title: Thermal infrared spectrometer for Earth science remote sensing applications-instrument modifications and measurement procedures.
abstract: In this article we describe a new instrumental setup at the University of Twente Faculty ITC with an optimized processing chain to measure absolute directional-hemispherical reflectance values of typical earth science samples in the 2.5 to 16 μm range. A Bruker Vertex 70 FTIR spectrometer was chosen as the base instrument. It was modified with an external integrating sphere with a 30 mm sampling port to allow measuring large, inhomogeneous samples and quantitatively compare the laboratory results to airborne and spaceborne remote sensing data. During the processing to directional-hemispherical reflectance values, a background radiation subtraction is performed, removing the effect of radiance not reflected from the sample itself on the detector. This provides more accurate reflectance values for low-reflecting samples. Repeat measurements taken over a 20 month period on a quartz sand standard show that the repeatability of the system is very high, with a standard deviation ranging between 0.001 and 0.006 reflectance units depending on wavelength. This high level of repeatability is achieved even after replacing optical components, re-aligning mirrors and placement of sample port reducers. Absolute reflectance values of measurements taken by the instrument here presented compare very favorably to measurements of other leading laboratories taken on identical sample standards.
authors.affiliation: Department of Earth Systems Analysis, Faculty of Geo-Information Science and Earth Observation (ITC), University of Twente, Hengelosestraat 99, P.O. Box 37, Enschede 7500AA, The Netherlands. hecker@itc.nl
authors.forename: Christoph
authors.initials: C
authors.lastname: Hecker
authors.forename: Simon
authors.initials: S
authors.lastname: Hook
authors.forename: Mark
authors.initials: M
authors.lastname: van der Meijde
authors.forename: Wim
authors.initials: W
authors.lastname: Bakker
authors.forename: Harald
authors.initials: H
authors.lastname: van der Werff
authors.forename: Henk
authors.initials: H
authors.lastname: Wilbrink
authors.forename: Frank
authors.initials: F
authors.lastname: van Ruitenbeek
authors.forename: Boudewijn
authors.initials: B
authors.lastname: de Smeth
authors.forename: Freek
authors.initials: F
authors.lastname: van der Meer