The design of an infrared spectrometer can generally be divided into optical system, control system, and signal and data processing system.

optical system

The optical system of an infrared spectrometer mainly consists of three parts: a light source, a spectral system, and a detector. In addition, there are components such as a diaphragm and a reflector.

1. Infrared light source

Infrared light sources can provide continuous infrared light within a certain wavelength range as needed. Generally, the wavelength range required for mid infrared is 2.5-25 μ m, and the wavelength range required for near-infrared is 0.8-2.5 μ m. Early infrared light sources used Nernst Glower lamps, but now most stable silicon carbide rods (Globars) are used as mid infrared light sources. Near infrared light sources generally use tungsten lamps, which can cover 0.8-2.5 μ m.

2. Spectral system

The spectroscopic system is the core of an infrared spectrometer. We know that Newton used prisms to split sunlight into rainbow bands of red, orange, yellow, green, blue, indigo, and purple. Early infrared instruments used prisms for spectroscopic analysis, but now it has basically disappeared and most people have never seen it. Later on, grating splitting was used to separate light by wavelength using a reflective grating, which is relatively less commonly used nowadays. Grating type infrared instruments are generally seen in specific fields. The above two types of spectrometers are called dispersive infrared spectrometers.

The infrared spectrometer commonly used nowadays is generally called Fourier Transform Infrared Spectrometer (FTIR), which has advantages such as fast scanning speed, high signal-to-noise ratio, and higher resolution compared to dispersive infrared spectrometers. It is currently the mainstream infrared spectrometer. Its core is the Michelson interferometer, consisting of a fixed mirror, a moving mirror, and a beam splitter, which generates interference light signals. A beam splitter is a semi transparent and semi reflective infrared optical material that divides light into two parts. The distance of the moving mirror determines the resolution, and the speed of the motion determines the sampling rate.

3. Detector

The detector is used to receive infrared light signals and convert them into electrical signals. Common types include pyroelectric detectors (DTGS), which operate at room temperature and can meet routine testing requirements. There is also a highly sensitive mercury cadmium telluride (MCT) detector that requires liquid nitrogen cooling (77 K) for detecting weak signals. The above are common detectors for mid infrared, while near-infrared generally uses electrically cooled indium gallium arsenide detectors.

The above three parts are the infrared light generated by the light source, which enters the spectral system for modulation, and then enters the detector. In practical use, we need to adjust the intensity of light, usually by adding a diaphragm between the light source and the spectral system.

control system

The control system mainly controls the power supply of the light source, the size of the aperture, the scanning speed, and the reception of detector signals through an electronic system. For example, in a Fourier transform infrared spectrometer, we can control the operating speed of the interferometer and adjust the size of the aperture through software. In addition, some instruments have richer configurations, such as multiple detectors or light sources, which can be automatically switched through the control system.

data processing system

The data processing system converts the signals received by the detector into the final infrared spectrum, including a signal processing unit, computer, and software. The signal received by the detector is amplified, filtered, and digitized by the signal processing unit. The preliminary signal digitized by Fourier transform infrared spectrometer is an interferogram, which will be further Fourier transformed into a single channel spectrum. Through software processing, we can obtain the infrared spectra such as absorption, transmission, and reflection that we need.