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High Photometric Accuracy in an Optical Null, Double-Beam Infrared Spectrophotometer
Volume 22, Number 5 (Oct. 1968) Page 578-581
Stine, K.E.; Weagant, R.A.
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.