PhRC NEWSLETTER PHOTONICS'La - Nanyang Technological ...

Now let’s take a look at the physics behind thermal
imaging.
Infrared Radiation
According to the Planck’s Law (as shown in Fig. 3), the
radiation of an object is determined by the object temperature.
A room-temperature object radiates at the peak
in the wavelength range of 7–14 microns, which is called
long-wave infrared (lwir) waveband. For high temperature
(∼ several hundred ◦ C ) object, the radiation peak
not only rises but also shifts to shorter wavelength, e.g.
the mid-wave infrared (mwir) waveband 3–5 microns).
The Stefan-Boltzmann Law relates the total radiant intensity
I (over the whole frequency range) of a blackbody
with temperature T by I = σT 4 ,whereσ is the Stefan-
Boltzamann constant (5.67 × 10 −8 Wm −2 K −4 ). Therefore,
in principle, the temperature of an object can be determined
from the radiation emitted by the object integrated
over all wavelengths. However, the Planck’s Law
describes the radiation from an ideal object with unity
coefficient of emissivity, the so-called blackbody. Real objects,
however, will emit less radiation as the emissivity
is less than unity. For example, the emissivity of human
body is commonly accepted as 0.95. One can determine
the nominal temperature from the measured radiation by
assuming the real body as a blackbody, but the emissivity
has to be known in order to determine the actual temperature.
Power Exitance [W/(cm 2 ·s·µm)]
0.1
0.08
0.06
0.04
0.02
600 K
500 K
400 K
300 K
0 5 10 15 20
Wavelength (µm)
Figure 3: Planck’s blackbody radiation law
Bolometer—the sensor of the thermal
imager [5]
There are two fundamental types of infrared detectors -
thermal detectors and photon detectors. In photon de-
tectors, photo-carriers are generated by absorbing the infrared
photons and the electric signal is proportional to
the number of the received photons. These detectors may
operate in photoconductive, photovoltaic, or photoelectromagnetic
modes. Because the infrared photon energy
(∼ 0.1 eV) is very small, narrow bandgap materials, such
as InSb, PbSnTe and HgCdTe, or multiple quantum wells
have to be used for infrared detection. In order to suppress
the current leakage due to thermionic emission, the
detectors have to work inside a cryogenic cooling system
which is bulky, costly and cumbersome to use.
In thermal detectors, the infrared radiation interacts
with the lattice of material and changes the material
temperature, which is subsequently converted to a measurable
electrical quantity. The electric signal is proportional
to the radiation power. Typical thermal detectors
are thermistors, thermocouples, and pyroelectric/ferroelectric
detectors. The thermal imagers in the
market today use the resistive microbolometer, where a
membrane, thermally isolated from the substrate, is used
to obtain a high temperature rise in response to the incoming
radiation, and an embedded temperature sensitive
resistor subsequently transfer the temperature deviation
into electric signal. A material with large temperature
coefficient of resistivity (tcr) is used to enhance the
sensitivity of the bolometer.
The most promising benefit of thermal detectors is
their ability to operate near room temperature (or an uncooled
environment), where only a temperature stabilizer
(i.e. thermo-electric cooler, tec) is needed rather
than the more complicated cryogenic cooling system,
thus lowering costs and increasing reliability. A commercial
imager uses an array of microbolometers, called Uncooled
Imaging Array, and a Germanium lens to focus the
IR radiation from the target object onto the sensors.
How accurate can an IR detector
measure body temperature?
This question has been raised in response to the rather
controversial claims made by some companies about the
accuracy of the imagers they are marketing. We asked our
expert, Prof Mei Ting, a member of PhRC, who has this
to say:
In thermal imaging cameras, the object temperature is
not measured directly, but indirectly through the IR radiation
incident on the detector in the imager. This received
radiation is determined not only by the object temperature,
but also by several other factors.
THERMAL IMAGING September 2003 23