantitative and prescriptive in nature, giving recommendations for solar protection solutions. They do not necessarily define a method to quantitatively assess the absence or presence of the risk of glare and test the effectiveness of the design strategies put in place. These criteria can be found in the table below:
LEED V4 BC+D
Option 1: if ASE1000,250 > 10% in regularly occupied spaces, identify how the space can be designed to address glare.
BREEAM International New Construction 2016
Implementation of a glare control strategy, whether through building form or building design measures in compliance with specific criteria. The solution should avoid increasing lighting energy consumption. The use or location of protection does not conflict with the operation of lighting control systems.
HQE Sustainable Building V3 (2019)
The number of credits allocated depends on the type of solar protection. Absence of the risk of glare must be confirmed by a qualitative assessment or DGP calculation.
DGNB New Construction Version 2018
A solar protection system must be implemented. The number of credits then depends on its performance class as assessed in accordance with DIN 14501.18
WELL V2 pilot (Q3 2018)
Option 1: solar protection on all exterior envelope glazing that can be set automatically or controlled manually by occupants to limit glare. If the system is manually controlled by occupants, all shades or other protection devices must be raised or retracted manually or automatically at least twice per week.
Use of a solar protection system whose performance is based on DGP limits that must not be exceeded for more than 5% of usage time. Assessed using a simplified method, by DGP calculation or measurement.
To ensure the visual comfort of the occupants of a building, take the risk of glare into consideration. More research is required to quantify this phenomenon with greater accuracy and to optimize prediction methods and make them more accessible. We can, however, use existing metrics and tools, keeping their limitations in mind. This assessment stage is key in promoting good use of natural light in our buildings while avoiding the impact of its negative effects.
The experts at SageGlass can help you run simulations to identify the risk of glare in your project and suggest suitable design strategies. Please contact us if you are interested.
Eloïse Sok is Concept Creator in the SageGlass Europe & Middle-East Team. She holds a Double-Degree in the Engineering field from Ecole Centrale (France) and Tsinghua University (China). Her main interests include sustainable architecture, daylighting and occupant’s comfort. Her motto: “Passion is our best strength!”.
- Maximum luminances and luminance ratios and their impact on users’ discomfort glare perception and productivity in daylit offices, A. C. Linney, 2008
- A critical literature review of spatio-temporal simulation methods for daylight glare assessment, S. Wasilewski et al, 2019
- Adaptation luminance is the luminance to which the eye has adjusted in a given moment. This is assessed either based on background luminance, i.e. average luminance in the field of vision excluding sources of glare (for smaller sources), or on vertical illuminance at eye level (for large glare sources occupying a wide area of the field of vision).
- Review of Factors Influencing Discomfort Glare Perception from Daylight, C. Pierson et al, 2018
- Cross-validation and robustness of daylight glare metrics, J. Wienold et al., 2019
- UDI>3000 lux represents the percentage of time in the year when the illuminance on the work surface exceeds a limit of 3,000 lux.
- ASE is defined as the fraction of the space exceeding a certain level of direct illuminance for more than a specific number of hours in the year, with the blinds open.
- LEED v4.1 Building Design and Construction
- UK Education Funding Agency Daylight Design Guide, 2014
- Evaluating a New Suite of Luminance-Based Design Metrics for Predicting Human Visual Comfort in Offices with Daylight, V.D. Wymelenberg et al, 2015
- Daylighting metrics: is there a relation between useful daylight illuminance and daylight glare probability?
J. Mardlajevic et al, 2012
- Lighting measurement #83, spatial daylight autonomy (sDA) and annual sunlight exposure (ASE). IESNA-Daylight Metrics Committee. 2012.
- The Application of Luminance Mapping to Discomfort Glare: A Modified Glare Index for Green Buildings, M. Hirning, 2014
- DESIGN RECOMMENDATIONS FOR PERIMETER OFFICE SPACES BASED ON VISUAL PERFORMANCE CRITERIA, I. Konstantzos et al, 2016
- Temporal effects in glare response, M. Kent, 2016
- EN 17037: Daylight in buildings – Annex E “Glare”
- Evaluation of high dynamic range photography as a luminance data acquisition system, M. Inanici, 2006
- DIN 14501: Blinds and shutters – Thermal and visual comfort – Performance characteristics and classification. This standard defines the performance classes associated with thermal and visual comfort for solar protection systems. Note: this does not apply to dynamic glazing