Like all measurement instruments, a spectrometer must undergo periodic wavelength calibration to maintain measurement reliability. The wavelength calibration of the s-Nova-850 spectrometer uses atomic spectral lines emitted from krypton and mercury-argon lamps. The wavelengths of these atomic spectral lines are well established and are provided by the National Institute of Standards and Technology (NIST).
Figure 1. Wavelength calibration of the s-Nova-850 using calibration light sources
As shown in Figure 1, preparation for wavelength calibration is simple: connect the calibration light source and the spectrometer to be calibrated using an appropriate optical fiber. Next, acquire the spectra of the krypton lamp and the mercury-argon lamp with the s-Nova-850. At this time, adjust the spectrometer’s exposure time and gain so that the spectral lines are distinguishable. The spectral lines and their wavelengths obtainable from the two calibration sources with the s-Nova-850 are shown in Figure 2.
Figure 2. (a) Spectral lines of a krypton (Kr) lamp measured with the s-Nova-850, (b) Spectral lines of a mercury (Hg)-argon (Ar) lamp
Determine the pixel positions of each spectral line by Gaussian fitting, and then perform a third-order regression using the pixel positions and the known wavelengths of the spectral lines to find the intercept and the first-, second-, and third-order coefficients. The regression analysis can be performed easily using Excel’s Data Analysis tool. Using the obtained intercept (A), first-order coefficient (B), second-order coefficient (C), and third-order coefficient (D), compute the wavelength for each pixel with Equation (1). Figure 3 shows the pixels and wavelengths of the spectral lines used for calibration; the black curve is the pixel wavelengths calculated with Equation (1).
Equation (1). Pixel wavelength calibration equation
Figure 4. In-house wavelength calibration certificate
The s-Nova series spectrometers from Meter-Lab have a high resolution of 0.1 nm. Therefore, precise wavelength calibration is essential. Before shipping, Meter-Lab performs the above calibration procedure and provides a wavelength calibration certificate like that in Figure 4, supplying the spectrometer specifications and the calibration equation coefficients together with the product. To maintain wavelength measurement reliability, we recommend performing periodic wavelength calibration of the spectrometer.
