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Optical prism is defined as a wedge-shaped transparent object that deviates light. The light deviates, or bends, toward the base and, when you are looking through the prism, the image moves toward the apex.
Unwanted prism can cause discomfort including visual distortion, headaches, nausea and blur. It is often caused by lab errors in surfacing, laying out or edging or optician errors in measuring the seg height or PD.
Lenses Contain Prisms
Ophthalmic lenses inherently contain prisms. Minus lenses are two prisms apex to apex and plus lenses are two prisms base to base. Depending on where light passes through the lens, it goes straight or bends. The optical center is where there is no prism and light goes straight through. When light passes through at any point other than the optical center, it bends and affects the vision. This is called prismatic effect.
The Major Reference Point (MRP) of the lens is the place where the prescription is its clearest and most accurate. The Pupillary Distance (PD) is the distance from center pupil of one eye to center pupil of the other eye. The Optical Center (OC) is the point on the lens where light passes straight through and where there is no prism. Ideally, the PD and the MRP are at the same place. If there is no prescribed prism, we want the PD, MRP and OC to be at the same place.
If the prescription calls for prism, the MRP and PD are at the place corresponding to the amount of prism required in each eye. If no prism is prescribed, but the MRP and OC are not positioned at the same place in the lens, prism is induced. This happens because when the wearer looks anywhere other than the OC, he is looking through some point in the prisms that make up the lens. When the wearer’s MRP is not positioned at the OC, he is looking through the lens prisms. Because he is looking through the lens prisms, his vision is altered. The farther away from the OC that the light passes through the lens, the more prism is induced.
The way to calculate how much prism is induced is by using Prentice’s Rule. (This is an approximation and becomes less accurate in lower lens powers.)
^ = D x d ÷ 10
D is the symbol for prism diopter
D is the symbol for power of the lens
d is the symbol for decentration in millimeters
The 10 is used to change the distance values from centimeters to millimeters. Since opticians are used to using millimeters, we change everything into millimeters to better understand the math. In the case of this formula, the end result is in prism diopters.
If the power of the lens is -6.50 and the eye is looking through it 2mm away from the optical center, the amount of prism is figured by Prentice’s Rule:
^ = D x d ÷ 10
D = 6.50 x 2 mm ÷ 10
D = 13 ÷ 10
D = 1.3 or 1.3^
Base Direction
As for the direction the light bends, called base direction, we consider whether the lens is minus or plus. You must consider where the eye is looking, not where the light is striking, to determine base direction.
Our eyes are sensitive to unwanted or unfamiliar prism. The more prism there is, the more discomfort it can cause.
Excessive Base Down Prism causes the sensation of standing in a bowl. The wearer feels like he is walking uphill and vertical objects appear taller than they really are.
Excessive Base Up Prism causes the sensation of standing on a hill. The wearer feel like he is walking downhill and vertical objects appear shorter than they really are.
Excessive Base In or Out Prism causes the wearer to see horizontal objects as slanted.
Canceling prism
When each lens contains the same amount of vertical prism and in the same direction, both base up or both base down, the prism in both eyes cancels each other out and there is no effective prism in either eye.
When each lens contains the same amount of horizontal prism and in opposite directions, one base in and one base out, the prism in both eyes cancels each other out and there is no effective prism in either eye.
Compounded prism
When each lens contains horizontal prism with the bases in the same direction, the prism is compounded, wherein the prism in both eyes is added together for a total amount of OU prism.
When each lens contains vertical prism with the bases in opposite directions, the prism is compounded, wherein the prism in both eyes is added together for a total amount of OU prism.
Split prism
To even out the lens thickness that prism can cause, the amount of prism can be split in half with each half assigned to a lens. Vertical prism in the prescribed lens will have the base direction prescribed, while the other lens will have the opposite base direction.
For example, the Rx calls for 8D of prism base down OD. To split the prism evenly, the OD lens will have 4D of prism base down and the OS lens will have 4D of prism base up.
Base direction for horizontal prism will be the same in each lens. For example, the Rx calls for 8D of prism base out OD. To split the prism evenly, the OD lens will have 4D of prism base out and the OS lens will have 4D of prism base out.
Yolked prism
Yolked prism is prism in the left and right lens that have the same base direction and do not cause the eyes to converge or diverge, either horizontally or vertically, relative to one another. Yolked prism is also used in progressive lenses to make the upper portion of the lens lighter and thinner. This is called prism thinning.
Image Jump
When a bifocal wearer’s gaze moves from the upper portion of the lens to the lower portion of the segment, the image suddenly appears closer and magnified. This is called image jump. This happens because prism is induced when looking away from the optical center of the upper portion to the OC of the lower portion. The amount of image jump depends on the placement of the optical center in the segment.
When the gaze moves from the upper portion of a plus lens to the top of the segment, base up prism is created and the image moves down slightly. The prism is base up because the base of the prism in the upper part of the lens is at the distance optical center. As the gaze continues to move down into the segment, the base direction of the prism in the segment determines whether there is image jump and how much.
When the gaze moves from the upper portion of a minus lens to the top of the segment, base down prism is created and the image moves up slightly. The prism is base down because the base of the prism in the upper part of the lens is at the outside of the lens. As the gaze continues to move down into the segment, the base direction of the prism in the segment determines whether there is image jump and how much.
Anisometropia
A difference in prescription or MRP between the lenses is called anisometropia. When the wearer lowers his gaze below the distance optical center anisometropia causes Vertical Imbalance, which is prismatic effect usually felt when the difference between lenses is greater than 1.5 prism diopters. The person affected by vertical imbalance is the bifocal wearer with good vision in both eyes. If one eye is defective or blind, the difference between the lenses will not matter.
Slab-off
When anisometropia causes vertical imbalance in a set of bifocal lenses, we can have prism ground into one lens’ segment to cancel out the prism created by the different prescriptions. Base up prism equaling the amount of prism caused by the Rx is ground into the reading segment of the lens with the more minus in the distance portion. This is called bi-centric grinding or slab-off. This method is limited to compensating for a maximum vertical imbalance of six diopters.
Verifying prim
To verify prism when the prescription is known, dial in the prescription on the lensometer and place the mires at the prescribed place for the prism and spot the lens. After doing this on both lenses, measure the distance between the dots and the result should match the PD.
When the prescription is unknown, we can estimate the amount of prism by placing the higher powered lens on the lensometer stage where the PD would most likely be and noting where the mires are positioned. Then we carefully shift the glasses to place the other lens on the stage. If the mires are positioned somewhere other than in the center, make a note of the position. The positions of the mires will indicate that there is prism and approximately how much between the two lenses.
Another method used to check for prism when the prescription is unknown is to place the stronger lens on the lensometer stage and center the mires. Then evenly shift to the other lens and note where the mires are positioned. This will tell approximately how much prism is present between the lenses, but not which lens it’s in or if the prism is split between the lenses.
To verify or check for prism in a progressive lens, we use the progressive mask that covers the lens when a job comes from the lab. Under the fitting cross is a dot and we put this dot at the center of the eyepiece on the lensometer. We check both lenses at this dot and if there is no prism, the mires for both lenses will be at the same place on the reticle. If there is prism, the mires of each lens will be at different places on the reticle.
Understanding prism may be a little daunting at first, but if you take the information one concept at a time, it will fall into place and you will better understand why prism affects vision — whether we want it to or not.
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