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 powders, liquids, and disks of powders and solid materials using a commercially available, matte gold-coated integrating 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 prepared by the National Institute of Standards and Technology (NIST), LabSphere Infragold and Spectralon standards, 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 reflectance 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 and samples measured, properly scale exclusion port are all bisected by the aforementioned vertical the measured DHR and diffuse reflectance spectra with respect plane. Light from the spectrometer comes to a focus at the flip to an appropriate NIST standard, provide an uncertainty mirror. The specular exclusion port is large enough to account budget for the resulting reflectance spectra, and better under- for the beam divergence of the spectrometer’s f ∕4.5 optics. stand the sources of these uncertainties. The results of these The centers of the sample and specular exclusion ports are measurements on five NIST reflectance standard materials, in the same vertical plane as the entrance port. The 1 cm diam- as well as Infragold, Spectralon, sulfur, talc, and water are pre- eter detector port is bisected by a horizontal plane that includes sented here. Comparisons among our measurement results, the center of the sphere and is centered on the sphere wall NIST standards’ calibrated values, and published results (where opposite the flip mirror handle (see below). The detector is available) for the other materials are presented, and from these, a 2mm× 2mm, 60 deg field-of-view, liquid-nitrogen-cooled estimates of random and systematic measurement uncertainties HgCdTe (MCT). Two diffuse gold-coated baffles (not shown are given. Corrections to the results to account for anomalies in in Fig. 1) are positioned near the detector port to prevent first- the sphere design, or the protocol used, are discussed. bounce light from either the bottom sample port or the specular 434 Vol. 57, No. 3 / January 20 2018 / Applied Optics Research Article

term on the right side of Eq. (1), which is referred to as the sphere multiplier, scales the radiance of the sphere due to multiple reflections as a function of sphere reflectance and port fraction. The incident power, radiance, and reflectances are functions of wavenumber (cm−1). Based on measurements, a better estimate of the matte gold reflectance for the Bruker sphere is given in the Discussion section. For comparison, visible near-infrared (VNIR) spectra were recorded for some of these materials using a dispersive spectrometer (Cary 5000) and pre-formed polytetrafluoroethylene (PTFE)-lined integrating sphere from LabSphere (DRA-2500). A detailed description of the VNIR measurements is given in [23]. For the VNIR sphere, the samples are held vertically, and a cover window is used to hold powdered samples in the sample cup. There is spectral overlap (ca. 1.4 μm–2.3 μm) between the VNIR system and the present infrared system, which is used for Fig. 1. Schematic of the A 562-G integrating sphere used by PNNL comparison purposes. for the work presented here. The sample, reference, entrance, and de- B. Methods tector ports are shown. The flip mirror is left of center. The light baffles Alignment of the sphere is critical, especially the positioning of are not shown. Dashed line with an arrow indicates the path of the the input light’s focus from the interferometer onto the Bruker light directed to the reference spot on the sphere wall. Cap in the ’ specular exclusion port is removed for diffuse reflectance measure- sphere s flip mirror, to obtain the quantitative reflectance values ments; it is kept in place for DHR measurements. The figure is given here. Selecting a point on the sphere wall that is baffled reproduced, in part, from the A 562-G operating manual, Bruker from the detector’s field of view, and using the same spot for Optics, Inc. both DHR and diffuse reflectance reference measurements are also important considerations. The procedures used to record DHR and diffuse reflectance measurements have been reported exclusion port from reaching the detector. A loose-fitting cap earlier [25]. Here, we provide more thorough descriptions of whose inward-facing, matte gold surface has the same curvature the methods used. In order to obtain quantitative reflectance as the sphere is used to cover the specular exclusion port when values, a reference spectrum relative to matte gold is required. needed for DHR measurements. The flip mirror is connected For this study, we have chosen the position where the bottom to the exterior of the sphere by a thin axle, which is connected edge of the light spot, as directed by the flip mirror, is approx- “ ” to a lever arm so that the mirror can be flipped from the up imately 1 cm above the bottom sample port on the wall of the “ ” position pointing at the specular exclusion port to the down sphere as our point for the reference measurements for both position pointing at the sample port. Two spacers are used to DHR and diffuse reflectance spectra. The sample is in the sam- delimit the travel of the lever, and these are adjusted to center ple port when the reference measurement is made. A spacer is the light in the top and bottom ports. With the mirror pointing placed between the mirror lever and a mechanical stop on the down, the angle between the incident light on the sample sur- side of the sphere to reproducibly position the mirror at this face and the surface normal is 14.8 deg. The detector and its reference point. With the light directed to this reference spot preamplifier are bolted to the sphere, and the entire unit, in on the sphere wall, the baffle on the wall of the sphere between turn, is bolted to a three-point kinematic baseplate that sits the sample port and the detector port is positioned to block in the sample compartment. The sample or sample cup is most of the first-bounce light from this reference position from pressed tightly against the bottom of the sphere, thus helping reaching the detector. (An alternate reference position for the to avoid artificially low reflectance values that