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.. For the lens of the eye.
Antique Microscopes
Compound microscopes,
left and right, use two or more sets of lenses to produce high
magnification. First conceived in the late 16th century, compound
microscopes were refined into elegant instruments in the 17th
and 18th centuries.
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- 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.
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- 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.
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- 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.
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