Ophthalmic prisms have been used in the treatment of binocular vision problems for more than 100 years. In mid 1960’s, the 150 years old Fresnel principle (introduced by a French physicist Augustin – Jean Fresnel) was applied to ophthalmic prisms for first time. The first ophthalmic Fresnel prism, known as “Wafer” prism, was molded of an acrylic resin, making it much lighter & thinner than the corresponding powers of the conventional prisms, thus extending the useful range of prism powers. However, this wafer prism could not be incorporated directly to a spectacle lenses, rather, it had to be clipped on or taped over the patient’s regular correcting glasses or used in a separate frame. Though, it remains a remarkable optical and technical achievement, it was used mainly in trial testing in the United States. Its cosmetic appearance and the difficulty to mount it to the patient’s spectacles prevented any large scale use as a therapeutic lens. In 1970, the press on prism, Fresnel Principle prism molded in flexible plastic, was introduced to optometrists and ophthalmologists. This thin, light membrane had a advantage of confirming and adhering to a smooth surface and thus could be affixed directly to a plano or base-curved correcting spectacle lens. This characteristic together with its construction using smaller prisms with less separation, improved considerably its utility and cosmetics. Thus, it appeared that at last there was an ideal therapeutic prism.
Fresnel prisms are made of plastic, consisting of a set of concentric annular rings called the grooves. In each grooves, the overall thickness is decreases compared to an equivalent conventional prism. Fresnel prism design allows a substantial reduction in thickness and thus mass and volume of materials, at the expense of reducing image quality and hence this may cause reduced visual acuity. Fresnel prisms are placed onto glasses to overcome problem of diplopia. These prisms try to join the two images when double vision is present, to make just a single image. This plastic prism is cut in the shape of glasses and placed on the back surface of the lens. It is fixed on by a layer of water between the lenses and the prism. It is placed on the lens with the lines and grooves going in a perpendicular direction, depending on the types of double vision. This prism is usually fitted on one side of the glasses, in front of the eye with the weakest vision. Occasionally, a prism may need to be fitted to both lenses of glasses. Initially, the prism was made in powers ranging from 0.50Δ to 15Δ. In 1971, this range was extended to 30Δ. Not only this prism is flexible but it also has advantage over the original wafer prism in that, it is thinner. Even at maximum power, it does not exceed 1 mm in thickness.
How to fit Fresnel Prism to Glasses
These prisms are attached to a boarder which tells about the base of prism
Detect the smooth side of the Fresnel prism. This is the side that attaches to the lens
If prism is to be fitted base out, place the base of prism on the inside of the lens with base near the arm of glass. The lines of the prisms will run vertically.
If prism is to be fitted base in, place the base of prism on the inside of lens with base near the nose pads. The lines of the prisms will run vertically.
If prism is to be fitted base down, place the base of the prism on the inside of lens with base near bottom rim of the lens. The lines of prisms will run horizontally.
If the prism is to be fitted base up, place the base of the prism on the inside of lens with the base near the top rim of the lens. The lines of prisms will run horizontally.
Holding the prism in place against the inside of the lens, draw on the rough side of the prism around the shape of the lens with non-permanent marker.
Carefully cut the prism out using a scissor to fit the lens.
Place some amount of water on the inside of the spectacle lens where the Fresnel prism is to be placed and push the smooth side of the Fresnel prism down onto the water.
Squeeze out any excess water and air bubbles and dry with a damp cloth.
How to clean the spectacles with attached Fresnel Prism
Fresnel prism can be cleaned as normally as clean your spectacles with a soft, dry cloth. You can use a soft tooth brush or nail brush to gently remove dust from the ridges of the prisms. Occasionally, you can clean your prism and glasses with water. You can remove prism by placing finger nail between the prism and glasses and prising it off, then washing it with warm water and soap.
How to replace the removed Fresnel prism after cleaning
It is essential that the lines on the prism are in the same direction as when the prism was fitted by the optometrist/orthoptist and that the prism is replaced on the same lens of the spectacle.
Whilst holding the spectacle glasses in shallow water or under gently running tap, press the smooth side of prism to the inside of the correct lens of glasses.
Remove the glass from water, check the lines of prisms are correctly positioned then squeeze out air bubbles under the prism with the help of dump cloth.
Affect of Fresnel prism on vision
There may be a slight blurring of vision in the eye looking through the prism, but with both eyes open one should have comfortable single vision. It may cause slight rainbow effect when looking at lights. This is caused by the prism splitting up the light into different colors. This prism will help to get single vision when looking straight ahead, but may still have double vision when look in different directions. So try to move head to look in different directions, rather than moving your eyes.
Author Sarbind Kumar Yadav, M.Optom Senior Consultant Optometrist Ramlal Golchha Eye Hospital Foundation Biratnagar, Nepal