Interference Comparator

an instrument for the metrological determination of lengths of standards in terms of the wavelengths of light or for the comparison of lengths of standards on the basis of light interference. End-block lengths up to 100 mm are measured with a Kösters interferometer. End blocks of greater length, as well as line standards, are measured by means of a universal interference comparator (see Figure 1).

Figure 1. Diagram of an interference comparator

Light from the source (1), in the form of a parallel beam of rays, is directed by a mirror (2) to a semitransparent glass dividing plate (3). The portion of the light reflected by the plate is incident on an end standard (4), which is mounted on a movable carriage. The front surface (A) of the standard is free, but a flat glass plate (C) is lapped to its rear surface (B). After reflection from the planes (A) and (B) the light passes through a plate (3) and is directed to a telescope (7) by a mirror (5). A portion of the light from mirror (2), after passing through plate (3), is directed by mirror (6) to mirror (7), which is rigidly attached to a photoelectric microscope (8). The miscroscope can be moved along a table (9), which has a line standard (10) set on it. The light reflected by mirror (7) is returned to mirror (6) and then directed to plate (3). The light reflected by the plate is also directed by mirror (5) to the telescope (7). The position of the table is recorded by means of an interference indicator (11), which is a Michelson interferometer having one mirror rigidly attached to the table.

When comparing the reference standard, the end block, and the line standard being measured, the position of the zero line on the line standard is fixed under the microscope, and the path lengths of both beams of rays formed by the reflection of the light from surface (A) of the standard and the mirror (7) are made equal by moving the carriage and its end block. Then, by shifting the mirror (7) and the microscope (8), the path lengths are made equal for both beams of rays formed by the reflection of the light from surface (B) and mirror (7). For this it is necessary that the shift of the mirror (7) and the microscope (8) be equal to the length of the end block. The new position of the mirror relative to the line standard is fixed by the microscope and the interference indicator. To compare the lengths of end blocks in a parallel beam of rays coming from a light source, a tubular étalon ( a Fabry-Perot étalon) is mounted and the mirror (7) is positioned so that its plane divides the standard being compared into parts that are multiples of the etalon’s length. The accuracy of measurement of lengths of standards on a universal interference comparator is about 1 X 10^-7.