Glare is one of the most consistently documented sources of visual discomfort in workplaces. It causes occupants to squint, reposition themselves, tilt screens, or raise physical barriers between themselves and light sources — all responses to a lighting condition that could have been addressed at the specification stage. The Unified Glare Rating (UGR) is the standardised method used in international lighting design to quantify this discomfort and to set thresholds below which glare is considered acceptable for a given task type.

For office environments — where workers engage in sustained visual tasks at screens and on paper across long working periods — the accepted threshold is UGR below 19. This figure appears in the principal international and national standards governing indoor workplace lighting, and it functions as a pass/fail criterion: lighting installations that cannot demonstrate UGR <19 in the relevant viewing conditions do not meet the standard for office use, regardless of how well they perform on other metrics such as illuminance or colour rendering.

What the UGR formula actually calculates

The UGR system was developed by the International Commission on Illumination (CIE) and published in CIE 117:1995. It provides a single numerical value summarising the glare contribution of all luminaires in a room as perceived by an observer at a defined position and viewing direction. The formula draws on four variables for each luminaire in the room: the luminance of the luminaire in the direction of the observer, the solid angle subtended by the luminaire at the observer's eye, the position index of the luminaire relative to the line of sight, and the background luminance against which the luminaire appears.

The relationship between these variables captures the perceptual reality of glare: a bright source that is large, close to the line of sight, and seen against a dark background causes more discomfort than the same source seen against a bright background, from a greater distance, or at a wider angle from the line of sight. UGR combines all of these contributions into a single logarithmic scale value. The scale runs from approximately 10 (imperceptible glare) to 30 (intolerable glare), with values increasing in steps of 3 that each correspond to a just-noticeable perceptual difference.

The UGR scale and what each threshold means in practice

The UGR scale is logarithmic, which means that a change of 3 points corresponds to a perceptible step in glare sensation. A change of 6 points represents a clearly noticeable difference that most observers would describe as a significant improvement or deterioration. This structure is important for interpreting specification requirements: the difference between UGR 19 and UGR 22 is not marginal — it is a clearly perceivable step that occupants of a space will notice over the course of a working day, even if they do not attribute their discomfort specifically to glare.

Where the UGR <19 requirement appears in international standards

The UGR <19 threshold for office lighting is not a guideline or a preference — it is a requirement embedded in the principal international and regional standards governing workplace lighting. Understanding which standard applies in a given project context, and how each frames the UGR requirement, is essential for compliance.

EN 12464-1, the European standard for lighting of indoor work places, specifies UGR <19 for all general office tasks including computer work, writing, reading, and data entry. This standard has been adopted across EU member states and is referenced in building regulations and green building certification schemes in Europe and many markets beyond it. ISO 8995-1 — the international equivalent — carries identical requirements for the same task categories, extending the UGR <19 threshold to workplace projects in jurisdictions that follow ISO frameworks.

In the United States, the Illuminating Engineering Society's RP-1 standard for office lighting addresses glare through visual comfort probability (VCP) and luminance ratio criteria rather than through a direct UGR figure, but the underlying discomfort thresholds translate to comparable conditions. LEED and WELL certification schemes used in the US and globally reference glare control requirements that align with UGR <19 for occupied workspaces.

Why UGR is stated as a rated value, not a measured installation value

A critical and frequently misunderstood aspect of UGR is that the figure published on a luminaire data sheet is not a measurement of the luminaire itself, but a calculated prediction of the glare condition that would result from a standard installation of that luminaire in a reference room of defined dimensions, reflectances, and layout. The CIE tabular method defines this reference room — typically 8m × 8m × 2.8m with standard reflectances of 0.7 ceiling, 0.5 walls, and 0.2 floor — and the UGR value on the data sheet tells you what the formula would yield in that specific scenario.

This means that a luminaire rated UGR <19 in the reference room will not automatically produce UGR <19 in every real installation. Room dimensions, ceiling height, reflectance values, fixture spacing, fixture count, and the observer's position relative to the fixtures all affect the actual UGR experienced. A narrower room, lower ceiling, or darker walls will generally produce a higher UGR than the reference room value for the same fixture. Conversely, a room with higher reflectance surfaces, greater ceiling height, or wider fixture spacing may produce a lower UGR than the rated value suggests.

