DC magnetic field effects on shadow mask color cathode-ray tube (CRT) performance

Electromagnetic interference testing of video equipment reveals that both monochrome and color cathode-ray tube (CRT) monitors are susceptible to picture distortions due to DC magnetic fields. The most common commercially available color CRTs use shadow mask technology. These CRTs, however, are the most susceptible to DC magnetic fields due to their precise alignment requirements for color convergence and registration. Testing on the CRT display systems was conducted to determine equipment compatibility to the magnetic field environment as defined in DOD-STD-1399, Section 070. Measurements were taken to determine the levels at which degradations begin to appear. Mu-metal and carbon steel shields were constructed and evaluated for their shielding effectiveness. Shadow mask color CRTs are more susceptible to DC magnetic fields than monochrome CRTs. The color CRTs have all the problems associated with the monochrome CRTs, with the additional problems due to their precise alignment requirements necessary for color convergence and registration. Preliminary results show that simple metallic shielding using magnetic materials, by itself, is only effective for static, low-level DC magnetic fields. The use of massive, multilayered shields in dynamic environments with fields larger than 400 A/m. is impractical and not cost effective. Deperming circuitry, along with the shielding, can be used to cancel the higher-level external fields.<>