To date, optical null infrared spectrophotometers have been capable of double-beam photometric accuracies of no better than 0.5% to 1.0%. When operated in the double-beam mode, optical null instruments measure the transmittance of a sample by optically attenuating the reference beam until its energy equals that of the sample beam. This operation is usually performed automatically by a servosystem which constantly repositions an optical attenuator (comb) in the reference beam to obtain a null balance condition. A pen servosystem then follows the comb movement from a linear potentiometer geared to the comb.
K. E. Stine and R. A. Weagant, "High Photometric Accuracy in an Optical Null, Double-Beam Infrared Spectrophotometer," Appl. Spectrosc. 22, 578-581 (1968)