Figure 7-1. Photoconductive detector.

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

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