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Calibrating the Spectrometer

To convert pixel axis to wavelengths or Raman shifts, the spectrometer needs to be calibrated. It is recommended to perform regular checks on the calibration, especially for sensitive measurements.

Calibration is done using a calibration lamp. Currently, Spectrum Analyzer supports two type of calibration lamp: Mercury Argon and Neon. For the Starter and Performance Editions, we recommend using a Neon calibration lamp.

To calibrate your spectrometer, first take a clean spectrum of your calibration lamp using both blank measurement and the baseline removal of the software. You should also allow the lamp to warm-up according to its manufacturer recommendations.

Once you have acquired a clean spectrum, open the calibration window by clicking the button in the toolbar. Adjust the number of peaks and sensitivity such that the software correctly identifies your peaks. Note that the software is robust to missing peaks but does not tolerate erroneous peak.

Next, set the algorithm parameters and click the “Calibrate!” button. The quality of the fit will be shown in terms of a RMS figure. A RMS much below the resolution of the spectrometer indicate a good fit quality. When you are confident in the quality of the fit, you can upload the calibration data to the camera.

The algorithm that was developed to calibrate the system is relatively robust but may need some tweaking. You can either choose from a Linear model or from a Cubic model. Cubic model offers more flexibility but is slower to fit. If the algorithm does not fit properly, you can try increasing the sampling parameter. Increased sampling will run slower.

The algorithm works by a global search method that will reject solutions that are outside boundaries. You can define boundaries for the following parameters:

  • Wavelength range of the spectrometer. By default, the algorithm will cover all the visible range of silicium detectors plus some margin. If the algorithm fails to converge, you can narrow these fields towards the theoretical value of your spectrometer design.

  • Minimum Span is how much wavelength your camera sensor will cover. By default, the software will cover a relatively large span.

  • Maximum distortion will limit the deviations to the linear model. This mode is available only in cubic model type. It is recommended to leave the minimum value at zero.

When tweaking these values, always use some safety margins. For example, if you spectrometer has a theoretical range of 550-670 nm, use values such as 520-700 nm or even more.

Calibration data will be saved with the file when saving in the Spectre file format. It is then possible to import the calibration from a Spectre file from the calibration window. When trying to import data from a file that contains a calibration performed with a different camera, the software will warn you that the calibration was not performed on the same system and that the result may therefore be incorrect.