What is glare?

Glare: Excessive Brightness Causing Visual Discomfort

What is Glare?

Glare is a photometric phenomenon that arises when the visual field contains regions of excessive brightness or extreme contrast, resulting in discomfort or a reduction in the ability to distinguish details. According to the International Commission on Illumination (CIE), glare is a “condition of vision in which there is discomfort or a reduction in the ability to see details or objects, caused by an unsuitable distribution or range of luminance, or by extreme contrasts.”

In lighting design, glare is a central parameter: it directly affects comfort, task accuracy, and safety. Glare can result from direct light sources, reflective surfaces, or even daylight entering through windows. In environments such as offices, factories, and air traffic control towers, glare can hinder productivity, health, and safety. Assessment of glare is both subjective (user perception) and objective (using photometric indices like UGR), enabling designers to optimize lighting for comfort and compliance.

Types of Glare

Glare is classified by its mechanism and effect on the observer:

Distinguishing between these types guides the choice of measurement metrics and mitigation strategies. For example, in offices, discomfort glare is the main concern, while in aviation or control rooms, disability glare is prioritized due to its impact on safety-critical tasks.

Photometric Foundations

Luminance

Luminance is the photometric quantity representing the brightness of a surface in a given direction (measured in candelas per square meter, cd/m²). High luminance, especially from small or direct sources, is a primary cause of discomfort glare.

Illuminance

Illuminance measures the total luminous flux incident on a surface (in lux, lx), ensuring adequate visibility. While important, illuminance is less directly linked to glare than luminance.

Unified Glare Rating (UGR)

UGR is the main international index for quantifying discomfort glare indoors. It accounts for source luminance, solid angle, background luminance, and the source’s position relative to the observer.

[ UGR = 8 \log_{10} \left( \sum \frac{L^2 \omega}{b p} \right) ]

• L: Luminance of each source (cd/m²)
• ω: Solid angle subtended by the source
• b: Background luminance (cd/m²)
• p: Position index (weighting factor)

UGR values typically range from 10 (imperceptible glare) to 28 (unbearable). Standards such as EN 12464-1 specify recommended UGR thresholds for different environments.

Causes and Mechanisms of Glare

Glare occurs due to an unsuitable distribution of luminance or extreme contrast within the visual environment. Key factors include:

• High source luminance: Directly visible or reflected into the line of sight.
• Low background luminance: Increases perceived contrast.
• Multiple small bright sources: Can collectively create discomfort.
• Spectral content: Blue-rich (high CCT) light is more glaring due to increased intraocular scatter.
• Reflective surfaces: Glossy finishes amplify indirect glare.
• Environmental context: Sun at low angles, wet surfaces, or poorly shielded fixtures.

Effects on Visual Performance and Well-being

Excessive glare has significant consequences:

• Reduced contrast sensitivity: Obscures task details, especially critical in precision work.
• Veiling reflections: Obscure text, graphics, and indicators.
• Eye strain and fatigue: Persistent glare leads to headaches, irritability, and lower productivity.
• Safety risks: In aviation and industry, glare can mask warnings or cause temporary visual impairment.
• Age-related sensitivity: Older individuals are more susceptible due to changes in the lens and cornea.

Visual comfort, defined by the absence of glare, is essential for health, safety, and productivity.

Measurement and Evaluation

Objective assessment is vital for compliant, comfortable lighting:

Unified Glare Rating (UGR)

• Calculated from source and background luminance, solid angle, and position.
• Used for electric lighting in interiors.

Daylight Glare Probability (DGP)

• Estimates the probability of discomfort from daylight and mixed sources.
• Integrates photometric data and user surveys.

Subjective Methods

• Adjustment method: Observer adjusts source luminance to just-perceptible discomfort.
• Category rating: Observer rates glare as “imperceptible,” “acceptable,” etc.
• Comparison methods: Observer compares two scenes for glare.

Measurement tools include luminance meters, imaging photometers, and simulation software to predict glare indices in complex environments.

Standards and Guidelines

Multiple standards regulate glare:

Key organizations and documents:

• CIE 117:1995, CIE 112:1994: Glare definitions and measurement.
• EN 12464-1: UGR limits for workspaces.
• IES (Illuminating Engineering Society): Glare recommendations.
• ICAO Annex 14: Aviation lighting and glare control.

Compliance is mandatory in many jurisdictions for new construction and major renovations.

Factors Influencing Glare Perception

Glare perception depends on:

• Source characteristics: Luminance, size, spectral content (CCT, SPD).
• Environment: Background luminance, surface reflectance, task type.
• User factors: Age, adaptation level, visual acuity, personal tolerance.
• Geometry: Angle of incidence, glare is worst within 30° of the line of sight.

Older adults and those with eye conditions are especially sensitive, making individualized assessment important.

Practical Use Cases

• Offices: Overhead LEDs or unshielded fixtures create discomfort glare, especially for computer users.
• Industry: High-bay lighting exposes workers to direct glare, raising accident risk.
• Retail: Spotlights reflecting off glossy displays reduce product visibility.
• Education: Large windows can cause both direct and reflected glare on boards and screens.
• Aviation: Cockpit glare from sunlight or instrument panels can impair safety-critical tasks.

Mitigation Strategies

• Choose luminaires with low UGR ratings.
• Use indirect lighting or diffuse sources.
• Install shades, blinds, or smart glass for daylight control.
• Select matte, non-reflective surface finishes.
• Arrange workstations to avoid direct or reflected glare.
• Regularly assess and adjust lighting as tasks or layouts change.

Glare Metrics in Practice

For most offices, a UGR of 19 or lower is recommended. In daylight-rich spaces, DGP is increasingly used. In aviation and transport, other metrics (e.g., threshold increment, contrast reduction) are relevant.

Conclusion

Glare is a fundamental consideration in lighting design, with direct implications for comfort, health, productivity, and safety. Through careful selection of luminaires, measurement and simulation, adherence to standards, and ongoing assessment, glare can be effectively managed in any environment.

For expert guidance on glare assessment and lighting design, contact our team or request a lighting audit .

Sources

• International Commission on Illumination (CIE), CIE 117:1995, “Discomfort Glare in Interior Lighting”
• European Standard EN 12464-1: “Light and lighting, Lighting of work places, Part 1: Indoor work places”
• Illuminating Engineering Society (IES) Handbook
• Boyce, P.R. (2014). Human Factors in Lighting, 3rd Edition.
• Veitch, J., & Newsham, G. (2006). “Determining acceptable levels of discomfort glare: A review and practical application of subjective evaluation methods.”
• Lighting Research Center, Rensselaer Polytechnic Institute