can arise from DHR spectra is to point the IR beam at the cap in the specular optical sample standoff, even from distances <1mm[29–31]. exclusion port. This approach gives DHR values comparable to The radiance, L, inside the sphere, when illuminated by the ones reported here, certainly within the expanded uncer- infrared light from the interferometer, is given by [32] tainties.) For the DHR measurements reported here (see Φ ρ Fig. 1), the sample is in the bottom port, and the diffuse gold L ; · (1) dome cap is in the specular exclusion port. The flip mirror πAs 1 − ρ1 − f directs the light from the entrance port to the reference point Φ V where is the incident power, which is 0.036 W, as measured to record the reference spectrum ( reference) and toward the V with a calibrated pyrometer (model E6, Eppley Laboratory, sample to record the sample spectrum ( sample). The two spec- A ’ −2 2 ρ ρ V ∕V Inc.), s is the sphere s surface area, 1.767 × 10 m , is tra are ratioed, sample reference, and scaled to a calibrated the reflectance of the matte gold surface of the sphere, assumed standard, viz. [24,28]: to be 0.95 here, and f is the exchange fraction, which is the ∕ ’ ρ Vsample Vsample-reference ρ ; ratio of the surface area of the open ports to the sphere s total sample-scaled ∕ standard (2) surface area. When the cap is in the specular exclusion port, Vstandard Vstandard-reference f L ∕ 2 V ∕V V ∕V 0.0227 and 8.7 W m sr and with the cap re- where ( sample sample-reference) and ( standard standard-reference) moved f 0.0677 and L 5.4 W∕m2 sr. The second are measurements made using the Bruker sphere of the sample, Research Article Vol. 57, No. 3 / January 20 2018 / Applied Optics 435 and its reference measurement, and a standard, and its reference Spectralon standard that is calibrated in the visible and ρ μ measurement, from, for example, NIST. standard is the reflec- near-infrared to 2.5 m was also measured. A DHR calibration tance spectrum of the standard as measured by the issuer of the spectrum was supplied with this standard, but a diffuse reflec- standard (NIST). tance spectrum was not available. To measure the diffuse reflectance (see Fig. 1), first the cap is To measure the reflectance of liquid water and the loose removed from the specular exclusion port. Next, with the sam- powder materials, the samples were placed in machined alumi- ple in the bottom sample port, the reference spectrum is re- num sample cups. The cups have an outside diameter of corded with the light directed to the reference point on the 3.0 cm, an outside height of 1.1 cm, an inside diameter of sphere wall, and the sample spectrum is recorded with the light 2.5 cm, and an inside depth of 0.9 cm for a sample volume directed onto the sample. The two spectra are then ratioed as of 4.4 cm3. The cups are sufficiently deep that infrared light ρ V ∕V d sample reference. The amount of light exiting the specu- does not penetrate to the bottom when filled with a sample. lar exclusion port is dependent on the bidirectional reflection Deionized water was used for reflectance measurements. The distribution function (BRDF) and the solid angle intercepted sulfur (purity >99.5%) was procured from Aldrich (part # by the port. The sphere used by NIST [24,33–35] to measure 84638-1KG, CAS # 7704-34-9). The sulfur was used without their standards, for example, has smaller port sizes, and it lets further purification but does show evidence of a hydrocarbon less light out during diffuse measurements. For a sample with a impurity and adsorbed water in its reflectance spectrum. The specular lobe in its BRDF, a NIST measurement will give larger talc was procured from Aldrich (part # 243604-500G, CAS # diffuse reflectance values compared with a diffuse measurement 14807-96-6). The powder samples were prepared for the infra- of the same sample using the Bruker sphere. Consequently, we red measurements by weighing out the powder for each use the DHR standard values as measured by NIST and the cup, placing it into the cup and smoothing the surface with DHR values of the standard as measured by PNNL to scale the blade of a spatula, so that the surface is uniform, and then the PNNL diffuse reflectance measurement using Eq. (2). re-weighing the cup. C. Materials D. Measurement Uncertainties Five reflectance standards were purchased from NIST To estimate the uncertainties in the PNNL reflectance mea- (Gaithersburg, MD, USA) in the fall of 2009 [25]. The stan- surements, we made multiple measurements of the standards dards are not official NIST standard reference materials but and materials to determine the random measurement uncer- were prepared and calibrated by NIST researchers as part of tainties (Type A) and used an optical ray tracing program to a multi-laboratory reflectance study [36]. Of the five make estimates of systematic uncertainties originating from 25 mm diameter disks, two are specular (PN/S25A02: highly the sphere design and measurement protocols (Type B). reflecting electroplated gold on a polished copper substrate and These uncertainties were then combined in quadrature with PN/S25B02: medium reflecting, polished, high-density silicon a coverage factor of k 2 to give an expanded uncertainty carbide [25]), and three are diffuse (PN/D25A02: highly re- as given for the NIST uncertainty described above [37]. flecting electroplated gold on nickel on arc-sprayed aluminum For each material examined here, at least six sets of DHR on a machined brass substrate, [25] PN/D25D02: medium- and diffuse reflectance spectra were measured and, unless stated reflecting Krylon silver paint on arc-sprayed aluminum on a otherwise, each reflectance spectrum consists of a ratio of 2048 machined brass substrate, and PN/D25C02: low-reflecting sample scans to 2048 reference scans. Six different samples were Nextel black paint on a machined brass substrate [25]). prepared for each of the “loose” compounds: hand-loaded sul- NIST calibration data from 555 cm−1 to 10;000 cm−1 are pro- fur and talc, and water. The solid disk standards from either vided for each of these standards, which include a DHR spec- NIST or LabSphere were placed in the bottom sample port trum, ρ, the diffuse spectrum, ρd , specular spectrum, ρs, and of the sphere and rotated to six different positions, 60 deg apart the specularity spectrum, defined as ρs∕ρ, where ρ ρs ρd to provide six different “samples.” Average DHR and average [36]. Each of these spectra has an associated NIST expanded diffuse reflectance spectra were calculated from the six samples uncertainty (coverage factor k 2, level of confidence 95%) for each material. Also, DHR and diffuse reflectance standard [37]. The D25X02 standards were recalibrated in 2014, and deviation spectra are calculated from the averaged reflectance the S25X02 standards were recalibrated in 2015. spectra. These spectra give a measure of the sample-to-sample A LabSphere 2-in. square Infragold standard (UIRT-094- random uncertainty. 