Resources | LpR Article | Quality | Optics | Mar 13, 2018

Glare Reduction Made Easy

A pleasant lighting atmosphere due to low glare at the workstation is not only pleasant and performance-enhancing, but also legally required according to EU standard EN 12464-1. But what does “glaring” mean here? Dr. Claudius Noack, Technical Director at NORKA Leuchten, explains if luminaires with the same luminous flux are glaring to the same degree and if the same luminaires are glaring to the same degree at different workstations. He answers the questions of what a manufacturer does and if manufacturers are lying to themselves and therefore to their customers when providing information on the glare rating.

There are many important questions and there is a huge uncertainty about glare that need to be discussed. For starters, the following should be clarified: whether light is perceived as glaring by a certain person in a certain environment cannot be predicted with certainty. Glare is a subjective feeling and therefore only the probability as to whether a certain luminaire will glare in the aforementioned environment can be predicted with quantitative values.

UGR - Unified Glare Rating

A person experiences glare when a high fluctuation in luminous intensity occurs in his or her field of vision. An easy example here is a flashlight. If you look directly into a flashlight from a distance of three meters in a dark room, you will certainly be blinded by glare. If you now look outside the range of the flashlight from the same distance, the light will certainly not seem as glaring. Instead of the flashlight, we can naturally also take a small spotlight as an example.

Why is this the case? The process in the human eye is the same as for a camera: If the overall brightness is high, the shutter is reduced so that less light falls on the sensor. The sensor corresponds with the retina and the shutter with the pupil, which is regulated by the two ciliary muscles at the incidence of brightness. If the overall brightness is low, as in a dark room, a small luminaire is perceived as more glaring because the pupil is open wide. Or more simply expressed: The relationship between direct and indirect light is decisive. A luminaire with an indirect percentage, e.g. a pendant luminaire or floor lamp that shines upwards and downwards, is therefore usually perceived as less glaring than a luminaire without an indirect percentage.


Figure 1: 
Photo of a flashlight in a dark room and in daylight

The so-called glare rating takes advantage of this circumstance. In fixed values, this rating reflects the probability that a luminaire in a certain environment is perceived as glaring. Among other things, a calculation is used to evaluate in which areas and at which ratio the luminaire emits its light. The generally applied Unified Glare Rating, UGR for short, additionally contains specified rooms for consideration.


From this apparently simple formula, the glare limitation for various areas is indicated in steps of three in accordance with standard DIN EN 12464-1.

The most common area for many people is writing or computer work. This includes all classrooms, offices and so on. For this area, a maximum value of UGR <= 19 must be observed. Let’s return to the original meaning of glare: the probability of whether a luminaire is perceived as glaring in this environment.

UGR <= 19 means that 65% of the test subjects in scientific studies did not perceive the luminaire as glaring in this area. If you select this luminaire in the respective constellation, there is a 35% chance that you will find your workstation to be glaring. At a low UGR value, this probability is therefore smaller.

Different Luminaires

The UGR calculation uses light distribution to evaluate individual emission ranges at various strengths. One would perhaps think that looking at the light distribution of the luminaire would provide information as to whether a luminaire is glaring. This is not always easy, however.

Figure 2: Light distribution of a narrow-beam, linear LED luminaire

On the one hand, a high luminous flux of a luminaire is understandably more glaring than a low one. It is therefore more probable that a luminaire with 3,000 lm is more glaring than a comparable luminaire with 1,000 lm. On the other hand, the emission angle is decisive.

Let’s return to our flashlight. As mentioned above, a flashlight can be equated with a built-in spotlight with a narrow emission angle. If you hold it directly in your face, you will perceive it as glaring, but do luminaires generally shine directly in your face? In the most frequent cases, the luminaire is mounted on the ceiling and shines downwards to illuminate the workstation. A luminaire with a narrow emission angle, like our built-in spotlight, now no longer shines directly in your eyes since you are generally looking forward or down during your work and not up at the ceiling. In general, you can also assume that a luminaire with a narrow emission angle is perceived as less glaring. At present, this circumstance is leading to difficulties for a major section of the luminaire industry: the LED panel market. Today’s market requires LED panels to be an extremely homogeneous lighting area. This requirement is frequently achieved using polymer planes made of PC or PMMA. These planes scatter the light of the individual LED in such a way that the lighting area is perceived as a homogeneous surface and not as individual points. The result, however, is that the emission angle of the LED panel is enlarged by this scattering. An LED panel with a homogeneous lighting area with an UGR of <=19 is therefore more difficult to develop and produce. Here, some manufacturers take advantage of a trick in the UGR calculation, which comprises an additional important factor: the size of the lighting area itself.

