Glare is a fact of life. All of us certainly know it when we see it. But believe it or not, glare is not so easy to define in technical terms. Several measurement systems, such as visual comfort probability (VCP) and unified glare rating, have been developed, but none has gained universal support from the lighting community. For the time being, most designers use a combination of one or more of the glare metrics, plus their own subjective assessment and common sense to address glare in lighting and daylighting design.
At present, the Illuminating Engineering Society (IES) defines glare as one of two conditions:
- Too much light
- Excessive contrast, meaning the range of luminance in the field of view is too great
Whether designing lighting or daylighting, the principal objective is to ensure that there is the right amount of light with appropriate limits to glare. Because glare is physiological and can cause intense physical response, there are occasions when glare is wanted, such as scanning a concert audience with a spotlight to heighten the excitement. In architecture, glare is desirable when a designer wants to cause extreme contrast that exhilarates the visual experience. But for most architectural lighting and daylighting, effective methods of glare control and prevention are essential to good lighting practices.
To better understand glare, it is important to consider that the human eye adapts to the average luminance of a visual scene. The eye's range is amazing, perceiving scenes with light levels of less than .01 lux (which equates to starlight) to more than 100,000 lux (the equivalent of a sunny day). But the eye can only adapt to a part of this range at one time, and it takes a few moments for the eye to adapt, such as when you enter a movie theater on a sunny day. When you first enter the theater, you are temporarily blinded because your eyes are adapted to daylight; but when you leave the theater, even the reflected outdoor daylight is temporarily glaring. In other words, your susceptibility to glare is a direct function of adaptation: The darker the scene to which you are adapted, the more likely you are to experience glare from a light source.
In practice, glare is almost always a situation where a source of unshielded light is at least 1,000 times brighter than the average visual field. For instance, since the night sky is dark, almost all outdoor light sources, such as a street luminaire or automobile headlight, cause glare. To evaluate glare, however, light can't be measured in lux or footcandles—one must use luminance, which typically is measured in candelas per square meter (cd/m2) or nits. It also is common, although not technically correct, to use the term brightness rather than luminance.
Good lighting design practice either diffuses the light to reduce the luminance or shields the source from view. The control of glare in electric lighting is generally called shielding. For natural light, however, the term shading is used. While technically they are almost the same thing, the slight difference in language is welcome to help differentiate between lighting and daylighting.
CONTROLLING TOO MUCH LIGHT Too much light from an electric lamp is not common. The brightest lamp we are likely to use is a 1000W high-intensity discharge or, perhaps, a high-wattage xenon arc lamp. It is hard to imagine that anyone would use these lamps without proper enclosures, optics, and shielding.
On the other hand, direct sunlight is, for most practical purposes, too much light—indoors or out. Regardless of the eye's ability to adapt to brightness levels, one cannot look directly at the sun without risking retinal damage. The sun's brightness must be mitigated by some type of shading device. In humans and other mammals, the shape of eye socket and eyebrow provides shading during the brightest part of the day. At low solar angles, the pain of too much light causes physical aversion and one must turn away or shield his or her eyes. Outdoors, the use of sunglasses and billed hats is strongly recommended for both comfort and to prevent long-term damage to the eyes. For instance, those who wear glasses or contacts have fewer cataracts and other aging eye issues because of the ultraviolet protection of corrective lenses.
In buildings, too much natural light is almost always manageable. The simplest roof provides shade during the brightest period of the day. Without direct sunlight, the diffuse light of the lower sky near the horizon, in combination with reflected light from the ground plane and vertical surfaces, produces adequate light. However, in the early morning and late afternoon, direct sunlight does penetrate into the structure. Shading, therefore, is needed to control glare.
CONTROLLING TOO MUCH RANGE OF LUMINANCE In electric lighting, a lot has been said over the years about the importance of maintaining “balance of luminance.” In recent years, the IES' office lighting recommendations and manufacturers' literature have promoted the concept of balanced luminance in which room surfaces should be no more than 10 times brighter than the task nor less than one-tenth of the task. Assuming that the average luminance of a computer screen is about 100 cd/m2, this means that the brightest surface of the room should be 1,000 cd/m2 or less, and the darkest should be 10 cd/m2 or greater.
