A flat form of lens that uses concentric rings of glass instead of one large piece to focus light.

Each ring can be considered a fragment of a more complete hypothetical lens, usually convex. When all lens fragments have the same focal length, they behave as one lens.

Fresnel lenses were first created for lighthouses. The problem was that they wanted a large lens with a relatively short focal length so that the lens could be placed close to the light source. However, such a lens would have to be very thick, and the ideal traditional lens could easily end up weighing more than a ton. Because of this, it would costing a fortune in materials and labor to transport and install in a tall, thin tower. Plus, if it got cracked, you had to replace the whole shebang.

The advantages of fresnel lenses were that they were lightweight compared to their single element cousins, and the increased cost in engineering was more than offset by the decrease in material required. The modularity was a plus too, as a cracked element in the structure would only lead to a small decrease in performance, and was (relatively) cheaply replaced.

Today, fresnel lenses can be simply stamped out of thick sheets of glass or plastic. You can see plastic fresnel lenses stuck to the back of some minivans in an attempt to remove the blind spots. Glass lenses are reserved for more high-end applications, such as stamp collecting and jewelry making.

In the eye-care field, the Fresnel lens principle is used as a quick means to relieve an individual from the discomfort of diplopia (double vision) which is usually caused by a deviating eye. It is always used as a temporary measure until the underlying cause of the double vision can be established and treated.

Someone with double vision needs to have the light rays re-directed through their spectacle lenses to place the image back on the primary visual axis of the deviating eye. This means adding a prism component to the lens power. Not quite the shape of the prism on Pink Floyd’s “Dark side of the Moon” album, but imagine one which is taller and with a smaller base. Incorporating this in the lens design would normally (a) necessitate the expensive manufacture of a temporary lens, and (b) result in a thick, heavy lens possibly with image distortion issues.

Enter the Fresnel prism. This ingenious way of refracting light has been around for centuries and is applied to produce a prism which is uniformly thin and pliable. Moreover it can be stuck onto the lens of an existing pair of eyeglasses. Using rows of tiny straight prismatic ridges moulded onto a thin sheet of flexible transparent plastic, each ridge is effectively the tip of a full sized prism of a certain power. Thus a high power prism can be manufactured in a form which is only around a millimetre thick. The ridges look like grooves on a vinyl record. A reasonable level of transparency is retained.

These thin sheets of effective prism can be ordered relatively inexpensively (in the appropriate power) by the eye-care practitioner. They are then trimmed to shape and attached at the required orientation to the back surface of the patient’s own eyeglasses. Single vision is thus restored, albeit with a slightly grainy image. Meantime, the underlying cause of the double vision can be investigated.

Here it should be mentioned that any sudden onset of persistent double vision can result from a serious vascular or neurological event for which appropriate medical advice should be obtained.

Like much of the optical technology in use today, the Fresnel lens design is not new. It dates back to the 18th century when it was developed to produce large lightweight lenses for lighthouse illumination systems.

Fres`nel" lens" (?). [See Fresnel lamp.] Optics

See under Lens.

 

© Webster 1913.

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