26/07/2013
Lighting technology: The UGR value
... is a non-dimensional index expressing the degree of direct glare from luminaires in interior locations. The UGR values are defined for the range 10 to 30, the lower this figure is, the less direct glare perceived by the observer.
Abbreviation: UGR (Unified Glare Rating).
The UGR value at a specific observer position can only be calculated mathematically, and it can neither be measured on the spot nor be defined in the form of a visual assessment.
Compliance with the UGR limit values (UGRL) is required in a number of different standards (e.g. EN 12464-1), depending on the area of activity, these being intended to define the maximum permissible glare from the lighting system affecting the observer.
DIAL perspective
The UGR value is not in any way purely a characteristic of a luminaire. The quality of glare control of a luminaire affects this value of course, however other aspects are also included in the evaluation. The observer position in the room and the observer’s typical line of vision also play a part in this because the observer cannot experience any glare if the luminaire is not in his field of vision.
A further aspect is the degree of reflectance of the room surfaces. Due to the strong contrast, a luminaire against a dark background creates a stronger glare than a luminaire positioned against a light background.
By definition it is not possible to specify a UGR value for lighting systems using luminaires that have an indirect ratio > 65% and where narrow-beam spotlights or luminaires with asymmetrical distribution are used.
According to EN 12464-1 (2011-08) light planners must provide evidence for the rating of direct glare of a lighting system by using the tabular CIE glare rating method.
This method is, however, anything but precise and quite cumbersome. First of all, the degrees of reflectance of floor, wall and ceiling underpinning this method are quite limited. For instance, according to the tabular method, the floor has a maximum degree of reflectance of 20 %. For walls it is 30% or 50% and ceiling 50% or 70%. “Normal“ white walls or ceilings as are frequently found in architecture have a degree of reflectance of approx. 75 – 90%, however.
Furthermore, the observer’s line of vision may be either transverse or longitudinal to the lamp axis. A diagonal line of vision is not taken into account here.
L-shaped or polygonal rooms are also not taken into account in the tabular method. It uses exclusively rectangular rooms as a basis.
A further disadvantage is that this method must be used for each type of luminaire individually if there are various luminaires in a room.
In practice you do not notice any visual difference between UGR values that are close together. For this reason, EN 12464-1 (2011-08) also refers to UGR steps which include a perceptible change. These steps are: 10, 13, 16, 19, 22, 25 and 28.
If a manufacturer states a precise UGR value for his product in the data sheet, this is the value that can be read from the table for degree of reflectance of 70% (ceiling), 50% (walls), 20% (floor) and the geometry of the room 4H, 8H. It should be noted that this value only applies to this specific application and does not in any way represent any “quality characteristic” of a product.
For the above reasons we recommend calculation of the UGR value with the aid of software ( e.g. DIALux). This takes into account the actual degree of reflectance and geometry of the room. In addition to this, all the luminaires in the room are included in the calculation.
