Wavelength accuracy and repeatability are one of the important quality indicators of spectrometers,which have a significant impact on the correct use of the instrument and even experimental results. In addition, due to external factors such as temperature, humidity, air pressure, bumps, as well as the increase in the use of the instrument itself, changes in internal factors such as the fiber optic emission angle, the diffraction ability of the grating, and the detection efficiency of the detector will affect the response of the spectrometer sensor. Therefore, the spectrometer needs to be calibrated regularly to obtain more accurate data.
Definition: Spectral calibration is to clarify the spectral response function of each channel of the imaging spectrometer, that is, to clarify the response of each pixel of the detector to different wavelengths of light, so as to obtain the central wavelength of the channel and the width of its spectral band.
In actual micro-fiber spectrometers, the wavelength of light wave is reflected by the CMOS pixels. Therefore, in actual measurements, the the influence of environment and time can cause changes between the wavelength of the light wave and the pixels. The actual wavelength of light wave corresponding to each CMOS pixel in the spectrometer must be accurately determined, otherwise the accuracy of the measurement will be reduced. Therefore, in order to obtain accurate measurement results, the spectrometer must be strictly calibrated before use to determine the correspondence between CMOS pixels and light wave wavelength.
As shown in Figure 1, the commonly used cross fiber spectrometer uses CMOS chips to collect spectral data. In order to obtain accurate measurement results, the spectrometer must be strictly calibrated before use to determine the correspondence between CMOS pixels and light wavelength.
Figure 1. Internal structure of a commonly used cross-type fiber optic spectrometer
The commonly used spectrometer wavelength calibration is to use the characteristic spectrum to find the corresponding positions on the corresponding pixel point of CMOS. For SR50C, the detection uses a 2048-unit linear array CMOS, and the measured spectrum is 200~1000nm. Each CMOS corresponds to about 0.4nm. The grating equation can be written as
Among them, m is the diffraction order, d is the grating constant, i is the incident angle (which can be considered as a constant), and θ is the diffraction angle. At small angles, it can be considered that (sinθ~θ~x). It can be seen that the wavelength and the diffraction order are approximately linearly related. Considering various problems such as large diffraction angles, we can use the least squares method to fit the third-order polynomial to obtain the minimum sum of squares of deviations.
In the formula, a0, a1, a2, a3 are the fitting coefficients, x1, x2,..., x6 are the measured pixel numbers, y1, y2,..., y6 are the fitted wavelengths. Use Matlab software to program and solve the fitting coefficients in y=a0+a1x1+a2x2+a3x3. Use mercury argon calibration light source for calibration.
Taking the SR50C micro-spectrometer developed by JINSP Technology as an example, the mercury-argon lamp spectrum of the spectrometer is shown in Figure 2.
Figure 2. Mercury-argon lamp spectrum of SR50C
According to the wavelength calibration results of SR50C, it can be seen that the product has a wide spectral range, supporting spectral customization within the range of 200-1000nm, and can achieve high-resolution spectral detection in the ultraviolet, visible light, and near-infrared bands.
The wavelength calibration of the fiber optic spectrometers is of great significance for their use. Choose JINSP Technology and get professional pre-sales and after-sales technical support to help you solve the wavelength calibration problem. Choose JINSP and choose high-quality service.
Link：Fiber Optic Spectrometers - JINSP Company Limited (jinsptech.com)
Post time: Jul-11-2024