WAVELENGTH CALIBRATION METHODS FOR LOW-RESOLUTION PHOTODIODE-ARRAY SPECTROMETERS

A wavelength calibration method for low-resolution diode array spectrometers is described. The method was developed for routine calibration of 0.1-meter-focal-distance spectrometers having 35- or 38-element silicon diode arrays, normally operated in the 340-700 nm spectral range. Each diode of the array is approximately 1 mm wide, giving an instrumental bandwidth of approximately 10 nm per diode. The calibration method requires two well-separated monochromatic spectral lines, their central image locations on the array, and the grating groove frequency. This method is compared with nonlinear regression (least-squares) methods, with multiple calibration lines fitted to quadratic or cubic polynomials. The predictive accuracy of the wavelength-pair method compares favorably with the regression methods. A calibration accuracy of approximately +/-1 nm is expected for the instruments considered here. The method described could, in principle, be applied to instruments with higher resolution, such as those having self-scanned photodiode arrays with 25-mum or 50-mum-wide pixels. For such instruments, however, a large number of calibration lines should be resolved. In this case, the regression method, which averages diode position measurement errors, is probably more accurate. The wavelength-pair method is most useful for low-resolution instruments, for which regression methods may not be practical.