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Hemispherical and Diffuse Reflectance Measurements Using an FTIR and a Commercial Integrating Sphere

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Hemispherical and Diffuse Reflectance Measurements Using an FTIR and a Commercial Integrating Sphere 432 Vol. 57, No. 3 / January 20 2018 / Applied Optics Research Article Methods for quantitative infrared directional- hemispherical and diffuse reflectance measurements using an FTIR and a commercial integrating sphere 1, 1 1 1,2 THOMAS A. BLAKE, *TIMOTHY J. JOHNSON, RUSSELL G. TONKYN, BRENDA M. FORLAND, 1 1 1 1 TANYA L. MYERS, CAROLYN S. BRAUER, YIN-FONG SU, BRUCE E. BERNACKI, 3 4 LEONARD HANSSEN, AND GERARDO GONZALEZ 1Pacific Northwest National Laboratory, 902 Battelle Blvd., Richland, Washington 99354, USA 2Current address: Red Rocks Community College, 13300 West 6th Avenue, Lakewood, Colorado 80228, USA 3Optical Technology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA 4Alecam FTIR Services and Consulting, The Woodlands, Texas 77381, USA *Corresponding author: [email protected] Received 27 September 2017; revised 7 December 2017; accepted 7 December 2017; posted 13 December 2017 (Doc. ID 307361); published 17 January 2018 We have developed methods to measure the directional-hemispherical (ρ) and diffuse (ρd ) reflectances of pow- ders, liquids, and disks of powders and solid materials using a commercially available, matte gold-coated inte- grating sphere and Fourier transform infrared spectrometer. To determine how well the sphere and protocols produce quantitative reflectance data, measurements were made of three diffuse and two specular standards pre- pared by the National Institute of Standards and Technology (NIST), LabSphere Infragold and Spectralon stan- dards, hand-loaded sulfur and talc powder samples, and water. Relative to the NIST measurements of the NIST standards, our directional hemispherical reflectance values are within 4% for four of the standards and within 7% for a low reflectance diffuse standard. For the three diffuse reflectance NIST standards, our diffuse reflec- tance values are within 5% of the NIST values. For the two specular NIST standards, our diffuse reflectance values are an order of magnitude larger than those of NIST, pointing to a systematic error in the manner in which diffuse reflectance measurements are made for specular samples using our methods and sphere. Sources of uncertainty are discussed in the paper. © 2018 Optical Society of America OCIS codes: (120.3150) Integrating spheres; (120.4800) Optical standards and testing; (120.5700) Reflection; (120.6200) Spectrometers and spectroscopic instrumentation; (300.6300) Spectroscopy, Fourier transforms; (300.6340) Spectroscopy, infrared. https://doi.org/10.1364/AO.57.000432 1. INTRODUCTION for material identification (wavelength dependence of spectral features) and surface temperature measurement (i.e., absolute The use of longwave infrared (IR) hyperspectral imaging of ter- – restrial and extraterrestrial targets to determine geological com- emissivity as a function of wavelength) [18 20]. position and surface temperature has increased many-fold over For geological materials, for example, spectral databases have the past three decades [1–8]. With the advent of both compact been populated using either of two methods: the first uses ’ sensor designs and low noise-equivalent ΔT focal plane array an integrating sphere to measure the materials directional- detectors, the sensing of environmental (geological and vegeta- hemispherical reflectance (DHR or ρ) spectrum and then uses tion) and manmade materials from satellites, aircraft, fence Kirchhoff’s law, ε 1 − ρ, to calculate the emissivity, ε [18]. lines, and handheld contact sensors has become possible For brevity, in this paper we will use “DHR” to mean and, in many cases, routine [9–15]. In addition to sensor “directional-hemispherical reflectance.” development, great progress has been made in data analysis Alternatively, the emissivity of the materials in the database (preprocessing, anomaly detection, target identification, etc.) can be measured directly by placing each on a temperature- [16,17] and in building laboratory spectral databases used regulated plate and using a conical mirror to direct the emitted 1559-128X/18/030432-15 Journal © 2018 Optical Society of America Research Article Vol. 57, No. 3 / January 20 2018 / Applied Optics 433 longwave light from the sample into a spectrometer [19]. 