Civil Engineering Reference
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work lenses, the computer worker can easily navigate the workplace while wearing the computer glasses.
This is important for most computer users and other workers with intermediate viewing needs —
because it is inconvenient to change glasses each time they move to or from the workstation.
Progressive addition lenses (Figure 18.5a) are another common lens design used to correct presbyopia.
These lenses are characterized by a smooth continuous change of power across the lens surface but with
undesired aberrations in the lower quadrants that limit viewing through those areas. PALs have cosmetic
advantages and also optical advantages compared to standard bifocals and are generally preferred over
bifocals by most wearers. However, even though progressive addition lenses provide a region with inter-
mediate power, this is the portion of the lens with the narrowest field of clear vision. If the worker
requires significant viewing at intermediate distances such as at a computer, then they must continually
find the small sweet spot on the lens and use their neck to move the head rather than moving their eyes.
This is inefficient and results in blur and musculoskeletal stress.
Occupational progressive lenses (OPL — Figure 18.5b) can very successfully meet the needs of pres-
byopic workers with extensive intermediate viewing needs such as computer users, assembly line workers,
assemblers, clerks, janitorial, general office work, etc. The lens design provides a reasonably large inter-
mediate viewing zone straight ahead, a wide near viewing zone in the bottom of the lens, and far-
intermediate vision on the top of the lens to enable the worker to navigate the workplace. The magnitude
of unwanted aberrations in an OPL is significantly less than in a PAL because the total power change is
less and the power poles are farther apart, resulting in wider viewing zones compared to the PAL. OPLs
are also a work pair of spectacles that do not meet general viewing needs outside of the workplace.
Workers with extensive near or intermediate viewing needs, especially at a wide workstation, benefit
from an Executive bifocal or trifocal (Figure 18.4b and Figure 18.4d). This design provides very wide
fields of clear, near, and intermediate vision, however, the complete near correction in the bottom of
the lens can make workplace navigation difficult.
Many presbyopic workers with near or intermediate viewing needs and superior gaze angles need
double segment lens designs such as in Figure 18.4e and Figure 18.4f. These jobs include pharmacists,
librarians, mechanics, drapery hangers, painters, etc. The double segment design enables better job per-
formance and posture.
Presbyopic workers are generally unaware of the mismatch between their visual needs at work and the
vision provided by their general wear glasses. Likewise such mismatches are not frequently identified
during routine eye examinations. The problems discussed above are not common among younger pres-
byopic individuals who still have some remaining accommodation because the lens power prescribed for
near viewing distances is relatively small and they have greater ranges of viewing distances through the
distance and near portions of their lenses. The problems noted earlier become most common for workers
aged 50 and above. When awkward postures are noted in presbyopic workers, the typical viewing dis-
tances and gaze angles should be noted and conveyed to the eye doctor for optimal occupational prescrip-
tion and lens design. In the situations discussed earlier, the spectacles are prescribed and designed to meet
the specific visual tasks in the workplace and should be provided by the employer.
18.2.3.3 Eye Movements
Of course, workers do not always look straight ahead. In order to fixate a noncentral target, the eyes
rotate so the image of the object falls upon the central or foveal portion of the retina. The maximum
magnitudes of ocular rotation superiorly are 42
8
(range 33-56
8
) and inferiorly are 50
8
(range 33-
62
) (Yamashiro,
1957). However, the eyes seldom rotate to these extremes. When a worker fixates a peripheral object,
the object is initially fixated nearly entirely with eye movement (Uemura et al., 1980), but the initial
eye movement is quickly followed by head rotation. The final resting state while fixating the object
involves significantly more of head than eye movement (Table 18.5). Similar findings apply to vertical
eye movements. Over a 39
8
), and the maximum lateral ocular rotation is approximately 55
8
(range 45-65
8
8
range of vertical computer display location, neck angle was found to
change by 25
8
, ocular rotation by 8
8
, and thoracic angle by 5
8
(Villanueva et al., 1997).
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