Scanning the plane of the retina yields 2-dimensional and hence 3-dimensional results like an ultrasound’s B-scan.

In Fourier- (also called spectralor frequency-) domain OCT, the reference beam mirror is fixed at one position. Interference fringe patterns, all mixed together, arise from the various tissue interfaces, but Fourier analysis enables them to be dissected apart. When the pattern arises from closer tissue interfaces, the fringe patterns’ undulations are spaced farther apart than those arising from deeper tissue planes, which yield fringes spaced more closely together. The more highly reflective tissue plane interfaces yield higher-amplitude fringe patterns. Thus, the spacing of the fringe pattern tells us how deep in the tissue it comes from, and its amplitude tells us how much the light is reflected by that tissue plane interface. In this manner, the A-scan of all the depths is obtained instantly without moving the reference mirror. Scanning across the retina yields 2-dimensional and hence 3-dimensional images. A swept-source version of the Fourier-domain OCT replaces the superluminescent diode’s band of frequencies with a laser; the laser emits different frequencies, one at a time, and the A-scan is performed for each frequency at each location.

Optical Aids

A variety of optical devices, including handheld and stand magnifiers, high-add spectacles, and telescopes, are available to assist patients with impaired vision and normally sighted individuals.

Magnifiers

The simplest low vision aid is a handheld magnifying glass (Fig 7-35). Lowto medium-power magnifiers can make continuous text reading possible for patients with mild to moderate vision loss. When function is more severely affected, stronger magnifiers may allow for shorter reading periods or for spot reading. However, the smaller field of view requires that the device be moved continuously along the reading material; this limits the feasibility of using a handheld magnifier for reading extended text passages for long periods of time. Newer magnifiers with LED illumination are excellent options for spot reading.

Figure 7-35 Simple hand magnifier. A +4 D lens mounted in a convenient handle, often described as a 2× magnifying

glass. (Courtesy of Scott E. Brodie, MD, PhD.)

The most commonly prescribed powers range between +5 D and +20 D. Above +20 D, the higher magnification and reduced field of view make it more difficult for the patient to maintain a steady focus, although some patients may do very well with a +24 D or +28 D magnifier.

The “power” or “magnification factor” of a magnifier is usually specified in terms of the relative angular size of the magnified image compared with the angular size of the original object at a standard reading distance. Most commonly, the reference distance is taken as 25 cm. In general, the maximal magnification will be obtained when the object to be viewed is placed at the anterior focal point of the magnifier. When the magnifier is used this way, the magnification factor is equal to the dioptric power of the lens divided by 4 (the dioptric equivalent of the reference distance of 25 cm). For example, the power of a +24 D magnifier is 6× (24 D/4 D).

Simple low-power magnifiers (typically around +4 D) are rarely used in this way, as it is difficult to hold a lens steady so far from the page, and this magnification factor convention is no longer appropriate. If such a lens is held with the text at half the distance to the anterior focal point, the virtual image seen by the user will be located at the anterior focal point and will be twice as large as the original text. These magnifiers are often casually described as 2×.

Patients with tremors, arthritis, paralysis, or poor hand–eye coordination often have difficulty holding handheld magnifiers steady as they scan along lines of continuous text. Typically, they will have improved performance with the same lens in a stand magnifier that rests directly on the page (Fig 7-36).

Figure 7-36 An illuminated stand magnifier placed flat against the page provides magnification, illumination, and stability. As with all magnifiers, the field of view decreases with increased magnification. (Courtesy of Darren L. Alb ert, MD.)

Telescopes

Tasks that require magnification for distance viewing are less common than those for near viewing, especially in older patients. Handheld monoculars, binoculars, and spectacle-mounted telescopes are available and allow the benefit of magnification at a greater distance, with the drawback of reduction in field of view, a narrow depth of field, and reduced contrast (Fig 7-37). In addition, patients cannot

wear a telescopic device when walking. Autofocus telescope models are available. A simple telescopic spectacle without a casing is available commercially and has become very popular, as it is lightweight and relatively inexpensive.

Figure 7-37 A, Top: Binocular, spectacle-mounted telescopes are available for prolonged distance tasks, such as watching a play in a theater, and/or near tasks such as reading. Bottom left: A high-power (6×), monocular, handheld Keplerian telescope may be difficult to hold steady and on target because of magnified motion and a narrow field of view. Bottom right: A low-power (2.8×), monocular, handheld Galilean telescope is ideal for intermittent distance tasks such as reading street signs or bus numbers. B, Both hand–eye coordination and training are required for successful use of

telescopic and other visual aids. (Courtesy of Darren L. Alb ert, MD.)

Loupes are spectacle-mounted telescopes set to focus at near points. They can provide an escape from the trade-off between high magnification and short working distance inherent in simple high-add reading glasses. However, the visual field is narrow and the depth of field small.

Bioptic telescopes are spectacle-mounted telescopes set to focus at distance, mounted in the upper portion of the lenses of carrier spectacles. These are allowed in many states for use while driving. The telescopic portion of the spectacles is positioned superior to the line of sight and used only briefly to read signs or look into the distance. The rest of the time, the individual drives looking through the regular prescription portion of his or her spectacles. Driving with a bioptic requires prescription of the device as well as device training and driver training on an individual basis.

Prisms

A variety of designs of prisms have been proposed to compensate for field loss by projecting the visual image onto the functioning portion of the retina or by redirecting the image of the object of regard onto the preferred retinal location (PRL) in patients with central macular dysfunction. Research is currently evaluating the efficacy of such devices compared to, or in conjunction with, training in systematic scanning.

High-Add Spectacles

High-plus reading glasses are an option for patients who can adapt to the closer working distance required. As a starting point, the clinician may estimate the required reading add power by using the