THE USE OF ASYMMETRIC INTERFEROGRAMS IN TRANSMITTANCE MEASUREMENTS

The asymmetric interferogram obtained with a transmission sample in one arm of a far infrared Michelson interferometer may be used to obtain the spectral transmittance of the sample both in magnitude and in phase. The possible accuracy of such measurements is illustrated with data obtained in testing the technique with a vacuum interferometer. The ordinary ray index of refraction of crystal quartz and the index of refraction of KBr have been measured with an uncertainty in n I of about 0.1 % and 1 % respectively in regions between 20 and 400 cm-1. The measurement accuracy is limited by the quality of the sample ; plane parallel surfaces and an accurately known thickness are required. A (( pseudo-coherence 1) effect is described and is illustrated by a measurement of n, no for crystal quartz without the use of polarizers. This (( pseudocoherence )) effect is shown to be a major problem for the determination of the extinctioncoefficient of samples from asymmetric interferograms. The effect of a non-linear detection system on the resulting spectrum is also described. R6sum6. L'interferogramme asymktrique qui est obtenu lorsqu'un echantillon est place sur l'un des bras d'un interferomktre de Michelson pour l'infrarouge lointain peut donner B la fois la grandeur et la phase du faisceau transmis par l'echantillon. Nous illustrons la prkcision que ces mesures peuvent donner au moyen de resultats obtenus en essayant cette technique avec un interfCrometre dans le vide. L'indice de refraction ordinaire du quartz et l'indice de rkfraction de KBr ont Ctk mesurks avec une erreur sur n I d'environ 0,l % et 1 % respectiverent dans des rkg'cns situCes entre 20 et 400 cm 1 . La pr6cision est limit& par la qualite de l'Cchantillon ; il est nkcessaire d'avoir des faces planes et paralldes et une Cpaisseur connue avec prkcision. Nous dkcrivons un effet de (( pseudo cohkrence n, qui est illustrC par une mesure de a, no pour du quartz sans polariseur. Cette (( pseudo cohkrence )) constitue un probleme majeur dans la dktermination du coefficient d'extinction d'Cchantillons a partir d'interferogramrnes asymetriques. Nous dkrivons Cgalement l'effet d'un systbme de detection non linCaire sur le spectre rCsultant. Introduction. The advantages to be derived from the use of Fourier transformation of two beam interferograms in spectroscopic work are well known. In practice the normal operation has been to arrange the interferometer so that the interferogram is symmetric about the central maximum, to record one side of the interferogram, and to use a cosine Fourier transformation to calculate the spectral power through the instrument. The sample is placed in the source or the detector arm of the interferometer in this symmetric mode of operation. Special uses for the asymmetric mode of operation, on the other hand, have been pointed out by Chamberlain [l, 2, 31 and Bell [4, 51. (*) This work was supported in part by a contract between the Air Force Cambridge Research Laboratories and The Ohio State University Research Foundation. In the asymmetric mode of operation the sample is placed in one arm of a Michelson interferometer, and an asymmetric interferogram is recorded. The asymmetric interferogram is processed by a full, sine and cosine, Fourier transformation so that both amplitude and phase information is obtained. This paper will consider some advantages and problems associated with the use of the asymmetric mode of operation to obtain the optical properties of solid samples in transmission. The data which will be presented were obtained with a Michelson interferometer (in the generic sense that a Michelson interferometer is a wave amplitude split, two beam device) which was described by Russell and Bell [6]. The sample is traverssd only once by the beam in this particular interferometer, and the sample is at an image point of the source and