Radiation safety

	Overview   |   Generation of the image   |   Standards   |   User Safety   |   Light emissions   |   Notes

Overview
The safety of users is a primary consideration when VDTs are introduced into the work environment. VDTs are essentially electrically identical to television sets. Both VDTs and television sets produce electromagnetic emissions (sometimes called radiation) from CRT (Cathode ray tube); the phosphor, which is the chemical composition that coats the inside of the CRT screen; and from associated electronic components.

All life is constantly exposed to various forms of radiation, which are emitted from natural and man made sources. Radiation is an energy transfer from one place to another in waveform, and some of its effects are essential to the existence of life on earth. The consequences of the transfer of radiated energy to living organisms are biological effects, many of which are useful and some injurious.

Electromagnetic radiation is arranged according to frequency in a continuum that is called the electromagnetic spectrum

Electromagnetic spectrum.

Radiation varies in wavelength and frequency. Wavelength is the distance from peak-to-peak of a wave. Frequency refers to the number of oscillations per second. The electromagnetic spectrum is divided into two primary regions: ionizing and non-ionizing radiation, depending on the wavelength and frequency.

Ionizing radiation (shorter wavelengths) has sufficient energy to disrupt and separate electrons from atoms which absorb the radiation. This produces "ions" which are atoms with a net electrical charge.

Non-ionizing radiation (longer wavelengths) has energy in sufficient quantity to excite atoms, or electrons, but not sufficient to remove electrons from their orbits or to cause the formation of ions.

In a CRT, a stream of electrons is released from a heated cathode within a vacuum and accelerated by a high voltage toward a screen coated with a fluorescent material called a phosphor. The phosphor glows as a reaction to the electrons in the incident beam striking it, thus producing the light seen as an image on the front of the CRT.

X-rays, developed on the inside of all CRTs, are of relatively low energy, limited by the high voltage available from the VDT power supply. The CRT picture tube employs integral X-radiation shielding.

Higher voltages are effectively prevented by the use of sensing circuits which turn off the power to a failing power supply before the voltage reaches a potentially dangerous level. The inherent limitations of electron energy and integral CRT X-radiation shielding explain why X-ray emissions from CRT-type VDTs are not detectable by usual measurement practices.

Figure 2 is a comparison of the amount of ionizing radiation from CRT-type VDTs to other common sources of radiation. The total ionizing radiation of all wavelengths from the use of a VDT eight hours daily represents a very small portion of the radiation we receive from other consumer products, such as ceramics, combustion of natural gas and fossil fuels. In fact, the ionizing radiation a person absorbs from the rocks, masonry and construction materials in many buildings is generally orders of magnitude greater than any emissions from a VDT. Even our bodies are sources of minute traces of radiation which come from radioactive materials contained in the food we eat and the air we breathe.

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