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Fresnel lens: from lighthouses to multimedia spheres

In the old days, approaching the shore for sailors was the most dangerous part of their journey. Due to unfavorable climatic conditions, shoals or coastal rocks could cause a shipwreck. Sailors saved the sailors, the best navigational designs of that time. For a long time, their fires were lit by bonfires, then kerosene lamps served as light sources until they started using electricity. In the 19th century, the light that saved life became the Fresnel lens, making the light of the beacon most vivid and visible from afar.

A composite complex lens was created by Augustin Fresnel, a French physicist who created the wave theory of light. The Fresnel lens is made up of individual small thicknesses of concentric rings adjacent to each other and forming a cylinder with a light source inside. In the section, the rings have the form of prisms. Each of the rings collects light into a parallel narrow beam of rays, diverging from the center. When the cylinder rotates around the light source, the light rays extend to the horizon. The color of the rays, their number, the time interval between them constitute a special unique handwriting of the lighthouse. A summary of the characteristics of the various lighthouses was on board ships, and it was on it that the seamen recognized the lighthouse in front of them.

Fresnel lenses mounted on lighthouses have become an important step in equipping them with powerful light sources. These complex composite lenses allowed to increase the concentration of luminous intensity to 80,000 candles. Before Fresnel's invention, it was possible to focus the light of a burning wick or lantern by placing the lantern in the focus of a collecting lens of a sufficiently large diameter or a concave mirror. For these purposes, a single-piece optical element of a large size was needed, which under the influence of its own gravity could burst. Therefore, dozens of concave mirrors were used, in the focus of each of them there was a separate lantern. This decision was uncomfortable.

The composite Fresnel lens helped to achieve an increase in the intensity of light, its concentration in a given direction. The assembly of individual optical elements did not reflect light, but worked on the lumen, rotating around a light source radiating in all directions, constant in intensity.

Since then, Fresnel designs remain an unsurpassed technical device used not only for river buoys and lighthouses. In the form of Fresnel lenses, glasses of various signal lights, traffic lights, car headlights, and details of lecture projectors were first made. Then loops were created in the form of rulers made of transparent plastic, with unobtrusive circular grooves, each of which was a miniature ring prism, and as a whole they were a collecting lens. The resulting lens is used as a magnifying glass to magnify an object, like a camera lens, creating an inverted image.

Over time, the scope of Fresnel lenses has significantly expanded. It includes the development of photographic equipment, various lighting devices, sensors for tracking security systems, an energy concentrator for solar collectors, mirrors used in telescopes. Optical properties of lenses are also used in the field of multimedia. Thus, DNP, the largest manufacturer of high-tech projection screens, creates Supernova screens on the basis of lenses. And in the back projection screens not only the Fresnel lens is used, but also other optical technologies, which allows to obtain the unique means of display.

Depending on the field of application, lenses can have different diameters, differ in type. Two types of lenses are known: annular and belt. The first are designed to direct the flow of light rays in one direction. Ring lenses have found application in manual work with small details, having superseded usual loops. Belt lenses that can transmit light beams in any given directions are used in the industrial sector.

The Fresnel lens can be positive (collecting) and negative (scattering). A negative polyvinyl lens with a short focus significantly increases the field of view. It is known as a Fresnel parking lens. Expanding the viewing angle, which it gives, allows you to see the obstacles that are below the car, not included in the field of view of the side mirrors or the rearview mirror. Such a lens greatly facilitates maneuvering when parking, towing a trailer and reversing, allowing you to avoid running into playing children, animals or other objects.

The Fresnel lens has become a multifunctional tool, its invention has played an important role in the development of the technological sphere.

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