Biomedical Engineering Reference
In-Depth Information
A
Red-green
eye chart
at optical
infinity
gr
B
Red-green
eye chart
at optical
infinity
FP
gr
Figure 15-19.
When a patient views a red-green acuity chart, images on the green
background (g) are focused in front of those on the red background (r).
A.
In a myopic
eye (or an emmetropic eye viewing through a plus lens), the optotypes on both the
red and green backgrounds are focused in front of the retina. Since red is imaged
closer to the retina, letters on this background appear clearer.
B.
The goal of the
red-green refraction technique is to straddle the retina with the green and red images
such that they are positioned equally distant from the retina and the optotypes on
both backgrounds are equally clear to the patient. This occurs when the appropriate
correction—in this case a minus lens to correct the myopia—is in place.
The Red-Green Refraction Technique
14
This commonly used refraction procedure takes advantage of the eye's longitudinal
chromatic aberration. When a patient views a visual acuity chart that is green on
one side and red on the other side, the green image is focused anterior to the red
image. If the patient is myopic, both the green and red images are focused anterior
to the retina and both will appear blurred (Fig. 15-19A). The optotypes on the red
background, however, appear less blurred because they are focused closer to the
retina than the optotypes on the green background.
When the duochrome test is performed in the clinic, plus lenses may be placed
in front of the patient's eye to ensure that both the green and red images fall ante-
rior to the retina.
15
Under these conditions, the optotypes on the red background
14. This is also called the duochrome or bichrome refraction technique.
15. Since the images in an uncorrected myopic eye fall anterior to the retina, it is not necessary to add
plus lenses. For an emmetropic or hyperopic eye, however, plus lenses must be placed in front of
the eye so that the images are focused anterior to the retina.
