By Mathew Musladin, ABOM 

Prism. What is prism? So mysterious! Everybody in the optical community seems to have their own particular spin on prism.

Prism can be a physical thing. It can be an abstract effect. It can be the name of an apartment complex in San Francisco! The discussion for this article will be prism in relation to ophthalmic dispensing.

In my experience in the lab, I've had final inspectors say this lens has prism. Those marking lenses ask how they can decenter for prism. Some ask about ground-in prism. For the patient, it all amounts to the same thing. And it's not some additive that people think the lens has in terms of optical dispensing. It's an effect. The effect is simply image displacement. What does the patient see through their line of sight through that lens?

Any line of sight other than through the optical center of a lens with dioptric power produces the effect we call prism. We can conceptualize this by using triangles. The bottom of a triangle or the base is the direction to which the light is refracted, and the apex or opposite corner of the triangle is where the image is displaced. Think of it as an arrowhead. The archer pulls the arrow back bending the bow and the bowstring. Light is also bent back toward the prism base. When the archer releases the arrow, it goes toward the point or the apex of the arrowhead. Similarly, prismatic effect directs the image towards the apex of the prism.

The primary issue would be regarding the verification of the glasses before they're dispensed. If the PD does not line up with the optical centers of the lens, there will be perceived image displacement by the wearer of the glasses. This may cause fusion problems where the brain cannot create a single image from the input of two eyes. The greater the power, the more important it becomes to have the patient’s line of sight coincide with the optical center of each lens in order to avoid unwanted prism.

So, what's this thing called prescribed prism all about? If a doctor prescribes prism, he wants image displacement so that an image can be fused in the brain. Usually this is due to an intraocular muscle imbalance that does not allow the eyes to fixate. Therefore, it is helpful to understand where the base of the prism is on the lens before you put it in a lensometer. The thickest part of a lens is the base for prismatic effect. Remember, plus lenses will have the center thickness as the prism base and for minus lenses it will be the edge.

What does this mean for the dispensing optician? Be practical! Guide customers with higher dioptric powered prescriptions toward smaller eyewires and frame PD’s that are closer to the patient’s actual PD. Also, suggest bridges with adjustable nose pads. Above all, be deliberate when you are verifying every pair of glasses you dispense. The “effect” will be greater satisfaction for your clientele and your practice!