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Prisms:
The Cornerstone of Clear Vision |
Everything we do as eyecare professionals involve prisms. Yet, understanding prism theory can be frustrating. Due to the vast amount of information available, the scope of this article will be narrowed down to the more basic concepts.
Prism can be visualized as a triangle. It is comprised of two angled planes that meet at a point (apex). The thicker, opposite side is the base.
When light moves through a prism, it is dispersed, deviated, and displaced.
Dispersion is the breaking up of white light into its spectral colors. When a light ray enters a prism, each wavelength is refracted at a different rate. The longer wavelengths such as red, orange and yellow are refracted the least. The blue, indigo, and violet wavelengths on the opposite end of the spectrum are refracted the most. Common examples of this phenomenon are the prisms that are hung in windows to catch sunlight.
Any object that is seen through a prism will appear in a different place than without the prism. This is displacement. The object will always be displaced toward the apex of the prism. This is why the eye will also be naturally drawn to the same position; to see the image.
For our purposes, the strength of a prism is measured in diopters.
The power of a prism can be expressed in the amount of displacement between where the angle of deviation strikes and where it would strike without the prism.
One diopter is the power necessary to deviate a ray of light 1 centimeter from the place it would otherwise strike at a point 1 meter from the prism.
All lenses can be seen as combinations of prisms. A minus lens can be simplified as two prisms, apex to apex. A plus lens can be simplified as two prisms, base to base. The power of a lens is the result of prismatic effect; lenses can also be seen as prisms of varying powers stacked on top of each other.
Slight increases and decreases of the prism power create the curvature of the lens. The displacement and deviation of a prism is instrumental in allowing a lens to be an effective method of correcting vision.
In most cases, the optical center of a lens is placed in front of the pupil. This is the point on a lens where light moves through the lens undeviated. If this point is decentered away from the pupil, the patient will experience unprescribed prism.
The eye is very sensitive to changes in prism. It doesn't take a large amount of prism to make the patient uncomfortable.
It is important to know how much prism is present in a lens. The Prentice's Rule Formula allows this to be calculated:
dD
P = -------
10
P = the amount of prism present at a particular point,
d = the distance of that point from the optical center, in mm,
D = the dioptric power of the lens.
Unwanted base down prism can cause the wearer to feel like they are in a bowl, or a valley. People, buildings, and other vertical objects seem taller. Conversely, unwanted base up prism causes vertical objects to seem shorter, and often the patient will feel as if they are standing on a hill.
These descriptions coincide with the fact that objects are displaced toward the prism apex.
The pupilary distance of a patient is a simple measurement that should not be overlooked. If the pupilary distance is set too wide or too narrow, the patient will have to deal with excessive horizontal prism.
Floors, tables and other horizontal objects will seem to slope. The side that seems higher is the base direction. Patients will also turn their head and view objects from an angle in an attempt to decrease the prismatic disturbance.
Vertical imbalance can be experienced by patients who look too far from the optical center. Often, the complaint is that images and lines of text jump between the lenses.
For single vision wearers, the solution may be to adjust the frame and head position to keep the gaze in the center of the lens.
Multi-focal wearers face a unique challenge, as they must look through the bifocal segment to read. For these patients, there are several solutions; having a pair of dedicated eye wear for reading and another for distance is an option. Bicentric grinding (slab-off ) can also be used; Base up prism is ground on half the lens with the most minus power. The canceling of the prism alleviates the power difference.
There are situations where prism is a prescribed component of a patient's prescription.
Strabismus and amblyopia combined make up the most common types of extra ocular muscle issues in children.
Strabismus is an apparent fixation in which the non-fixating eye is turned in a different direction, causing retinal images to fall on non-corresponding points on the retina. This condition can lead to amblyopia, a condition in which one eye deteriorates.
The refractionist may use the Maddox rod to evaluate if a phoria (tendency to turn) or a tropia (definite turning) are present, and how much prism would be needed for correction.
Being that the eye will always turn toward the apex, it can be manipulated to turn in any direction by placing a prism in front of it.
A relieving prism is placed in front of a weak muscle to displace the image in the same direction that the eye turns. An adverse prism is placed with the apex over the weak muscle, exercising it.
It is important to determine how much prism a patient is dealing with between the two lenses. Problems can arise from both eyes rotating both the same amount and in the same direction, as well as rotating in different amounts and in different directions.
Patients with a significant prescription difference between the two eyes are more susceptible. If left untreated, the patient may develop fixation issues such as diplopia and amblyopia, or, at the very least headaches, eye fatigue, and visual distortion.
Prism amounts and positions can compound and cancel:
Canceling Situations
Base Up and Base Up
Base Down and Base Down
Base In and Base Out
Compounding Situations
Base Up and Base Down
Base In and Base In
Base Out and Base Out
Prism splitting is exactly that; splitting prism between two lenses. The eye in which the prism was prescribed will keep the given base direction. The remaining prism in the other eye should be in the compounding base direction.
Prism splitting should be discussed with the refractionist or doctor. There may be instances where prism splitting is not recommended.
If a patient is bothered by any unwanted prism in their lens, then correcting the error would prove beneficial. However, if they are satisfied with their current eyewear, the best course of action may be to leave it or change it only slightly.
The advanced study of prism theory can no doubt be daunting. However, comfort can be gained in that the basic facts do not change, only the uses and potential of prisms advance.
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