Radiation safety
|
Generation of the image
The electron beam in a CRT moves across the screen in a series of horizontal
lines starting at the top left and moving down left-to-right, much like the
way a typist fills a typewritten page.
The circuitry that controls the horizontal movement of the beam is called
the horizontal deflection system which operates at a frequency of 15.000 to
85.000 cycles per second (15-85 kHz) and is the major source of non-ionizing,
very-low-frequencey (VLF) electric and magnetic fields in the VDT.
The time required to "paint" the screen from top left to bottom
right once is approximately sixteen one-thousandths of a second (16 milliseconds).
The beam then returns to the top left corner in approximately one millisecond
and the process begins once again. The circuitry that controls the vertical
movement of the beam is called the vertical deflection system. This circuitry
operates at a frequency of 50 to 80 cycles per second (50-80 Hz) in the majority
of VDTs and is the major source of non-ionizing, extremely low frequency (ELF)
electric and magnetic fields in the VDT.
In addition, the high static positive voltages on the inside of the front
of the CRT produce a charge on the front of the CRT surface which is highly
variable, depending on such widely diversified factors as room humidity, the
clothing worn by the user and the floor covering material.
Figure 3 is a comparison of the amount of non-ionizing radiation from VDTs
to other common sources of non-ionizing radiation.
 |
FM |
65.90% |
| High VHFTV |
14.70% |
| UHFTV |
10.60% |
| Land/mobile phone |
0.14% |
| VDT at 25cm distance |
0.04% |
| Low VHFTV |
8.60% |
Average personal exposure to non-ionizing radio frequency radiation in urban
areas.
|
