Do you feel blinded by the high beams of oncoming cars? Do you feel that the lights are too bright when you look at the whiteboard in class? This is caused by glare.
True lighting quality extends to visual comfort, and glare management is an important aspect of this. The Unified Glare Rating (UGR) is a global benchmark for quantifying uncomfortable glare. Let's understand the factors behind glare and high UGR values, and learn how to control them.
Understanding Glare and UGR
To better understand UGR, let's first know about what glare is. Glare is the visual sensation caused by excessive brightness or illogical distribution of light within the field of vision. It can be caused by high-brightness luminaires shining directly into the human eye or by light reflecting off flat surfaces (such as computer screens, window glass, etc.).
The Unified Glare Rating (UGR) is a metric that quantifies the discomfort glare that observers may experience in indoor lighting.
Discomfort glare can cause eye fatigue, headaches, and other physical problems because our sight needs to be constantly adjusted in these spaces. The UGR measures discomfort glare–the annoying factor–rather than disabling glare, which damages vision.
The UGR value ranges from 10 (almost no glare) to 28 (strong discomfort). Higher values indicate more visual discomfort. European standard EN 12464-1 recommends maximum UGR limits for various tasks–UGR in offices and classrooms should be ≤19.
Is UGR a Luminaire Property or an Environmental Characteristic?
A common misconception in the lighting industry is that UGR is a fixed property of a luminaire, like its wattage or color temperature.
UGR is essentially a measurement of an installation in an indoor environment (not an outdoor environment), not a measurement of a single luminaire. Its value is dynamic and changes depending on the interaction of various factors in the entire environment and the position of the observer. Therefore, strictly speaking, there is no isolated "UGR19-compliant luminaire".
The UGR calculation formula is as follows. This explicitly includes several variables that depend on the installation environment and the observer, reflecting its environmental characteristics:
Lb = Background Luminance
L = Luminaire Luminance
P = Guth Position Index（The likelihood of glare based on the luminaire's position relative to the observer's eye）
ω = Solid Angle（The following figure shows an example）
Based on the above formula, we can divide the influencing factors into luminaire-specific and environmental factors. See below for details.
Luminaire-Specific Factors Driving UGR
The inherent design of LED luminaires and their optical systems have a significant impact on UGR.
Lumen Output and Luminance
Luminaires with higher lumen packages tend to contribute to higher UGR values. To alleviate this situation, some strategies can be adopted. For example, reducing the lamp wattage.
Optical Design and Shielding
The effectiveness of glare control is significantly enhanced by well-designed shielding and optical components. These elements help to direct light precisely where it is needed and prevent direct light from reaching the observer's eye at high, uncomfortable angles.
Prismatic diffusers, for instance, are often preferred over opal glass for their superior ability to control light distribution and reduce direct glare, especially when viewing the luminaire directly.
Luminous Surface Area
Choosing luminaires with larger luminous surfaces helps to distribute the light more broadly and evenly. This reduces the concentration of brightness from any single point, thereby lowering the perceived glare and UGR.
For instance, a larger LED panel light will generally have a lower UGR than a smaller, more intense downlight with the same total lumen output.
Environmental Factors Amplifying UGR
In addition to the luminaire itself, other factors within the space also affect UGR.
Room Geometry and Dimensions
The physical characteristics of the space itself are fundamental determinants of UGR. Larger rooms typically result in higher UGR values because a greater number of luminaires are likely to be within the observer's view.
Conversely, a light fixture with a UGR of 17 in a large room might increase to a UGR of 19+ or higher if the same fixture is placed in a smaller room with a lower ceiling height.
Luminaire Spacing
Tighter luminaire spacings can help achieve desired light levels with less individual brightness per fixture, which can contribute to a lower overall UGR.
However, wider spacing can also reduce glare by decreasing the number of visible sources.
The optimal spacing depends on the overall design and desired uniformity.
Observer's Position and Viewing Angle.
Glare is most pronounced when a light source is directly within the observer's line of sight or at a sharp angle. The Guth position index (P) in the UGR formula mathematically accounts for this angular displacement.
Variations in actual observer height and viewing direction (e.g., crosswise or endwise to a luminaire) can also influence perceived glare.
The above necessitates the use of advanced lighting simulation software to model different observer positions and viewing directions within a space, ensuring comfort for all occupants, not just a theoretical average.
Surface Reflectances and Background Luminance
The reflective properties of interior surfaces define the background luminance.
The UGR formula directly incorporates background luminance. Higher reflectances on walls, ceilings, and floors increase the overall ambient brightness, reducing the contrast between the light sources and their surroundings. This lower contrast results in reduced perceived glare and a lower UGR value.
Strategies to Control and Optimize UGR Values
Achieving optimal UGR values requires a multi-faceted approach, integrating careful luminaire selection with intelligent lighting layout and thoughtful material choices for interior surfaces.
Strategic Luminaire Selection
Choose larger panels or fixtures with wider light-emitting surfaces rather than compact, high-intensity point sources. This approach helps distribute light more evenly, minimizing the perception of glare.
Prioritize fixtures equipped with advanced optical designs such as prismatic diffusers, louvers, or deep-recessed light sources that control light distribution.
Consider indirect lighting systems. By projecting some light onto the ceiling or walls, you raise the ambient luminance and lower the contrast of the direct luminaires.
Intelligent Lighting Layout and Placement
Designers should carefully consider the placement and aiming of luminaires to avoid directing light into the typical sight of observers. Angling luminaires away from common viewing angles, or placing them higher on the ceiling or in corners, can effectively mitigate direct glare.
Strategic spacing between luminaires can help achieve uniform illumination with less brightness per individual fixture.
Increasing the mounting height can reduce the perceived intensity of glare sources by increasing the distance between the light source and the observer's eye.
Material Choices for Interior Surfaces
Utilize high-reflectance matte finishes. This increases background luminance, which in turn reduces the contrast between the light sources and their surroundings, thereby lowering the UGR.
Minimize the use of highly reflective, glossy materials on desks, floors, or walls, as these can create distracting reflections and amplify glare.
Managing UGR is essential for creating visually comfortable indoor environments. By understanding the factors that influence UGR and employing targeted strategies, lighting designers can ensure that spaces are well-lit without causing discomfort.
