What is fiber optic spectrometer?
Spectroscopy is a technique for measuring the intensity of light in the ultraviolet, visible, near-infrared and infrared wavelengths. Spectroscopic measurements are used in a wide variety of applications, such as color measurement, concentration detection of chemical components, or electromagnetic radiation analysis. Fiber optic spectrometer usually uses optical fiber as signal coupling devices to couple the measured light into the spectrometer for spectral analysis. Due to the convenience of optical fiber, users can build a spectral acquisition system very flexibly. The advantage of fiber optic spectrometers is the modularity and flexibility of the measurement system.
Portable spectrometer is the main component of optical instruments. Due to its advantages of high detection accuracy and speed, it has become an important measuring instrument used in spectroscopy measurement and is widely used in agriculture, biology, chemistry, geology, food safety, chromaticity calculation, environmental testing, medicine and health, LED testing, semiconductor industry, petrochemical industry, etc.
Working principle of fiber optic spectrometer
The compact structure of fiber optic spectrometer
includes an incident slit, a collimating objective, a grating, an imaging reflector, a color filter and an array detector, and also includes a data acquisition system and a data processing system. The optical signal is projected onto the collimating objective lens through the incident slit, the dispersive light is reflected onto the grating, and the spectrum is presented on the receiving surface of the array receiver by the imaging reflector after dispersion, forming the spectral surface. The spectral surface is both the sequence of monochromatic light arrangement (with sub-spectral influence) so that the entire spectrum of any one tiny spectral band irradiation to the corresponding detector image elements, where the light signal into electronic signals, after analog to digital conversion, A/D amplification, and finally by the electrical system control terminal display output. Thus, various spectral signal measurements and analyses are completed.
Feature of fiber optic spectrometer
a. Fiber optic spectrometer is a derivative of the introduction of fiber optic technology. It allows the object to be measured to be removed from the confines of the sample cell, and the sampling method becomes more flexible, using fiber optic probes to direct the spectral source of the sample away from the spectroscopic instrument. It can adapt to the complex shape and position of the sample to be measured. The introduction of an optical signal by optical fiber can also isolate the interior of the instrument from the external environment, which can enhance the resistance to harsh environments (humid climate, strong electric field interference, corrosive gases), ensuring the long-term reliable operation of the spectrometer and prolonging its service life.
b. The fiber optic spectrometer uses a charge-coupled device (CCD) array as a detector, and the scan of the spectrum does not have to move the grating. It can perform transient acquisition with very fast response (measurement time is 13-15ms) and real-time output through a computer.
c. Fiber optic spectrometer uses holographic grating as the spectroscopic device, with low stray light, which improves the measurement accuracy.
d. The application of computer technology in fiber optic spectrometer has greatly improved the intelligent processing capability of the spectrometer.
Composition of fiber optic spectrometer
The basic configuration of a fiber optic spectrometer includes a slit, a grating and a detector. The parameters of these components must be detailed when purchasing a spectrometer. The performance of the fiber optic spectrometer depends on the precise combination and calibration of these components. After calibration of the fiber optic spectrometer, these components cannot be changed in any way.
Fiber optic spectrometer parts
A. Optical grids
The choice of grating depends on the spectral range as well as the resolution required. For fiber optic spectrometers
, the spectral range is usually between 200nm and 2500nm. If the resolution requirement is high, it is difficult to obtain a wide spectral range; at the same time, the higher the resolution requirement, the lower the luminous flux will be. For lower resolution and wider spectral range requirements, a grating of 300 lines/mm is the usual choice. If a higher spectral resolution is required, this can be achieved by choosing a 3600 lines/mm grating, or by selecting a detector with more pixel resolution.
A narrower slit increases the resolution but has a smaller luminous flux; on the other hand, a wider slit increases the sensitivity but loses resolution. In different application requirements, the appropriate slit width is chosen in order to optimize the overall test results.
The detector determines in some respects the resolution and sensitivity of a fiber optic spectrometer. The light-sensitive area on the detector is in principle limited and it is divided into many small pixels for high resolution or into fewer but larger pixels for high sensitivity. Usually, the back-sensitive CCD detector sensitivity is better, so you can somewhat in the case of no sensitivity to obtain a better resolution.