2. EXPERIMENTAL Twenty years ago, Salisbury et al. demonstrated [21,22] that A. Instrumentation the two laboratory techniques produce equivalent emissivity re- The infrared DHR and diffuse reflectance measurements sults for the majority of geological materials that they studied, reported here utilized the same setup as previously reported, both as powders and as disks. The majority of databases used [25] which consists of a commercial integrating sphere for disentangling field data are populated with DHR data. The (Bruker model A 562-G, hereafter referred to as “the Bruker NASA/Jet Propulsion Laboratory ASTER Spectral Library sphere”) and Fourier transform infrared spectrometer (Bruker database [18], for example, consists of reflectance data of miner- model IFS 66S) configured to operate in the mid-infrared using als, rocks, vegetation, soils, manmade materials, snow, water, ice, a 1500 K silicon carbide light source, germanium-coated, etc. Additionally, Christensen and co-workers have developed a f ∕ μ – μ potassium bromide beamsplitter, 10 mm aperture, and 4.5 longwave infrared (5 m 45 m) emission spectral database of optics [27]. The instrument resolution was set to 4.0 cm−1.A rocks and rock-forming minerals in support of several thermal Cooley-Tukey fast Fourier transform using a Blackman-Harris mapping missions to Mars [19]. Reflectance databases used for − apodization function and Mertz phase correction, at 8cm1 analysis of specific classes of materials, such as industrial process phase resolution was used to transform the interferograms to chemicals, explosives, etc. tend to be more limited, impromptu, − spectral space (cm 1). The transformed data were saved and not widely distributed [12–14]. Given the growing interest in − − between 600 cm 1 and 7500 cm 1. Bruker’s OPUS software this field, we present here a practical guide for the use of a com- (version 5.5) was used to run the spectrometer and also to mercial integrating sphere and Fourier transform spectrometer for collect and display the data. Further details of the operation quantitative DHR and diffuse-(ρd ) reflectance measurements of of the FTIR spectrometer are given elsewhere [27]. In most solids and powders in the infrared (1.3 μm–16.7 μm). We have cases, 2048 double-sided interferograms were averaged for both required from the outset a horizontally oriented sample cup, so the reference and sample single-channel spectra. The integrat- that a cover window for the powders would not be needed. This ing sphere was mounted in the sample compartment separated also provides for easy sample exchange and avoids étalons and from the interferometer by a thin, wedged KCl window. other spectral artifacts associated with either internal or external However, no window is used over the sample. While this window reflections [23]. can potentially lead to sphere contamination, it avoids certain Notations, terminology, and definitions used here follow artifacts such as reflections, étalons, etc. The interferometer and those summarized by Hanssen and Snail in their comprehen- integrating sphere interior were under dry nitrogen purge; the sive review paper [24] on integrating spheres used for reflec- door to the sample compartment was, however, left open for tance measurements in the infrared and near-infrared. In these measurements. When the single-channel sample and that paper, they point out that the sphere wall coatings, detec- reference spectra were ratioed, many of the water and CO tors, and standards for the infrared are not as developed as those 2 spectral features were removed, and most—though not all— of their ultraviolet, visible, and near-infrared counterparts. remaining features were removed using an atmospheric They also go on to describe several absolute measurement tech- compensation routine in the OPUS software. niques for making integrating sphere measurements in the in- The Bruker sphere has an inner diameter of 7.5 cm with a frared, including the technique used by the National Institute nominally matte gold interior surface that displays some specu- of Standards and Technology (NIST, USA) to measure DHR lar characteristics [28]. A schematic of the sphere is shown in spectra of the five standards presented in this paper. In a pre- Fig. 1. A vertical plane located at the center of the sphere bisects liminary study at the Pacific Northwest National Laboratory a 2.0 cm diameter entrance port where light from the interfer- (PNNL), [25] we reported DHR and diffuse reflectance mea- ometer enters the sphere. A small flip mirror is positioned in- surements of three NIST reflectance standards, and side the sphere, so that light entering the sphere can be directed ’ LabSphere s Infragold [26] and Spectralon materials, and con- downward to a 1.9 cm diameter sample port in the bottom of cluded that good agreement between our measurements and the sphere, back out the entrance port, upward to a 3.2 cm calibration or reported values for these materials could be diameter specular exclusion port in the top half of the sphere, achieved. Subsequently, measurements have been made using or to any point on the sphere wall along an arc joining the cen- our sphere and spectrometer that have allowed us to expand ters of these three ports. The mirror, sample port, and specular the number of standards
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