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7 Light in Biology and Medicine
7.16 Physical Infrared Detectors
Just as semiconductors can be used to generate IR light, they also can be used to
detect various wavelengths of IR radiation. The basis for this technology is the
fact that valence electrons in a semiconductor are bound to atoms, but are not
very tightly bound, so that relatively small inputs of energy will release electrons
to become conducting. This energy requirement per electron is typically in the IR
photon energy range (but also in the visible light range, and with germanium, up to
UV light).
A bolometer is a device for measuring the temperature change due to incoming electromagnetic radiation. Bolometers for detection and measurement of the
intensity of infrared radiation are a good example. This is often done by using a
blackened thin sheet of metal which is insulated from the environment except for
a thermal conductor. The metal can act as a thermistor, i.e. change resistance by
temperature changes, or the metal can be thermally coupled to a separate thermistor.
By comparing the resistance of the exposed metal-thermistor to that left at a fixed
colder temperature, the heating effect of the radiation can be measured. In 1880,
the inventor Samuel Pierpont Langley constructed a bolometer which could detect
a cow a quarter of a mile away.
7.17 Infrared Instruments
7.17.1 IR Absorption Spectroscopy
Absorption spectroscopy has become an indispensable tool for the analysis of
complex molecules. Table 7.4 shows the range over which absorption IR spectra
are obtained.
For chemical solutions, absorption is measured by passing light through a given
concentration of the substance to see how much of the light gets absorbed, and then
compensating for the absorptivity of the solute and for the light that is scattered
and not absorbed. The schematic design of an infrared spectrometer (also called an
IR spectrophotometer) is shown in Fig. 7.8. The reference cell contains the solution
without the absorbing solutes, so that the solution spectra can be subtracted from
the sample spectra. The diffracting grating spreads the spectra into its component
frequencies, one of which is picked by the slit.
Table 7.4 IR spectroscopy range
Wavelength
Frequency
Frequency/c
Photon energy
Far IR
<1 mm
3.00 × 10 11 Hz
10 cm −1
0.00124 eV
Near IR >700 nm
4.28 × 10 14 Hz
14286 cm −1
1.77 eV
7 Light in Biology and Medicine
7.16 Physical Infrared Detectors
Just as semiconductors can be used to generate IR light, they also can be used to
detect various wavelengths of IR radiation. The basis for this technology is the
fact that valence electrons in a semiconductor are bound to atoms, but are not
very tightly bound, so that relatively small inputs of energy will release electrons
to become conducting. This energy requirement per electron is typically in the IR
photon energy range (but also in the visible light range, and with germanium, up to
UV light).
A bolometer is a device for measuring the temperature change due to incoming electromagnetic radiation. Bolometers for detection and measurement of the
intensity of infrared radiation are a good example. This is often done by using a
blackened thin sheet of metal which is insulated from the environment except for
a thermal conductor. The metal can act as a thermistor, i.e. change resistance by
temperature changes, or the metal can be thermally coupled to a separate thermistor.
By comparing the resistance of the exposed metal-thermistor to that left at a fixed
colder temperature, the heating effect of the radiation can be measured. In 1880,
the inventor Samuel Pierpont Langley constructed a bolometer which could detect
a cow a quarter of a mile away.
7.17 Infrared Instruments
7.17.1 IR Absorption Spectroscopy
Absorption spectroscopy has become an indispensable tool for the analysis of
complex molecules. Table 7.4 shows the range over which absorption IR spectra
are obtained.
For chemical solutions, absorption is measured by passing light through a given
concentration of the substance to see how much of the light gets absorbed, and then
compensating for the absorptivity of the solute and for the light that is scattered
and not absorbed. The schematic design of an infrared spectrometer (also called an
IR spectrophotometer) is shown in Fig. 7.8. The reference cell contains the solution
without the absorbing solutes, so that the solution spectra can be subtracted from
the sample spectra. The diffracting grating spreads the spectra into its component
frequencies, one of which is picked by the slit.
Table 7.4 IR spectroscopy range
Wavelength
Frequency
Frequency/c
Photon energy
Far IR
<1 mm
3.00 × 10 11 Hz
10 cm −1
0.00124 eV
Near IR >700 nm
4.28 × 10 14 Hz
14286 cm −1
1.77 eV