Linearity Characterization of NIST's Infrared Spectral Regular Transmittance and Reflectance Scales

Published

January 1, 2003

Author(s)

Leonard M. Hanssen, Simon G. Kaplan

Abstract

System Linearity is a fundamental characterization performed on spectrophotometers. Yet it is one that is not adequately performed on Fourier transform instruments, because of the lack of a method for linearity characterization that will work sufficiently well in a rapidInterferometer. The two primary traditional methods for monochromator instruments are the double aperture method and the rotating sector wheel method. But these methods do not work well for FT-IRs because of the nature of the interferometer beam and modulation. [Chase has proposed a sector wheel study on a step scan FT]. Other methods in which the measured flux is varied are limited by the uncertainty with which the relative level of the flux can be independently measured.We present the results of a careful linearity study of an FT-IR spectrophotometer system that uses an integrating sphere for transmittance and reflectance measurements. The linearity characterization method employed has been successfully used in detector linearity studies. Inthis method, the transmittance of a spatially uniform filter, such as a Si wafer sample, is measured at various different flux levels. Since the sample transmittance is independent of input flux, one can establish system linearity if one measures a constant value oftransmittance. In our case, a large set of apertures placed at an intermediate focal point of the FT-IR beam is used to vary the measured flux. For each aperture setting, the spectral transmittance is measured in both near- and mid-infrared configurations.Over the entire range of flux levels of approximately two decades, the system response is shown linear to within O.1%. In a typical spectrometer sample compartment such a result would be very difficult to demonstrate because of the size and range of a wide varietymeasurement errors not related to detector linearity, including inter-reflections between components, interaction of sample, beam detector non-uniformities, and other errors. However, the region of the integrating sphere that is used for the FT-IR beam measurehas ver good spatial uniformity, and the low throughput of the sphere places even its photoconductive MCT detector in a linear regime.

Citation

Handbook of Vibrational Spectroscopy

Volume

4826

Publisher Info

John Wiley & Son, Hoboken, NJ

Pub Type

Books

Keywords

Fourier transform infrared spectrometer, linearity, photometric accuracy

Citation

Hanssen, L. and Kaplan, S. (2003), Linearity Characterization of NIST's Infrared Spectral Regular Transmittance and Reflectance Scales, John Wiley & Son, Hoboken, NJ (Accessed April 19, 2024)