As indicated in Art. 15.11, quantity and quality of light are actually inseparable in contributing to good lighting, although they are treated separately, for convenience of presentation, in this section. Illumination should meet the requirements of visual tasks for safe, efficient performance, esthetic reasons, and the purpose of attracting attention. Factors that affect visual performance of a task include:
Luminance, or brightness, of the task
Luminance relation between task and surroundings
Color rendering of the light
Size of details to be detected
Contrast of the details with their background
Duration and frequency of occurrence of the task
Speed and accuracy required in performance of the task
Age of workers
The influence of these factors on visibility is described in Art. 15.10.
Dim lighting is sometimes desirable for mood effects. For merchandising, however, a pattern of brightness is depended on to capture the attention of potential customers. For task lighting, sufficient lighting must be provided on the work area if the task is to be executed without eyestrain and fatigue. Higher than the minimum required level of illumination usually improves visibility but often with greater energy consumption and increased life-cycle costs for both lighting and cooling the building. Consequently, for task lighting, illumination should be kept to the minimum necessary for maintenance of adequate quantity and quality of lighting.
For many years, single values for minimum illuminance, or level of illumination, fc, developed by the Illuminating Engineering Society of North America (IES) and listed in tables in the IES Lighting Handbook, have been widely used in the United States. In 1979, however, the IES revised its criteria to a recommended range of target illuminances. These recommended values take into account many of the factors listed above. Following is an example and abbreviated tables to illustrate the use of the tables in the IES handbook.
To determine the target illuminance, start by ascertaining the type of activity, or illuminance category, for the space to be illuminated. (The IES Lighting Handbook contains a detailed table correlating specific areas or activities with categories labeled A to I. For some of these categories, the effects of veiling reflections should be evaluated, for which purpose equivalent-sphere-illumination, ESI, calculations may be used, as indicated in Art. 15.10.5.) The first column of Table 15.7, which is based on descriptions in Selection of Illuminance Values for Interior Lighting Design, IES RP-1SA, gives a general description of these categories.
Next, determine the appropriate weighting factors W from Table 15.5 to adjust for loss of visual acuity with age, for importance of speed and accuracy in performing tasks, and for reflections of task background. For categories A to C, for which there are no task activities, use W = 0 as the weight for speed and accuracy.
Then, add the weighting factors. From Table 15.6, determine the visual condition number corresponding to W and the category. Decrease the condition number by one if the task is of short duration or occurs infrequently. Finally, select the target illuminance, fc, from Table 15.7 corresponding to the category and visual condition.
The IES recommends that if information is available on the effect of changes in illuminance or equivalent sphere illumination, ESI, on task performance, the data may be used to determine if a variation from the design illuminance or ESI will be meaningful in terms of increased or decreased productivity, which may be used in a cost-benefit analysis.
See Art. 15.20, Bibliography.