How Important is Glare from Reflections of Light Sources?
Bright light sources may create glare by reflection from objects in the viewer’s field of vision. Most task areas include objects that can pick up reflections of light sources, for example, computer screens, the polished surfaces of machine tools, crystal paperweights, etc. Whether reflections of the light sources will cause an objectionable amount of glare depends on these factors:
• the surface texture of the illuminated objects. Smooth surfaces produce specular (mirror-like) reflections that concentrate glare. On the other hand, diffuse surfaces cannot create reflected glare. For example, reflections of the light fixtures are more troublesome for a machinist handling metal objects than for a sales clerk handling stuffed toys.
• the curvature of illuminated objects. If smooth surfaces within the visual field are highly curved, they reflect little of the light from the source into the eyes of the viewer. For example, reflections from the curved handle of a coffee cup are insignificant. On the other hand, fairly flat surfaces focus much more light into the viewer’s eyes. They can cause extreme glare, as in sheetmetal work.
• the orientation of illuminated surfaces. In order for glare to occur, the surface must be oriented to reflect the image of the fixture into the eyes of the viewer. If the surfaces are fixed, you can avoid glare by laying out the lighting geometry to avoid it. Trouble arises when the surfaces are movable, for example, when reading magazines.
• the steadiness of the viewer’s gaze. The importance of reflected glare depends on how much the viewer’s gaze is moving. For example, reflections from a computer monitor are serious because the user stares at the screen. In contrast, reflections from the bright stainless steel surfaces in a kitchen are tolerable to the cooks because they shift their gaze continually.
•the brightness of the light source. Annoyance increases with the surface brightness of the light source. For a given amount of light delivered to the task, average surface brightness is inversely proportional to the surface area of the light source. A naked incandescent bulb is much more annoying than a fluorescent fixture with a diffuser. Of these factors, the only one that you can control in the lighting design is the last one. If the task area contains objects that can reflect an image of the light sources into the eyes of the viewer, reduce the brightness of the light sources by increasing the surface area of the fixtures.