You are probably already familiar with dispersing prisms that produce rainbows or spectra. Refractive index varies with frequency (or wavelength) of light, a phenomenon known as dispersion (discussed in Chapter 2). When light containing a mixture of frequencies traverses a dispersing prism, each frequency is deviated by a different amount, producing a spectrum. Ophthalmic prisms help minimize the separation of colors by using materials that have nearly the same refractive index for all frequencies so that all the light is deviated by essentially the same amount.
When a ray traverses a prism, the ray is deviated in accordance with Snell’s law. The same prism can produce a range of deviations. The angle of deviation is greatest when the ray strikes one face of the prism at normal incidence (the Prentice position, Figure 1-3A). The angle of deviation is least when light passes through the prism symmetrically (the minimum deviation position, MAD; Figure 1-3B).
Prism Power
Prisms are labeled with a prism power—its strength, or amount of deviation produced as a light ray traverses the prism. That labeled power is correct only if the prism is positioned in front of the patient in a manner consistent with its labeling. Glass prisms should be held in the Prentice position and plastic prisms or plastic prism bars in the MAD position. You can approximate the latter by holding the prism with its back surface perpendicular to the direction of the fixation object, which for distant objects corresponds to the frontal plane position (Figure 1-3C).
The power of prisms combined with surfaces adjacent is not additive (Figure 1-4A). To verify this, look at the interface between 2 such combined prisms (Figure 1-4B). Notice, however, that the net effect of 2 prisms placed over the 2 eyes separately is additive. It is thus preferable to split the prisms between the 2 eyes when you measure large strabismic deviations.
The angular deviation a prism produces is measured not in degrees or radians but in prism diopters. A prism diopter is the number of centimeters of deviation at 100 cm from the prism (Figure 1-5). This is equal to 100 times the tangent of the angle of deviation. The prism diopter is indicated by a delta symbol (∆). Thus, 15∆ is a deviation of 15 cm at 1 m.
A prism deviates light toward its base. Therefore, the virtual image that the eye sees is shifted toward the apex of the prism (Figure 1-6).
The orientation of a prism is designated by its base, as base up, base down, base in, or base out. A patient may have both a vertical and a horizontal deviation, in which case prism power adds as vectors. For instance, a 4∆ base-out prism combined with a 3∆ base-up prism in front of the right eye produces a net 5∆ base at the 143° meridian, base up-and-out. (Meridians are discussed in the Quick-Start Guide.) Clinically, it is rarely necessary to be concerned with this detail. You can prescribe the prism by specifying the individual base-up and base-down powers; the optician will perform the calculation. However, be aware that the lens will be ground with a single prism at an orientation that is neither base up nor base down.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.