What does “glare” mean?
One of the factors that increase employee performance is visual comfort, which should be ensured by a correct lighting design. Neglecting these principles in lighting design can have a negative impact on occupational safety and overall productivity. And that is not all – low glare is also required by the European standard EN 12464-1. But what exactly does “glare” mean?
It makes a difference whether a luminaire emits luminous flux X at an angle α onto a particular workplace, or whether the same luminaire with the same beam angle emits the same luminous flux X onto a different workplace. An observer may perceive the light as glaring in one environment, but we cannot assume with certainty that this will be the case in any other environment. Glare is a subjective sensation, so the probability that a particular luminaire will cause glare under the conditions described above can only be predicted using quantitative values.
Glare is a condition that disturbs visual comfort or impairs vision, possibly to the point of making it impossible to see. We are not even aware of mild glare, but it is often the cause of unnecessary eye strain.
UGR – Unified Glare Rating
We experience glare when there is a large change in light intensity within our field of vision. A simple example is a pocket torch. If you look directly into it from a distance of 3 m in a dark room, you will be dazzled. But if you looked at it from the same distance on a sunny day, its light would most likely not dazzle you at all.
Why does our eye behave like this? The human eye works much like a camera. When the surrounding luminance is high, the aperture closes so that less light reaches the sensor. In our case, the sensor is the retina and the aperture is the pupil. The pupil is regulated by two muscles of the iris, which narrow it as luminance increases and widen it as it decreases. When the surrounding luminance is low, for example in a dark room, a small luminaire is perceived as more glaring, because the pupils are wide open.
Put simply: what matters is the ratio of direct to indirect light. A luminaire with an indirect light component, such as a pendant luminaire that shines both up and down, is therefore perceived as less glaring than a luminaire without an indirect component.
Discomfort glare caused directly by the luminaires of an interior lighting installation must be assessed using the tabular method of the Unified Glare Rating (UGR). The UGR value reflects the probability that a luminaire will be perceived as glaring in a particular environment.
| Type of space | UGR |
| Technical drawing | 16 |
| Reading, writing, computer rooms | 19 |
| Reception areas | 22 |
| Archives | 25 |
| Circulation areas and corridors | 28 |
UGR values are most commonly applied to writing and computer work. For this area, the maximum value is set at UGR ≤ 19. But let us return to the original meaning of glare: the probability that a luminaire will be perceived as glaring in a given environment. UGR ≤ 19 means that in 65 % of environments the luminaire was not perceived as glaring. In 35 % of cases, however, you may find your workplace glaring. With a lower UGR value, this probability is even lower.
What to look out for when choosing a luminaire
The UGR calculation uses the luminous intensity distribution curve of the luminaire to assess individual emission components under different conditions. Some might think that the distribution curve alone tells you whether or not a luminaire will cause glare. However, this is not always clear-cut.
On the one hand, a luminaire with a high luminous flux will of course cause more glare than one with a low flux. A luminaire with a luminous flux of 3,000 lm is therefore more likely to cause glare than a comparable luminaire with 1,000 lm. On the other hand, the beam angle is also decisive.
Let us go back to the pocket torch. Instead, imagine a classic recessed spotlight with a narrow beam angle. If you pointed it straight at your face, it would dazzle you. But do you shine a luminaire straight into your face in practice? In most cases the luminaire is mounted in the ceiling and shines down into the space below. At a normal viewing angle, a luminaire with a narrow beam angle does not shine directly into your face. In general, a luminaire with a narrow beam angle will therefore appear less glaring.
Today this causes problems on the LED panel market. The market expects LED panels to have extremely homogeneous light emission. Manufacturers usually meet this requirement with a diffuser made of polycarbonate (PC) or polymethyl methacrylate (PMMA). The diffuser scatters the light from the LED chips so that the resulting surface looks homogeneous and the individual chips cannot be seen. However, the scattering widens the beam angle of the LED panel, which makes it harder to develop a homogeneous LED panel with UGR ≤ 19. Some manufacturers therefore use a trick in the UGR calculation that involves an important factor: the size of the luminaire’s luminous area.