020, AS-02792-020) was purchased (November 2013) as a To determine the systematic uncertainties in our reflectance mid-infrared standard. There are no reflectance calibration data measurements from both the sphere and the methods used to for this particular standard. LabSphere lists DHR values for make those measurements, a 3D CAD drawing from Bruker of Infragold between 1.0 μm and 2.5 μm in 0.05 μm steps the integrating sphere was used in conjunction with an optical [38]. Using spectral values between 2.5 μm and 13 μm from engineering software program (TracePro) to ray trace light a LabSphere-calibrated Infragold standard produced in 2001, moving through the sphere. To keep the calculations tractable, together with the aforementioned near (NIR) and shortwave the sphere and sample surfaces were assumed to have a infrared (SWIR) values, we produced a composite reference Lambertian BRDF with 97% reflectance. One million parallel, spectrum that approximates the original calibration. LabSphere monochromatic starting light rays were introduced into the affirms that it has been making Infragold in the same manner sphere through the entrance port. The radiance reaching the for decades and that the calibration values are persistent. detector for a DHR measurement is determined for the sphere LabSphere 99% (SRS-99-020, AS01161-060) reflecting configured, as it is used compared to an idealized sphere. So, for 436 Vol. 57, No. 3 / January 20 2018 / Applied Optics Research Article

Table 1. Fractional Uncertainties Used for Calculating of the standard as measured by the issuer of the standard Systematic Uncertainties When Using the Bruker Sphere (NIST, for example). The δRi are the expanded uncertainties a for DHR and Diffuse Reflectance Measurements as given in Eq. (3). All these values are functions of wavenum- −1 Source of Systematic DHR Diffuse ber (cm ). A NIST traceable polystyrene film on a KBr window (NIST Uncertainty Configuration Configuration Standard Reference Material 1921) was used to regularly check Knife edges/flat specular 0.0294 0.0135 the wavenumber scale of the FTIR. For each measured trans- port edges mission spectrum of the film, the wavenumber positions of a Curved sample/flat sample 0.0020 0.00106 Detector FOV and baffle 0.0078 0.0155 dozen spectral features are compared with those in NIST 1921, position and a root mean square of the differences is calculated. The − FTIR baseline drift 0.0020 0.0020 average RMS difference is 0.25 cm 1. aFractional uncertainties for the sphere are calculated from ray trace simulations of the Bruker integrating sphere assuming a Lambertian wall 3. RESULTS R interior and Lambertian sample surface both with 0.97. For specular A. NIST Standards samples, the fractional uncertainty associated with knife edges and port edges is assumed to be zero. Measurement results for NIST standards D25A02 and S25A02 are shown in Figs. 2 and 3, respectively. In these figures, we show the PNNL ρ and ρd spectra as the uncorrected example, the radiance reaching the detector when the incident V ∕V light is directed toward a curved sample within a sphere with sample reference data. flat specular port edges is simulated and compared with the In the upper panel of Fig. 2 are shown, on an expanded radiance reaching the detector when the incident light is scale, our DHR (blue) and diffuse (green) reflectance spectra directed to a curved sample within a sphere that has knife edge of the NIST diffuse gold reflectance standard, D25A02 [25]. The expanded uncertainties of the PNNL ρ and ρd spec- ports. A fractional change between these two radiances is cal- ρ ρ culated, and this is used as the fractional uncertainty introduced tra are shown in the bottom panel. The NIST (black) and d by the flat specular port edges. Similar calculations are per- (red) spectra, which were recorded in June 2014, are also shown formed to determine the fractional uncertainties introduced in the upper panel and the NIST expanded uncertainties [37] by flat samples and combined detector field of view and baffle are also shown in the lower panel. The NIST DHR and diffuse position. The process is repeated with the sphere configured for spectra for this material are coincident with one another diffuse reflectance measurements. Modeling of the sphere with indicating a highly Lambertian material [36,39]. The PNNL a specular sample shows that reflected light in the sphere does DHR and diffuse spectra are not coincident for much of not significantly interact with the port edges. Consequently, the the wavenumber range shown, pointing out an important dis- fractional uncertainty for the port edges is set to zero for specu- tinction between the NIST and PNNL data sets. The NIST lar samples. The other fractional uncertainties remain the same as the diffuse sample case. The fractional uncertainty related to the baseline drift of the FTIR is estimated to be 0.002. The fractional uncertainties are summarized in Table 1. i δR i The expanded uncertainty at wavenumber bin , exp ,is calculated from δR i k μ R i 2 μ R i 2 exp 1 2 μ R i 2 μ R i 2 σ i 2 1∕2; 3 4 rndm (3) where k 2 is the coverage factor for a 95% confidence level, μj are the Type B fractional uncertainties, Ri is the measured σ i DHR or diffuse reflectance, and rndm is the random sample- to-sample standard deviation. The DHR and diffuse reflectance expanded uncertainties are plotted with their respective reflectances for each