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Patent drawings for Hartness screw-thread optical comparator (numbering removed for clarity).[1]

An optical comparator (often simply called a comparator in context) is a device that applies the principles of optics to the inspection 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 and Russell W. Porter.[2] 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 and telescope-building) to the problem of screw 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.[3]

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


  1. ^ Hartness, James; Porter, Russell (1929), United States Patent 1703933: Optical comparator, U.S. Patent Office,  
  2. ^ Roe 1937, p. 43-45.
  3. ^ Smith, Kennedy (2002). "Shedding light on optical comparators—How much better can this type of system get?". Quality Digest: QCI International. Retrieved 2008-05-12.  


  • Roe, Joseph Wickham (1937), James Hartness: a Representative of the Machine Age at Its Best, New York: American Society of Mechanical Engineers  


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