The larger the luminous area of a luminaire with the same luminous flux, the less glaring it appears. Compare a recessed spotlight with an LED panel. If both had the same beam angle and a luminous flux of 2,000 lm, the LED panel would appear less glaring.
Let us look at two examples. A homogeneous LED panel measures 620 × 620 mm and has a 40 mm frame. The effective luminous area is only 580 × 580 mm, which means it is 25 % smaller than the luminaire itself. When we compare the dimensions of the luminous area and of the luminaire in the Eulumdat file, we can see that some manufacturers state the outer dimensions of the luminaire instead of the actual luminous area. As these values were entered by the manufacturer and not measured by an automated system, the glare and the UGR value are likely to be higher.
Another example of misleading information is an LED panel that does not have a homogeneous luminous area, but 32 separate LED chips with a chip-free area in the middle. The manufacturer states that the luminous area is 325 × 325 mm, which is also the outer size of the luminaire.
However, the luminaire has an area in the middle with no LED chips. The luminous area is therefore 6 % smaller and the risk of glare and the UGR are higher than the manufacturer specifies. Lighting designers and buyers should therefore always study the available Eulumdat data carefully and compare the luminous area with the outer dimensions.
The picture shows a diagram of an LED panel with individual light points and a “free” inner surface.
Different standardised rooms – which one is right?
Whether a luminaire is perceived as glaring does not depend on the luminaire alone. It depends above all on the surroundings and on how the luminaires are arranged. Ultimately, the ratio of direct to indirect light is decisive. In a very low room, where the luminaires are in the observer’s horizontal line of sight, they will cause more glare than if they were mounted higher and the light reached the observer’s eyes vertically.
The room surfaces are also a very important factor. If the ceiling, walls and floor are highly reflective, the ratio between the surrounding luminance and the luminous intensity of the luminaires decreases, and the luminaires appear less glaring. In a dark, matt room, on the other hand, the probability of glare is very high. Because of these differences, 95 standardised rooms have been defined for UGR, differing in geometry and reflectance. A UGR value can be calculated for each of them.
A detailed lighting calculation also takes into account the size and amount of furniture and other large objects.
Luminaire manufacturers generally give a single UGR value in their datasheets. A room measuring 4H × 8H with ceiling, wall and floor reflectances of 70 %, 50 % and 20 % has become a kind of “standard” room, and many manufacturers state the UGR value for exactly this room. Because the other data are missing, luminaires can no longer be compared in terms of glare. The manufacturer can simply choose the value that suits the application best.
An example:
A manufacturer states that a luminaire is “suitable for lighting technical drawing workplaces”, where UGR ≤ 16 is required. In the “standard” 4H × 8H room, however, the UGR reaches 18.9, which is well above the maximum permitted value. For a 2H × 2H room, the UGR is better than required – 15.5. The manufacturer kept its promise, but only for this particular room. Claiming that the luminaire is suitable for technical drawing is therefore very misleading for lighting designers and lighting buyers alike.
UGR tables list 19 reference rooms for different X and Y side ratios. The width (X) and length (Y) of the room are multiples of H, the distance between the light centre of the chosen luminaire type and the horizontal plane at eye level.
As mentioned above, UGR also depends on the resulting luminous flux. These values therefore have to be adjusted for the specific luminaire, which in our case has a luminous flux of 3,397 lm. After the adjustment, the UGR comes out at 23.1. For this luminaire, the probability of glare is much higher than stated, which means it may be perceived as glaring in some rooms.
Correct lighting design matters
If glare is a high priority for you and you want to take all these factors into account, lighting design software is essential – DIALux, RELUX or similar programs that can work with the luminous intensity curves of luminaires in a room modelled in detail. This means that we have to take into account the reflectance of surfaces, the furniture and all larger objects.
In addition, the UGR value also depends on the likely position and viewing direction of the observer. It is not enough to include only the working plane in the calculation. Professional lighting designers take this into account. For a simple estimate of the probability of glare, it is enough to look at these details:
- a high proportion of indirect light,
- a large and correctly stated luminous area,
- a narrow beam angle.
Where a standard must be met or glare reduction is part of the project, a detailed lighting design with a correct UGR calculation must be prepared.
Do you need help with a lighting calculation? Get in touch with us.
