A double beam, optical null infrared spectrometer has been adapted for digital recording without interfering with the normal operating modes of the instrument. A photoelectric dial reader operates a solid state switching circuit at each calibration mark of the wave number dial to cause a digital voltmeter to read the voltage across the linear transmittance slidewire of the spectrometer, while the spectrometer operates in the absorbance recording mode. The digital voltmeter operates a paper tape punch, permitting computer processing of the spectra.