material in Fig. 2 through Fig. 9 [37]. For those sample reflectance spectra scaled using Eq. (2), the uncertainty associated with the standard used for scaling must be taken into account. To do this, the propagation formula Fig. 2. DHR (blue) and diffuse (green) PNNL uncorrected reflec- given in Eq. (4) is used: sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi tance spectra of NIST D25A02 standard. The standard is an electroplated gold on nickel on arc-sprayed aluminum on machined brass δR 2 δR 2 δR 2 δR R su stdPNNL std ; substrate. The PNNL spectra were recorded with the disk in six differ- ss ss R R R (4) su stdPNNL std ent positions; for each position, 2048 scans were co-added. Data from R R the six positions were averaged (upper plot); the expanded uncertain- where ss is scaled reflectance of the sample, su is the unscaled ties are shown in the lower plot. The NIST DHR and diffuse reflec- reflectance of the sample as measured using the Bruker sphere, tance measurements of this standard are shown as the black and red R stdPNNL is the reflectance of the standard as measured by traces, respectively. The NIST uncertainties—DHR and diffuse—lie R PNNL using the Bruker sphere, and std is the reflectance on top of one another. Research Article Vol. 57, No. 3 / January 20 2018 / Applied Optics 437

As with the D25A02, both the D25C02 [25] and the D25D02 exhibit small organic and adsorbed water features in their reflectance spectra. In contrast with D25A02 and D25D02, whose average PNNL DHR values are within the uncertainty range of the average NIST DHR values (roughly 3%), the PNNL D25C02 [25] is slightly out of the NIST uncertainty range. Nonetheless, the comparisons between the PNNL and NIST DHR (shown in Table 2) are quite good considering that no correction or scaling has been made for the reflectance properties of the Bruker sphere wall. In terms of percent change, the average PNNL diffuse reflectances for D25A02 and D25D02 are below their NIST values by 1.93% and 4.9%, respectively, but the average PNNL diffuse reflectance for D25C02 is above that of the NIST value by 4%. As will be described below (see the Discussion section), the D25A02 and the D25D02 have specular lobes that escape from the Bruker sphere to a greater extent than from the NIST Fig. 3. DHR (blue) and diffuse (green) PNNL reflectance spectra sphere during diffuse reflectance measurements. The BRDF of NIST S25A02 standard. The standard is electroplated gold on pol- of the D25C02 is flat. ished copper substrate. The PNNL spectra were recorded with the disk Spectra of the NIST specular reflectance standard, S25A02, in six different positions; for each position, 2048 scans were co-added. shown in Fig. 3, are flat across the infrared. Similarly, the Data from the six positions were averaged (upper plot); the expanded −1 uncertainties are shown in the lower plot. The NIST DHR and diffuse S25B02 standard [25] is spectrally flat above 2000 cm . reflectance measurements of this standard are shown in black and red The NIST-PNNL DHR agreement for the two specular traces, respectively. standards is very good with percent changes of 1.1% and 3.3% for S25A02 and S25B02, respectively, and absolute differences of ρ ∼0.01 reflectance units (see Table 2). Although the PNNL diffuse reflectances for these two standards have and PNNL data were collected using different spheres and tech- ρ < niques, and these differences, which are described in Section 4, d 0.01, they are approximately an order of magnitude larger can lead, in some cases, to a divergence in reflectance values. than those of the NIST spectra. This discrepancy is explained In both the PNNL and NIST spectra, weak organic features in the Discussion section. −1 near 2900 cm —probably due to the solvent used in making B. Gold Standards — the standard are visible, and broad, adsorbed water features LabSphere’s Infragold is a commonly used diffuse infrared −1 −1 near 1600 cm and 3400 cm are also visible. The PNNL reflector with a DHR reflectance of about 0.94 from the far- diffuse spectrum is not as level as that of the DHR, and there infrared through the SWIR. Our preliminary DHR and diffuse −1 appears to be a pivot point at 1850 cm at which the diffuse reflectance measurements for a 200 × 200 square of the material spectrum slopes away below the DHR at higher wavenumbers. were presented in [25]. In that work, we made several DHR The steep increase in the PNNL-measured DHR and diffuse and diffuse reflectance measurements along one side of the −1 reflectance below about 1200 cm and that the diffuse crosses square and then rotated the square 180 deg and made more over the DHR in this wavenumber range indicates that the measurements on the side opposite the previous measurements. D25A02 is more Lambertian in this spectral region than We noted that there was little difference in the DHR spectra for the sphere wall material, which is used as the reference (see the two orientations of the square, but there was a statistically the Discussion section). Diffuse materials, with respect to their significant difference between the diffuse reflectance spectra. reflectance spectra, appear to be less diffuse with increasing This result points to an anisotropy in the BRDF of Infragold wavelength [36]. when light is incident at 14.8 deg. Here, we have scaled the

Table 2. Average DHR and Diffuse Reflectance and Percent Change Between PNNL and NIST Measurement of the Five NIST Standardsa

b b c Sample Standard Type ρ(PNNL) ρ(NIST) %Δρ ρd (PNNL) ρd (NIST) %Δρ D (PNNL) D25A02 Diffuse 0.971(59) 0.985(28) −1.42 0.965(40) 0.984(28) −1.93 0.993(74) D25C02 Diffuse 0.0344(23) 0.0324(13) 6.17 0.034(2) 0.0323(12) 4.02 0.975(75) D25D02 Diffuse 0.863(53) 0.844(24) 2.25 0.792(35) 0.831(24) −4.9 0.917(69) S25A02 Specular 0.999(17) 0.988(3) 1.11 0.0072(6) 0.0006(4) 1100 0.0072(6) S25B02 Specular 0.1859(30) 0.180(1) 3.28 0.0059(6) 0.0006(5) 880 0.033(3) aAverages calculated between 1000 cm−1 and 7000 cm−1. The values in parenthesis after the reflectances are the two standard deviation (k 2 coverage factor) expanded uncertainties. b Δρ ρ − ρ ∕ρ Percent change, % f PNNL NIST NISTg × 100%. c −1 −1 The diffuseness spectrum is defined as D ρd ∕ρ and measured by the Bruker sphere. D is averaged between 1000 cm and 7000 cm to give the values shown. 438 Vol. 57, No. 3 / January 20 2018 / Applied Optics Research Article

