ETS-Lindgren HI-3603 VLF Survey Meter - Manual (Page 30)

HI-3603 VDT/VLF Survey Meter
© ETS-Lindgren, August, 2005
Revision E, Part # H-600042
30
CHARACTERIZING VDT EMISSIONS AND
OPERATOR EXPOSURE
In the section labeled VDT Characteristics above, the effect of
the presence of the operator on field emissions was described.
Because of this perturbing influence on measured electric field
strength values near VDT's, it is important to distinguish between
assessments of operator exposure and basic emission
characteristics of VDT's. Relative to electric fields, these two
properties are not the same.
CHARACTERIZING VDT EMISSIONS
On occasion it is desirable to characterize the electromagnetic
emissions of a number of VDT's, such as in a large office
situation, to establish the general emission levels of these VDT's
for comparison with other VDT emission data. Such
measurements can be used to determine unusual operating
characteristics of particular VDT's within a group. To collect this
type of data, it is helpful to minimize unnecessary, extraneous
environmental factors. In this way electric field measurements
obtained on the VDT's will be as reproducible as possible and
can be compared to electric field data collected from other
similar VDT's with a maximum of consistency.
Emission characterizations should therefore be performed
without the operator present. Although the literature contains
numerous methods by which emission data have been obtained,
the principal difference lies in the locations about the VDT at
which measurements are performed. An exploration of the
surfaces of a typical VDT will reveal areas of particularly intense
fields, but these areas are usually on the sides or top of the VDT
and are not directly applicable to frontal area exposure where the
operator would be positioned. Because of this, a nearly universal
measurement location positioned at a point 30 cm directly in
front of the VDT screen has been most commonly used and
recommended in emission characterizations (FDA, 1984).
Measurement distances of 50 cm and 1 m have also been used.
The value of 30 cm actually represents a quite close distance
when compared to the viewing distance used by many VDT
operators. In fact, a minimum viewing distance closer to 36 cm