EM 1110-2-2907
1 October 2003
mosphere, ultraviolet light is greatly absorbed by ozone (O3) and becomes an important
tool in tracking changes in the ozone layer.
(2) Visible Light. The radiation detected by human eyes is in the spectrum range
aptly named the visible spectrum. Visible radiation or light is the only portion of the
spectrum that can be perceived as colors. These wavelengths span a very short portion of
the spectrum, ranging from approximately 0.4 to 0.7 m. Because of this short range, the
visible portion of the spectrum is plotted on a linear scale (Figure 2-8). This linear scale
allows the individual colors in the visible spectrum to be discretely depicted. The shortest
visible wavelength is violet and the longest is red.
(a) The visible colors and their corresponding wavelengths are listed below
(Table 2-2) in micrometers and shown in nanometers in Figure 2.8.
Table 2-2
Wavelengths of the primary colors of the visible spectrum
Color
Wavelength
0.40.446 m
Violet
0.4460.500 m
Blue
0.5000.578 m
Green
0.5780.592 m
Yellow
0.5920.620 m
Orange
0.6200.7 m
Red
(b) Visible light detected by sensors depends greatly on the surface reflection
characteristics of objects. Urban feature identification, soil/vegetation discrimination,
ocean productivity, cloud cover, precipitation, snow, and ice cover are only a few exam-
ples of current applications that use the visible range of the electromagnetic spectrum.
(3) Infrared. The portion of the spectrum adjacent to the visible range is the infra-
red (IR) region. The infrared region, plotted logarithmically, ranges from approximately
0.7 to 100 m, which is more than 100 times as wide as the visible portion. The infrared
region is divided into two categories, the reflected IR and the emitted or thermal IR; this
division is based on their radiation properties.
(a) Reflected Infrared. The reflected IR spans the 0.7- to 3.0-m wavelengths.
Reflected IR shares radiation properties exhibited by the visible portion and is thus used
for similar purposes. Reflected IR is valuable for delineating healthy verses unhealthy or
fallow vegetation, and for distinguishing among vegetation, soil, and rocks.
(b) Thermal Infrared. The thermal IR region represents the radiation that is
emitted from the Earth's surface in the form of thermal energy. Thermal IR spans the 3.0-
to 100-m range. These wavelengths are useful for monitoring temperature variations in
land, water, and ice.
trum from 1 m to 1 m (bands are listed in Table 2-3). Microwave radiation is the longest
2-9
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