|
Ophthalmology => Cataract => Lens
Lens
INTRODUCTION
Lens (optics), in optical systems, glass or other transparent substance so shaped that it will refract the light from any object and form a real or virtual image of the object. Contact lenses and lenses in eyeglasses correct visual defects. Lenses are also used in the camera , microscope, telescope, and other optical instruments. Other imaging systems may be effectively used as lenses in other regions of the electromagnetic spectrum, such as the magnetic lenses in electron microscopes. For the design and use of lenses, see Optics. For the lens of the eye, see Eye.
LENS MANUFACTURE
Most lenses are made from special types of high-quality glass, known as optical glass, that are free from internal strains, bubbles, and other imperfections. The process of making a lens from a block of optical glass involves several operations. The first step is to saw a lens blank from the glass block. For this purpose the glass is held against a thin, revolving, circular plate of metal, the edge of which is charged with diamond dust. The blank is next brought roughly to shape by grinding on a flat cast-iron plate covered with a mixture of abrasive grains and water. To form the rounded surface of the lens, the glass is then ground on concave or convex iron tools charged with abrasive. A convex lens surface is formed by a concave tool and a concave surface by a convex tool. Commonly, two or more tools are employed in this grinding process, using successively finer grades of abrasive. The final process of finishing the lens surface is polishing, which is accomplished on a pitch-covered iron tool coated with jeweler's rouge and water. After polishing, the lens is "edged" by grinding the edge until the physical center and the optical center of the lens coincide. In this process, the lens is mounted in the headstock of a lathe, so that its optical center is on the axis of revolution, and the edges are trued with a strip of brass charged with abrasive.
LENS MEASUREMENT
The optical characteristics of simple, or single, lenses or compound lenses (lens systems containing two or more individual elements) are determined by two factors: the focal length of the lens and the ratio of the focal length to the diameter. The focal length of the lens is the distance from the center of the lens to the image it forms of an object at an infinite distance in front of the lens. Focal length is measured in two ways: ordinary units of length, as for example 20 in. or 1 m; or units called diopters, equal to the reciprocal of the focal distance measured in meters. A 1-diopter lens has a focal length of 1 m (3.28 ft); a 2-diopter lens has a focal length of 0.5 m (1.64 ft). The ratio of the focal length to the diameter of a lens determines its light-gathering power or "speed." This ratio is the so-called f-number of the lens.
HISTORY
The earliest lenses, which were known to the Greeks and Romans, were glass spheres filled with water. These water-filled lenses were used as burning glasses. True glass lenses were not known in classical times; they were probably first manufactured at the end of the 13th century in Europe. The processes used in lens manufacture have not changed essentially since the Middle Ages, except for the utilization of pitch as a polishing medium, introduced by the English mathematician Sir Isaac Newton.
The recent development of plastics and of special molding processes has led to an increasing use of that material for the manufacture of lenses. Plastic lenses are cheaper, lighter, and less fragile than the glass ones.
Back
|
