handbook of modern sensors

434 14 Light Detectors
In applications, where high accuracy is not required, such as thermal motion
detection, the bias resistor can be replaced with one or two zero-biased parallelopposite
connected silicon diodes.
For the detection of thermal radiation, a distinction exists between two cases in
which completely different demands have to be met with respect to the pyroelectric
material and its thermal coupling to the environment [7]:
1. Fast sensors detect radiation of high intensity but very short duration (nanoseconds)
of laser pulses, with a high repetition on the order of 1 MHz. The sensors
are usually fabricated from single-crystal pyroelectrics, such as lithium tantalate
(LiTaO 3 ) or triglycinesulfate (TGS). This assures a high linearity of response.
Usually, the materials are bonded to a heat sink.
2. Sensitive sensors detect thermal radiation of low intensity, but, with a relatively
low rate of change. Examples are infrared thermometry and motion detection
[8–10]. These sensors are characterized by a sharp temperature rise in the field
of radiation. This generally requires a good thermal coupling with a heat source.
Optical devices, such as focusing lenses and waveguides, are generally employed.
A heat transfer to the environment (sensor’s housing) must be minimized. If well
designed, such a sensor can have a sensitivity approaching that of a cryogenically
cooled quantum detector [7]. Commercial pyroelectric sensors are implemented
on the basis of single crystals, such as TGS and LiTaO3, or lead–zirconate–titanate
(PZT) ceramics. Polyvinylidene fluoride (PVDF) film is also occasionally used
because of its high-speed response and good lateral resolution.
14.6.4 Bolometers
Bolometers are miniature resistive temperature detectors (RTDs) or thermistors (see
Section 16.1.3 of Chapter 16) or other temperature-sensitive resistors which are
mainly used for measuring root-mean-square (r.m.s.) values of electromagnetic radiation
over a very broad spectral range from mid-infrared to microwaves. Applications
include infrared temperature detection and imaging, measurements of local
fields of high power, the testing of microwave devices, radio-frequency (RF) antenna
beam profiling, testing of high-power microwave weapons, monitoring of medical
microwave heating, and others. The operating principle is based on a fundamental
relationship between the absorbed electromagnetic signal and dissipated power [11].
The conversion steps in a bolometer are as follows:
1. An ohmic resistor is exposed to electromagnetic radiation. The radiation is absorbed
by the resistor and converted into heat.
2. The heat elevates the resistor’s temperature above the ambient.
3. The temperature increase reduces the bolometer ohmic resistance.
A temperature increase is a representation of the electromagnetic power. Naturally,
this temperature differential can be measured by any suitable method. These methods
are covered in Chapter 16. Here, we briefly outline the most common methods of
bolometer fabrications which evolved quite dramatically since Langley first invented
a bolometer over 100 years ago.