Straightness errors are orthogonal deviations from an ideal linear motion. Quantifying and modeling those errors is an important element of precision engineering. Simple position-sensitive detectors (PSDs) enable measuring both the horizontal and vertical straightness simultaneously. Interferometers provide high precision displacement measurements, especially when using corner reflective cubes to avoid angular errors. This paper proposes a cascaded PSD-Double-pass Interferometer system to measure the two-dimensional straightness error and the position simultaneously. All is possible due to the use of polarizing optics. In the first pass, the light travels through the interferometer arm and is redirected upon return to the PSD units. There, a portion of the light is used for the straightness measurement, and the remainder is reflected back to the interferometer arm. During the second pass, the light follows the same path in the interferometer arm as before. This beautiful effect is common in double-pass interferometers and is only made possible with polarizing optics. The inventive detail: a Faraday rotator in the center of the instruments ensures that outgoing and returning beams have a perpendicular polarization—redirecting the light without losses to the detector of the heterodyne interferometer.

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