Glare assessment is a key aspect of lighting design, as excessive brightness or high-contrast sources can cause discomfort or even pose safety hazards.
Over the decades, lighting researchers and standards bodies have developed a series of quantitative glare metrics to predict visual comfort under different conditions. Each metric has specific definitions, strengths, weaknesses, and use cases.
With a wide range of glare metrics such as UGR, VCP and DGP, how do you choose? This guide comprehensively compares 6 main glare metrics, and the end section provides a selection reference for you.
- Visual Comfort Probability (VCP): The Early Standard
- Discomfort Glare Index (DGI): A Simpler Alternative to VCP
- CIE Glare Index (CGI): A Contrast-Centric Approach
- Glare Rating (GR): Illuminating Outdoor Spaces
- Unified Glare Rating (UGR): The Indoor Workhorse
- Discomfort Glare Probability (DGP): A Probabilistic View of Daylight
- How to Choose the Right Glare Metric for Lighting Applications?
Visual Comfort Probability (VCP): The Early Standard
The VCP is a statistic-based metric to quantify the probability that a group of people will find a lighting installation free of discomfort glare. It represents the percentage of observers expected to be comfortable under specified conditions.
The metric was widely used in mid-to-late 20th-century lighting design. Higher VCP means comfortable, while lower VCP means high glare.
VCP formula is intricate. It requires specialized lookup tables or software, limiting its practicality. Also, it doesn't handle complex glare patterns well.
In terms of usage, VCP was historically used to check comfort in open offices or classrooms. It has largely been superseded by newer metrics (especially UGR) in contemporary standards. Nevertheless, it can still serve as a conservative check of human comfort in similar indoor settings.
Discomfort Glare Index (DGI): A Simpler Alternative to VCP
The DGI is an early quantitative metric for glare in electric lighting installations. It aims to estimate the amount of visual discomfort due to glare from luminous sources, without directly predicting observer acceptability.
It was a simpler alternative to the VCP metric. It is calculated from each visible luminaire’s luminance and solid angle, compared to a small background area’s luminance. Higher DGI indicates more glare. However, DGI assumes all sources are direct-point or symmetric, which limits use with complex luminaires or daylight.
DGI can be useful for comparing two simple lighting setups mathematically, but practitioners seldom rely on DGI today except for academic exercises or legacy comparisons.
CIE Glare Index (CGI): A Contrast-Centric Approach
The CGI is a metric defined by the CIE (International Commission on Illumination) as a variant of glare assessment. Often, “CGI” stands for “CIE General Glare Index” in older CIE documents.
It, similar to CGI, quantifies discomfort glare from electric light sources. But the numerical value itself has no absolute meaning without context; it’s used comparatively.
Being a CIE metric, CGI enjoyed some official standing. It was defined in broad CIE terminology but never became dominant in practice. The formula is essentially the same idea as DGI, so it shares the same limitations.
Glare Rating (GR): Illuminating Outdoor Spaces
The GR is a glare evaluation method developed by the CIE in 1994, for applications in outdoor sports lighting and general area lighting. A lower GR value signifies better glare restriction.
The primary benefit of GR is to ensure no excessive glare in outdoor environments. But for certain high-luminance and small-aperture LED products used outdoors, designers should consider UGR extensions or other specialized models.
Like other metrics above, GR is largely of historical interest. Practitioners now use UGR or DGP instead of GR.
Unified Glare Rating (UGR): The Indoor Workhorse
The UGR is designed to give a single glare value for a luminaire arrangement viewed from a given position. It unifies earlier glare metrics and is widely used in architectural lighting design.
Higher UGR values indicate more visual discomfort. In practice, standards like EN 12464-1 impose maximum UGR values for different indoor applications.
UGR's primary strength lies in its widespread acceptance and standardization internationally. Its calculations can be performed for specific rooms, typical layouts, or even individual luminaires, offering flexibility in design assessment.
However, UGR is not used in outdoor products. Its accuracy in daylight environments (windows, sunlight) is also limited. Besides, UGR’s logarithmic scale and formula can be non-intuitive, and small errors in input can change the result significantly
Discomfort Glare Probability (DGP): A Probabilistic View of Daylight
The DGP is a metric specifically intended for daylit spaces. It estimates the probability that a person will find the daylight environment uncomfortable due to glare. It is analogous to a discomfort probability (0%–100%), with higher values meaning more discomfort.
It is grounded in experimental data for daylight situations with actual human subjects and considers both brightness and contrast of all glare sources (sun, sky, reflections). Therefore, it is widely regarded as the most robust and advanced daylight glare metric.
However, DGP requires full scene luminance data, not just discrete sources, and is therefore computationally intensive and can involve “thousands of radiation simulations”.This means it is best suited for detailed design stages or academic research rather than routine, rapid assessments.
DGP is used in the analysis and research of natural light space, particularly for indoor design with significant window area. For example, it guides where to place shading devices or what facade treatments to use to keep glare acceptable.
How to Choose the Right Glare Metric for Lighting Applications?
Selecting an appropriate glare metric depends on the lighting type, environment, and regulatory requirements:
Electric Light vs. Daylight
For primarily electrically-lit interiors, especially with fixed luminaires, UGR is generally preferred due to its wide acceptance and inclusion in standards.
For daylit spaces where windows, skylights, or the sun are significant glare sources, Daylight DGP should be used, as it specifically models human response to natural glare.
Interior Space vs. Exterior Space
Except for GR, all the listed metrics focus on interior or controlled environments. None is ideal for outdoor glare, where specialized rating methods apply.
If evaluating glare in common indoor spaces, use UGR for electric lighting. In residential or commercial buildings with large windows, consider both UGR (for ceiling fixtures) and DGP (for window/daylight glare).
Regulatory Compliance
Many building or lighting standards specify particular metrics. For example, European and international lighting standards (CIE, ISO, EN) typically mandate UGR for artificial lighting installations .
Green-building programs or comfort guidelines may reference DGP for natural light glare.
Practical Tools and Data
UGR is the only metric that many lighting software packages compute automatically for luminaire layouts. DGP requires advanced daylight simulation tools like HDR fisheye images.
VCP, DGI, CGI, and GR are seldom supported by modern lighting software, so using them would require manual calculation or custom scripts. Thus, for ease of implementation, UGR and DGP are preferred in current practice.
