A parallelism measurement optical system module includes a polarization beam splitter, a mirror positioned on a first surface of the polarization beam splitter, a first quarter wave plate positioned on a second surface of the polarization beam splitter that is perpendicular to the first surface, and a second quarter wave plate positioned on a third surface of the polarization beam splitter that is perpendicular to the first surface and parallel to the second surface.

A gradient of gravity is defined by a change in the optical path length required to maintain equality in optical path lengths of two beam arms which direct light beams to impinge upon and reflect from two freefalling test masses.

A non-contact optical metrology system to measure simultaneously the relative piston and the relative inclination in two axes between two low curvature reflective surfaces using partially coherent light interferometry. The system creates an interference pattern with partially coherent light from the linear phase change induced by a double prism system or equivalent, which allows the measurement of the relative piston and the inclination in two axes between the reflective surfaces. The relative piston between the mirrors is measured from the position of the interference pattern, the relative inclination in one axis from the distance between the fringes, and the relative inclination in the other axis from the inclination of the fringes. Relative piston measurements and relative inclination measurements in two axes are decoupled and can be extracted with simple morphological operations, without the need for marginal processing algorithms.

The present invention relates to an objective (3) for a confocal system (1) of spectral interferometric measurement, comprising: a source hole (14); a second beam splitter (12) having a partially reflective face (12a), a first beam splitter (10) having a face which is configured to form a reference surface (6) and being located between the source hole (14) and the second beam splitter (12); andlenses (11, 13). The first and second beam splitters are positioned in the objective (3) such that an optical distance (d.sub.ref) between the reference surface (6) and the partially reflective surface (12a) is substantially equal to an optical distance (d.sub.m) between the partially reflective surface (12a) and a focal plane of the objective (3).

A method to determine an absolute position of a first movable object using an interferometer system is described, said method comprising: providing first and second beams with a first light frequency from a first light source: providing further first and further second beams with a second (tunable) light frequency from a second light source: guiding the first and further first beams along a first axis to a reflective surface of the first object to obtain a first interferometer signal and guiding the second and further second beams along a second axis to a reflective surface of a second object to obtain a second interferometer signal, while changing the tunable frequency, detecting the first and further first interferometer signals detecting the second and further second interferometer signals, determining a first count offset and/or a further first count offset using a non-linear equation, and determining the absolute position of the first object.

A measurement apparatus for measuring a thickness of a semiconductor wafer includes: an optical system configured to perpendicularly irradiate a sample wafer and a reference wafer with light, and receive interference signals of the light reflected on front and back surfaces of the respective wafers; a signal processor configured to perform frequency analysis of the interference signals received by the optical system to obtain peak positions of a point spread function of the respective wafers; and a calculator configured to calculate a thickness tsample of the sample wafer based on the peak position x of the sample wafer and the peak position y of the reference wafer obtained by the signal processor, and a thickness treference of the reference wafer.