EXTERNAL POWER FREQUENCY MAGNETIC-FIELD-INDUCED JITTER ON COMPUTER MONITORS

Power frequency magnetic fields with flux densities greater than 0.5 mut are not uncommon in offices. This level has been shown to induce jitter on VDT monitors. In the present project, these magnetic field-induced disturbances have been studied in the laboratory in order to establish a firm technical basis for future studies of the disturbance's influence on eye strain in VDT workers. Eight volunteers judged the occurrence of distortion when an applied external magnetic field was varied both in amplitude and frequency for 8 investigated VDT screens. The level of the external 50 Hz magnetic field when the distortion was detectable ranged from 0.6 to 1.1 muT. If the screen was viewed through a stereomicroscope (25 x magnification), the corresponding level was in the order of 0.2 muT. If the frequency difference between the external magnetic field and the refresh rate of the screen is only +/- 1-2 Hz, the disturbance is noticeable at even lower flux densities.