Optical Caliper

an instrument for measuring linear dimensions by the relative method in which an optical lever mechanism is the conversion member. In this mechanism, a tilting mirror is the lever and an autocollimating telescope is the optical converter. A tilting mirror was first used in measuring instruments in 1837 by the German engineer I. Saxton. An instrument that used both a tilting mirror and an autocollimating telescope was first built in 1925 by the Zeiss company in Germany. Vertical and horizontal optical calipers are manufactured; they differ only in the construction of the base.

The optical converter of an optical caliper is a tube that may have eyepiece or projection readout (see Figure 1). In a tube with projection readout, a plate with a scale marked from the center line on one side and a fiducial mark on the other is illuminated by a lamp. In the eyepiece tube, the plate is illuminated by a spot of light reflected from a special mirror. The image of the scale strikes first the fixed mirror and then a mirror that tilts, assuming various angular positions depending on the position of the measuring pin. A tube with eyepiece readout has no fixed mirror. After the image of the scale is reflected by the mirror, it strikes the second half of the plate—that is, it is superimposed on the index. If the measuring pin moves, the secondary image of the scale is displaced with respect to the fixed index; the image is projected by means of mirrors onto a screen in the projection tube of the optical caliper or is viewed through an eyepiece. The tube of the optical caliper has a scale with divisions of 1 micron (μ), and the range of measurement of the scale is ±ρ100 μ.

Figure 1. Optical caliper with projection readout: (1)lamp, (2) plate with scale and index, (3) screen, (4) projection mirrors, (5) fixed mirror, (6) tilting mirror, (7) measuring pin

Optical calipers with scale divisions of 0.2 μ and a range of measurement of ±25 mm are called optical microcalipers. They differ from the arrangement described above in that the scale is reflected twice from the movable mirror. This increases the length of the optical lever and makes it possible to make the scale divisions smaller.

Optical calipers are equipped with interchangeable accessories, such as attachments for measuring the mean diameter of screw threads, the dimensions of wires, and the length of gauge blocks. Other accessories include projection attachments for the eyepiece tubes and electric contact heads for measuring apertures of 1.0–13.5 mm (horizontal optical calipers).