Civil Engineering Reference
In-Depth Information
TABLE 4.2
Illuminance Recommended by the
IESNA for Industrial Tasks
Type of Task
Range of
Illuminance (lux)
Workplaces where visual tasks are only occasionally performed
100-200
Visual tasks of high contrast or large size: printed material, rough bench
and machine work, ordinary inspection
200-500
Work at visual display terminals for extended periods of time*
300-500
Visual tasks of medium contrast or small size; e.g., penciled handwriting,
difficult inspection, medium assembly
500-1000
Visual tasks of low contrast or very small size; e.g., handwriting in hard
pencil on poor-quality paper, very difficult inspection
1000-2000
Visual tasks with low contrast items and very small size over a prolonged
period; e.g., fine assembly, highly difficult inspection
1000
Performance of exacting visual tasks such as extra fine machine work,
exacting assembly and manual crafts, precision arc welding
3000
*This recommendation is from ANSI/HFS 100 (Human Factors Society, 1988).
Adapted from Kaufman and Christensen (1984). The upper values in the range are for individuals
aged over 55 years and the lower values are for individuals younger than 40 years.
Society (IES) publishes recommended values of illumination (Table 4.2). Depending
upon the size of the visual task and the contrast of the task, different levels of
illumination are required. These guidelines also take into account the worker's age, the
importance of speed and accuracy, and the reflectance of the task background. The upper
end of the recommended range in the table should be used to accommodate older workers
and the lower values are for younger workers. It is also suggested that local task lighting
rather than general ambient illumination be used, particularly if the illumination at the
workplace is above 1000 lux.
4.11
THE AGING EYE
For older individuals there are several physical changes in the eye. The most important is
the loss of focusing power (accommodation) of the lenses in the eye (Safir, 1980). This is
because with increasing age the eye lenses lose some of their elasticity, and therefore
cannot bulge or flatten as much as before.
Figure 4.11 illustrates that the average accommodation for a 25-year-old is about 11
diopters, but for a 50-year-old it is only 2 diopters and for a 65-year-old it is 1 diopter.
The number of diopters translates into a range of clear vision that is defined by its far
point and its near point. Assume that for the 25-year-old the far point is
