An optical comparator is a device that applies the principles of optics

Optics

Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it. Optics usually describes the behavior of visible, ultraviolet, and infrared light...

 to the inspection

Inspection

An inspection is, most generally, an organized examination or formal evaluation exercise. In engineering activities inspection involves the measurements, tests, and gauges applied to certain characteristics in regard to an object or activity...

 of manufactured parts. In a comparator, the magnified silhouette of a part is projected upon the screen, and the dimensions and geometry of the part are measured against prescribed limits.

The first commercial comparator was a creation of James Hartness

James Hartness

James Hartness was an American inventor; a mechanical engineer; an entrepreneur who mentored other inventors to develop their machine tool products and create a thriving industrial center in southeastern Vermont; an amateur astronomer who fostered the construction of telescopes by amateurs in his...

 and Russell W. Porter

Russell W. Porter

Russell Williams Porter was an American artist, engineer, amateur astronomer and explorer. He was a pioneer in the field of “cutaway illustration" and is sometimes referred to as the "founder" or one of the "founders" of amateur telescope making."-Biography:Russell W...

. Hartness' long-continuing work as the Chairman of the U.S.'s National Screw-Thread Commission led him to apply his familiarity with optics (from his avocations of astronomy

Astronomy

Astronomy is a natural science that deals with the study of celestial objects and phenomena that originate outside the atmosphere of Earth...

 and telescope

Telescope

A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...

-building) to the problem of screw thread

Screw thread

A screw thread, often shortened to thread, is a helical structure used to convert between rotational and linear movement or force. A screw thread is a ridge wrapped around a cylinder or cone in the form of a helix, with the former being called a straight thread and the latter called a tapered thread...

 inspection. The Hartness Screw-Thread Comparator was for many years a profitable product for the Jones and Lamson Machine Company, of which he was president.

In subsequent decades optical comparators have been made by many companies and have been applied to the inspection of many kinds of parts. Today they may be found in many machine shops.

The idea of mixing optics and measurement, and the use of the term comparator for metrological

Metrology

Metrology is the science of measurement. Metrology includes all theoretical and practical aspects of measurement. The word comes from Greek μέτρον , "measure" + "λόγος" , amongst others meaning "speech, oration, discourse, quote, study, calculation, reason"...

 equipment, had existed in other forms prior to Hartness's work; but they had remained in realms of pure science (such as telescopy and microscopy

Microscopy

Microscopy is the technical field of using microscopes to view samples and objects that cannot be seen with the unaided eye...

) and highly specialized applied science (such as comparing master measuring standards). Hartness's comparator, intended for the routine inspection of machined parts, was a natural next step in the era during which applied science became widely integrated into industrial production.