Magnetic fields and VDUS - emissions and susceptibility

Abstract

As computers become more and more a part of our everyday lives, the number of people sitting at Visual Display Units (VDUs) continues to increase. This increase has coincided with concerns amongst the general population about possible health effects from low frequency electromagnetic radiation. The combination of these two factors has resulted in market pressure which compels VDU manufacturers to measure and control magnetic and electric emissions so that VDUs do not add appreciably to the general office ambient levels. Despite the lack of any convincing medical, scientific or epidemiological evidence these market pressures are likely to spread beyond VDUs to all office equipment. The principle of operation of the CRT is that one or more beams of electrons are accelerated towards the rear of the viewing faceplate where they strike a phosphor coating which then emits light. A complete image is formed by using magnetic `deflection' fields to sweep the electron beam vertically and horizontally across the faceplate and it is largely the leakage fields from this process that can be detected outside a VDU. The light emitted from a point on the phosphor starts to decay once the electron beam has moved past it so the image must be refreshed at a suitable rate to minimize flicker. Typical vertical and horizontal scanning frequencies are 70 Hz and 50 kHz respectively, waveforms are approximately sawtooth.