Linearity Characterization of NIST's Infrared Spectral Regular Transmittance and Reflectance Scales
Leonard M. Hanssen, Simon G. Kaplan
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.