Don't be dazzled by UGRs - Understanding and controlling glare from light fittings

What is glare?

Glare is the unpleasant visual sensation caused by an excessive and uncontrolled brightness within a person's field of view. It can prevent you from distinguishing objects and detail.

Glare is a highly subjective sensation and sensitivity to it can differ greatly from person to person. Older people are generally more sensitive to glare, due to the effects of aging on their eyes.

Glare ranges from simply being distracting or uncomfortable - discomfort glare - to being seriously disabling or dangerous - disability glare.

Discomfort Glare

Discomfort glare is defined as perceived discomfort, annoyance or, in extreme cases, pain caused when the eye is unable to adapt to the excessive brightness of a light source.

The degree of discomfort depends on several factors, such as:

• The dimensions and luminance of the source of the glare
• The position of the source in relation to a person's line of sight
• The contrast between the source and the area surrounding it

Disability Glare

Disability glare is defined as directly impairing vision. Put very simply, it is caused by bright light sources being scattered within the eyes of the observer.

Outdoors, a classic cause of disability glare (and road accidents) is the bright headlamps of a vehicle travelling towards you over the brow of a hill. Indoors, it can be caused by sunshine through a window, or the direct brightness of an artificial light source.

Direct and indirect glare

Both discomfort and disability glare can be caused by two directional types of glare - direct glare and indirect glare (reflected glare).

• Direct glare is caused by high levels of brightness from a light source which is directly within the field of vision.
• Indirect glare is caused when the direct light from the source is reflected off a bright surface before entering the eye.

In lighting terms, glare is determined and classified by the Unified Glare Rating, or UGR.

What is the UGR?

The UGR is the method of calculating the glare from light fittings (as well as light through windows and other bright light sources).

It helps to determine how likely the artificial light source is to cause discomfort to the people around it, and to what degree.

Glare in the workplace

Glare is a common problem in interior workplaces. It can be caused by inappropriate light fittings emitting excessive brightness directly into a person's line of sight, or as a result of being bounced off reflective surfaces such as computer screens or whiteboards.

This can cause workers to experience headaches and eye trouble and result in significant disruption which, ultimately, can have a negative impact on a business due to employee absences.

Calculating the UGR

The UGR calculation for indoor lighting is made in accordance with the CIE document CIE 117.

It takes into account a number of factors that may contribute to glare, such as the angle of the light fitting, the likelihood of glare and the luminance value according to the UGR method.

The UGR limits range from 5 to 40 and the lower the number, the less glare will be an issue. For example, a low UGR of 10 means the glare will go unnoticed, while a UGR of 30 will cause much distraction.

The values of glare and their meaning are defined as:

• ≤ 10 Imperceptible
• ≤ 13 Just perceptible
• ≤ 16 Perceptible
• ≤ 19 Just acceptable
• ≤ 22 Unacceptable
• ≤ 25 Just uncomfortable
• ≤ 28 Uncomfortable

UGRs in the workplace

In commercial offices, for a lighting installation to be classified as ‘low glare’ it must achieve a UGR of equal to or below (≤) a value of 19 when measured at the eye height of the workers - which is normally assumed as being 1.2 metres above finished floor level (FFL).

Any calculated value above 19 may cause discomfort, so it is important that any interior lighting is designed to achieve a UGR of below 19.

There are multiple UGR limits relating to different work applications, which should not be exceeded. Examples include:

• ≤ 13 The glare is usually unnoticed
• ≤ 16 Technical drawing
• ≤ 19 Reading, writing, training, meetings, computer-based work
• ≤ 22 Craft and light industries
• ≤ 25 Heavy industry
• ≤ 28 Railway platforms, foyers

You can find a comprehensive summary of maximum UGR limits for interior applications in the British Standard document BS EN 12464-1:2011.

Expert support and advice

UGR ratings are many and varied and installation might not be as simple as it first appeared.

For example, an original lighting design - and its UGR - can be altered if changes are subsequently made to the office environment, for example changing the dimensions of the office space or workstation layout, or shifting the location or orientation of the light fittings.

So, it is best practice to select fittings whose light distribution achieves the lowest possible UGR.

A lighting design specialist can help you deliver an installation which achieves the required UGR value, based on the original specification. The 4lite team will be happy to help. You can contact us here sales@4liteuk.com

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