What Are Autocollimators?

Autocollimators are high precision instruments that use internal optics to measure angles. This instrument resembles a telescope, but it is positioned in a horizontal orientation for testing purposes. The main purpose of the instrument is to align external mechanical devices, such as industrial machines, into a perfectly parallel line. These devices come in two different types, including visual and digital versions.

The autocollimator functions by emitting perfectly horizontal parallel light beams out to a reflective object, like a mirror. After reflecting off the object, the light returns to the instrument's internal optics. Any aberration from a perfect parallel position is displayed on a predetermined scale within the autocollimator. The scale is normally measured in arc seconds — also referred to as angular distance — to determine the amount of error in the mirror.

Applications using autocollimators are highly diverse. Fiber optic manufacturing companies use these instruments to ensure that the optical lengths are perfectly straight for the best operation. Leveling laser systems, such as those used to create a smooth concrete roadway, use autocollimators for calibrating the laser's beam plane. In addition, production lines with strict alignment needs, like in the aerospace industry, use autocollimators to line up machinery and final production models.

A visual autocollimator depends on the human eye to determine angular error. The instrument is equipped with an eyepiece. Once the reflective surface and autocollimator are set up across from one another, the person views the reflected light beam across the instrument's scale through the eyepiece. Any angular error that is out of tolerance for that reflective surface must be adjusted; the person can modify the surface's angle in comparison to the autocollimator to reduce the angular error. This calibration process continues until the visual error is acceptable for the particular reflective surface.

Digital autocollimators function in the same manner as visual instruments; however, the device has an internal photodetector to sense the angular error rather than relying on the human eye. The error is electronically converted into a visual value on a liquid crystal display (LCD). Technicians or repair workers can modify the error and run the autocollimator again to ensure proper alignment.

The main benefit to digital autocollimators is data storage and traceability. In addition, the visual error interpretation can vary widely, based on the person's eyesight and angle of vision. Digital error interpretation relies on precision electronics with no interpretation discrepancies. As a result, high precision industries can prove that their machinery is aligned to specification, especially if the accuracy comes into question because of a product defect or failure.