Unified Glare Rating
European Standard EN 12464 regulates the lighting of indoor workplaces. In doing so,
the unified glare rating method is used to evaluate (psychological) glare. UGR is based on a glare formula. This formula takes account of all the luminaires in the system that contribute to the sensationof glare. UGR levels for luminaires are determined using the tabular method according to CIE 117.
The UGR limits are specified in the EN 12464 standard and the UGR limits (UGRL) that must not be exceeded:
≤ 16 Technical drawing
≤ 19 Reading, writing, training, meetings, computer-based work
≤ 22 Craft and light industries
≤ 25 Heavy industry
≤ 28 Railway platforms, foyers
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The luminance limiting curve method assesses the mean luminance between 45°
and 85°, and formerly even the glare effect of a luminaire.
The new European standard sets UGR = 19 as the maximum permissible value for
offices, which is equivalent to the luminance limiting curve for 500 lx in Quality class 1.
UGR values are available for each individual product in the corresponding photometric data sheet.
the maximum UGR ('Unified Glare Rating'). The UGR is an approximate model that expresses the chance of direct glare by luminaires. The higher this figure, the greater the chance of glare. For each luminaire, a standardised table can be calculated with UGR values. The parameters are the size of the room, the reflection factors and the observer's orientation in the room. The appendices of the standard specify the UGR value that may not be exceeded for each application.
Typical limits are applied in the standard.
Glare is a visual impression created by the presence of illuminated area in the field of view. Preventing glare is important from the work safety point of view: it can cause tiredness, errors and injuries. It makes a viewer feel uncomfortable, but does not necessarily cause eye strain. It makes reading a computer screen or paper documents more taxing and difficult. The prevention can be achieved by several means.
First of them is correct placement of luminaires. The light from them should be directed to workspace in such a way that light reflected from visible objects would not be directed into the eyes of a worker when he is in the usual seating or working position. When glare is present for a long time, eye strain occurs and can even lead to health problems. The rules of workplace safety aim to prevent such outcomes and great care is taken to diminish the potential glare to a minimum.
Glare is a visual impression created by the presence of illuminated area in the field of view. Preventing glare is important from the work safety point of view: it can cause tiredness, errors and injuries. It makes a viewer feel uncomfortable, but does not necessarily cause eye strain. It makes reading a computer screen or paper documents more taxing and difficult.
Light sources with too high luminance can cause glare and complicate visibility of objects. To prevent this, the light source should be covered or partially obstructed and windows should be covered by protective shutters. The covering of the light source should provide that it is not directly visible in a 65-degree angle.
The human eye is highly adaptive and regenerative, provided the glare is only of a short duration and low intensity. When glare is present for a long time, eye strain occurs and can even lead to health problems. The rules of workplace safety aim to prevent such outcomes and great care is taken to diminish the potential glare to a minimum.
The prevention can be achieved by several means. First of them is correct placement of luminaires. The light from them should be directed to workspace in such a way that light reflected from visible objects would not be directed into the eyes of a worker when he is in the usual seating or working position.
The second recommendation is to use large luminaires with low luminance. Surface finishes that diffuse and scatter light should be used instead of glossy ones that create strong reflections. Last but not least, luminaires with appropriate distribution of luminous density should be used. A butterfly-shaped luminosity curve is desired with maximum luminosity in the angled parts of the curve.
Long-term eye discomfort due to insufficient or low-quality illumination may lead to eye strain. Its symptoms include irritated or itching eyes, headaches, diplopia (commonly referred to as double vision), spasms of facial muscles, conjunctivitis (otherwise known as pink eye), hot flushes, watering, increased nervousness and consecutively lower work performance.
If employees feel several of such symptoms the work safety rules demand a medical assessment of their condition. Repeated eye strain is a cause to re-evaluate the lighting system in the workplace. The overall tiredness is just a result of long-term eye strain, leading to loss of focus and attention. This can result in incorrect work practices and even injury. To prevent overall tiredness and consequent injuries, the standards in EN 12 464-1:2011 set up a framework for lighting depending on the demands of the job being carried out.
The probability of psychological glare can be estimated by the so-called UGR (Unified Glare Rating) method defined by International Commission on Illuminations (CIE, from the French term Commission Internationale de l'Éclairage).
where L stands for luminance of lighting parts of every luminaire in the direction of the eye (in candelas per square meter). Ω is a cut-off angle of a luminaire relative to the eye of an observer (in sr). p is a Guth factor of of spatial position of every single luminaire relative to the field of view. Finally, Lδ expresses background illuminance (in candelas per square meter).
The maximum allowed UGR according to the EN 12 464-1 standard is 19 for most activities, with technical drawing demanding even more strict 16. Higher ratings are allowed for less critical environments such as reception (22) or archives (25). This method by its very definition takes into account all the luminaires in a given space.
In special circumstances, such as when using screen with high-gloss finish additional care has to be taken to prevent direct or reflected glare. Work areas using such devices might need an individual approach: separate window shutters or even a specific setting of a lighting system. LQS awards a maximum of 5 points in the rating to the solutions which provide UGR below 16.