as 4, 5, and 6. The scaled average (averaged between 1000 cm−1 and 7000 cm−1) DHR for samples 4, 5, and 6 is 0.956(87), and for samples 1, 2, 3, and 7 is 0.954(87); the DHR average from LabSphere is 0.943 [38] (see Table 3). The values in parenthesis after the reflectances are the two standard deviation (k 2 coverage factor) expanded uncertainties. The percent change between the average PNNL DHR values and the LabSphere DHR reflectance are 1.38% and 1.17% for the 4, 5, 6 and 1, 2, 3, 7 samples, respectively. The agreement between the scaled PNNL infrared measurements and the LabSphere reference is thus very good. The scaled average diffuse reflectance for samples 4, 5, and 6 is 0.944(74), and for samples 1, 2, 3, and 7 is 0.935(74) (see Table 3). Unfortunately, there are no calibrated diffuse reflectance spectra to which we can compare our results. The calculated diffuseness spectrum, defined as −1 −1 D ρd ∕ρ and averaged between 1000 cm and 7000 cm , Fig. 4. DHR and diffuse reflectance spectra of 200 × 200 LabSphere is 0.987(119) for samples 4, 5, and 6 and 0.980(121) for sam- Infragold square, part # UIRT-94-020 are shown in the plot. ples 1, 2, 3, and 7 (See Table 3). The diffuseness, D, is analo- Reflectance measurements were made with the Infragold square in the gous to the “haze factor,” b, used in visible transmission or bottom sample port. Four sets of measurements, samples 1, 2, 3, and 7 fenestration measurements [40,41]. Shepherd reported a specu- were made from the “left” side of the square. The square was then turned “ ” lar reflectance component for Infragold that increases across the 180 deg and three sets of measurements were made from the right side ∼ μ ∼ μ of the square, samples 4, 5, 6. For each position, 2048 scans were infrared from 2% at 2 mto 5% at 12 m[42,43]. recorded for DHR and diffuse reflectance measurements. The left side To determine if moving the sample increases the sample- measurements were averaged, as were the right side measurements. The to-sample standard deviation, six sets of DHR and diffuse re- spectra were scaled using the NIST D25A02 standard. The expanded flectance spectra were measured for the Infragold with the uncertainties are shown in the lower panel. The LabSphere DHR square fixed in one position on the “right” side (see above) Infragold reference data are shown in the blue trace. for all the measurements. The results were again scaled using the NIST D25A02 standard. The average scaled DHR is 0.957 (87), and the scaled diffuse reflectance is 0.949(74) and D Infragold results using the NIST D25A02 standard. The DHR 0.992120 for this set of measurements (see Table 3). The and diffuse reflectance spectra are shown in Fig. 4 along with averages for the DHR and diffuse reflectance and the expanded the expanded uncertainties and LabSphere’s DHR reference uncertainties are nearly identical to those for the spectra spectrum. Measurements made on the “left” side of the square (samples 4, 5, 6) shown in Fig. 4, so moving the sample does are labeled as 1, 2, 3, and 7 and those made on the “right” side not appear to increase variability. The LabSphere Infragold

Table 3. Average Scaled DHR and Diffuse Reflectance Values for Infragold, Bruker Diffuse Gold, Spectralon 99% Reflectance Standard, Hand-Loaded Sulfur, Hand-Loaded Talc, and Watera

b c Sample ρ(PNNL) ρ(Calib.) %Δρ ρd (PNNL) D (PNNL) Infragold (Right, 4, 5, 6)d 0.956(87) 0.943 1.38 0.944(74) 0.987(119)d Infragold (Left, 1, 2, 3, 7)d 0.954(87) 0.943 1.17 0.935(74) 0.980(121)d Infragold (Right, fixed)d 0.957(87) 0.943 1.48 0.949(74) 0.992(120)d Infragold (VNIR)e 0.958 0.941 1.81 0.952 0.993d Bruker Goldd 1.007(91) ––0.914(60) 0.909(112)d Spectralon 99% f 0.961(88) 0.972 1.13 0.961(63) 0.998(113)g Sulfur (Hand-Loaded)d 0.969(88) ––0.967(65) 0.998(113)d Sulfur (VNIR)h 0.964 ––0.946 0.982h Talci 0.882(80) ––0.887(58) 0.994(112)c Talc (VNIR)i 0.856 0.841 0.982i Waterj 0.0232(7) 0.0216 7.41 0.005(4) 0.23(80) j aAverage DHR values for calibration spectra, where available, and percent change with the PNNL results are given. Calculated diffuseness values are also given. The values in parenthesis after the reflectances are the two standard deviation (k 2 coverage factor) expanded uncertainties. b Δρ ρ − ρ ∕ρ Percent change, % f PNNL calib calibg × 100%. c The diffuseness spectrum is defined as D ρd ∕ρ and measured by the Bruker sphere. D is averaged over the wavenumber ranges noted to give the values shown. dAveraged between 1000 cm−1 and 7000 cm−1. eAveraged between 5000 cm−1 and 9000 cm−1. fAveraged between 4000 cm−1 and 7000 cm−1. gAveraged between 1500 cm−1 and 7000 cm−1. hAveraged between 5000 cm−1 and 7500 cm−1. iAveraged between 5000 cm−1 and 7000 cm−1. jAveraged between 1000 cm−1 and 5000 cm−1. Research Article Vol. 57, No. 3 / January 20 2018 / Applied Optics 439

[23,31]. The NIST D25A02 standard was used to scale the reflectance spectra shown in Fig. 6. The figure shows both the scaled and unscaled spectra. Because of the highly Lambertian nature of Spectralon, the DHR and diffuse reflectance spectra are nearly coincident. The diffuseness spectrum, D, averaged between 1500 cm−1 and 7000 cm−1, is 0.998(113). The average DHR for the LabSphere calibration between 4000 cm−1 and 7000 cm−1 is 0.972, and the PNNL scaled DHR for this material averaged over the same range is 0.961(88) with a relative percent change between the two of 1.13%. The scaled diffuse reflectance is 0.961(63) over the same range (see Table 3)[38,42,43]. We observed that there is a slight anisotropy in the specular fraction on the surface of the Spectralon disk: as we rotated the disk to different positions in the sample port the spread between the DHR and the diffuse in the shortwave infrared would increase to 1% (maximum) for several positions and Fig. 5. DHR and diffuse reflectance spectra of the Bruker flat dif- then begin to grow closer until the DHR and diffuse spectra fuse gold plug. Ten sets of reflectance measurements were made with were lying on top of one another. By