LENS
TIPS
Understanding and
Utilizing Prism
By Alex Yoho, ABOM
Prism is a subject that is often misunderstood. In optical terminology, it simply means at the point a patient looks through the lens. It is wedge-shaped and has a base or thick edge, and an apex or thin edge. Prism is used in lenses to deviate an image for the patient. For instance, in a case with double vision because of a weak eye muscle, a prism might be placed with its base over the weak muscle to bring the images of each eye to fusion.
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Aligning Optical Centers |
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When spotting the optical centers of a pair of eyewear, it is important to check the strongest lens first. The weaker a lens is, the less the target will move in the lensmeter, so it is possible to get a false reading of prism when checking from weakest to strongest. If the PD is not as specified, it is necessary to determine just how much prism will be induced. The easiest way is to use a progressive layout chart and mark the patient�s PD on the lenses. Then put the lenses in the lensmeter and raise the inker to touch the spot you made with the chart (this will make sure it is exactly centered). The amount of prism showing in the lensmeter at that point is the amount of prism induced. A.N.S.I. standards allow 1/3 prism on each eye tolerance in the horizontal plane and 1/3 vertical disparity. |
Prism can also happen by accident when lenses are fabricated. The amount of prism in a lens or between a pair of lenses is determined by how much there is as the patient is looking at the distance of intended use. This is important to comprehend since all lenses with power have the capability to induce unwanted prism. The amount is proportionate to the lens power and the deviation of the optical center of the lens from the specified pupilary distance.
As you consider the shape of a lens, it is either thin in the center and thick on the edge for a minus lens, or thick in the center and thin on the edge for a plus lens. In either case, when the lens is placed slightly off center from the pupil, prism is induced with its base in the direction of the thicker part of the lens.
As you might guess, most of the time a person is not looking directly through the optical center of the lens. Often, both lenses are close enough in power that the amount of prism and base direction are the same, so it does not induce a disparity between the eyes and no discomfort is noticed.
If the corrections differ in each eye, the stronger lens will induce prism faster as the eye deviates away from the optical center of the lens. The patient will experience discomfort if this is not addressed. As the eyes deviate from the optical center of the lenses, the lens with more minus will encounter base down prism. If the other eye is a lesser minus or a plus lens, it will encounter less induced prism, so the disparity for the pair of eyes will cause discomfort and, at times, vertical double vision.
This is the reason for ordering a slab-off lens�a lens with a compensating prism ground in the near area only. When there is minus in one eye and plus in the other, a slab-off is nearly always necessary.
Since it is so important that the eyes are aligned with the optical centers of the lenses (see below), particular attention should be paid to the PD. When checking at the patient�s PD, you should be within the closest circle to the center in the lensmeter�s reticule. EB
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