Improved Wavelength Calibration by Modeling the Spectrometer

Abstract

Wavelength calibration is a necessary first step for a range of applications in spectroscopy. The relationship between wavelength and pixel position on the array detector is approximately governed by a low order polynomial and traditional wavelength calibration involves first-, second-, and third order polynomial fitting to the pixel positions of spectral lines from a well known reference lamp such as Neon. However, these methods lose accuracy for bands outside of the outermost spectral line in the reference spectrum. We propose a fast and robust wavelength calibration routine based on modelling the optical system that is the spectrometer. For spectral bands within the range of spectral lines of the lamp we report similar accuracy to second- and third-order fitting. For bands that lie outside of the range of spectral lines we report an accuracy 12-121 times greater than that of third-order fitting and 2.5-6 times more accurate than second-order fitting. The algorithm is developed for both reflection and transmission spectrometers and tested for both cases. Compared with similar algorithms in the literature that use the physical model of the spectrometer, we search over more physical parameters in shorter time, and obtain superior accuracy. A secondary contribution in this paper is the introduction of new evaluation methods for wavelength accuracy that are superior to traditional evaluation. 

Broadband coherent anti-Stokes Raman spectroscopy for small scale microplastic detection

“…Wavelength calibration is the first step required before performing intensity calibration, and is achieved using the method of polynomial fitting implemented by Liu and Hennelly. 17 The intensity calibration method used to correct the spectral intensity values of the recorded spectra is found in the work of Hutsebaut et al 18 Performing wavenumber calibration is required if the wavelength of the excitation source is not accurately known. This involves using a known reference spectrum with known peak positions, such as 4-acetamidophenol, and assigning a particular wavenumber value to each detector axis value.…”

Comparison of broadband-CARS and spontaneous Raman scattering for blood cell analysis

“…12, which first involves wavelength calibration using the method described in Ref. 13 followed by determining the wavelength dependent sensitivity of the Raman spectrometer by recording the spectrum of a NIST-calibrated white light and comparing the resultant spectrum with the spectrum provided by NIST. Each recorded spectrum is intensity calibrated by correcting the intensity values using this sensitivity.…”

Wavelength calibration with deep-learning using long short-term memory architectures

“…This work is based on a recent contribution in which we have used this model to augment traditional wavelength and Raman wavenumber calibration. 10 In summary, the parameters of interest that define the relationship between the wavelength axis λ, and the detector pixel axis (x, include the diffraction grating period d, the focal length of the parabolic mirrors f , the pixel width of detector T , the grating rotation angle θ d , the angle of the optical axis α, the misalignment of the centre of the detector array with respect to the optical axis C. The relationship between wavelength and pixel is given as follows:…”

“…It is the most popular and widely used method because of its efficiency and simplicity. Wavelength standards, such as neon and krypton are well characterised and provide excellent references for wavelength calibration using polynomial fitting [7][8][9][10] and similar references exist for direct raman wavenumber calibration. 6,11 A low-order (first, second or third order) polynomial is fit to a series of and pixel position and associated known reference wavelength coordinates.…”