Modern infrared spectrophotometers have the convenience of direct recording on a chart marked with a wavelength or wavenumber scale. Many laboratories, as ours does, own the older Perkin-Elmer model 13 spectrophotometer, or other instruments using the same monochromator. This instrument records on a Leeds and Northrup potentiometer recorder with no simple means of directly reading spectral positions on the curves. In order to avoid the tedious procedure for assigning wavelengths to absorption spectra by chart measurement and reference to a calibration curve and to allow observation of the 2 to 15 microns recorded spectrum directly by inspection, our instrument recorder was modified and a transparent plastic overlay with wavelength, wavenumber, and "drum turn" scales was prepared. Since many of these instruments are in use and some are still being sold, it is felt that the procedure for doing this would be of interest to other users.
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