Civil Engineering Reference
In-Depth Information
Amplitude of accommodation (diopters)
5
4
3
Subjective measurements
2
1
Objective measurements
0
40
45
50
55
60
Age (years)
Figure 27
FIGURE 23.1 Average magnitude of available accommodation by age, measured subjectively and objectively.
(Adapted from Hamasaki, D., Ong, J. and Marg, E., Am. J. Optom. Physiol. Opt., 33, pp. 3-14, 1956.)
focused) or presbyopia (farsightnedness where near stimuli cannot be brought
in focus) (e.g.,
Soderquist, 2002).
For example, a 15-yr-old healthy user normally has approximately 10 diopters of accommodation
(or is able to focus at 1
10 of a meter). However, that same person at age 40 may show less than one-
third of that accommodation. Figure 23.1 illustrates how the ability to accommodate deteriorates
even further for people over the age of 40 yr, both objectively and according to subjective ratings.
This reduced accommodation ability can be compensated for, to a limited extent, by applying more
force to the muscles that control accommodation. Still, it cannot be overcome completely and therefore
needs to be considered in the design of interfaces that are intended to be used by a range of diverse users.
After passing through the lens, visual stimuli reach the retina at the back of the eye. The retina consists
of layers of cells that contain two major types of photoreceptors: rods and cones. These two types of recep-
tors contain different light-processing chemicals, called photopigments. Rods, which contain the photo-
pigment rhodopsin, are much more numerous than cones. There are approximately 100 to 120 million
rods in the retina, compared to only 6 to 7 million cones.
Rods and cones differ considerably with respect to the perceptual functions they support. Rods are
highly sensitive to light but they are not sensitive to color nor do they support high-acuity vision.
Thus, rods are particularly important for night vision and mostly ineffective during daylight because
of saturation. Rods also support contrast sensitivity, a perceptual phenomenon that is of critical import-
ance to human factors professionals. Contrast sensitivity has been defined as “the reciprocal of minimum
contrast between lighter and darker spatial areas that can just be detected” (Wickens et al., 1998). Contrast
sensitivity thus is a prerequisite for detecting and recognizing shapes. Several factors influence contrast
sensitivity, including the illumination of an object where lower illumination reduces sensitivity, as illus-
trated by our difficulties with reading under low lighting conditions or detecting objects at night. Contrast
sensitivity is also reduced when the object of interest is moving and with increasing age due to factors
such as cataracts (increased clouding of the lens).
When stimulated, rods rapidly lose their sensitivity to light and require a long time to regain it. For
example, when entering a dark room, we cannot distinguish any objects around us at first. Over
several minutes, the visual system adapts to the ambient light and objects become increasingly visible.
This dark adaptation can take as long as 20 to 30 min if a transition from photopic (cone-based
vision) to scotopic (rod-based vision) conditions is required. Note that during the first 7 min in dark-
ness, the cones require less light to perceive a visual stimulus. After that time period, the cones are
more sensitive. This point is referred to as the rod-cone break. The reverse process, light adaptation
from darkness to bright light, occurs significantly faster than dark adaptation. It requires only about
2 to 3 min. Dark and light adaptation need to be considered in the design of workspaces that involve
rapid changes in illumination (see Purves and Lotto, 2003).
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