changed by the availability of a variety of imaging sensors with 
one or more of the following characteristics: 
e Image space or object space line scan à 
e Narrow spectral bandwidth sensitivity 
e Sensitivity outside the visual portion of the electro- 
magnetic spectrum 
e Multiband sensitivity 
e Digital or analog encoding and recording of sensor response ( 
e Complex interaction of the object with reflected and emitted 
energy | 
The imagery often is not easily interpreted by visual inspection. If 
recorded on black-and-white film, the observed gray tones may have 
little relation to the familiar experience of the interpreter. And 
due to radiometric and geometric errors, the film density may have 
little absolute relation to the spectral energy emitted or reflected 
from the scene. 
Some instruments include radiometric reference sources that aid 
in calibrating the sensor response. Some also incorporate roll compen- 
sation and scan linearization to improve geometry. While such improve- 
ments are extremely useful, they do not completely solve the problems. 
For example, thermal mapping of powerplant discharges often requires 
an absolute temperature reference within a limit of less than 1° for 
the water surface. The scanner is affected by the varying atmospheric 
path length depending on local weather conditions, mainly humidity. 
Thus a carefully distributed series of ground instruments must sample 
water temperatures simultaneously with the aircraft overflight to 
provide a surface reference. The change of temperature with depth © € 
must also be obtained with ground instruments if volume computations 
are required. 
Similarly the roll compensation and scan linearization do not 
correct for pitch and yaw changes of the aircraft. The imagery is 
seldom referenced to ground control, causing a serious problem in 
comparing successive sets or examples of imagery. Since the neces- 
sity for computer analysis of sensor data is increasing, the geometric 
register and reference of imagery are critical. Register is the cap- 
ability to overlay one sensor output on another; reference is the 
capability to position all parts of the image in a defined ground 
coordinate system. 
Instrument calibration is more than verifying the manufacturer's 
specifications; rather, it is a determination of the systematic and 
random characteristics of an instrument under controlled, statistically 
valid conditions. Calibration results are of vital importance to the | 
data user. Occasionally the information can be extracted from tests 
conducted during manufacturing; more often, separate tests are required 
on the completed instrument. With photogrammetric cameras, for example, 
the nominal focal length and limiting distortion are interesting for 
planning purposes, but the calibrated focal length and calibrated dis- 
tortion are essential for accurate metrology. Likewise, the specified 
linearity of a television system gives a manufacturing goal, but the