since we took for f. € the value 0,1 + 0.01 as stated in the 
; ch — 
literature (9). 
C) Atmospheric correction 
When electromagnetic radiation is traversing the atmosphere, 
it may interact with atmospheric constituents in three fundamen- 
tal ways. It may be reflected, scattered or absorbed and reemit 
ted. Many techniques have been used to evaluate the total effect 
of atmospheric influence, ranging from simple methods to some 
very elaborated. 
Pickett (15) described a simple empirical method to correct 
infrared radiation measurements. He found that the correction 
formula was a linecr function of flight altitude and air tempe- 
rature, being these the most important factors introducing errors 
in the remotely measured surface temperatures. The correction 
formula he developed is: 
C = 1,54 + 0.00046 z - 0,043 T (11) 
where 
C = environmental correction 
z s altitude of flight (in feet), and 
T air temperature at 1,000 feat (in °C) 
Pickett concluded that the errors introduced by the atmospheric 
effects can be reduced by applying this empiricel correction. 
Figure 3 contains a number of correction factors in degrees 
centigrade, for a series of different temperatures at flight 
level and for different altitudes. These corrections should be 
added to the radiant temperature readings. 
For tke purpose of this paper, however, it wes felt that a 
more accure&te correction shouid. be used. The atmospheric path 
absorbs and reemitts at the air temperature; the slight opacity 
of thc atmosphere in the window region is due to absorption by 
water vapor, atmospheric gases, and aerosols. However, the 
absorption and reemission by water veper in this narrow spectral 
band (9.5 « 11.3 pm)excead that by any of the other species 
except aerosols under very dry and hazy conditions. The trans- 
mission error if not considered this influence is about a feu 
tenths of degree. 3esides being this error almost constant 
during a flight over a large area (if the flights are restricted 
to clear sky conditions)chenges of altitude are desirable in 
order to evaluate atmospheric peth disturbance. Vertical profiles 
of atmospheric temperature and humidity are also needed. 
If we consider the form of the radiative transfer equation 
for an absorbing and emitting but not scattering atmosphere, we 
may establish: 
e 1 ea ius 
Toa = [bo 8 (ve), Cu) du + Tf 4e 8,0) dss 
ó Wo