SYS11000OM Rev G
[UF 350-HD, Generic Template]
FLIR Systems Polytech AB
Page
55
of
70
Antennvägen 6
SE-187 66 Täby
Sweden
1.12.2
Atmospheric Attenuation
Thermal radiation does not pass cleanly through the atmosphere to the detector.
Radiation can be scattered or attenuated as it passes through the atmosphere.
The net effect of this attenuation is to reduce the contrast of the target / background as it
is received by the detector. Two factors dominate the attenuation. The first is by
absorption which is primarily due to water vapor.
The second is scattering in dense fog, very dense smoke and dusty conditions. By using
a detector that operates in the 8 to 12 micron band, the effects of atmospheric
attenuation can be minimized. The result of Atmospheric attenuation is to reduce the
image quality and under severe conditions the image will appear washed out and lacking
in contrast reducing the detection range of targets.
1.12.3
Diurnal Cycle
The effect of the diurnal cycle is a major source of Infrared energy for thermal imaging
systems. In simple terms, objects warm up during the day as they absorb solar radiation
and cool at night as they relinquishing their radiation back to space. This continuous
cycle (taking radiation from space then giving it back) changes the surface temperatures
of all target and background objects. The rate at which the surface heats or cools is
dependent on the surface emissivity, its size, heat capacity, conductivity, and the
convective heating / cooling effects.
The surface of an object such as a military tank, even if it has the same emissivity as the
terrain, may heat and cool at a rate much slower than the terrain. This is due to the fact
that the tank’s higher conductivity replenishes lost surface heat more efficiently than the
soil replenishes the terrain. The soil surface is often largely decoupled from the main heat
reservoir that is the earth, and thus it heats and cools more rapidly than the tank surface.
The thermal contrast between the terrain and tank will pass through zero twice during a
24hr period. This thermal crossover occurs for most objects during a relatively short
space of time during which the detected thermal image will also lack contrast and clarity.
The operator will see the effect of the diurnal cycle as a reduction in image quality for a
short period twice a day.
