Triangular linetype wavelength calibration process for micro-spectrometers with multichannel detectors

An advanced wavelength calibration process with higher wavelength accuracy is developed based on the conventional calibration method of micro-spectrometers with multichannel detectors. In the conventional method, the precision of the wavelength position on the CCD for calibration is one pixel. One pixel usually has the length of a few microns, but the spectral line's peak is usually at a point in the pixel. Therefore, the location of a spectral line's peak can't be measured accurately. The error of the wavelength-peak location measurement results in an error of the pixel-wavelength calibration. A new method, applying triangular linetype to approximate the real spectral linetype, which improves the wavelength-peak location accuracy, is introduced in this article. The method is proved useful in theory and a formula for calculating the accurate pixel location is derived from the method. Two steps are added to the conventional method before the final pixel-wavelength fitting. Firstly, three-pixel data collection is carried out. In the conventional method, only the pixel of the wavelength-peak position is chosen for calibration, but it is not enough for getting more precise calibration result. In the new method, three pixels, including the original one and its two neighbors, are collected for the more precise wavelength-peak location process. Secondly, sub-pixel calculation is applied to improve the wavelengthpeak location accuracy. On the assumption that a triangular linetype can approximate the real spectral linetype in theory, the wavelength-peak location is calculated more accurately and the precision of the result is better than 0.5pixel. The experiment was carried out on a compact spectrometer HR4000 with the crossed Czerny-Turner optical design, which was produced by Ocean Optics Company. Its manufacturer's calibration precision is 0.24nm. Mercury lamp's line spectrum were used as wavelength standards. The results show that by applying the new method, the wavelength-peak location precision improves from 1 pixel to around 0.5 pixel, and the wavelength accuracy improves from above 0.2nm to around 0.1nm.