autocollimation

Autocollimation is an optical alignment and angular measurement technique that determines small angular displacements by using a collimated light beam that is directed onto a reflective surface and returns along the same path. When the system is perfectly aligned, the outgoing and returning beams overlap, making the reflected image coincide with a reference mark seen through an eyepiece or reticle.

Principle and setup: A light source and lens produce a collimated beam that travels through an autocollimator toward a mirror or reflective target attached to the object under test. The reflected beam passes back through the optical axis to the observer. Any angular misalignment between the optical axis and the reflective surface causes the return image to shift relative to the reticle or scale. The measured displacement translates into an angle through simple geometry, with the sensitivity depending on the geometry and magnification of the instrument.

Variants and instrumentation: Geometric autocollimation uses a plane mirror to generate a return beam, while optical autocollimation may employ additional reticles or scales to provide a direct angular readout. Common instruments include autocollimators and specialized collimators coupled with a telescope or viewing eyepiece. The setup is often used in calibration and alignment tasks requiring high angular precision.

Applications: Autocollimation is widely used in precision metrology, optical component alignment, machine-tool and coordinate-measurement equipment alignment, telescope and instrument collimation, and the verification of angular accuracy and flatness of surfaces. It enables non-contact, high-sensitivity measurements of angular errors in a compact, relatively simple arrangement.

History and notes: The method developed in the 19th century and matured with advances in light sources and optics. Modern autocollimators can achieve very high angular sensitivity but require stable setups and clean reflective surfaces to minimize error.