The Production and Calibration of Foucault Test Objects of Various Contrasts*

Abstract

A method is described for the production and calibration of resolution test objects of a type similar to those introduced by Foucault. These test objects consist of alternate white and gray bands of equal width. The targets are photographic copies of impressions made from inked ruled zinc plates. In order to compensate for the lateral diffusion of the white bands in the photographic processes, the zinc plates are ruled to produce black bands twenty percent wider than the white bands. In the copying process each photograph consists of alternate exposures to the zinc plate impression and to a white card, such that the total exposure for each photograph is constant. A series of such photographic copies is simultaneously printed on one sheet of photographic paper. These prints constitute a set of test objects of various contrasts in which the white and gray bands are practically equal in width. The test objects are calibrated by means of a photoelectric mechanism which records the reflectivity of the white and gray bands and indicates the number of white bands per inch. Using the method outlined above, test objects have been produced and calibrated in which the number of lines varied up to 120 per inch and which had contrasts as low as 1 percent.