noting the position with a the plug in the bottom sample port. For each set, 256 scans were re- notch on the side of the Spectralon disk, we noticed that the corded for the DHR and diffuse measurements. All sets were averaged positions where the DHR and diffuse spectra merged and sep- to give the spectra shown in the upper plot. The spectra were scaled using the NIST D25A02 standard. Both the scaled and unscaled spec- arated were repeatable as the standard was rotated in the sample tra are shown. The expanded uncertainties are shown in the lower port. Furthermore, the disk is not entirely flat: one half of the panel. disk’s surface slopes down approximately 0.4 mm with respect to the other. This may explain the slight difference in directional–hemispherical and diffuse reflectance of the disk, as it is rotated in the sample port. reference data and the DHR and diffuse VNIR reflectance spectra of the same standard were measured using our dispersive spectrometer and PTFE integrating sphere. For the VNIR results, three different sets of measurements were made, each time the Infragold square was rotated 120 deg. No discernible difference was observed for the spectra in the three different positions, and the spectra were averaged. The PNNL VNIR measurement of the Infragold square’s DHR and diffuse reflectance values are 0.958 and 0.952, respectively, giving D 0.993, averaged between 5000 cm−1 and 9000 cm−1. The scaled and unscaled DHR and diffuse reflectance spectra of a flat, diffuse gold plug are shown in Fig. 5. The plug is sold with the Bruker sphere and fits in the bottom sample port. Using Eq. (2), the data were scaled using the NIST D25A02 standard. Scaling pushes the DHR and diffuse reflectances up about 0.02 reflectance units throughout much of the mid-and short-wave infrared. The scaled DHR is 1.007(91), averaged between 1000 cm−1 and 7000 cm−1, suggesting a slight over- estimate in the scaling factor, and the scaled diffuse reflectance is 0.914(60) with the average of the diffuseness spectrum at Fig. 6. DHR and diffuse reflectance spectra of the LabSphere 0.909(112), as seen in Table 3. The finish on the flat, matte Spectralon 99% standard are shown in the plot. Six sets of reflectance gold plug is highly reflective but is far less Lambertian measurements were made with the standard in the bottom sample (D 0.909) than Infragold (D 0.987119)[28]. port. The standard was rotated 60 deg for each new set of measurements. For each set, 2048 scans were co-added for the DHR and 2048 C. Spectralon scans were co-added for the diffuse reflectance measurements. All sets Our DHR and diffuse infrared reflectance spectra for a 99% were averaged to give the unscaled spectra shown in the upper plot: the reflecting disk of Spectralon (LabSphere) are shown in blue trace is the unscaled DHR spectrum and the purple trace is the unscaled diffuse reflectance spectrum. The spectra were scaled using Fig. 6 [25]. A calibration DHR spectrum that came with the the NIST D25A02 standard: the black trace is the scaled DHR disk is plotted as the green trace in this figure. Spectralon is a spectrum and the red trace is the scaled diffuse reflectance spectrum. useful and convenient Lambertian reflecting material in the vis- The expanded uncertainties are shown in the lower plot. The green ible and near-infrared wavelength regions, but strong absorp- trace is the LabSphere DHR calibration spectrum that came with tions begin in the mid-infrared and extend into the far-infrared the standard. 440 Vol. 57, No. 3 / January 20 2018 / Applied Optics Research Article

range between 0.54 and 0.63 [45]. Egan and Hilgeman [46] examined the DHR reflectance of sulfur powder using an integrating sphere, though they do not specify a reference surface, and report an average reflectance of 0.94 between 900 cm−1 and 3300 cm−1; the resolution of their spectrograph was sufficiently low (R λ∕Δλ 33) that no sharp features were observed. Our average DHR reflectance for the hand-loaded sulfur in the short to midwave infrared is 0.95, and at the local minimum at 938 cm−1 the reflectance is 0.60, so our reflectance values are consistent with previous results. Also shown in Fig. 7 are our VNIR DHR (blue trace) and diffuse reflectance (green trace) spectra of sulfur powder [23]. These spectra overlap with the infrared measurements between 5000 cm−1 and 7500 cm−1. Because the sample in the VNIR sphere is held vertically, a quartz window must be used to hold the sample in place, and, while the specular nature of the window has been Fig. 7. DHR and diffuse infrared reflectance spectra of six different corrected for [23], the sulfur powder, pressing against the win- hand-loaded sulfur samples. The average packing density of the sulfur dow, may introduce effects that make the material slightly more ∕ 3 is 0.78 g cm . For each sample, 2048 scans were recorded for DHR specular than the hand-loaded powder samples used for the and diffuse measurements. All sets were averaged to give the spectra infrared measurements described here. In the overlap region, shown in the upper plot. Black (DHR) and red (diffuse) traces have been scaled using the NIST D25A02 standard. The expanded uncer- the infrared DHR spectrum is, on average, about 0.52% above tainties are shown in the lower panel. The blue and green traces are the the VNIR spectrum, and the infrared diffuse reflectance spec- DHR and diffuse reflectance visible/near-infrared (VNIR) measure- trum is 3.4% above that of the VNIR. Overall, the agreement ments of sulfur in which the sulfur was packed into a sample cup between the two methods is very good, lending further cre- and a glass cover was placed over the sulfur in the cup. The VNIR dence to the hardware and protocols used for the infrared measurements were made using a Cary 5000 grating spectrograph measurements. Such good agreement between IR and VNIR and LabSphere DRA-2500 PTFE integrating sphere. measurements (<3%) for PTFE have also been reported by Tsai et al. [47]. E. Talc D. Sulfur Talc is a common phyllosilicate that is chemically stable, and, if The average DHR and diffuse reflectance spectra of the hand- kept dry in a desiccator box when not in use, can be used as a