Ever see a lighthouse beam and wonder how it shines so brightly across vast distances? Or perhaps you’ve used a thin plastic magnifier to enlarge tiny text. You may thank a Fresnel prism lens for this magic.
A prism is a wedge-shaped piece of transparent material cut with precise angles and plane faces. The path of light entering through a prism will be affected / changed as it passes through the prism and then exits the prism. How much change will be dependent upon the prism material and the prism angles and the angle the light first enters the prism surface. Fresnel prisms are made from a thin transparent plastic sheet consisting of multiple angular grooves on one side. These prisms are much lighter than the regularly used prisms of the same power. It can be thought of as a series of tiny plastic prisms placed on a plastic platform that is thin and adjacent to each other in a format that the base of one is attached to the apex of the next prism. The magnification effect seen with conventional prisms is minimal with Fresnel prisms.
After 1960, the Fresnel principle, which existed for more than 150 years, was used for the first time to manage strabismus. In 1970, the Optical Scientific Group from San Rafael, California designed a combination of a series of Fresnel press on prisms. These prisms are made up of plastic membranes with powers ranging from 0.5-30 prism diopters. The Fresnel prism is derived from the concept of hand-ground lenses prepared by the physics and French specialist Augustine Fresnel in 1921.
Initially, Fresnel prisms were intended for use in lighthouse beacons. Prisms have been used extensively in the field of ophthalmology, especially related to neuro-ophthalmology and strabismus. Prisms are used extensively to increase the binocular field, relieve diplopia, or expand the field of vision. Fresnel prisms are thin transparent plastic membranes with multiple tiny prisms attached from base to base. Augustin Fresnel articulated the optics of Fresnel prisms.
The angle of refraction of a prism depends upon the angle of the surface and the refractive index of the material of which the prism is made. The angle of refraction is independent of the thickness of the prism. A Fresnel prism offers the advantage of being lightweight and easy to handle. The most common use is to temporarily relieve the diplopia in cases like nerve palsies secondary to trauma, blowout fractures, decompensated phoria, divergence insufficiency, or convergence insufficiency.
Fresnel prisms are most commonly used as temporary prisms. Traditionally, prisms have also been used in patients with stroke and homonymous hemianopia. The aim is to shift the peripheral image towards the central retinal meridian.
How Does it Stay on the Lens?
The 3M Press-On™ prism is applied to the inside of the carrier lens with just water.
The 3M Press-On™ prism is attached to a border which identifies the base, if the border is discarded the base may identified easily by running a finger across the prisms, it will slide smoothly towards the base and jagged away from the base.
The smooth side of the prism is attached to the lens.
Fitting a 3M Press-On™ Prism
- Holding the prism in place against the inside of the lens with the base oriented as required.
- Draw on the rough side of the prism around the shape of the lens with a non-permanent marker. It is best if the lens can be removed from the from frame to ensure the prism alignment is correct.
- Cut the prism using a small pair of scissors or Exacto knife.
- Cut the prism a bit smaller than the lens so no portion will overlap the frame and be pulled off.
- Ensure the carrier lens is clean a free of dust and oil. Do not use cleaner with alcohol as this will damage the prism. There are dish soaps that contain alcohol.
Applying a 3M Press-On™ Prism
- Put a few drops of water on the inside of the lens where the Fresnel prism is to be placed. Push the smooth side of the Press-On™ prism down on the water.
- Using your thumb squeeze out any excess water and air bubbles and gently pat dry.
The prism may appear cloudy or hazy for the first 24 hours until all the water has dissipated – it may be easy to slide during this time, so care is encouraged. - After it has dried the prism will no longer move. To remove the prism for the carrier lens, use the edge of your fingernail to carefully pull away.
The Fresnel Press-On™ prism can be cleaned using a non-alcohol lens cleaner and a soft micro-fiber cloth. Pull the cloth in the direction of the prisms vs against.
What Powers are Available?
The 3M Press-On™ Prisms are available in 17 Diopter Powers: IMAGES MISSING, may be on site
I found a few I put in folder.