But if the sunlight on the floor near the window is 10,000 cd/m2, is that OK? I suggest that maybe it is and maybe it isn't, depending on whether the work being done in the space and the overall intent of the design is harmed in some way by the extremes in condition. After all, contrast is drama. While I would hate to have an overly dramatic workspace, I often create borderline-excessive contrast to draw attention to certain aspects of the architecture and to appeal to people's visual interest, especially for retail and other dramatic types of spaces.
When it comes to daylighting, the extreme contrast of sunlight to the luminance of interior surfaces (whether by natural or electric light) mandates shading. This is not a new issue: throughout history, mankind has learned to shade interiors from direct sun, devising a number of exterior solutions, such as awnings and overhangs, and interior solutions ranging from curtains and sheers to blinds, louvers, and roller shades. Because the sun moves while the structure remains stationary, almost all static shading systems are imperfect. These work well during certain seasons or during certain hours of the day, but they are ineffective during others. Moreover, on cloudy days the shade often is undesirable in order to get enough light indoors. The need for dynamic shading has resulted in a number of clever solutions, from motorized exterior louvers to automated roller shades and blinds.
In the 20th century, improvements in window glass enabled the windows themselves to shade the interior by letting in a controlled amount of direct sunlight. Today, glass can be:
- Tinted to absorb a specific amount of light energy. (Note that the absorbed energy becomes heat and will cause the glass to expand, with the threat of cracking.)
- All of these treatments allow windows to shade the interior space. Since they are static, they will produce darker interiors on darker days. But this often is a good solution, especially for multi-story buildings with relatively large areas of glass, as the treatments reduce excessive light and heat gain.
- Treated with a reflective mirror-like coating, redirecting a portion of solar energy away from the building. Even if the building appears as a mirror, the reflection is not total and a controlled amount of light enters the space.
- Coated with a pattern of white reflective ceramic elements called frits. Frits allow some direct light penetration while reflecting unwanted light. Frits are not diffusers—they simply reduce the amount of light equal to the ratio of fritted to unfritted surface area.
Many projects today employ perforated roller shades as a primary means of glare control. The percentage of openness—meaning the density of the weave of the shading material—determines the amount of light penetration and is a function of the visible light transmission of the glass. For instance, from tests performed on the New York Times building, the highly transmittive glass (more than 70 percent) requires 1 percent to 2 percent openness before people find direct sunlight too glaring. If glass with half the transmission (35 percent to 40 percent) were used, the openness could be doubled.
These shades are used on the south, east, and west faces to control direct solar glare; shades on the north side of the structure can be far more open, if they're needed at all. I typically recommend 10 percent openness on the north façade to reduce the luminance (brightness) of the sky without cutting out all of the light. The best thing about roller shades is that they can be easily automated so that they can be raised to harvest as much daylight as possible on cloudy days.
SHADING AS DESIGN Current daylighting design in the U.S. usually involves a combination of exterior, interior, and self-shading elements that alternate to play the roles of controlling direct sunlight, controlling solar gain, and controlling glare. In my daylighting practice, I work from the outside in to develop shading solutions. Exterior shading is studied first and is generally used to prevent direct overhead sun penetration during the cooling season; the shading of the glass is used to control the total light entering the space and to control sky brightness during most of the day; and the interior shading is used primarily for morning, afternoon, and winter daytime direct solar glare control. This is a relatively foolproof way to design shading for a building with good solar orientation.
In a current project in Chicago with complex interior lighting requirements and having east and west facing façades, my firm's design uses a combination of dark-tinted low-E glass, frits in a pattern to reduce solar glare and sky luminance, and motorized perforated roller shades to control the brightness when the sun is rising and setting. We developed calculations for both lighting performance and solar energy performance. A mock-up is being developed to test the theories against worker acceptance of the light levels. Because projects with complex luminance requirements are almost impossible to predict, this is a situation where the architect, lighting designer, and owner will know the right solution when they see it.