loaded sulfur samples are shown in Fig. 7. The average packing reference material for infrared integrating sphere performance. ∕ 3 density of the sulfur in the cups was 0.783 g cm , and the The average DHR and diffuse reflectance spectra for six hand- μ mean sulfur particle size was 40(26) m as determined by loaded samples are shown in Fig. 8. The talc DHR (black trace) optical microscopy [44]. The DHR (black trace) and diffuse and diffuse reflectance (red trace) were scaled using the NIST reflectance (red trace) were scaled using the NIST D25A02 D25A02 standard. The average packing density for these standard. As with the Spectralon 99% reflectance standard, samples is 0.733 g∕cm3, and the particle size is less than the DHR and diffuse reflectance spectra are nearly coincident 10 μm. Between 5000 cm−1 and 7000 cm−1 (i.e., between D across the entire infrared, with 0.998113 averaged from the talc v OH stretch and OH stretch overtone), the aver- −1 −1 4 1000 cm to 7000 cm (see Table 3). As noted previously, age DHR is 0.882(80), the average diffuse reflectance is 0.887 [44] the infrared reflectance measurements for sulfur show a (58) and D 0.994112 (see Table 3). The VNIR measure- nearly perfect Lambertian behavior for the hand-loaded ment of the DHR (blue trace) and the diffuse reflectance (green material, but pellets of sulfur made with a die and hydraulic trace) of a talc powder sample held behind a glass window are ∕ 3 press (pellet density 1.797) g cm ) have a significant specular also shown in Fig. 8. component. To our knowledge there is no definitive sulfur reflectance F. Water reference spectrum. Nash examined sulfur from different sup- In the past, researchers have used water as a specular, low pliers using an FTIR and a biconical reflectance accessory [45]. reflectivity standard [21]. To examine its suitability for this pur- He states that those samples were powders with particle sizes pose, the DHR and diffuse spectra of six deionized samples less than 100 μm and that these were lightly packed, though were recorded at 23°C and the results plotted in Fig. 9. The he does not give a specific packing density. In the NIST S25B02 reflectance standard was used to scale the data; 1430 cm−1–5000 cm−1 region, his observed reflectances for both the scaled and unscaled spectra are shown in the plot. The his various sulfur samples are all ∼0.90 referenced to a piece scaled diffuse spectrum is essentially flat across the entire infra- of 600-grit sandpaper that had been gold coated. Because red with an average reflectance of 0.005(4) in the 1000 cm−1 to Nash’s interest was in examining reference reflectance materials 5000 cm−1 range. Over the same range, the average scaled μ for CO2 lasers operating at about 10.6 m, he notes that his DHR is 0.0232(7). As expected, the spectrum shows two sulfur reflectances at 938 cm−1, which is at a dip in the reflec- major absorptions at 1600 cm−1 and 3200 cm−1. As a refer- v v tance corresponding to the 1 5 sulfur combination band, ence, we calculated the bidirectional reflectance spectrum of Research Article Vol. 57, No. 3 / January 20 2018 / Applied Optics 441

upon the water’s surface. The calculated curve has an average reflectance over the 1000 cm−1–5000 cm−1 range of 0.0216. In terms of percent change, the measured DHR is 7.41% higher than that of the calculated bidirectional reflectance. Unfortunately, Downing and Williams do not present their raw reflectance data in their paper [48] and do not provide uncertainties for their n and k values, so it is not possible to determine if our values lie within their error bars.

4. DISCUSSION The integrating sphere we have used for producing the results reported here is convenient to use, fits easily into most FTIR sample compartments, and, as demonstrated, yields good quantitative DHR and diffuse reflectance values. We emphasize that these results were achieved only after careful alignment and use of the sphere. Comparing the absolute differences between the Fig. 8. DHR and diffuse infrared reflectance spectra of six different average PNNL DHR and NIST DHR measurement results hand-loaded talc samples. The average packing density of the talc is given in Table 2, we see that these differences are <0.02 re- ∕ 3 0.73 g cm . For each sample, 2048 scans were recorded for DHR and flectance units in all cases, corresponding to a percent change diffuse measurements. All sets were averaged to give the spectra shown agreement for the PNNL values to within 4% of the NIST in the upper plot. The black (DHR) and red (diffuse) traces have been scaled using the NIST D25A02 standard. The expanded uncertainties values except for the low diffuse reflecting standard D25C02. are shown in the lower panel. Blue and green traces are the DHR and Comparing the percent changes between the PNNL and NIST diffuse reflectance visible/near-infrared (VNIR) measurements of talc average diffuse reflectance results, we see that the PNNL values in which the talc was packed into a sample cup and a glass cover was are within 5% for the diffuse standards. The measured placed over the talc in the cup. The VNIR measurements were made PNNL diffuse reflectances for the specular standards S25A02 using a Cary 5000 grating spectrograph and LabSphere DRA-2500 and S25B02 are an order of magnitude larger than the NIST PTFE integrating sphere. values for these standards. For the Bruker sphere with a specular sample in the sample port, the specular exclusion port cap removed, and the infrared light directed to the sample, our ray water (green trace) using Fresnel’s equations and the n and k tracing results show that no reflected light reaches the detector. values for water (27°C) given in [48]. We use an angle of in- When we examine the sample channel data for this arrange- cidence of 14.8 deg and assume unpolarized light is incident ment for both S25A02 and S25B02, we see that the signals are indeed small, though non-zero. The reference channel signals are considerably larger than those of the sample channel, though, on the interferometer’s A/D scale, still less than one. For S25A02 at 2125 cm−1, for example, the ratio of the sample and reference signals is 0.00011∕0.0211 0.0052. An alternate