Measurements of Electromagnetic Emissions from Video Display Terminals at the Frequency Range from 30 Hz TO 1 MHz

Abstract

Electric (E) and magnetic field (B) strengths or flux densities were measured at distances of 30 and 50 cm from the screen of video displays at a frequency range from 30 Hz to 1 MHz. The measurement system consisted of an optically coupled active dipole for E-fields, a magnetic field meter, a digital oscilloscope, a portable computer for data storage and a laboratory computer for Fourier analysis of the recorded signals. Comparison of measurement results with available broadband exposure standards or proposed standards indicated that magnetic flux density should be measured at both the (ELF) frequency range from 30 Hz to 300 Hz and at the (RF) frequency range from 10 kHz to 500 kHz. Alternatively, time derivative of magnetic flux density may be measured. Nor should the measurement of the electric field strength at the RF range be neglected. These conclusions, however, are valid only in relative terms. In all cases the exposure is at least one decade below the most stringent exposure limit. The maximum relative exposure 0.077 was obtained by applying the ACGIH standard for magnetic fields at a distance of 30 cm from the screen. The field strengths decrease by a factor varying from 2.5 to 3.5 at a distance of 50 cm, which is a more realistic distance when considering actual working conditions.

Rate of occurrence of transient magnetic field events in U.S. residences

“…There are several potential sources of such steady-state ®elds in residences, including television sets, computer monitors,¯uorescent tubes, and switching power supplies used in computers and possibly in some types of appliances. Of these, only computer monitors [Harvey, 1983;Guy, 1987;Jokela et al, 1989;Sandstro Èm et al, 1993] and television sets [Gauger, 1985;Preece et al, 1997;Kaune et al, in press] have received much attention. Data presented later will show the presence at a low level of steady-state signals in the 3.3 nT, but not in the 33 nT, channel.…”

Effects of low-frequency magnetic fields on embryonic development and pregnancy.

“…Video display terminals with a cathode ray tube display emit extremely low-frequency magnetic fields at 50 or 60 Hz and very low-frequency magnetic fields at 15-30 kHz. The strength of the extremely low-frequency magnetic field is a few amperes per meter (a few microteslas) at the surface of the terminal and about 0.1-0.3 Aim (0.1-004 IlT) at the operator's position (48,49). In addition to low-frequency magnetic fields, video display terminals emit several other types of electromagnetic energy, including optical radiation, radiofrequency radiation, and static electric fields.…”

The Time and Frequency Domain Measurements of the Magnetic Fields Emitted by Video Display Terminals

“…Especially for the estimation of the biological effects it is necessary to know many field parameters (e. g. peak to peak value, root mean square value, the harmonics content of the B). Because of the wide use of the VDT, the measurement of their generated fields constitute a challenge [2], [3]. The main sources of the magnetic fields of the VDT are the vertical and horizontal deflection systems.…”