Figure 6-5.-Photoconductive detector circuit.
Energy-Matter Interaction
There are two basic types of energy-matter
interaction.
They are the thermal effect and the
photon effect.
THERMAL EFFECT.— The thermal effect type
of energy-matter interaction involves the absorption
of radiant energy in the detector. This results in a
temperature increase in the detector element. The
radiation is detected by monitoring the temperature
increase in the detector. Both the elemental and
imaging forms of detectors use the thermal effect.
PHOTON EFFECT.— In the photon effect type
of energy-matter interaction, the photons of the
radiant energy interact directly with the electrons in
the detector material. Usually, detectors using the
photon effect use semiconductor material. There are
three specific types of photon effect detection. They
are photoconductivity, photoelectric, and photo-
emissive.
1. Photoconductivity.
Photoconductivity is the
most widely used photon effect. (See figure 6-5.)
Radiant energy changes the electrical conductivity of
the detector material. An electrical circuit is used to
measure the change in the conductivity.
2. Photoelectric (also referred to as photo-
voltaic). In the photoelectric effect (fig. 6-6), a
potential difference across a PN junction is caused by
the radiant signal.
The photocurrent (current
generated by light) is added to the dark current
(current that flows with no radiant input). The total
current is proportional to the amount of light that falls
on the detector.
3. Photoemissive.
The photoemissive effect
(fig. 6-7) is also known as the external photo effect.
The action of the radiation causes the emission of an
electron from the surface of the photocathode to the
surrounding space. The electron is photoexcited from
the Fermi level above the potential barrier at the
surface of the metal.
INFRARED IMAGING SYSTEMS
An infrared imaging system has the following
components: detectors, a scene dissection system,
front end optics, a refrigeration system, and an image
processing system.
Detectors
Detectors convert the IR radiation signal into an
electrical signal that is processed into information
used by the operator. Detectors can be arranged in
many different configurations for their use in an IR
imaging system.
DETECTOR ARRAY.— Only a small portion of
the image scene is taken by a detector (or detectors) to
Figure 6-6.-Photoelectric effect.
Figure 6-7.-Photoemessive effect.
6-5
