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A. Ikehata
Sample
Reference
Detector
Light
Source
Spectrometer
White Light
Monochromatic
Light
(a)
Detector
Spectrometer
Sample
Reference
White Light
Monochromatic Light
(b)
Light
Source
Discrete detector
or
Array detector
Discrete detector
Fig. 9.1 Device configuration of NIR spectrometers. a Monochromatic light irradiation, and b white
light irradiation
(b) spectrometers, the sample is positioned just after the light source and irradiated
with white light. This configuration is often used for NIR applications, and achieves
high-speed detection by use of an array detector. However, type (a) is better than (b)
from the viewpoint of preventing damage to the sample by irradiation with light. A
sample and a reference are measured alternately, and a spectrum is calculated from
the ratio.
9.1.2 Near-Infrared Light Sources
9.1.2.1 Thermal Radiation
A light source that uses thermal radiation generated by heating a filament by passing
an electrical current through it (resistive heating) is the most popular light source in
the NIR region. Halogen lamps using tungsten filaments are inexpensive, constitute
a stable thermal radiation source, and are widely adopted in NIR spectrometers
on the market. Halogen lamps emit light with high brightness across the visibleto-infrared range by a reaction called the halogen cycle. To stabilize the halogen
cycle, not only the filament but also the inner wall of the lamp must be heated to a
sufficiently high temperature. Since the temperature is very high, care must be taken
to prevent failure of the seal and deterioration of the socket. The emission spectrum
of a halogen lamp is generally explained by Planck’s law, and has an emission peak
of approximately 1 μm. Higher filament temperatures result in higher brightness and
larger emission intensities of short wavelengths. Heated nichrome wire (wavelength
2–5 μm) and Glover (silicon carbide; Wavelength 1–50 μm), which are often used as
light sources for mid-and far-infrared wavebands, are also used in the NIR waveband.
Since emission characteristics follow Planck’s law, it is necessary to use a high
temperature by passing a large amount of current to produce the NIR waveband.
A. Ikehata
Sample
Reference
Detector
Light
Source
Spectrometer
White Light
Monochromatic
Light
(a)
Detector
Spectrometer
Sample
Reference
White Light
Monochromatic Light
(b)
Light
Source
Discrete detector
or
Array detector
Discrete detector
Fig. 9.1 Device configuration of NIR spectrometers. a Monochromatic light irradiation, and b white
light irradiation
(b) spectrometers, the sample is positioned just after the light source and irradiated
with white light. This configuration is often used for NIR applications, and achieves
high-speed detection by use of an array detector. However, type (a) is better than (b)
from the viewpoint of preventing damage to the sample by irradiation with light. A
sample and a reference are measured alternately, and a spectrum is calculated from
the ratio.
9.1.2 Near-Infrared Light Sources
9.1.2.1 Thermal Radiation
A light source that uses thermal radiation generated by heating a filament by passing
an electrical current through it (resistive heating) is the most popular light source in
the NIR region. Halogen lamps using tungsten filaments are inexpensive, constitute
a stable thermal radiation source, and are widely adopted in NIR spectrometers
on the market. Halogen lamps emit light with high brightness across the visibleto-infrared range by a reaction called the halogen cycle. To stabilize the halogen
cycle, not only the filament but also the inner wall of the lamp must be heated to a
sufficiently high temperature. Since the temperature is very high, care must be taken
to prevent failure of the seal and deterioration of the socket. The emission spectrum
of a halogen lamp is generally explained by Planck’s law, and has an emission peak
of approximately 1 μm. Higher filament temperatures result in higher brightness and
larger emission intensities of short wavelengths. Heated nichrome wire (wavelength
2–5 μm) and Glover (silicon carbide; Wavelength 1–50 μm), which are often used as
light sources for mid-and far-infrared wavebands, are also used in the NIR waveband.
Since emission characteristics follow Planck’s law, it is necessary to use a high
temperature by passing a large amount of current to produce the NIR waveband.