One of the most important functions of a lighting system is to support visual performance by ensuring appropriate visibility of tasks and objects the designer wants seen. The human eye can adjust to a wide range of light levels, including about 10,000 footcandles on a sunny day to about 0.01 footcandles under full moonlight. As Americans spend 85-90 percent of their time indoors, it is essential that interior lighting provide adequate light levels for people to perform tasks with desired speed, accuracy and safety.
Formula for task visibility
The visibility of a given task is determined by the contrast of its brightness or color against its surround. Other important factors include the size of the task, movement, time available to complete the task, and age of the person completing the task.
In short, the stronger the color contrast of a task against its surround, the easier it is to see. The larger the task is, the easier it is to see. Tasks that are motionless are easier to see than tasks that are moving, especially if that movement is fast or unpredictable (although movement can aid in detection, such as a person raising their hand during a seminar). If the user has a longer time to complete a visual task, less light may be needed on the task.
These factors are interdependent, so if one factor is strengthened, it can make up for weakness in another. They are also typically out of the lighting designer’s control, however. But there is an important factor the designer does control, and that is luminance.
The critical role of luminance
Again, luminance is light intensity radiated in a given direction after light exits a light source and falls on a task. The eye perceives light as brightness, the subjective experience of the light the eye sees. The higher a task’s luminance relative to its surround, the more visible it will be. Note that the eye takes time to adjust to luminance changes, during which vision may be impaired to an extent, which may be a factor in some applications such as building exits. The eye typically adapts to higher luminances quickly and lower luminances slowly.
Luminance is determined by the interaction of light level and task reflectance. Luminance contrast is determined by source/task/eye geometry.
Generally speaking, increasing light levels falling on a task will increase luminance, which will increase visibility. As light levels are a function of lumen output, the lighting designer can influence task visibility by controlling light levels. Different tasks call for different vertical and horizontal light levels based on different combinations of size, movement, time, etc.; the Illuminating Engineering Society (IES) has developed light level recommendations for a wide range of tasks. For a description of the light level description procedure and recommended values, consult the latest edition of the IES Handbook.
Task surface reflectance, expressed as a percentage of light reflected versus absorbed, is also important. The reflectance of the task should enable resulting luminance to be within recommended luminance ratios for the task versus its immediate surround. Walls, ceilings and objects in the space, meanwhile, should have a high reflectance to produce desirable interreflections and improve average light levels and visual comfort. Reflectance should not be confused with glossiness; glossy surfaces are certainly brighter, but that brightness is more likely to produce unwanted glare. For walls and ceilings, light-color matte finishes are often desirable.
Source/task/eye geometry—or the angular relationships between the light source, task and the user—is a determinant of luminance contrast. This is important to understand because sometimes lighting problems are not always problems with the lighting system. In some cases, users may be located or oriented in such a way they are more exposed to glare; the problem may be solved by simply relocating or reorienting the task.
Other considerations
When ensuring good visibility, lighting designers must take care to avoid visual fatigue, glare and shadows. Visual fatigue and glare are covered in the next section. Shadows can be useful for defining details but unwanted shadows can obscure tasks and thereby impair task visibility. Point sources are more likely to produce shadows. Linear lighting and localized supplementary task lighting can help reduce shadows.
A final factor that is important to ask visibility is the age of the user. As people age, the pupil grows smaller for a fixed ambient light level. The eye exhibits a progressively lesser ability to focus on near objects, a condition called presbyopia. An average 50-year-old may need double the light level of an average 20-year-old to transmit the same amount of light onto the retina. What’s more, older eyes are more susceptible to experiencing disability glare and take longer to adjust to changes in light levels (such as from one room to the next).
As can be seen, the luminous environment is much richer and more complex than simply delivering enough lumens to a horizontal workplane. In many retrofit situations, a considerable amount of trust is given to the original system designer, as existing light levels are either maintained or, in the case of older systems, reduced to current standards. In a relighting or new construction project, the lighting designer will review the application with a fresh eye and optimize task visibility without a requirement for allegiance to the existing design.
