called the electromagnetic spectrum (Figure 1-2). Visible light, for example, is only a minute
portion of this spectrum. All other forms of electromagnetic energy can only be seen and
measured by mechanical means.
8. IR radiation is divided into three regions based on the location in the frequency spectrum
in relation to the visible light range. These regions are:
Near --0.7 to 1.2 microns.
Intermediate --1.2 to 7.0 microns.
Far --7.0 to 1,000 microns.
IR radiation wavelengths of .09 to 1.3 microns can be detected by and recorded on
IR film used in standard aerial cameras.
An overlap occurs on the visible light portion of the spectrum. For example, on a hot
day, while driving up an incline, heat appears as water on the surface of the pavement. This is
a small portion of IR that overlapped into the visible light portion of the electromagnetic
9. Semiconductors. The emergence of solid state semiconductors after World War II has
permitted greater sensitivities and response to longer wavelengths. Current operational IR
sensors employ semiconductor detectors to record selective infrared emissions from 0.3 to
16.0 microns. IR sensitive film detects radiation reflected by an object; therefore, it required
an external light source such as sunlight. The semiconductor-type detectors detect inherent
IR radiation from objects. Hence, semiconductors can be used day or night with no external
Detectors used in semiconductor-type IR sensors depend upon that portion of the IR
region under surveillance. Semiconductor IR sensors detect emitted energy and
convert it to proportional electrical signals. These signals are in turn amplified and
converted to usable imagery.
10. IR systems. The IR detecting system used in the Army National Guard is the AN/AAS-24
IR mapping system, mounted in the OV-1D Mohawk aircraft. The US Air Force has the
AN/AAD-5 IR reconnaissance set, mounted in the RF-4C aircraft. These systems scan the
terrain directly below the aircraft and record detected temperature variances. The US Air
Force also has the AN/AVQ-26 Pack Tack line-of-sight, forward-looking IR reconnaissance
sensor; it can be mounted under the RF-4C, F-4E or F-111F aircraft. The AN/AVQ-26 has two
basic operating modes of search and track. The search mode is used to locate the target.
The track mode is used to accurately track targets after acquisition. These systems do not
emit signals like radio or radar. For this reason, IR detecting systems are considered