Didymium glass filter

As in the case of dispersive Raman spectrometers (cf Section 4.4.1), it is necessary to calibrate the wavelength scale of dispersive UV/VIS spectrometers. The most accurate standards for checking the UV/VIS wavelengths are lasers of various types. The inexpensive helium-neon laser can be used to check at 632.8 nm. For spectrometers with a deuterium source, spectral lines at 486.6 and 656.1 nm can be used for calibration. A common method for wavelength calibration is the use of optical filters. A filter of didymium glass has many sharp absorption peaks, which can be used as a second wavelength standard (precision within 0.5 nm). 

Glass filters are preferable to liquid absorbance standards. Vandenbelt has reported on a collaborative study of Cary spectrophotometers in the visible region using glass filters and on a 5-mm didymium glass and a 2-mm Corning H. R. Red Purple Ultra No. 587 in nine different Cary Model 14 spectrophotometers in the near-infrared region. 

Fig. 14.5 A variety of typical filters, a An interference bandpass filter showing the typical mirrored surface of these types of filter b two square silvered glass neutral density filters and a round neutral density filter of a colloidal dispersion in glass c a series of round, coloured absorption cut-off filters, and d a pale blue glass heat filter e a Wratten filter of dyed gelatin, which is flexible and easily cut to shape f a polariser g a didymium absorption wavelength standard. The 5p UK coin, included top left to give some idea of scale, has a diameter of 18 mm...

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