What is glare?

Glare is a visual sensation that occurs when two competing light sources are present simultaneously in a person’s eye line, for instance, the headlights of oncoming vehicles can hinder your ability to see the road ahead when driving at night. A similar situation can occur in an office environment, when the visibility of a computer screen is hindered by glare because of poorly designed lighting or a badly planned lighting scheme. However, with a professional lighting design this situation that can be avoided as glare is a quantifiable phenomenon, i.e. it can be measured, and its values can be utilised to predict levels of glare in a lighting scheme before any installation is considered.

Glare can be experienced in two different ways: directly and indirectly. Direct glare is caused by an immediate light source such as looking at the sun, whereas indirect glare is caused by a reflection or contrast in lighting. By differentiating which type of glare is apparent, you may decide to optimise productivity by rearranging the desk layout to avoid glare on computer screens from the windows. Whether it is experienced directly or indirectly glare influences the body which can result in fatigue, eye strain and even migraines. These effects can be measured in two ways:

Disability glare: This is defined as a situation where vision can be impaired, preventing you from seeing an object or completing an activity, for instance looking at a computer with the sun shining on it makes it particularly difficult to see.

Discomfort glare: This is used to describe times where excessive luminance results in an automatic need to look away and a feeling of discomfort.

The main type of glare that is experienced in the office is discomfort glare, usually caused by an overbearing back light around a computer screen or a strong overhead light. By adjusting the level of glare that is emitted by these fittings to a more comfortable level for the eye, an optimum working environment can be achieved.

How is glare measured?

The photometric data that is provided with a luminaire will contain a table of UGR values that relate to different applications the product may be used in. These figures specify the amount of glare generated by the luminaire in relation to the size and surface reflectance values of a given room.

European Standard BSEN 12464: 2002 specifies the ideal UGR values for typical environments, e.g.:

UGR <16 Technical Drawing
UGR <19 Reading, writing, training, meetings, PC based work.

A UGR of <10 will produce minimal glare and will be virtually unnoticeable, where as anything higher than UGR 30 would severely impair visibility. In accordance with the Lighting of indoor work places standards, BSEN 12464: 2002, “glare must be limited to avoid errors, fatigue and accidents.” It states that a glare rating should be calculated by the UGR tabular method and must be below the rating listed for the application.

The UGR calculation

Prior to installation a lighting scheme designer would use sophisticated software such as Relux or Dialux to calculate the lighting requirements for every room, i.e. quantity of luminaries and mounting positions. Luminaire manufacturers provide electronic files which the software uses to perform the necessary calculations.

The software is used to create a 3D model of the room and from there to simulate the behaviour of any given luminaire within that room.