Didymium filter

Figure 2-13. Visible absorbance spectrum of a didymium filter.

Figure 3-1 i Spectra transmittance curve of a didymium filter. (Perkin-Elmer Model 35 spectrophotometer, 8-nm nominal spectral bandwidth.)... 

With broader-bandpass instruments, a didymium filter may be used to verify wavelength settings. This filter should show a minimum percent transmittance at 530 nm against an air blank (Figure 3-11). Because didymium has several absorption peaks, the setting should be verified grossly by... 

Following the development of these methods of standardization, it was found that a simpler method of standardization could be employed for the NIR Systems line of instrumentation. Because wavelength alignment is accomplished with internal polystyrene and didymium filters within the instrument,... 

The most important spectral interferences arise from barium, sodium, caesium and rubidium. Of these the most serious is likely to be that of sodium but this can be reduced considerably by using a dense didymium filter which absorbs strongly at 5890 A and therefore reduces the light from the main sodium lines. 

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). 

They are again dissolved in water, and precipitated with potassium hydrate. The precipitate is didymium oxide hydrate. When filtered on paper it is bluish violet. During filtration and wash it rapidly takes up carbonic acid and thereby changes its colour to a slightly reddish violet. After ignition didymium oxide is obtained... (Berzelius 1844a,b). 

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...

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. 

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