decreases, additional inward decentration of the bifocal segment is required. Interpupillary distance. The wider the interpupillary distance, the greater the convergence requirement and, correspondingly, the need for inward decentration of the segments.
Lens power. If the distance lens is a high-plus lens, it will create a greater base-out prism effect (ie, induced exophoria) as the viewer converges. Additional inward decentration of the segments may be helpful. The reverse is true for high-minus lenses.
Existing heterophoria. As with lens-induced phorias, the presence of an existing exophoria suggests that increasing the inward decentration would be effective. An esophoria calls for the opposite approach.
Prescribing Special Lenses
Aphakic Lenses
The problems of correcting aphakia with high-plus spectacle lenses are well known and were described eloquently by Alan C. Woods. They include
magnification of approximately 20%–35%
altered depth perception resulting from the magnification pincushion distortion; for example, doors appear to bow inward difficulty with hand–eye coordination
ring scotoma generated by prismatic effects at the edge of the lens (causing the “jack-in-the- box” phenomenon)
extreme sensitivity of the lenses to minor misadjustment in vertex distance, pantoscopic tilt, and height
in monocular aphakia, loss of useful binocular vision because of differential magnification
In addition, aphakic spectacles create cosmetic problems. The patient’s eyes appear magnified and, if viewed obliquely, may seem displaced because of prismatic effects. The high-power lenticular lens is itself unattractive, given its “fried-egg” appearance (Fig 3-36).
Figure 3-36 Aphakic lens with magnification and pincushion distortion. (Courtesy of Tommy Korn, MD.)
For all these reasons, intraocular lenses and aphakic contact lenses now account for nearly all aphakic corrections. Nevertheless, spectacle correction of aphakia is sometimes appropriate, as in bilateral infantile pediatric aphakia.
Refracting technique
Because of the sensitivity of aphakic glasses to vertex distance and pantoscopic tilt, it is nearly impossible to refract an aphakic eye reliably by using a phoropter. The vertex distance and the pantoscopic tilt are not well controlled, nor are they necessarily close to the values for the final spectacles. Rather than a phoropter, trial frames or lens clips are used.
The trial frame allows the refractionist to control vertex distance and pantoscopic tilt. It should be adjusted for minimal vertex distance and for the same pantoscopic tilt planned for the actual spectacles (approximately 5°–7°, not the larger values that are appropriate for conventional glasses).
Another good technique is to refract with clip-on trial lens holders placed over the patient’s existing aphakic glasses (overrefraction). Take care that the center of the clip coincides with the optical center of the existing lens. Even if the present lens contains a cylinder at an axis different from what is needed, it is possible to calculate the resultant spherocylindrical correction with an electronic calculator, by hand, or with measurement of the combination in a lensmeter.
Guyton DL. Retinoscopy: Minus Cylinder Technique, 1986; Retinoscopy: Plus Cylinder Technique, 1986; Subjective
Refraction: Cross-Cylinder Technique, 1987. Reviewed for currency, 2007. Clinical Skills DVD Series [DVD]. San Francisco: American Academy of Ophthalmology.
Absorptive Lenses
In certain high-illumination situations, sunglasses allow for better visual function in a number of ways.
Improvement of contrast sensitivity
On a bright, sunny day, irradiance from the sun ranges from 10,000–30,000 foot-lamberts. These high light levels tend to saturate the retina and therefore decrease finer levels of contrast sensitivity. The major function of dark (gray, green, or brown) sunglasses is to allow the retina to remain at its normal level of contrast sensitivity. Most dark sunglasses absorb 70%–80% of the incident light of all wavelengths.
Improvement of dark adaptation
A full day at the beach or on the ski slopes on a sunny day (without dark sunglasses) can impair dark adaptation for more than 2 days. Thus, dark sunglasses are recommended for prolonged periods in bright sun.
Reduction of glare sensitivity
Various types of sunglasses can reduce glare sensitivity. Because light reflected off a horizontal surface is polarized in the horizontal plane, properly oriented polarized lenses reduce the intensity of glare from road surfaces, glass windows, metal surfaces, and lake and river surfaces. Graded-density sunglasses are deeply tinted at the top and gradually become lighter toward the lens center. They are effective in removing glare from sources above the line of sight, such as the sun. Wide-temple sunglasses work by reducing glare from temporal light sources.
Use of photochromic lenses
When short-wavelength light (300–400+ nm) interacts with photochromic lenses, the lenses darken by means of a chemical reaction that converts silver ions to elemental silver. This process is similar to the reaction that occurs when photographic film is exposed to light. Unlike that in photographic film, however, the chemical reaction in photochromic lenses is reversible. Current photochromic lenses incorporate complex organic compounds in which UV light changes the molecules into different configuration states (ie, cis to trans); this process darkens the lenses (Fig 3-37). Photochromic lenses can darken enough to absorb approximately 80% of the incident light; when the amount of illumination falls, they can lighten until they absorb only 20% of the incident light. Note that these lenses take some time to darken and, in particular, take longer to lighten than to darken. This discrepancy can be problematic in patients who move frequently between outdoor and indoor environments. Because automobile glass and the window glass in many residences and commercial buildings absorb light in the UV spectrum, photochromic lenses do not darken inside cars or buildings. In colder weather, patients should also be warned that these lenses darken more than usual, especially during a cloudy day. Nevertheless, when darkened, photochromic lenses are excellent UV absorbers.
