ETS-Lindgren HI-3637 VLF Magnetic Field Meter - Manual (Page 44)

Page — 36 HI n 3637 Manual
measurements should at least include this distance
among possible others.
VDT emission data reported in the literature show that in
most instances a fixed screen condition has been used to
promote more repeatable measurements. For example, a
commonly reported method involves filling the screen
with a single character such as an E or M and adjusting
the brightness and contrast controls to their maximum
position. In contrast to these precautions, it has also
been reported that these measures often seem to have
very little, if any, impact on the resulting measured values
of electric and magnetic field strength (Roy, et al, 1983).
Nevertheless, because of peculiarities of some VDTs, a
check of the effect of varying the brightness and contrast
controls should be made. Roy, et al (1983), suggest that
CRT performance, which decreases with age, and the
type of video generating system used are two possible
factors responsible for this phenomenon.
Measurements of the magnetic field strength are
considerably less difficult since the presence of the
human body does not perturb the magnetic field.
Distances are measured between the screen surface and
the center of the probe for magnetic field values. As the
distance between the probe and the screen is decreased,
greater error will exist in the indicated value of magnetic
field strength. This spatial averaging error diminishes
rapidly with distance from the VDT since the field rapidly
becomes more uniform.
Because the magnetic field gradients are so great near the
VDT, significant error may occur if extra care is not
exercised when attempting repeated measurements at a
specific location. This is apparent when holding the
instrument without a tripod very near the screen and
attempting to obtain a constant reading of field strength.