An ophthalmic prism is a transparent optical element, bounded by two planar, nonparallel refracting surfaces. These surfaces are inclined at an angle to one another, intersecting along the thinnest edge, called the apex of the prism. The thicker edge opposite the apex is called the base of the prism. When light traverses the prism, its rays are always bent (refracted) towards the base of the prism (Figure 1).
In 1890, Charles Prentice introduced the prism diopter (∆) as a measure of the power of a prism. The power of a prism in prism diopters is equal to the displacement in centimeters (cm) of a light ray passing through the prism, measured 100 cm from the prism (see Figure 1). In other words, a prism with a power of 10∆ deviates a light ray a linear distance of 10 cm, perpendicular to the direction the light ray was originally travelling, measured 100 cm beyond the prism.
Effect on the Image
Figure 2 illustrates the effect of a prism on an image. Consider a “real” image of a point of light formed by a lens (Figure 2a). If a prism is introduced, the real image is displaced toward the base of the prism, the same as the light rays (Figure 2b). However, if one turns the light around, makes the image the object, and views it through the prism, a “virtual” image is seen. This virtual image is displaced toward the apex of the prism (Figure 2c). Therefore, although the light rays are always bent toward the base of prisms, virtual images viewed through prisms are displaced toward their apex.
Normal Spectacle Lens as Prism (Prentice’s Rule)
When a single ray of light passes through a lens, it follows the same path as if it were passing through a prism whose refracting surfaces are tangential to the curved surfaces of the lens at the points where the ray enters and exits. Thus one can imagine a simplified lens in cross section as a series of prisms, whose power increases from the center towards the edge. A plus lens, for example, can be considered as an arrangement of two prisms with their bases touching (Figure 3a), while a minus lens can be simplified to two prisms with their apices touching (Figure 3b). At the optical center of a lens, the planes tangent to the lens’ surfaces are parallel, so any ray of light passing along the optical axis experiences no deviation. In other words, a lens has no prismatic power at its optical center.
Although this is a simplification of the true nature of a lens, the refractive power of lenses can be thought of as being due to the prismatic power of the lens at various distances away from the optical center. More precisely, the prismatic power of a lens (∆) at any point on the lens is equal to the distance of that point from the optical axis in centimeters (hcm) multiplied by the power of the lens in diopters (D). This relationship is known as Prentice’s Rule (Equation ):
Δ = hcmD
Recently, it has become increasingly apparent that Prentice’s Rule is an oversimplification and, in certain situations, notably in anisometropic corrections, more complex equations are required for accuracy.
Measurement of Prism
Detection of Prism by Inspection
The presence of prism in glasses may be detected simply and quickly by viewing a horizontal or vertical edge through the lens (Figure 4). A good edge to view is the edge of a laminated countertop. If the lens—at the position of the patient’s pupil on the lens—breaks the continuity of the edge of the countertop, prism is likely present in the glasses.
Neutralization With Loose Prisms
The ability of prisms to refract light without changing its vergence makes prisms ideally suited to neutralize strabismus. Neutralization with prisms refers to optical correction of the deviation. For instance, if a patient has an eye deviation, prism can be used to correct this deviation so that diplopia is prevented and both eyes see a single image in the same place.
If two identical prisms—two prisms of the same power—are stacked together with the base-apex lines in the same direction, so that the base of the first prism is adjacent to the apex of the second prism, the combined prismatic power is zero. This method forms the basis of hand-neutralizing a prism, a method for determining the power of a prism that is unknown.
Measurement With the Lensmeter
The presence of prism in glasses can also be readily detected by means of a lensmeter. If there is no prism present in the spectacle lens, the center of the cross-line target will be in the center of the lensmeter reticule. If, however, the intersection of the target lines is away from the center (Figure 5), and the lens has to be shifted away from the normal viewing position of the patient to center it in the reticule of the lensmeter, then one knows that prism is present in the spectacle lens. Note that if the intersection of the target lines is decentered horizontally, horizontal prismatic power is present, whereas a vertical shift indicates the presence of vertical prism power, with the shift in the direction of the prism base. Figure 5 shows a vertical displacement of the target lines away from the center, indicating the view that is seen when vertical prism power, base-up, is present.
Occasionally the prism power is so high that the displacement exceeds the scale of the lensmeter. If this occurs, adding a neutralizing prism of a known power anywhere between the spectacle lens and the lensmeter telescope may help to bring the lines back onto the scale.
Especially with high-power lenses and with anisometropic corrections, it is always best to mark the spectacle lens with either a water-based marker or a triangular piece of tape at the center of the patient’s pupil, and to measure the prism at this point.
Problems With Measurement of Vertical Prism
Measure Prism at Center of Patient’s Pupil
The prismatic power prescribed should act in the line of sight of the patient when looking straight ahead. In this way, the effective amount of prism is the same as the prescribed prism. Unfortunately, there is no universal standard reference location at which to measure vertical prism. Many methods currently employed do not take into account the vertical position of the glasses on the patient’s face, so that the effective prism amount can be different from the amount of prism prescribed and dispensed.
A pupil-based method is recommended not only when measuring prism, but also when dispensing glasses and when verifying the prescription of an existing pair. This takes into account the vertical position of the glasses on the patient’s face. This method involves marking the position of the patient’s pupils on the lenses while the patient is fixing on the ipsilateral eye of the examiner through the distance portion of the lens.
When changing the amount of vertical prism in a prescription, it is best not to state the resultant amount, but rather to ask the optician to add the desired amount of additional prism to whatever the optical lab measures to be in the glasses already.