There are two basic generic styles of optical encoder: i) Incremental ii) Absolute The incremental encoder provides a pulse each time the shaft rotates through a predefined distance while an absolute encoder provides a “whole world” output with a unique code pattern representing each position. Optical encoders tend to follow one of two principles of operation; they consist of either a system of coded tracks consisting of transparent & opaque sections & associated lamps and photo cells to detect the corresponding switching sequence, or they rely on the use of moiré fringe techniques, capable of much higher resolution when used for incremental measurement. The absolute digitizer comprises an assembly consisting of a Gray-coded pattern photographically reproduced on a glass disc mounted on the input shaft. The code consists of ten annular tracks each with a pattern of opaque and transparent sections. The code reading system employs a filament lamp and collimating lens from which light passes through the disc and a narrow radical slit, to be detected by ten photovoltaic cells. Depending on the angular position of the shaft, certain cells receive light from the transparent portion of the disc and enable the outputs from all ten cells to reproduce the shaft position directly in parallel – Gray – Coded from. The output, which is noise free, is suitable for amplification and subsequent processing for use in digital servo systems, computers, data logging and visual displays. Fig. shows the shaft encoders using straight binary and Gray codes.