Page 332 - Sensors and Control Systems in Manufacturing

Advanced Sensors in Pr ecision Manufacturing
(photons) to systems that control high-speed welding and cutting 289
lasers that produce kilowatts of optical power.
Sensors exist for almost any application imaginable: from a pho-
tomultiplier tube, which gives a large voltage pulse for every photon
it detects, to cooled thermopiles that absorb kilowatts of power pro-
viding a thermocouple voltage proportional to the optical power
absorbed. The following describes the most popular light sensing
technologies. Their characteristics are summarized in Table 6.2.
Photomultiplier tubes are special vacuum tubes that have a light
sensing surface, the photocathode, which absorbs incoming light
photons and emits secondary electrons. These secondary electrons
are accelerated and multiplied within the photomultiplier tube by
dynode plates. Each time an electron strikes a dynode, it has gained
enough momentum to create a larger number of secondary electrons.
This multiplication process continues for each dynode within the
tube. Tubes with 10 to 12 dynodes can easily generate multiplications
of more than a million, resulting in sufficient current to develop hun-
dreds of millivolts across an output 50-ohm load resistor for a single
incident photon.
Photomultiplier tubes provide the ultimate in detection sensitiv-
ity. They can sense the smallest amount of optical energy there is—an
individual photon. When cooled, it can be essentially noise-free, with
at most one false photon pulse in a one-second time period.
However, these light sensor detectors have a few drawbacks:
• Τhey are mechanically fragile.
• They need an extremely stable high-voltage power supply.
• They are relatively expensive.
• Shapes and sizes are very limited.
• They are susceptible to external magnetic fields.
• The available photocathodes are limited to sensing ultraviolet
to near-infrared wavelengths between 190 to 900 nm, and can
be extended to 1100 nm.
Photomultiplier tubes are generally used to detect the lowest
light levels where the application demands their superior sensitivity,
such as in high-speed spectroscopy applications.

6.5 Photodiodes
Photodiodes are light-sensitive semiconductor devices that are man-
ufactured in essentially the same way as semiconductor diodes used
in conventional electronic circuits. The primary differences are that
photodiode chips are larger and are packaged to allow light onto the
sensitive area of the diode.