Optical Measuring Instruments

(in machine building), instruments in which sighting (alignment of the boundaries of the dimension being checked with a reference line or with cross hairs) is done or a dimension is determined by means of a device with an optical principle of operation.

Three groups of optical measuring instruments are distinguished. In the first group are instruments that use an optical method for sighting and a mechanical or other nonoptical method for reading the displacement. Instruments of the second group use optical methods for both sighting and reading, and instruments of the third group have mechanical contact with the object being measured and optical reading of the displacement of the contact points.

Among the instruments of the first group, projectors for measuring and checking’small parts of complex shape, such as templates and clockwork parts, have become common. Projectors used in machine building have magnifications of 10, 20, 50, 100, and 200X and screens 350–800 mm in diameter or along one side. Projection attachments can be mounted on microscopes, metalworking machines, and various instruments. Toolmaker’s microscopes are most often used for measurements of screw thread parameters. Large models of toolmaker’s microscopes are usually equipped with a projection screen or a binocular head to facilitate sighting.

The best known instrument of the second group is the universal measuring microscope. In this instrument the part being measured is moved on a longitudinally moving carriage, and the microscope head moves on a transverse carriage. Sighting of the boundaries of the surfaces being checked is accomplished using the microscope head. The dimension being checked (the movement of the part) is read from a scale, usually with a reading lens or microscope. In some models of the universal measuring microscope, the reading device is of the projection type. Interference comparators also belong to this group of instruments.

The instruments of the third group are used to compare the linear dimension being measured with known measures or scales. These instruments as a group are usually called comparators. The group includes optical calipers, opticators, gauging machines, contact interferometers, and optical length gauges. The contact interferometer, which was developed in 1947 by I. T. Uverskii at the Kalibr Plant in Moscow, uses a Michelson interferometer whose movable mirror is rigidly fastened to the measuring pin. During measurement, the measuring pin is displaced and causes a proportional displacement of the interference fringes, which can be read from a scale. These instruments, which may be of the horizontal or vertical type, are most frequently used for relative measurements of the length of gauge blocks and for their certification as standards. In an optical length gauge, such as the Abbe length gauge, the scale moves together with the measuring pin. For measurements by the absolute method, a dimension equal to the displacement of the scale is determined by readings through an eyepiece or, using a vernier, on a projection device.

A promising trend in the development of new types of optical measuring instruments is the use of electronic readout devices, which simplify the reading of data and the sighting and make it possible to obtain data that have been averaged or processed in a specific way.