Whatever the application, a very basic principle always applies: a lighting installation needs to be visually comfortable for its users. But some applications are especially demanding from a visual comfort perspective. Take offices, for example. Due to the intense concentration that’s required of office workers, and to the fact that they typically do their work in front of computer screens, providing the highest level of visual comfort is crucial.
But what do we mean by visual comfort and how can we achieve it?
Let’s start by first defining the opposite term: visual discomfort. Visual discomfort is associated with both psychological and physiological effects. It involves eye fatigue (which can manifest itself in sore, red, and itchy eyes) and can lead to headaches and to neck and shoulder pain. It can also have a negative impact on performance, mood, and motivation, and boost the amount of sick leave employees take. If you’re interested in reading more about visual discomfort and its possible effects, we recommend you take a look at the Human Factors in Lighting publication.
Visual discomfort can have various root causes, alone or in combination. Too little or too much illuminance can cause it, as can strong illuminance contrasts or distracting light patterns. Flicker or stroboscopic effects (temporal lighting artefacts, as they’re technically known) can cause it. And so can glare…
Just what is glare, actually?
Glare is a phenomenon caused by extremely bright light sources or by strong brightness contrasts in the visual field. (Brightness, for its part, is the perceptual sensation caused by a luminous source or reflecting surface.) Glare sources include windows, glossy magazine pages, computer screens, or incorrectly designed and installed artificial lighting.
The impact of glare is linked to the luminance of a source or surface – that is, to its light intensity in a given direction per surface area, measured in candela per square meter. But it also depends on other parameters, like luminance contrast in the visual field; and on the viewer’s age, iris color, and individual sensitivity to light.
Disability glare vs. discomfort glare
Excessive brightness can have a serious direct impact on visibility and on our ability to perceive objects. When brightness has such an effect on us, it’s subjecting us to disability glare. A typical example of high-level disability glare is when, driving on the highway at night, you’re dangerously blinded by the bright headlights of the car coming at you from the opposite direction. Inside a building, bright sunlight entering through a window can cause disability glare.
Discomfort glare, on the other hand, occurs when people don’t feel comfortable with their given lighting conditions (because, again, the light source might be too bright), yet don’t find their vision actually impaired. Discomfort glare is also referred to as “psychological glare,” as physiological tests can’t measure it and physiological values can’t account for it. Discomfort glare is measured and evaluated exclusively on the individual level. Despite many decades of research, the phenomenon and exact causes of discomfort glare still aren’t fully understood.
Measuring glare: The UGR value
Numerous methods for measuring direct discomfort glare have been proposed over the years. Until recently the most widely used was the Unified Glare Rating (UGR), proposed by Sorensen in 1987 and adopted by the International Commission on Illumination (CIE) in 1995 before being recommended for international application.
The UGR adds up the contributions of all glare sources in the visual field, taking into account source luminance, source size, source position and background luminance. UGR value may also vary depending on the size of the room, the reflection factors of the ceiling, walls, and floor, and the position and viewing direction of the observer in the room. There are 5 UGR classes with limiting UGR values of 16 (lowest glare), 19, 22, 25, and 28 (highest glare).
To give an idea about the recommended UGR value for a given activity, the below table indicates the limiting UGR values for some typical indoor applications. These norms were established by the EN-12464-1.
What is indirect glare and why does it matter?
In addition to its direct form, glare has an indirect variety, which can also be a source of visual discomfort. Indirect glare results from high luminance reflections off of polished or glossy surfaces in the field of view. It usually is associated with reflections from within a visual task or from areas in close proximity to the region being viewed, according to the Illuminating Engineering Society. Indirect glare should be taken into consideration in offices especially.
Screen displays are a leading cause of indirect glare – or reflected glare as it’s also called – in offices today. It’s important to select appropriate luminaires and to locate and arrange them correctly to avoid it. CEN, the European Committee for Standardization, specifies luminance limits for luminaires that could reflect off computer monitors, causing indirect glare. These limits are specified at elevation angles of 65° and above from the nadir, in the area surrounding the luminaire. Depending on screen luminance, maximum luminance limit of luminaires above 65° is 1500cd/m2 (medium luminance screen < 200cd/m2) or 3000cd/m2 (high luminance screen > 200cd/m2).
How to incorporate glare requirements when selecting light sources in office applications
Glare is one of the first elements requiring attention in environments like offices, where both direct and indirect glare can impact visual comfort. The table above makes clear that when it comes to actual working area in offices, light sources need to have a UGR value of 19 or below. In line with this, and also so as to provide a variety of options in terms of archetype, design, and connectivity, the Philips office portfolio includes a variety of luminaires, such as SmartBalance, SlimBlend, and TrueLine (office-compliant version). All have <UGR 19 and eliminate the effects of indirect glare, thanks to their having a value of L65<3000 cd/m2.
Even though a UGR value of 19 is typically accepted in offices, particularly attention-intensive activities such as technical drawing or control room work require lower UGR values for better visual comfort and quality. The Philips PowerBalance luminaire, with UGR 16 and L65<1500 cd/m2, suits such activities with an optimized light distribution that doesn’t compromise lighting design efficiency.
Applying a light source with the correct, and lowest possible, UGR value is an important step towards minimizing the impact of glare in offices. To enhance comfort and limit the phenomenon of glare further, lighting professionals can also consider applying smart control applications such as Philips ActiLume, which automatically adjusts the brightness of artificial light depending on the amount of natural light coming from outside. This further reduces the luminance that can potentially reflect off surfaces such as computer screens or glossy furniture.
In conclusion, visual comfort is crucial to our well-being. In offices and other work environments it’s even more important, as it makes us more productive. Glare, a complex phenomenon that comes in both direct and indirect forms, is a chief cause of visual discomfort – and even visual disability. With the right lighting, however, lighting professionals can mitigate or even eliminate it, improving the environments they’re charged to light and the way that people interact and work in them.
