G. J. Buist University of Surrey Guildford Surrey GU2 5XH England
Interference Filters for Wavelength Calibration of UV-Visible Range Spectrophotometers
In their review of wavelenpth calibration methods1 Alman and Rillmeyer refer tu an in&erenre filter ronsistinx of a pair of halt-nluminized elass ~ l a t e separated s hv a metal foil spa. cer. Such a filter is not readily available in most laboratories, and it is the purpose of this note to point out that an ordinary infrared cell with silica or calcium fluoride windows will produce interference fringes when placed in the beam of a uvvisible range spectrop~otometerhaving moderate resolution. We have carried out tests with a demountable infrared cell fitted with silica or calcium fluoride windows and a Teflon spacer 25 thick. With two silica prism spectrophotometers of moderate resolving power, the Pye-Unicam SP800 and SP8000, interference fringes were obtained from 220-850 nm, almost the full range of the instruments. The calcium fluoride windows gave the better results; the fringes were of greater amplitude compared with thase obtained from silica windows. The spacing of the fringes is constant in terms of wavenumber (200 cm-' for a 25-u s ~ a c e r but ) in terms of waveleneth i t varies from 1nm at 220nm & ' 14 nm at 850 nm. Thii variation suits the characteristics of a silica ~ r i s ms~ectro~hotometer very well; the resolution is good at the shoit wavelength end, but Door a t the lone wavelength end. (corresvondindv, . . the manhcturer's tuleianre in t h i wavelen@h raiibration of the above s~ertr0~hotomrtt:ri is r0.4 nnl and f. nm at 200 2nd 850 ni, resp~ctively).Calibration can be achieved by re-
cordine absomtion Deaks of holmium or didvmium glass filters ou rhe'same chart the interference fringes: rhenihe rurrect wuwlenrth ran be found at any point. Theerror in the method is abou
