What is UGR?

UGR stands for Unified Glare Rating. It is an objective measure of glare that is used by lighting designers to help control the risk that occupants of a building will experience glare from the artificial lighting.

UGR values range from 40 (extremely high glare) to 5 (very low glare). In most situations, the less glare the better, so a low UGR is better than a high UGR.

International standards such as EN12464 recommend maximum UGRs for different situations. UGR<19 is recommended for many office and classroom settings.

UGR is measured for an installation, not for a light fitting. However, the design of a light fitting can have a significant impact on the UGR of the installation where it is used.

What is UGR?

UGR is an expression of the relative intensity of the light from a light fitting compared with the intensity of the light from the surrounding area, as perceived by the viewer.

A UGR can only be calculated for an indoor lighting installation. It cannot be calculated for an exterior installation (such as street lighting) and it cannot be calculated for a light fitting on its own.  

Here is the formula for calculating UGR. It is not as complicated as it looks, so follow the notes and you will have a much clearer understanding of this important measure of the quality of a lighting installation.

How is UGR calculated?

UGR is calculated in widely used lighting design programmes such as Relux, Dialux and AGi32.

The data required to be input include the photometric files of the fittings to be used in the lighting scheme, the room geometry, the surface reflectances and the number and spacing of the fittings. From these data the lighting design software will calculate the UGR.

How can I reduce the UGR of an installation?

There are several steps you could take during the design of an installation to reduce the UGR:

• Consider the position of the room occupants in relation to the position of the light fittings. For example, if the layout of a classroom is known, position the lights so they are not in the direct line of the students’ sight as they look towards the teacher.

• Consider the choice of light fittings.

  • If they are suspended, can they be suspended at a lower height?

  • Can you introduce an uplight component? Projecting light onto the ceiling will decrease the contrast between the fittings and the background and this will reduce the UGR.

  • For ceiling mounted fittings, choose ones with a narrower beam angle. This will reduce the luminous intensity as perceived by the viewer. Take care – this could decrease the uniformity, so consider lowering the wattage and adding more fittings to counteract this.

  • Consider using fittings where the light source or light emitting surface (such as the diffuser) is recessed behind a bezel.

   

In summary, to reduce the UGR you need to:

• Increase the background luminance

• Decrease the luminance of the luminaire as seen by the viewer (narrow the beam angle or decrease the wattage)

• Angle the luminaires away from the viewer so they are not shining in their eyes

• Avoid positioning the fittings in the viewer’s direct line of site

What is a UGR<19 light fitting?

Strictly speaking, there is no such thing. UGR relates to an installation not a light fitting. However, many manufacturers sell light fittings that are described as “UGR<19” – so what does this mean?

It means that the product has a light distribution that will assist the lighting designer to deliver a UGR of <19 if the fitting is used at typical heights and spacings in the environment for which it is intended. Therefore, there is no guarantee that using a “UGR<19” light fitting in an office or a classroom (for example) will result in a UGR of <19, but your chances will be improved by selecting fittings with the appropriate lighting distribution.

Generally speaking, fittings for which the “UGR<19” claim is made will have a strong downward component, little lateral output (so the beam angle is relatively narrow) and some might have an uplight component.

What UGR levels are recommended?

BS EN12464 Light and lighting – lighting of work places, recommends maximum UGR levels for different applications. In all, the standard lists more than 280 different types of work areas in almost every type of working environment from classrooms, offices and hospital wards to indoor car parks and industrial premises.A summary of BS EN12464’s recommendations are below.

Are specific UGR values mandatory? Are they a legal requirement?

No. It is not a legal requirement that the UGR levels in BS EN12464 are implemented. They are recommendations only, but following them is generally regarded to be good practice.

With the best of intentions, a lighting designer could select fittings and design a lighting scheme to deliver a specific UGR value. Then, when it was implemented, a change of ceiling paint selection and a change in the position of the desks or the height of the seating could result in a different UGR being experienced by the room occupants from that which was designed.

However, this does not mean that UGR should be ignored. Far from it. It would still be good practice to select fittings whose light distribution tended to reduce UGR and the use of suspended fittings with an uplight component, for example, would be a good way to do this.

A summary of the maximum UGR levels recommended by BS EN12464