Linear encoder using diverging light beam diffraction

A linear encoder characterized as having a few number of parts including a light source that directs a beam of light onto the grating of a movable scale where reflected diffracted beams from two locations on the scale interfere with one another to form light fringe bands on a plane. In one embodiment, a Ronchi grating is located on the plane having lines with a spacing equal to the spacing of the light fringe bands. The combination of the Ronchi lines and the bands of the fringe pattern form a Moire pattern which is transmitted through the Ronchi grating and whose spatial distribution of intensity moves as the scale is moved and is detected by a photodetector array. The photodetector array converts the signal from the Moire pattern to a sinusoidal expression of displacement whose phase is used to determine displacement of the scale.