"Anoptical comparator(often called just acomparatorin context) is a device that applies the principles ofopticsto theinspectionof 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 againstprescribed limits. The measuring happens in any of several ways. The simplest way is thatgraduationson the screen, being superimposed over the silhouette, allow the viewer to measure as if a clear ruler were laid over the image. Another way is that various points on the silhouette are lined up with thereticleat the centerpoint of the screen, one after another, by moving the stage on which the part sits, and adigital readoutreports how far the stage moved to reach those points. Finally, the most technologically advanced methods involve software that analyzes the image and reports measurements. The first two methods are the most common; the third is newer and not as widespread, but its adoption is ongoing in the digital era.
The first commercial comparator was developed byJames HartnessandRussell W. Porter.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 ofastronomyandtelescope-building) to the problem ofscrew threadinspection. 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 termcomparatorformetrologicalequipment, had existed in other forms prior to Hartness's work; but they had remained in realms of pure science (such as telescopy andmicroscopy) 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."