FRESNEL PRISM

Fresnel prism varies in construction and thickness compared to normal lenses and prism .They are highly versatile and are very useful in certain specific circumstances. A Fresnel prism attempts to circumvent thickness by building a tower of small, wide prisms .For better understanding, Imagine cutting off the tops of a large number of equally powered prisms and gluing them, one above the other, onto a thin piece of plastic. They are cut and easily applied to the patient’s spectacles lenses. They are available in power up to 40 prism diopters. Most often, Fresnel prisms are thought of as temporary, but because of their thin profile, patients often tend to continue with the Fresnel prisms rather than grinding an ophthalmic prism into their lens. A Fresnel prism is only 1mm thick.

Pros of Fresnel prisms:
There are several advantages of a Fresnel prism .first, it is very thin and extremely light weight .it is flexible and can be applied to an existing spectacle lens without an in house optical laboratory.
Conventional prisms have a large increases in thickness from apex to base a high powered prism is troubled by magnification differences and changes in power across the lens. Although Fresnel prisms do not eliminate this problem, they do reduce magnification difference considerably.
Fresnel prisms are easily removed and cleaned with warm soapy water, then reapplied.

CONS OF FRESNEL PRISM:
Highly powered prisms will cause a slight decrease in visual acuity .most of this is due to the chromatic aberration and distortion associated with prisms. Fresnel prism also cause a slight loss of visual acuity caused by reflections at the prisms facets, especially under certain sources of illumination.
They do tend to degrade and get brittle over time, so they need to be replaced periodically.

WHEN TO PRESCRIBE FRESNEL PRISMS?
Because of its thickness advantage, Fresnel prism is especially useful for high amounts of prism where normal prism can be prescribed till 6 prism diopter only in spectacles.

SECTORAL APPLICATION:
A partially paralyzed extra ocular muscle may result in a different amount of prisms needed for different directions of gaze. Patient usually reports with either vertical or horizontal .some may with combination of both (torsional diplopia).
It is always advised to do a trial before prescribing Fresnel prisms. Trail kits are available which will help us to assess the outcome with Fresnel prisms.
In vertical diplopia cases, patient may require a correction for vertical imbalance .Fresnel prism can be applied to existing spectacles to see if a vertical imbalance correction of a certain amount will be helpful before it is actually ordered.
If the patient complains of diplopia at near only, then Fresnel prisms can be cut accordingly and applied to the lower portion of the lenses only.

FRESNEL PRISM AS A FIELD EXPANDER:
A person may have a visual filed defect where the right half of the visual field is blind for right and left eyes. This defect is known as homonymous hemianopia. Fresnel prism can be used to increase the patient field by applying them to the peripheral sides of the lens. Fresnel prisms are placed on the back side of the lens with the base away from the center. The concept is to have the patient look into the prism, which shifts the image of the view towards the center, reducing the distance the patient has to move their eyes to locate an object.
There are several techniques using Fresnel prisms. The first is to apply base out only to the temporal side for hemianopia. This brings objects centrally from the periphery approximately 1° for every 2 prism diopters of Fresnel prism .for example, wearing a 40 prism diopter lens would expand field by 20°. the patient’s scanning movement is therefore reduced by an equal amount. the nasal prism will be base in, the temporal prism base out, the inferior prism base down and the superior prism base up. Decisions for the use of each will be determined by the specific needs of the patients.
These spectacles will often only be used for ambulation and not for any near or reading tasks. If a patient has good scanning techniques, prism may interfere and cause confusion.

PRISM TRIAL
Have the patient view a distant object. Introduce an opaque index card into the temporal side of the field until the patient can first detect it .This indicates where the central edge of the prism should be applied.
Apply the prism base out repeat the procedure for nasal field, then the inferior and the superior fields. Have the patient scan the room and adjust the prism to give the patient a comfortable central corridor .the patient should be encouraged to make eye movements into prism to experience the change in field position and then return to the central corridor of the lens. The process is then repeated on the second eye as needed and adjusted accordingly to resolve any binocular conflicts. Train the patient to use the prism while performing both near and distance tasks. A home trial will be the next step, with adjustments made at the follow up after a week or two of wear. Training and wearing time are considered good predictors of continue use and satisfaction.
Problems with Fresnel prism can happens when inappropriate placement within remaining field of the patient, visual acuity loss through the prism ,glare through the prism, and difficulty in keeping the prism clean.
Fresnel prisms are also used for enhancing the cosmetic appearance in blind or prosthetic eye. In case of nystagmus they helps in slow down the movement.

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Location

Mahathma Eye Hospital Private Limited
No. 6, Seshapuram,
Tennur, Tiruchirappalli-620 017.
Phone : 91-431-2740494 , 2740494, 2741198

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93447 77411



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