External magnetic field compensator for a CRT.

摘要

Ligh sensors [8] with red filters [9] are located at four corners of a cathode ray tube [CRT] out of view of the operator. A red raster is generated in the corners of the CRT, the pattern of the raster corresponding to the area covered by the sensors. A pair of coils [3, 4] is located about the face of the CRT in a Helmholtz configuration and are driven by a current generator [8] in response to the sensor output to maximise sensor output, thereby generating a compensating magnetic field which compensates for external axial magnetic fields affecting the CRT.