Optical encoder using diffraction imagery in a reflective mode

Claim 1

In an encoder which measures the movement of a first radiation transmissive plate, having thereon periodic markings of alternating transmissive and opaque increments relative to a second reflective plate, having thereon periodic markings of alternating reflective and non-reflective increments, the improvement comprising the encoder and a readout system in the encoder being optiMally designed while taking into account diffraction effects of radiation in the encoder and comprising: a. a first transmissive plate having thereon periodic markings of alternating transmissive and opaque increments, with the length of one transmissive increment - opaque increment cycle being S1; b. a second reflective plate side having thereon periodic markings of alternating reflective and non-reflective increments, with the length of one reflective increment - nonreflective increment cycle being equal to one-half S1 to satisfy diffraction theory; c. a radiation source for producing radiation having a mean wavelength lambda and for directing the radiation through said first transmissive plate onto said second reflective plate where it is reflected and directed back through said first transmissive plate, with relative movement between said first plate and second plates causing modulation of the radiation; d. a detector means for detecting the modulation of radiation by said first transmissive and said second reflective plates; and e. means for optimally positioning said first and second plates in accordance with diffraction theory and including means for positioning said first plate relative to said second plate with a gap (Z) between them substantially satisfying the following equation, Z nS12/2 lambda wherein S1 and lambda have already been defined and n equals an integer of 1 or greater.