Biomedical Engineering Reference
In-Depth Information
A
B
′
A
B
C
A
′
C
′
B
B
′
A
B
C
A
′
C
′
br
br
br
Figure 15-18.
A.
Points
A
,
B
, and
C
result in the images
A
′
,
B
′
, and
C
′
.
B.
Upon closer
inspection, we see that each of the images (
A
′
,
B
′
, and
C
′
) suffers from longitudinal
chromatic aberration. For the image
C
′
, blue (b) is focused closer to the retina, and
for the image
A
′
, red (r) is focused closer to the retina. The visual system may use
this information to determine if accommodation should be increased (as is required to
focus on
C
) or decreased (as is required to focus on
A
).
linear distance of 0.93 mm (Kruger et al., 1993). Although we are not normally
aware of longitudinal chromatic aberration, it is thought to be a stimulus to
accommodation.
In Figure 15-18A, an eye is focused on point
B
. The image of point
A
(i.e.,
A
)
is anterior to the retina and produces the same amount of retinal blur as the image
of point
C
(i.e.,
C
′
), which is posterior to the retina. Suppose the subject wishes to
change his or her fixation to point
A
. Since this point's image (
A
′
′
) has the same
amount of retinal blur as
C
, how does the accommodative system know if it should
increase or decrease its power?
Figure 15-18B shows the longitudinal chromatic aberration present in the
images
A
′
, the shorter wavelengths are focused closer to the
retina than the longer wavelengths. For
A
′
, B
′
, and
C
′
. For
C
′
, the longer wavelengths are focused
closer to the retina. If the accommodative system were able to use this infor-
mation, it could accommodate in the correct direction (i.e., increase its power
for the near object and decrease its power for the far object). Research suggests
that chromatic aberration may be a cue to accommodation. For instance, the
ability to accommodate accurately is impaired under monochromatic conditions
(Aggarwala et al., 1995).
′
