Fresnel

Augustin Jean Fresnel developed the first Fresnel lens in the early nineteenth century. Today, his invention can be found in optical systems where design criteria call for light weight or small size.

Unlike traditional lenses, Fresnel lenses do not employ smooth-surface contours to focus rays of light. Instead, the surface of a Fresnel lens is molded into many circular, concentric ridges. The symmetry of these concentric ridges is similar to that of a dart board (Figure A-7).

Figure A-7: A Fresnel lens looks like a dart board when viewed straight on, but it has a zigzag cross-section.

These circular ridges give the Fresnel lens a zigzag or sawtooth cross-section. Each sawtooth creates a tiny prism. By choosing appropriate powers for these prisms, designers can define the focal length and control image quality.

Fresnel lenses can be molded from precision optical-grade acrylic. Typical applications are condensing systems with small amounts of residual heat and for which a large aperture or thin profile is critical.

A common application for Fresnel lenses can be found in microfilm and microfiche readers. These products often are built to fit on a desk or tabletop. Fresnel lenses help to minimize the size of the illumination system.

Overhead projectors incorporate a Fresnel lens as part of their illumination systems. Usually the Fresnel lens is visible just below the glass platen onto which the transparencies are placed. In order to illuminate evenly the image of the transparency on the screen, the condensing system must direct light through all parts of the transparency into the projection lens, which is mounted on the arm above the platen. A more conventional lens could be used for the task, but its weight and expense would be prohibitive if it were made with the requisite 81/2 by 11-inch aperture.

Another application for a Fresnel lens can be found in the design of large-format view cameras. A view camera contains a large ground-glass focusing screen. In order to improve uniformity of brightness across the entire screen, a Fresnel lens can be placed against the ground glass on the side facing the camera’s lens. There, the Fresnel element functions as a field lens by intercepting light already focused at the screen and refracts it toward the viewer’s eye. Rays that would be lost because of their large angles of incidence near the edge of the focusing screen are thus captured and contribute to image brightness.

Other applications include TV projection, LCD projection units, 3 dimensional photography, solar energy systems, infrared alarms and detection systems, point of sale scanners, etc.

JML offers several different types of Fresnel lenses. They are defined and organized in our on-line catalog as: