Fresnel Lens

May 17, 2013

in Energy

A fresnel lens is a concentrating or magnifying lens that uses a series of concentric ridges or grooves that focus light. Each ridge/groove acts as a magnifying lens, bending light toward the center and compounding the concentrating power into a smaller area than a simpler smooth lens. Small fresnel lenses are often made from plastic and are inexpensive, but historically, large glass fresnel lenses were used to focus the light from lighthouses. The fresnel lens is named after Augustin-Jean Fresnel, a French optical physicist who contributed to the development of the first lighthouse lens around 1823. Fresnel lenses are in common use in projection devices (TV, overhead projectors), headlight and taillight lenses in cars, and many other light control applications.

In a survival scenario, a cheap, pocket sized fresnel lens can be used as a firestarter, as long as strong sunlight is available. A plastic lens about the size of a credit card that costs less than a dollar can easily be included in any survival gear bag or even carried in a wallet or pocket. Larger lenses can be used to cook, heat water or even boil water to purify or distill it.

Water is mostly transparent and even concentrated sunlight passes through clear water without much thermal transfer. Using a concentrating lens to heat water requires focusing the lens on an object that can absorb the heat and transfer it to the water. This can be accomplished by placing a water container on top of a dark metal object that is heated, or by placing a dark metal object inside the water and heating it. The best way will depend upon the materials available and the working environment.

Solar ovens and solar stills can be made more effective with the addition of a fresnel lens. But the lens only concentrates solar energy from the area of the lens into a smaller area and does not add any additional energy. For cooking, a slower, more diffuse heating process may be better than heating a small area faster and if the lens is not bigger than the solar oven, it may not improve the process.

It’s important to know that common fresnel lenses are made in two main categories. Linear lenses focus light into a band or bar shaped area and spot lenses focus light into a spot. A spot lens is better for the purpose of starting fires, while a linear lens might be useful for heating water in a pipe. listing of fresnel lenses:

WARNING – concentrating a large amount of solar energy into a small space may create danger from strong light to eyes and danger from moving focal points that can start fires. It may be necessary to use protective eyegear. It is wise to not leave concentrating lenses unattended.

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