Due to the multi-level diffraction effect of the spectrum itself, the use of filters reduces the interference of multi-level diffraction. Unlike conventional spectrometers, fiber optic spectrometers
are implemented by coating the detector, and this part of the function needs to be installed in place at the factory. This coating also has an anti-reflection function to improve the signal-to-noise ratio of the system. The performance of a spectrometer is mainly determined by the spectral range, optical resolution and sensitivity. Changes to one of these parameters will usually affect the performance of the others.
The main challenge of a spectrometer is not to maximize all parameters at the time of manufacture, but to make the spectrometer specifications in this three-dimensional space choose to meet the performance requirements for different applications. This strategy allows the spectrometer to meet the customer's needs for maximum return on a minimal investment. The size of this cube depends on the specifications to be achieved by the spectrometer, and its size is related to the complexity of the spectrometer and the price of the spectrometer product. The spectrometer product should be fully compliant with the technical parameters required by the customer.
E. Spectral range
Spectrometers with a small spectral range usually give detailed spectral information, in contrast to large spectral ranges which have a wider visual range. Therefore, the spectral range of a spectrometer is one of the important parameters that must be clearly specified. The main factors affecting the spectral range are the grating and the detector, which are selected accordingly to the different requirements.
It is important to distinguish between sensitivity in photometry (the minimum signal intensity that can be detected by a spectrometer) or sensitivity in chemometrics (the minimum difference in absorbance that can be measured by a spectrometer).
Optical resolution is an important parameter to measure the spectroscopic capability. If you need a high optical resolution, it is recommended that you choose a grating with 1200 lines/mm or higher line logs, along with a narrow slit and a CCD detector with 2048 or 3648 pixels.
Fiber optic spectrometer application
A. Color measurement
In general, color measurements of objects and thick liquids can be performed using different experimental layouts. For example, reflective fiber optic probes or integrating spheres are used. For this measurement, a spectrometer with a wavelength range of 380 to 780 nm and a resolution (FWHM) of 5 nm can be used; in addition, a white continuous light source and a white reflective tile are required. Different fiber optic probes can be used for different applications such as measuring textiles, paper, fruit, wine, bird feather colors, etc.
B. UV/visible absorption spectroscopy measurements
UV/Visible Absorption Spectroscopy Measurements
Absorbance measurements of liquids can be achieved with different experimental layouts and wavelength ranges, such as in-line absorbance measurements using an immersion fiber probe or flow cell, or absorbance measurements of samples using a sample holder. For spectrometers measuring in the UV/visible wavelength range, the wavelength range 200-1100 nm with a resolution of 1.4 nm (FWHM) can be selected. A deuterium-halogen lamp is also required as a light source. Different fiber optic probes can be selected for different applications.
C. Oxygen concentration sensor
The oxygen concentration sensor consists of a fiber optic fluorescence probe with a proprietary technology film coating on the probe surface and a blue LED as the excitation source and a highly sensitive miniature spectrometer. The sensor applies fluorescence technology to measure the absolute content of oxygen, and the fluorescence generated by the sample is reflected back to the detector. When oxygen in a gaseous or liquid sample diffuses onto the membrane layer of the probe, it causes a fluorescence burst, the degree of which is correlated with the concentration of oxygen in the sample.
D. Gemstone composition testing
Color is one of the determining factors in determining the color of a diamond, and natural and man-made diamonds can be detected with light in the wavelength range of 400-750nm. The characteristic wavelengths of 415nm and 478nm can be found in the absorption spectrum of natural Class LA diamonds, while man-made diamonds have no absorption peaks at this wavelength. Wavelengths of 592 nm and 741 nm can be detected in synthetic diamonds. Moreover, the difference in the amplitude of the absorption peaks between natural and synthetic diamonds is nearly 10 times. Of course, other gemstones can also be detected by this method, such as ruby, alexandrite, sapphire, etc.
E. Fluorescence measurements
Fluorescence detection techniques are required in many applications such as biology (chlorophyll and carotenoids), biomedicine (fluorescence diagnosis of malignant diseases) and environmental applications. Fluorescence detection usually requires high-sensitivity spectrometers. In most applications the fluorescence energy is only 3% of the excitation light energy, the wavelength is longer than the excitation light, and the light is scattered. In a fluorescence measurement system, it is important to avoid excitation light entering the spectrometer.
How to buy fiber optic spectrometer?
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