9 NIR Optics and Measurement Methods
223
9.1.4.3 Lead Sulfide (PbS)
Lead sulfide (PbS) is a type of photoconductive element that decreases in resistance
when light is incident on it. PbS is often used in NIR spectrometers because of its
sensitivity across the range 900–2500 nm. However, since the decrease in resistance
varies with the area exposed to light, it is necessary to design it so that the beam’s
diameter does not fluctuate depending on the sample. Lead selenide (PbSe) is also a
NIR detector that can be used across the 1500–4500 nm range. In both cases, pink
noise (1/f noise) occurs when temperature rises. For this reason, it is recommended
to remove low-frequency components using an optical chopper. However, it is more
effective to speed up the wavelength sweep when used for NIR spectroscopy. Highspeed photovoltaic pn junction compounds of indium arsenide (InAs) and indium
antimonide (InSb), having the same wavelength range as PbS and PbSe, are also
available.
9.1.4.4 Photomultiplier Tube
In general, photomultiplier tubes (PMTs) are effective only for shorter wavelengths
in the NIR region because the photoelectric effect cannot be generated by long
wavelength light. The PMTs using a GaAs photocathode can be used across the
range 500–800 nm and PMTs utilizing In/InGaAs semiconductors cover the NIR
range up to 1700 nm. Since PMT was originally used for detection of faint light,
PMTs are rarely used for quantitative analysis.
9.1.4.5 Image Sensor
An image sensor is a multi-channel detector with multiple sensors. An array of photodiodes arranged in a row is called a linear image sensor, and a two-dimensional array
spread on a plane is called an area image sensor (in contrast, an element with a single
sensor is called a discrete semiconductor). There are charge coupled device (CCD)
and complementary metal–oxide–semiconductor (CMOS) image sensors, and these
devices have different coupling methods. As described above, a dispersive spectroscope can distribute light of different wavelengths to different angles. Therefore, an
image sensor can simultaneously capture a spectrum of a certain wavelength range
without separating the wavelengths individually, using a slit. A spectrometer that
takes out one wavelength at a time using a discrete detector is called a monochromator, and a spectrophotometer that measures multiple wavelengths simultaneously
is called a polychromator. A device using an image sensor does not require mechanical parts, so it can be made robust and downsized. Since the two-dimensional area
image sensor is used for a camera, it can be applied to spectral imaging in combination
with LCTFs and AOTFs (Chap. 22).
223
9.1.4.3 Lead Sulfide (PbS)
Lead sulfide (PbS) is a type of photoconductive element that decreases in resistance
when light is incident on it. PbS is often used in NIR spectrometers because of its
sensitivity across the range 900–2500 nm. However, since the decrease in resistance
varies with the area exposed to light, it is necessary to design it so that the beam’s
diameter does not fluctuate depending on the sample. Lead selenide (PbSe) is also a
NIR detector that can be used across the 1500–4500 nm range. In both cases, pink
noise (1/f noise) occurs when temperature rises. For this reason, it is recommended
to remove low-frequency components using an optical chopper. However, it is more
effective to speed up the wavelength sweep when used for NIR spectroscopy. Highspeed photovoltaic pn junction compounds of indium arsenide (InAs) and indium
antimonide (InSb), having the same wavelength range as PbS and PbSe, are also
available.
9.1.4.4 Photomultiplier Tube
In general, photomultiplier tubes (PMTs) are effective only for shorter wavelengths
in the NIR region because the photoelectric effect cannot be generated by long
wavelength light. The PMTs using a GaAs photocathode can be used across the
range 500–800 nm and PMTs utilizing In/InGaAs semiconductors cover the NIR
range up to 1700 nm. Since PMT was originally used for detection of faint light,
PMTs are rarely used for quantitative analysis.
9.1.4.5 Image Sensor
An image sensor is a multi-channel detector with multiple sensors. An array of photodiodes arranged in a row is called a linear image sensor, and a two-dimensional array
spread on a plane is called an area image sensor (in contrast, an element with a single
sensor is called a discrete semiconductor). There are charge coupled device (CCD)
and complementary metal–oxide–semiconductor (CMOS) image sensors, and these
devices have different coupling methods. As described above, a dispersive spectroscope can distribute light of different wavelengths to different angles. Therefore, an
image sensor can simultaneously capture a spectrum of a certain wavelength range
without separating the wavelengths individually, using a slit. A spectrometer that
takes out one wavelength at a time using a discrete detector is called a monochromator, and a spectrophotometer that measures multiple wavelengths simultaneously
is called a polychromator. A device using an image sensor does not require mechanical parts, so it can be made robust and downsized. Since the two-dimensional area
image sensor is used for a camera, it can be applied to spectral imaging in combination
with LCTFs and AOTFs (Chap. 22).