Position measuring through the use of moire fringe multiplication

Claim 1

Apparatus in which the position of a table is determined by moire fringe multiplication comprising a table and base device, a coarse scale grating attached to one element of said device comprised of said table and its base, an index member in the form of a diffraction grating of frequency approximately a whole number Beta , exceeding three, times the frequency of said scale grating fixed to the other element of said table and base device in juxtaposition to said scale grating, an illumination means radiating a systematic bundle of rays obliquely upon said index member, whereupon said index member causes two dominant components of said rays to emerge with approximately symmetrical obliquities in the space between said index member and said scale grating, said two dominant components being subsequently diffracted by said scale grating, causing them to emerge from said arrangement of scale grating and index member as two sets of subcomponent rays, with a multiplicity of pairs of said subcompoNent rays, each pair comprised of a member from each set, traveling in a systematic sequence of diffracted directions, optical filtering means which subsequently intercepts said pairs of subcomponent rays and allows only one pair to proceed, namely, a favored pair that proceeds in a direction approximately normal to the plane of the scale grating, observing means that intercepts the said favored pair and senses the state of interference generated by said favored pair, said state of interference being manifest as fringes called moire fringes, and interpretation means to compare and indicate relative position of said table in relation to said base from changes of the said moire fringes.