What Is a Fresnel Lens

A Fresnel lens is a type of lens invented by French physicist Augustin-Jean Fresnel. Originally developed for lighthouses, the design enables the construction of lenses of large aperture and short focal length without the weight and volume of material which would be required in conventional lens design. The conventional magnifying glass lens is thick in the middle and thin at the edges. It would not be very easy to make a big magnifying glass lens because it is thick and heavy. A Fresnel lens overcomes this problem. It is flat on one side and ridged on the other. The basic idea behind a Fresnel lens is simple. Imagine the lens is sliced into a hundred concentric rings. Each ring is slightly thinner than the next and focuses the light toward the center. Now modify each ring so that it's flat on one side, and make it the same thickness as the others. To retain the rings' ability to focus the light toward the center, the angle of each ring's angled face will be different. Now if you stack all the rings back together, you get a Fresnel lens.

History

The earliest stepped-surface lens was suggested in 1748 by Count Buffon, who proposed to grind out material from the plano side of the lens until he was left with thin sections of material following the original spherical surface of the lens. The Fresnel lens was invented in 1822 by Augustin Jean Fresnel (1788–1827), a French mathematician and physicist. Fresnel’s original lens was used in a lighthouse on the river Gironde; the main innovation embodied in Fresnel’s design was that the center of curvature of each ring receded along the axis according to its distance from the center, so as practically to eliminate spherical aberration. Fresnel’s original design, including the spherical-surfaced and aspheric-surfaced central section. The early Fresnel lenses were cut and polished in glass – an expensive process, and one limited to a few large grooves. In the early 1900’s, only weak incandescent oil vapor lamp is equipped in the lighthouses before the invention of high brightness light sources. Fresnel lens with large aperture and low light absorption was especially important in this case. Until 1950’s, quality Fresnel lenses were made from glass by the same grinding and polishing techniques developed in 1822. Cheap Fresnel lenses were made by pressing hot glass into metal molds; because of the high surface tension of the glass surface, Fresnel lenses made by this way lost the necessary detail, and were poor indeed. In the past forty years, the advent of optical plastics, compression and injection molding techniques, and computer-aided manufacturing have significant improved the optical quality and broader the applications of Fresnel lens. Modern computer-controlled machining methods can be used to cut the surface of each cone precisely so as to bring all paraxial rays into focus at exactly the same point, avoiding spherical aberration. Better still, newer methods can be used to cut each refracting surface in the correct aspheric contour.

Theory

A Fresnel lens replaces the curved surface of a conventional lens with a series of concentric grooves. The grooves act as individual refracting surfaces, like tiny prisms when viewed in cross section, bending parallel rays in a very close approximation to a common focal length. Because the lens is thin, light lost by absorption is negligible. Fresnel lenses are a compromise between efficiency and image quality. High groove density allows higher quality images, while low groove density yields better efficiency. The Fresnel surface model is used to simulate flat surfaces which have been etched to have a spherical (or optionally aspheric) profile on a small scale. The surface intercept is determined by computing the intersection of the incoming ray with a plane. Once the plane intercept points are found, the surface is then treated as spherical (or aspherical) for the purposes of refraction into the next medium. This is only an approximation to a real Fresnel lens, however. The real Fresnel lens has grooves which may alter the exact intercept point. The model used here is adequate for Fresnel lens which has fine grooves (the groove depth is very shallow compared to the aperture).

Applications

Fresnel lenses are most often used in light gathering applications, such as condenser systems or emitter/detector setups. Fresnel lenses can also be used as magnifiers, projection lenses and LCD screen; however, due to the high level of image, this is not recommended.

Benefit

Compared to conventional glass lens, the Fresnel lens is much thinner, lighter, larger aperture, passing more ray and lower cost.

Material

Optical plastics are widely used to make Fresnel lens. PMMA is the best material for fresnel lens as magnifier. It is a good general-purpose material in the visible. Its transmittance is nearly flat and almost 92% from the ultraviolet to the near infrared. Its other physical properties, as rigidity, service temperature, weatherability, are very fit for us to manufacture optical lens.