Image drift correction apparatus

Apparatus for continuously and accurately maintaining the location of the raster produced by an energy beam on a display surface of a visual system, during operation of said visual system for its intended purpose, comprising:

• optical sensing means mounted to said display surface outside of the viewing area for continuously monitoring the position of a selected portion of the path of a raster line also outside of the viewing area on said display surface during operation of said visual system for its intended purpose, said optical sensing means for determining the position of said selected portion of a raster line with respect of the location of said optical sensing means and for providing an electrical signal representative thereof;
• said optical sensing means having a support structure, a photosensitive device having a multiplicity of linearly arranged photosensitive elements mounted in said support structure, said photosensitive device being oriented such that said linear arrangement is substantially at a right angle with respect to said monitored raster line path;
• means mounted to said support structure between said display surface and said photosensitive device for imaging said monitored raster line path onto an area of said multiplicity of photosensitive elements, said area varying according to changes in the location of said monitor raster line path on said display surface;
• circuitry for detecting which of said elements said image of said monitored raster line path impinges, and for providing said signals representative of the position of said selected portion of said raster line;
• means for receiving said electrical signal, and for comparing said electrical signal with a predetermined stored reference signal representative of said selected portion of the path of a raster line properly located on said display surface, and for generating a control signal representative of the difference between said reference signal and said electrical signal;
• and
• means for combining said control signal with sweep drive signals which are produced during operation of said visual system for its intended purpose for deflecting said energy beam to produce resulting sweep drive signals which maintain said raster scan in a selected location to provide a scene properly positioned on said display surface.