TM 5-6675-330-12&P
Effects of Prisms and Atmosphere on
Cube corner prisms, normally the type of reflector
used as a target for light emitting instruments, have
the property of reflecting light rays back in the same
direction as the incoming rays. This eliminates critical
angular alignment with respect to the EDM instrument,
The quality of the prism, defined by the flatness of the
surfaces and the accuracy of the 90 degree angles,
and the total number of prisms have a square law
effect on the maximum distance that can be
measured. For example, decreasing the quality of the
prisms ‘-y a factor of two increases by a factor of four
the number of prisms required to give the same
maximum range, or, for a given quality prism,
doubling the range would require a factor of four
increase in the number of prisms required.
Atmospheric factors such as absorption, scattering,
refraction, and background radiation tend to reduce
the maximum distance that can be measured.
Absorption and scattering relate to particles such as
dust, fog, smoke, rain/snow, etc., in the air and have
the effect of reducing maximum range in an
exponential manner. This atmospheric condition is
normally defined in terms of sea level visibility and,
for a given number and quality of prisms, can cause
wide variations in the maximum range achievable.
Variations in atmospheric pressure and temperature,
which cause localized changes in the index of
refraction of air, result in the bending of light rays and
can cause a reduction in range capability for a
given number of reflector prisms, These localized
variations are generally described as scintillation or
heat shimmer and are more pronounced along lines
having low ground clearance, As a general rule,
high scintillation conditions can be considered as a
square law function, thus requiring four times the
number of prisms to double the range, Light ray
bending due to more gradual gradients in the index
of refraction, especially over long ranges, can result
in an apparent change in a target’s position, This, in
the case of low ground clearance along a measured
line, could result in the loss of a target when going
from daytime to night time conditions, Back-ground
radiation, in the visible and infrared wave lengths,
has the affect of increasing electronic noise internal
to the instrument and may cause wide variations in
readings for a given distance, This condition normally
exists only when working with the EDM instrument
pointed close to the sun and should be avoided
because of possible damage to internal instrument
components.
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