The term infrared is a Latin word meaning
beyond the red. Infrared is commonly shortened to
IR. The process of detecting or sensing infrared
radiation from a target without being in physical
contact with that target is known as remote sensing.
Active and passive systems are used for remote
Active systems send a signal to the target and
receive a return signal. Radar sets are examples of
active systems. Passive systems detect a signal or
disturbance originating at the target. The signal may
be emitted either by the target or another source.
Photography using natural light is an example of a
Humans can see only a small part of the entire
electromagnetic spectrum. However, even though we
cannot see them, other parts of the spectrum contain
useful information. The infrared spectrum is a small
portion of the entire electromagnetic spectrum. IR
radiation is a form of electromagnetic energy. IR
waves have certain characteristics similar to those of
light and RF waves.
These characteristics include
reflection, refraction, absorption, and speed of
IR waves differ from light, RF, and
other electromagnetic waves only in wavelengths and
frequency of oscillation.
The IR frequency range is from about 300
gigahertz to 400 terahertz. Its place in the
electromagnetic spectrum (fig. 6-1 ) is between visible
Figure 6-1.-Electromagnetic spectrum.