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Page Title: Figure 7-1. Photoconductive detector.
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TM 9-258
limited to the very near infrared. The most elemental
change in conductivity of a photoconductor upon
form of photoemissive cell is the vacuum photocell,
exposure to radiation, it is also necessary to supply a
consisting of a photo cathode and an anode in an
bias battery and a load resistor. The photoconductor of
evacuated enclosure.
dark resistance R is placed in series with a load resistor
c.  The photon effect most widely used in infrared
RL and a bias battery Vb. Infrared radiation, which is
detection is photoconductivity. The reason for this is that
modulated  or  chopped,  is  absorbed  by  the
photon effects in general are faster and more efficient
photoconductor, thereby causing its  resistance to
than thermal effects, and photoconductivity is the most
decrease with a corresponding increase in the current
flowing in the circuit and a greater portion of Vb to appear
simple of the photon effects. Photoconductive detectors
across RL. The voltage appearing across RL is coupled
as a class are easier to produce and easier to use than
through capacitor c to the input circuit of the amplifier,
detectors based upon the other photon effects.  The
and  represents  the  radiation  absorbed  by  the
photoconductive detector (fig 7-1) is placed in the input
photoconductor.
circuit of an amplifier.
In order to detect the
Figure 7-1. Photoconductive detector.
d.  Another internal photo effect of interest in
power which they absorb. The most well known forms of
thermal  detectors  are  the  bolometer  and  the
known as the photovoltaic effect. As the name implies,
thermocouple.  The bolometer has seen widespread
the action of photons in this case produces a voltage
military use, while the thermocouple has found extensive
which can be detected directly without need for bias
commercial application.
supply or load resistor. In addition to the photovoltaic
f.
A bolometer is a radiant power detecting
detector, the operation of the photodiode and the
device, the operation of which is based upon measuring
phototransistor are based upon this effect. That is, as
the temperature change in resistance of a material due
long as radiation falls upon the p-n junctions within the
to the heating effect of absorbed radiation (fig 7-2). The
body of a semiconductor, electronhole pairs will be
simplest form of bolometer is a short length of fine wire
formed and separated by the internal field at the junction.
whose resistance at a given temperature is known.
If the semiconductor ends are short circuited by an
Radiation allowed to fall upon the wire is partially
external conductor, a current flow in the circuit. On the
absorbed, causing a rise in temperature. The resistance
other hand, if the ends are open circuited by a high
change due to the change in temperature is a measure
impedance voltage measuring device, a voltage will exist
of the radiant power absorbed. Because bolometers are
as long as radiation falls upon the p-n junction.
e.  Thermal detectors make use of the heating
power detectors, they are capable of detecting radiation
of all wavelengths and are widely used as detectors of
effect of radiation. They are simply energy detectors and
microwave power.
their responses are dependent upon the radiant
7-2

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