A Fresnel Lens was invented by French physicist Augustin-Jean Fresnel. Pronounced (fre-NELL) in scientific and lighting applications, the Fresnel Lens was originally developed for lighthouses. The large aperture and variable focal lengths allowed for a lighter weight version requiring much less material compared to earlier lens designs. because theFresnel Lens is much thinner than curved glass, more light passes through the lens and allowing lighthouses to be visible over much longer distances.
When placed in the sun, the Fresnel Lens will act as a giant magnifying glass concentrating sunlight to a small area. Over the years Dan Rojas has modified optical lenses to maximize the solar throughput increasing thermal concentration exceeding 3233 degrees Fahrenheit. This will cause wood to instantly catch on fire, zinc and aluminum to vaporize and copper to melt. Some of our more powerful lenses can boil 12 ounces of water in under 30 seconds with the aid of an evacuated solar tube and bring one gallon of water to a raging boil in under 30 minutes. One common misconception about the Fresnel Lens is that they "amplify" light. A Fresnel Lens "concentrates" or redirects light so the larger the lens, the more collection area achieved equaling more "total power." This is the reason small page magnifiers DO NOT WORK for solar applications.
The sun is always moving across the sky due to Earth's rotation so the angle of the lens must also change for maximum results or the beam will become "off axis" resulting in a drop in optical performance. A mounting stand is required for optimal angular adjustments with simpler applications like cooking and metal forging. A heliostat should be considered for professional installations requiring 100% automation of lens adjustments.
Never place your hand directly in the beam or instant, serious burns will result. Exposed surfaces and objects can heat to over 2000 degrees fahrenheit. Skin contact with these surfaces will send you straight to the ER. Never experiment without protective gear including fire retardant gloves and solar eye protection. Never EVER leave a lens in the sun unattended or you could start a fire. Never let kids play with the lens and do not cook or heat objects while pets or small children are active in your work area. The optical concentration spot illuminating lighter surfaces like concrete or shiny metal objects will exceed the lux (light output) of staring at direct sunlight. Without proper eye protection, vision loss is possible. Exposed objects like cook wear and cement blocks have retained extreme temperatures for over an hour without additional exposure. The power of the sun can be deceiving and anyone using a Fresnel Lens for solar collection should get in the habit of treating the Fresnel Lens like a stove, furnace, or blowtorch. When finished with a project, store the lens covered, preferably in a dark room or closet, AWAY from windows that experience direct sunlight. This is important to remember as annual sun positioning changes with the seasons. Used correctly, a Fresnel Lens is a wonderful scientific tool and a perfect solution to off grid cooking.