1.2 x wavelength x altitude 
  
GRE = 
diameter 
3.1. CAMERAS 
Cameras operate from the ultraviolet up to the near in- 
frared part of the electromagnetio spectrum. The collector 
is a glass lens system, but the other components vary with 
the pyte of camera. Most photographic sensors record reflec- 
ted sumlight on film which are sensitive up to about l.l mi- 
crone. This limit is due to the decrease in the number of 
photons emitted by the sun in the infrared. On the ultravio- 
let side of the electromagnetic spectrum, the limit is cau- 
sed by the camera lens which absorbs the ultraviolet radia- 
tion and by the atmospheric ozone that absorbs the ultravio- 
let sunlight both as it enters and leaves the atmosphere. 
3.2. RADIOMETERS 
radiometer measures the difference between the sour- 
radiation incident on the radiometer detector and ra - 
diant energy reference level. Both absolute and relative ra- 
diation levels may be obtained with & radiometer. Since ab- 
solute levels of radiation are defined with respect to abso- 
lute zero, the level or radiant power incident upon the de- - 
tector must be compared with a known reference level to de- 
rive the absolute radiant power level. 
Radiometers that measure the difference in radiation 
"rom two neighboring spatial positions provide relative in- 
formation only as no reference exists. Radiometers, used to 
compare any element of a large area to the average radiation 
associated with the entire area, can supply an absolute mea- 
surement, providing the average intensity is known and used 
as a reference level. Similarly, in the time domain, the po- 
wer level due to radiation at one instant may be compared 
to that of previous instant or to an average associated 
with all past measurements. 
3.3. SCANNERS 
A scanner basically is a camera or radiometer type of 
sensor vhose collector oscillates across the ground track, 
5