Prism can be a little complicated and hard to understand. I will assure you that when you are done reading this article, you will get quite the understanding regarding prism correction.
First off…What is prism?
Prism can be used to correct vision for an individual whose eyes are not perfectly aligned as with, for example, a patient with strabismus. When the eyes are not aligned, the right and left eye see different images resulting in blurred or double vision. Sometimes the brain can even “shut off” one eye, in an attempt to remedy the vision, resulting in monocular vision and loss of depth perception. Prism can often be used to align the images seen by both eyes, so the eyes can fuse or see the same image, restoring visual clarity and depth perception.
Prism, like lens power, is also measured in diopters (Δ), but measured differently. One diopter of prism is equal to the prism required to divert a ray of light 1 cm from its original path, measured at a distance of 1 m from the prism.
As important as the amount of prism, is the direction of prism. The prism must displace viewed objects in the proper direction to achieve the desired visual correction. Prism direction can be specified in two ways, either using the prescriber’s method or the 360 method.
The prescriber’s method specifies the direction if the prism in terms of the base, using base-up, base-down, base-in, and base-out (base-in referring to the direction of the nose and base-out referring to the direction of the temple). Often prescriptions will include a combination of directions to achieve the proper resultant prism. For example: 2 Δ base-in and 1 Δ base-up.
Labs however, use a 360° or 180° method of describing base direction. Using the 360° method, when a lens is viewed from the front, a prism with a base direction to the right (base-in for the right eye and base-out for the left) becomes 0°. Likewise, a prism with a base direction to the left (base-out for the right eye and base-in for the left) becomes 180°. Base-up then becomes 90° and base-down 270°. Using this method, prism directions other than base-in, base-out, base-up, and base-down can be specified at a single angle e.g. 2.7 Δ base 64°. The 180° method is similar, however, as the name suggests, only 180° are used, consequently, an up or down direction must also be specicfied.
Prism specified in using the presciber’s method consisting of multiple base direction components can easily be converted to the 360° or 180° methods by using a prism chart or simple trigonometric formulae.
∆ is the symbol for Prism
BI (BASE IN)
BO (BASE OUT)
BU (BASE UP)
BD (BASE DOWN)
Prism by Decentration
To better understand why there is induced prism, the cross-section of a plus lens can be likened to two prisms base-to-base, as the lens is thicker in the middle and thinner at the edges. Likewise, a minus lens can be likened to two prisms apex-to-apex, thinner in the middle and thicker at the edges.
This induced prism can actually be used to the advantage of the lab when prism is called for in a prescription. If the lens power is sufficient, to induce the prescribed prism, the lens can simply be cut off-center to achieve the required results. This is known as prism by decentration. If the power is insufficient, however, the prism must be cut into the surface of the lens.
A simple equation can be used to calculate the prism induced by decentration. Prentice�s rule states that prism in diopters (Δ) is equal to the decentration distance (c) in centimeters multiplied by the lens power (D).
Δ = cD