Image Processing Reference
In-Depth Information
12
Chapter 2
sphere is several hundred kilometers thick, most of the atmospheric mass is located
close to the earth's surface. For light propagation, the earth's atmosphere is transpar-
ent over a wide range of wavelengths of light, which is most important since we rely
on the thermal and visible radiation that comes from the sun to survive.
The atmosphere is commonly divided into four main regions known as the tro-
posphere, stratosphere, mesosphere, and thermosphere, each separated by a layer of
constant temperature (Hardy 1998) based on thermal changes in the layers as
shown in Fig. 2.1, which illustrates atmospheric temperature and pressure varia-
tions with height. From an optical perspective, it is important to know the tempera-
ture and pressure of the atmosphere because the index of refraction of the earth's at-
mosphere depends on them (Hardy 1998):
(
)
(
)
Pr
Tr
(
)
17910
6
−
nr
=+
⋅
,
(2.1)
where
n
is the index of refraction;
P
and
T
represent the pressure and temperature at
the point
r
, respectively. Clearly, the effects of temperature and pressure are small;
Figure 2.1
Plot of the atmospheric pressure and temperature variation with height.
The solid line shows the pressure in kPa; the dashed line, the temperature in degrees
Centigrade. The narrow dip in the temperature plot identifies the mesosphere layer.
