Overlapping common-path interferometers for two-sided measurement

An interferometric system for measuring test parts having first and second side surfaces, comprising:

• a first interferometer having a first reference surface and a first optical pathway conveying a first measuring beam for measuring optical path length differences between the first side surface of the test parts and the first reference surface;
• a second interferometer having a second reference surface and a second optical pathway conveying a second measuring beam for measuring optical path length differences between the second side surface of the test parts and the second reference surface;
• the first optical pathway of the first interferometer extending beyond the first side surface of the test parts to the second reference surface of the second interferometer for measuring optical path length differences between the first and second reference surfaces;
• and
• a processor that combines the measures of the respective optical path length differences between the first and second side surfaces of the test parts and the first and second reference surfaces with the measures of the optical path length differences between the first and second reference surfaces for measuring distances between the first and second side surfaces of the test parts.

An interferometric system for measuring the opposite sides of a test part both individually and relative to each other, comprising:

• first and second interferometers arranged for measuring the opposite sides of the test part;
• the interferometers having at least one light source for producing first and second measuring beams and an imaging subsystem for imaging the opposite sides of the test part with the first and second measuring beams;
• the first interferometer having a first reference surface that divides the first measuring beam into a first reference beam and a first test beam for measuring optical path length differences between the first reference surface and a first of the opposite sides of the test part and between the first reference surface and the second reference surface of the second interferometer;
• the second interferometer having a second reference surface that divides the second measuring beam into a second reference beam and a second test beam for measuring optical path length differences between the second reference surface and a second of the opposite sides of the test part;
• the first test beam having a primary transverse area that reflects from the first side of the test part and a secondary transverse area that reflects from the second reference surface of the second interferometer;
• the second test beam having a transverse area that reflects from the second side of the test part;
• and
• the imaging subsystem arranged for imaging interference patterns formed between (a) the first reference surface and the first side of the test part by the first measuring beam, (b) the second reference surface and the second side of the test part by the second measuring beam, and (c) the first reference surface and the second reference surface by the first measuring beam for locating the first and second sides of the test part with respect to both the first and second reference surfaces.

A method of measuring opposite side surfaces of a test part and of locating the opposite side surfaces of the test part with respect to each other, comprising steps of:

• mounting the test part between a first reference surface of a first interferometer and a second reference surface of a second interferometer;
• propagating a first measuring beam through the first interferometer;
• propagating a second measuring beam through the second interferometer;
• reflecting a first reference beam portion of the first measuring beam from the first reference surface;
• transmitting a first test beam portion of the first measuring beam through the first reference surface;
• reflecting a primary transverse area of the first test beam from a first side surface of the test part;
• reflecting a secondary transverse area of the first test beam from the second reference surface;
• simultaneously imaging interference patterns formed by the first measuring beam between the first reference surface and the first side surface of the test part and between the first reference surface and the second reference surface;
• reflecting a second reference beam portion of the second measuring beam from the second reference surface;
• transmitting a second test beam portion of the second measuring beam through the second reference surface;
• reflecting a transverse area of the second test beam from a second side surface of the test part;
• imaging an interference pattern formed by the second measuring beam between the second reference surface and the second side surface of the test part;
• and
• processing the two interference patterns formed by the first measuring beam and the interference pattern formed by the second measuring beam for measuring the first and second side surfaces of the test part and for locating the first and second side surfaces of the test part with respect to each other.