Autocollimator

An optical instrument that measures small angular deviations by reflecting a collimated beam from a mirror and detecting its return. It converts the beam’s displacement at a detector into an angle.

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Optical collimator: An optical device that uses a lens or mirror to produce a beam of nearly parallel light rays. Its parallel beam is the reference that the mirror reflects back through the instrument.
Optical alignment: The adjustment of optical components so their axes and surfaces have intended positions and orientations. Autocollimators reveal small tilt errors while optical systems are being aligned.
Theodolite: A surveying instrument that measures horizontal and vertical angles using a telescope and graduated circles. A theodolite measures angles over a broad range, while an autocollimator excels at tiny deviations.
Reflection (physics): The return of a wave from a surface into the medium from which it arrived. The returning beam depends on the law of reflection at the mirror.
William Fox Talbot: An English scientist and inventor known for developing negative-positive photography and research in optics. Talbot described an early autocollimation method for testing optical surfaces in the nineteenth century.
Plane mirror: A flat reflective surface that forms an image through specular reflection. Its rotation changes the direction of the returning beam by twice the mirror’s angular change.
Straightness measurement: The assessment of how closely a surface or motion path follows a straight line. A mirror and autocollimator can track angular changes along a machine guideway.
Optical lever: A mirror-based instrument that magnifies angular motion into a measurable displacement of a reflected light spot. Both use reflected light, but an optical lever measures spot displacement over a propagation distance.
Law of reflection: The rule that the angles of incidence and reflection are equal when measured from the surface normal. It explains why a mirror’s rotation changes the reflected ray’s direction by twice that rotation.
Autocollimation: An optical method in which a collimated beam is reflected back through its originating optical system. The instrument’s name and operating principle come from this optical method.