Page 336 - Sensors and Control Systems in Manufacturing

292
S i x
Cha p te r

than photodiodes because they have built in gain: the absorbed light
creates a current in the base region of the phototransistor, resulting in
current gains from 100 to several thousands. Photodarlingtons have
two stages of gain, with net gains that can be greater than 100,000.
The built in gain allows the phototransistor to be coupled with a
load resistor to accommodate TTL level voltages for a wide range of
light levels. Because of their ease of use, low cost, and TTL-compati-
ble signal levels, phototransistors have become popular for applica-
tions where there is more than a few hundred nanowatts of available
optical power.
These devices however, do have some drawbacks compared to
photodiodes. The frequency bandwidth and linearity are relatively
limited and spectral response is restricted to between 350 and 1100 nm.
In addition, there are very large variations in sensitivity between
individual devices and few standard package options.

6.7 Photoconductive Sensors
A photoconductive sensor is a thick film semiconductor material
whose electrical resistance decreases with increasing incident light.
These rugged assemblies that can withstand hundreds of volts are
typically smaller than 0.25 inches in diameter.
Photoconductive sensors based on cadmium sulfide (CdS) have
sensitivity curves that closely match the sensitivity of the human eye.
Accordingly, they are useful in applications involving human light per-
ception such as headlight dimmers and intensity adjustments on infor-
mation displays. These sensors can be designed for measuring micro-
watts to milliwatts of optical power and are very inexpensive at high
volume (less than $0.10 each). These characteristics make CdS photo-
conductors the sensor of choice in applications such as street light con-
trol and in the toy industry where economy is a major consideration.
There are, however, considerations that limit the use of CdS pho-
toconductors in more sophisticated applications requiring sensitivi-
ties over a wide spectral range, small variations between individual
parts, or no history-dependent response. The resistance of these sen-
sors depends on the thick-film microstructure, so the resistance spec-
ification has a wide tolerance—a max/min ratio of 3 is not uncom-
mon. The resistance also has long-term memory that depends, at any
given time, on the amount of light actually incident on the sensor
plus the sensor light history for the past several days.
Photoconductors made from materials other than CdS such as
lead telluride and mercury cadmium telluride are also available.
These materials have spectral sensitivities that cover the range that
photodiodes cannot: above 2 µm out to 15 µm. This longer wave-
length sensitivity is very important for infrared imaging cameras and
for long wave instrumentation such as that used to monitor carbon