It belongs to dispersion spectrometer, and its monochromator is prism or grating, which belongs to single channel measurement, that is, only one narrow-band spectral element is measured at a time. After rotating the prism or grating and changing its orientation point by point, the spectral distribution of the light source can be measured.
With the development of information technology and computer, a new infrared spectrometer featuring multi-channel measurement has emerged, that is, in one measurement, the detector can simultaneously measure the information of each spectral element in the light source. For example, in the hard human transformation spectrometer, the entrance or exit slit is replaced by a coded template based on the grating spectrometer, and then the signal measured by the detector is processed by a computer. Compared with grating spectrometer, the signal-to-noise ratio of hard human transformation spectrometer is higher.
2. Fourier transform infrared spectrometer
It is non-dispersive, and its core part is a double-beam interferometer (as shown in the dotted box in Figure 4), commonly used Michelson interferometer. When the moving mirror moves, the optical path difference of two coherent beams passing through the interferometer changes, and the light intensity measured by the detector also changes, thus obtaining the interferogram. After the mathematical operation of Fourier transform, the spectrum B(v) of the incident light can be obtained:
Where I(x) is an interference signal; V is the wave number; X is the optical path difference between two beams of light.
The main advantages of Fourier transform spectrometer are:
Fourier transform infrared spectrometer
① Multi-channel measurement improves signal-to-noise ratio;
(2) There is no limitation of incident and emergent slits, so the luminous flux is high and the sensitivity of the instrument is improved;
③ Based on the wavelength of He-Ne laser, the accuracy of wave value can reach 0.01cm;
④ Increasing the moving distance of the moving mirror can improve the resolution;
⑤ The working band can be extended from visible light to millimeter level, thus realizing the measurement of far infrared spectrum.
The infrared spectrometer can measure emission spectrum and absorption or reflection spectrum. When measuring the emission spectrum, the sample itself is used as a light source; When measuring absorption or reflection spectrum, tungsten halogen lamp, Nernst lamp, silicon carbide rod and high-pressure mercury lamp (used in far infrared region) are used as light sources. The detectors used mainly include thermal detectors and photoelectric detectors. The former includes Golay battery, thermocouple, triethyl sulfate, triethyl deuterated sulfate, etc. The latter includes mercury cadmium telluride, lead sulfide and indium antimonide. Commonly used window materials are sodium chloride, potassium bromide, barium fluoride, lithium fluoride and calcium fluoride, which are suitable for near-infrared and mid-infrared regions. Polyethylene sheet or polyester film can be used in far infrared region. In addition, metal coated mirrors are often used instead of lenses.