ETS-Lindgren HI-3637 VLF Magnetic Field Meter - Manual (Page 43)

HI n 3637 Manual Page — 35
CHARACTERIZING VDT EMISSIONS AND OPERATOR
EXPOSURE
Introduction
Because of the perturbing influence of the 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.
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 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 has been recommended
(Diffrient, et al, 1981). Nevertheless, because the value
of 30 cm has been so often reported in the literature,