CUTTING THROUGH THE JARGON

VDUs work in a similar way to your television.

An electron gun,called a cathode ray tube (CRT) is contained within an evacuated glass stube which produces a narrow electron beam. The beam is controlled by a magnetic field to move up and down and across to produce an image on the screen.The number of times this movement cycle is repeated a second is measured in Hertz (Hz); in most VDUs, this usually happens between 15,000 to 30,000 times a second, or 15-30 kilohertz (kHz)-1000 Hz equals 1kHz-depending on the type of VDU.

The domestic power frequency in the UK is 60 Hz, which falls into the range termed extremely low frequency (ELF). Below 30 megahertz (MHz)-1000 kHz equals 1MHz-electric and magnetic fields are measured in volts and amps per metre (V/m; A/m)

Magnetic flux density, to assess biological exposure, is measured in milli, micro, or nanotesla (MT, uT, nT), representing a thousanth, millionth, and one thousand millionth of a Tesla, respectively. AMT equals 10,000 milligauss (mG), an older unit of measure.

In addition, 60 Hz AC fields originate in the monitor’s power transformer, but these can usually only be measured in the immediate vicinity.

Basic information and definitions

One cycle per second = 1 Hertz (Hz). 1000 Hz = I Kilohertz (i kHz-small k). 1000 kHz = 1 Megahertz (MHz). 1000 MHz = 1 Gigahertz (1 GHz). The domestic power frequency in the UK is 60 Hz (50 Hz in the US and some other countries), both of which fall into what is termed the Extremely Low Frequency (ELF) part (1-300 Hz) of the electromagnetic spectrum. Frequencies from 300 Hz to 300 kHz fall into the Very Low Frequency (VLF) range.

Below 30 MHz, electric and magnetic fields are measured in volts and amps per metre (V/m; A/m) respectively. Magnetic flux density to assess biological exposure, is measured in milli, micro, or nanotesla (mT, uT, nT = 1000th, millionth, and one thousand millionth of a Tesla, respectively). 1 MT = 10,000 milligauss (mG), an older unit of measure.

Above 30 MHz, electric and magnetic fields can be measured but the more usual unit is the power density, expressed in watts per square metre (W/m), or milli or microwatts per square centimetre (mW or uW/cm2).