Correction of Refractive Errors 1651 Eyeglass Lenses

Monofocal Lenses

There are two basic types.

❖ Spherical lenses refract light equally along every axis.

❖ Toric lenses (known as cylindrical lenses) refract light only along one axis. Spherical and toric lenses can be combined where indicated.

The refractive power of the lenses is measured manually or automatically with an optical interferometer. The measured refraction is specified as spherocylindrical combination. By convention, the specified axis of the cylindrical lens is perpendicular to its axis of refraction (Fig. 16.15c and d). The orientation of this axis with respect to the eye is specified on a standardized form (Fig. 16.16).

Example: + 4.00 diopters -2.00 diopters/90 degrees means that the lens represents a combination of converging lens (+4 diopters) and cylindrical lens (-2 diopters) with its axis at 90 degrees.

Eyeglass lenses exhibit typical characteristics when moved back and forth a few inches in front of one's eye. Objects viewed through minus lenses appear to move in the same direction as the lens; objects viewed through plus lenses move in the opposite direction. A cylindrical lens produces image distortions when turned.

Multifocal Lenses

Multifocal lenses differ from the monofocal lenses of uniform refractive power discussed in the previous section in that different areas of the lens have different refractive powers. These lenses are best understood as combinations of two or more lenses in a single lens.

Bifocals: The upper and middle portion of the lens is ground for the distance correction; the lower portion is ground for the near-field correction (Fig. 16.17 a and b). Patients are able to view distant objects in focus and read using one pair of eyeglasses, eliminating the need to constantly change glasses. The gaze is lowered and converged to read. This portion of the lens contains the near-field correction. This near-field correction can be placed in a different part of the lens for special applications.

448 16 Optics and Refractive Errors — Eyeglass prescription. -

Eyeglass prescription for Mr./Mrs./M__

Spherical

Cylindrical

Axis

Prism.

Base

Vertex distance

F

R

+ 4,0

-2.0

90°

4

«Cotm

14

L

0

N

R

o

L

o

Typ of spectacles: Comments:

Signature

Fig. 16.16 The refraction values for the right eye have been filled in. The cylindrical axis has also been entered (red line). The diagram specifies the position of the cylindrical axis with respect to the eye. A perpendicular cylindrical axis (red line) corresponds to 90 degrees on the standard form.

Multifocal lenses.

Multifocal lenses.

Fig. 16.17 a and b Bifocals. c Trifocals.

Fig. 16.17 a and b Bifocals. c Trifocals.

Trifocals: These lenses include a third refractive correction between the distance and near-field portions. This intermediate portion sharply images the intermediate field between distance vision and reading range without any need for accommodation (Fig. 16.17 c).

Progressive addition lenses: These lenses were developed to minimize abrupt image changes when the gaze moves through the different correction zones of the lens while maintaining a sharp focus at every distance (Fig. 16.18).

These eyeglasses also offer cosmetic advantages. They produce well focused images in the central region but have a high degree of peripheral astigmatism. However, many patients learn to tolerate this peripheral distortion.

Presbyopic patients tolerate progressive addition lenses better when they still have only slight presbyopia and have not previously worn bifocals.

— Progressive addition lenses.

— Progressive addition lenses.

Fig. 16.18 These lenses provide a smooth transition between the distance correction (upper portion), intermediate correction (middle portion), and near-field correction (lower portion), with continuously increasing refractive power. Clear vision is limited to only the light blue area. The stronger the Add in the near field, the narrower this light blue corridor becomes. At the same time, this increases the peripheral optical distortion (dark blue) outside the corridor.

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