cific range of wavelengths for which there is a 
sensing capability. Those broad spectral bands 
which are of greatest importance in remote 
sensing research, listed in order of increasing 
wavelength, are: Gamma rays, X-rays, ultra- 
violet, visible, near infrared, thermal infrared, 
microwave, and radio waves. The energy 
within these specific wavelength bands that is 
transmitted, reflected, or emitted from objects 
is “what is being sensed.” 
r SENSOR SYSTEMS 
GAMMA RAYS 
fF SCINTILLATION COUNTERS, GAMMA- 
RAY SPECTROMETERS 
SOFT X-RAYS yp SCANNERS WITh FILTERED PHOTO- 
| MULTIPLIERS; IMAGE ORTHICONS 
SO 
| CONVENTIONAL CAMERAS & FILMS, 
SCANNERS WITH FILTERED PHOTO- 
| MULTIPLIERS; IMAGE VIDICON 
ULTRAVIOLET 
LIGHT VISIBLE LIGHT 
NEAR IR | 
INTERMEDIATE IR 
= CAMERAS _ WITH IR-SENSITIVE FILMS; 
=] SOLID-STATE DETECTORS IN SCAN- 
NERS & RADIOMETERS 
FAR f— SOLID-STATE DETECTORS IN SCAN- 
INFRARED | NERS & RADIOMETERS 
SEES 
PPP PPP 
MILLIMETER 
WAVES 
CENTIMETER 
WAVES 
ULTRA HIGH 
ft— RADAR, RF RECEIVERS IN SCANNERS 
& RADIOMETERS. 
}— ELECTROMAGNETIC PULSE TECHNI- 
ques 
FREQUENCIES 
10 KM — — | 
100 KM ina 
THE ELECTROMAGNETIC SPECTRUM 
FIGURE 1. The electromagnetic spectrum with appropri- 
ate sensor systems. 
What is the Source of Electromagnetic 
Energy? 
The sun has been the main source of energy 
for remote sensing. In addition, all matter at 
temperatures above absolute zero (0° K.) ra- 
diates (emits) electromagnetic energy and 
hence is an energy source. Sensing devices 
which record reflected and emitted radiation 
from natural sources are called passsive sensing 
systems. Cameras and thermal infrared scan- 
ners are examples of passive sensing devices. 
Artificial sources, such as flash bulbs and re- 
sonating tubes, also can be used as a source of 
energy. Devices which sense this energy are 
called active systems. Radar—which generates, 
transmits, and receives and records its own en- 
166 
ergy pulses—is an example of an active sen- 
sing system. 
What happens to Electromagnetic Energy? 
When photons of specific energy interact 
with matter, mass and energy are conserved in 
accordance with basic physical principles, and 
energy is either (a) absorbed, i.e., given up 
largely to heating matter; (b) emitted or re- 
emitted by the matter as a function of temper- 
ature and structure (at the same or different 
wavelength); (c) reflected, i.e., returned un- 
changed to the medium; (d) transmitted, i.e., 
propagated through the matter or the atmo- 
sphere; or (e) scattered, i.e., deflected by at- 
mospheric particles and lost ultimately to ab- 
sorption or further scatter, or (f) some combi- 
nation of two or more of these phenomena. 
It is important to understand that when 
electromagnetic energy of specific wavelength 
interacts with matter, a characteristic energy 
response occurs which is specific for that parti- 
cular kind of matter. 
By means of energy detecting and measuring 
devices, such as spectroradiometers, spectro- 
photometers, and precision radiation thermom- 
eters, one can determine the energy relation- 
ships between specific wavelengths of energy 
and various kinds of matter. Energy response 
curves can then be prepared as a function of 
wavelength. Comparison of these data will re- 
veal the wavelength band (i.e., portion(s) of 
the electromagnetic spectrum) where the en- 
ergy response for objects of interest are simi- 
lar and where they are different. Using this in- 
formation one can discriminate among signifi- 
cant objects on the resulting remote sensing 
data by selecting a sensing device which rec- 
ords in the wavelength bands showing greatest 
energy response difference for those objects. 
(On the remote sensing records these energy 
response differences are represented by gray 
tones, color, or signal output strength.) 
In some specific wavelength bands, called 
“molecular absorption bands,” strong absorp- 
tion by such atmospheric gases as ozone, water 
vapor, or CO. may preclude remote sensing of 
the earth’s surface from aircraft or spacecraft. 
Also, atmospheric dust and haze particles may 
preclude sensing in various portions of the 
electromagnetic spectrum. Haze particles, for 
example, cause extreme scattering of ultravi- 
olet energy and thus severely limit sensing 
capabilities in this band. Sensing in the visible 
spectrum may be precluded by fog, cloud cover, 
and dense smoke. Thermal infrared energy, 
however, can be sensed night or day through 
light fog or dense smoke. Microwave energy 
can penetrate nearly any weather condition, 
night or day, thereby permitting radar devices 
to obtain interpretable imagery. 