Table 1: A few examples of typical visual tasks

The larger the lighting area of a luminaire at the same luminous flux, the less it is perceived as glaring. In this case, as well, a comparison can be made between the spotlight and the LED panel. If both luminaires have a luminous flux of 2000 lm, the LED panel is perceived as less glaring with the same light distribution. But how large is the lighting area of a luminaire now? Let’s look at the following two examples. Our example LED panel with a homogeneous lighting area has dimensions of 620x620 mm and a perimeter frame of 40 mm. Effectively, the lighting area therefore only amounts to 580x580 mm, which means that the lighting area is 25% smaller than the luminaire itself. If you compare the value for the lighting area in the Eulumdat-file, which contains the light distribution, with the real area, you can see that some manufacturers list the outer dimensions of the luminaire and not the lighting area itself. Since the manufacturer itself enters these values and they are not measured by an automated system, the probability of glare and therefore the UGR value are considerably higher than specified.

A further example of incorrect information is a panel available on the market that does not have homogeneous emission area, but rather 32 individual light spots.

Figure 3: Schematic diagram of an LED panel with individual light spots and a “free” internal surface

This manufacturer specifies the lighting area of this luminaire as 325x325 mm, i.e., the outer edges of the light spots. The luminaire, however, has a center section that does not contain light spots. The surface is therefore about 6% lower and the risk of glare and the UGR are higher than specified.

This behavior of a few black sheep on the LED market is comparable to the current, well-known diesel scandal of the automobile industry, in which individual values have been manipulated to achieve the desired values demanded by the respective standards. A user and/or buyer of a luminaire should therefore closely examine the freely available Eulumdat-files and compare the lighting area.

Different Standard Rooms - Which Is the Right One?

Whether a luminaire is perceived to be glaring, however, does not depend on the luminaire itself, but rather on the environment and arrangement of the luminaires. In this case, as well, the direct and indirect lighting effect is a key factor. In a very flat room in which the light of the luminaires enters the eyes of the occupant horizontally, the occupant is dazzled more than when the luminaires are mounted higher and the light is therefore more likely to enter the occupant’s eyes vertically. Not only the geometry of the rooms, but also the surfaces inside the room are decisive for the glare behavior. If ceilings, walls and floors strongly reflect the light, a higher individual percentage of the light arises and the light seems to be less glaring. In a matt, black room, the probability is high, on the other hand, that occupants will find the light to be glaring. For these different circumstances, 95 standard rooms that differ in geometry and degree of reflection have been defined for UGR. For each of these standard rooms, a UGR value can now be calculated. All values are listed in the UGR table of a luminaire.

Table 2: Uncorrected UGR table using the example of an LED panel

The manufacturers of luminaires generally limit their data sheets to the publication of an individual value. A room with a geometry of 4H8H and reflectance’s of ceiling = 70%, wall = 50% and floor = 20% has been established as quasi-standard. Many manufacturers specify the UGR value for this room in their data sheets. Very few manufacturers, however, indicate the room to which their UGR values concretely refer. Due to this missing reference, a comparison is no longer possible. The manufacturer can select the best value for its application. In the example of the UGR table above, the manufacturer, for example, indicates that this special luminaire is suitable for technical drawing; i.e., it has a UGR value of <=16. If you check the UGR for the “quasi-standard room” 4H8H in the table, this value, at 18.9, is considerably higher than the permitted limit value. The UGR for room 2H2H, however, is in fact within standard at 15.5. Here, the manufacturer does not break its promise; but extremely limits it without telling the consumer.

In addition, only the uncorrected UGR values are listed in the displayed table. This means that only the values for a theoretical luminous flux of 1,000 lm are listed. Since, as described above, the luminous flux of a luminaire is also important and the luminaires shown here have a luminous flux of 3,397 lm, the UGR value must once again be adapted using the following formula.


As a result, a UGR of 23.1 and therefore <= 25 develops for the standard room from the specification “Suitable for technical drawing,” which requires a UGR of <= 16. For this luminaire, the probability of glare is therefore considerably higher than specified. Since this calculation was performed as described for standard rooms, however, this does not necessarily mean, that the light of this luminaire is perceived as being glaring in a certain room as only a few rooms can be mapped exactly to the standard rooms.

Figure 4: Modern, detailed lighting calculation takes furniture and other large objects into account

Proper Lighting Design

To take all these circumstances correctly and extensively into consideration, especially if glare reduction is a high priority for you, you should design your lighting with DIALux, RELUX or a similar lighting design tool in which the correct light distribution can be implemented in a room that is as detailed as possible. This means that furniture and other large objects must also be taken into account.

In addition, the glare values at the probable observer positions, and not just at the working area must be included in the lighting calculation. It is ultimately not of use to you if you know that your workstation does not have glare, but your visual range does. Professional lighting designers take this aspect into consideration, but even the lighting design departments of the manufacturers, which are increasingly offering such services, often require training in this area.

Conclusions

For a simple estimation of the probability of glare, it suffices to observe the following things:

  • High indirect percentage of light
  • Large and correct lighting area
  • Narrow emission angle

If legal standards must be observed or if glare reduction is an important part of a project, a detailed lighting design with a correct UGR calculation must also be performed.

(c) Luger Research e.U. - 2017

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