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).
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).
Figure 1-3 Positioning of prisms. A, Prentice position. The prism is held with its back surface perpendicular to the line of sight. Use this position for glass prisms. B, Minimum deviation position, MAD. The prism is held such that the line of sight makes an equal angle with 2 faces of the prism. C, Left: glass prisms should be used in the Prentice position. Right: plastic prisms should be held in the frontal plane position. The prism is held with its rear surface in the frontal plane (perpendicular to the direction of the fixation object). This position approximates the minimum deviation position, which is difficult to estimate.
(Parts A and B are courtesy of Edmond H. Thall, MD; part C is an illustration developed by Edmond H. Thall, MD, and Kevin M. Miller, MD, and rendered by C. H. Wooley.)
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.
Figure 1-4 Prism power is not additive. A, The power of 2 prisms in contact is not equal to the sum of the powers of the individual prisms. The resulting deviation is much larger. Prisms should never be combined in this way. Look at the interface between 2 stacked glass prisms: B, with prism 1 in the Prentice position, the light ray is perpendicular to the first surface of prism 1; with prism 2 nowhere near the Prentice position, the light ray enters at an angle far from perpendicular.
(Part A developed by Edmond H. Thall, MD. Part B from Irsch K. Optical issues in measuring strabismus. Middle East African Journal of Ophthalmology. 2015;22:265–270.)
Figure 1-5 Definition of a prism diopter. One prism diopter is a deviation of 1 cm at a distance of 1 m from a prism. This prism is a 15∆ prism, so light is deviated 15 cm toward the base, measured 100 cm away from the prism.
(Courtesy of Kristina Irsch, PhD.)
Figure 1-6 Effect of prisms on image position. A, Original real image formed by a lens. B, Base-down prism added. Because light passing through a prism is always refracted toward the prism’s base, the original real image is also displaced toward the base, in this case downward. C, By turning the light around, we see that a virtual image viewed through a prism is always shifted toward the apex of the prism.
(Reproduced from Guyton DL et al. Ophthalmic Optics and Clinical Refraction. Baltimore: Prism Press; 1999.)
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.