For accurate UGR prediction in a specific installation, the full UGR formula must be applied using the actual room parameters, or lighting calculation software must be used to compute the predicted UGR at the relevant observer positions and viewing directions. The data sheet value provides a starting point for product selection, not a guarantee of compliance in every room configuration.

Fixture design characteristics that determine UGR performance

The UGR contribution of a luminaire in an installation is governed primarily by the luminance of its light-emitting surface in the directions corresponding to typical observer viewing angles — generally between 45° and 85° from the vertical (nadir). A luminaire that restricts its luminance in these off-axis directions will produce lower UGR contributions regardless of how much total light it emits. This is the fundamental design variable that differentiates low-glare luminaires from standard ones.

The relationship between UGR and screen-based work

The UGR <19 threshold for offices has particular relevance in environments where the primary work task involves viewing screens. Screens introduce two glare-related problems that are distinct from the discomfort glare addressed by UGR but are worsened by the same lighting conditions that produce high UGR values.

The first is veiling reflections — the reflection of bright luminaires in the screen surface itself. When a luminaire is bright enough and positioned such that its reflection falls within the observer's normal viewing angle to the screen, the reflected image of the luminaire overlays the screen content, reducing contrast and legibility. This effect occurs independently of the observer's direct view of the fixture and is not captured by the UGR formula, but it is more severe when luminaire luminance is high — precisely the same characteristic that drives high UGR values.

The second is disability glare from direct or reflected light sources near the screen, which temporarily reduces the eye's ability to distinguish contrast. Again, the UGR formula quantifies discomfort glare rather than disability glare, but the design measures that control one tend to control the other. A luminaire with well-managed off-axis luminance is less likely to produce either form of screen-related glare than one with poor glare control, regardless of whether the UGR formula captures both effects precisely.

For screen-intensive environments — call centres, trading floors, control rooms, or any office where screens dominate the task surface — specifiers often apply a more stringent UGR limit than the standard UGR <19, targeting UGR <16 to provide an additional margin against both discomfort glare and screen interference.

How to verify UGR compliance for a specific installation

For any office lighting project where UGR <19 compliance must be demonstrated, the appropriate process involves more than selecting a product whose data sheet carries a UGR <19 designation. The data sheet value is calculated for a reference room; compliance in the actual project room requires calculation using the actual room parameters.

Lighting calculation software — DIALux, Relux, and similar tools — can compute UGR at specified observer positions and viewing directions using luminaire photometric data (IES or LDT files) and the actual room geometry and surface reflectances. These calculations should be run for the most critical observer positions: typically the positions closest to and most directly below the luminaires, and those with the longest sightline across the luminaire array. If these critical positions meet UGR <19, the installation as a whole is likely to comply; if they do not, fixture type, layout, spacing, or room surface treatment must be adjusted until they do.

UGR in the context of other visual comfort parameters

UGR addresses discomfort glare from luminaires in the visual field. It is one of several parameters that together define the visual comfort quality of a lighting installation, and it does not operate in isolation. Specifying UGR <19 while neglecting the other parameters will not produce a comfortable visual environment; conversely, a well-designed installation that meets all related criteria and achieves UGR <19 provides a robust foundation for sustained visual work.

Illuminance uniformity — the ratio of minimum to average illuminance across the task area — affects whether the lighting feels even or patchy. Very uneven illuminance causes the eye to repeatedly adapt between bright and dark zones, which is fatiguing over extended periods. EN 12464-1 specifies a minimum uniformity ratio of 0.6 for the task area and 0.1 for the immediate surround area in office environments.

Luminance ratios between the task, the immediate surround, and the wider field of view determine whether the visual environment is balanced. Very high luminance contrasts between the work surface and the surrounding walls or ceiling — even when UGR is within limits — can cause fatigue through repeated adaptation. Standard guidance recommends that the luminance ratio between the task and surrounding areas not exceed 3:1, and between the task and the wider background not exceed 10:1.

Flicker — rapid, periodic variation in light output — does not appear in the UGR formula at all, but has documented effects on visual comfort and headache incidence in office environments. It is characterised separately through flicker index, percent flicker, or stroboscopic visibility measure, and is determined by the driver and control system rather than the optical design of the fixture. A low-UGR fixture driven by a poorly filtered driver may still produce a visually uncomfortable environment through flicker alone.