method for measuring the diffuse reflectance of specular samples will have to be found. Using a comparison sphere, Shepherd gives results for a limited number of materials: Infragold, a silver mirror, and a sandblasted copper disk [42]. The Newport Corp. specular silver mirror exhibits a DHR reflectance spectrum close to 1.0 across the infrared, which is in agreement with the manufacturer’s specification, and a diffuse spectrum close to zero, as expected. For Infragold, Shepard measures a DHR of 0.94 and diffuse reflectance of 0.91 at 4 μm, and these values drop at longer wavelengths: ρ 0.93 and ρd 0.89 at 8 μm. Shepherd’s DHR values are thus similar to ours; however, he has lower diffuse reflectance values leading him to emphatically conclude that Infragold is not Lambertian. Shepherd’s sandblasted cop- Fig. 9. DHR and diffuse reflectance spectra of six different deionized per disk does appear to be far more Lambertian across the infra- water samples, 23°C, averaged. For each sample, 2048 scans were re- red from 2 μmto12μm, with ρ ρd 0.84 at 8 μm, though corded for the DHR (blue) and diffuse (pink) unscaled measurements. All sets were averaged to give the spectra shown in the upper plot. The details on how the disk was prepared were not given [42]. scaled DHR (black) and diffuse (red) reflectance measurements were Due to the paucity of diffuse reflectance data for many of scaled using the NIST S25B02 standard. The expanded uncertainties these materials, we have made some qualitative comparisons are shown in the lower plot. The green trace is a calculated DHR between our diffuse reflectance measurements and those made reflectance spectrum using the n and k values from [48]. by others using the same or similar materials. Our Cary VNIR 442 Vol. 57, No. 3 / January 20 2018 / Applied Optics Research Article spectrometer and LabSphere DRA-2500 integrating sphere al- and reference dependent but contain dependencies on sphere low us to make some comparison measurements. However, our design and coatings as well. 100 diameter NIST standards and the Bruker diffuse gold plug In addition to the sulfur reflectance results of Nash [45] are too small to fit properly in the sample port of the VNIR discussed above, Haner and Menzies [50] have measured the integrating sphere, and it would be preferable to make compar- in-plane reflectance of two preparations of sulfur at isons without the intervening window over the powder samples 10.6 μm. In one sample, a cast of sublimed sulfur was prepared in the VNIR measurements. For the Infragold, the Cary 5000/ by making a slurry of the sulfur with acetone and allowing this DRA-2500 measurements show a near-perfect Lambertian ρ to dry in the sample cup; in the other, dry sublimed sulfur was 0.958 and ρd 0.952 at θi 8°, D 0.993 in the SWIR loaded and pressed in the sample cup; the packing densities for without a cover window over the standard, and for the infrared these preparations are not given. Using a CO2 laser to illumi- measurements of the same standard ρ 0.95787 and nate the surface of the sulfur samples, they observed a sharp ρd 0.94949 at θi 14.8°, D 0.992120. These values retro-reflection at the angle of incidence, and, while the slurry are for the fixed position Infragold measurements and after preparation showed a near-perfect Lambertian reflectance spec- scaling using the D25A02 standard. trum over a wide sweep of reflection angles, the pressed sulfur Balling reports [43] BRDF values at wavelengths 544 nm, sample showed a distinct specular reflection peak. Oppenheim μ 632.8 nm, and 3.39 m for Infragold, Spectralon, and the and Feiner [51] repeated the Haner and Menzies experiment in NIST DxxA, DxxC, and DxxD materials. He describes the ex- 1994 using only the sulfur/acetone slurry preparation and ob- tent, if present, of any spectral lobing for these materials over a served that sulfur “is probably the most Lambertian surface in range of incident angles. From his BRDF values, he calculates the IR.” Assuming that the hydrocarbon impurities can be re- the DHR for these materials, and these calculated values agree moved and the sulfur dried, this statement is certainly true at μ well with our measured DHR values at 3.39 m[25]. wavelengths shorter than 5 μm, though working with sulfur It was of interest to find that the Bruker diffuse gold plug powder can be inconvenient. Our infrared results, after scaling that comes with the integrating sphere has a significant specular with the NIST D25A02 standard, show the DHR and diffuse component (ca. 9%, see Fig. 5). Both the DHR and diffuse reflectance traces nearly coincident in the shortwave-infrared reflectance curves for this material are spectrally flat across and well into the mid-infrared and longwave-infrared, as seen the infrared, and the DHR average is 1.007(91) after scaling in Fig. 7. Our VNIR reflectance spectra for sulfur powder, also to the D25A02 NIST standard. If we assume that the sphere shown in Fig. 7, show similar reflectance values with the infra- wall was prepared in the same manner as the surface of the plug, red measurements; however, the VNIR results have a small sep- and, because we are referencing our spectra to the sphere wall aration between the DHR and diffuse reflectance traces. This before scaling, this high value is not surprising. With the flip difference could be a residual effect not removed when we cor- mirror pointing at the wall reference spot, and the diffuse gold rected for the specular reflectance from the glass sample cover, plug in the bottom sample port, we have measured the detector or possibly the sulfur is being slightly compressed by the glass signal ratio without and with the cap in the specular exclusion window as discussed above. port. From Eq. (1), we can calculate the ratio as Finally, the water’s DHR spectrum leaves some doubt as to V − ρ − f its usefulness as a reflectance standard. As expected, the diffuse w∕o 1 1 w ; V − ρ − f (5) reflectance spectrum is essentially flat and near zero across the w 1 1 w∕o infrared. (The deionized water was carefully pipetted into the sample cups; we did not observe a meniscus on its surface. f f where w 0.0227 and w∕o 0.0677 are the exchange frac- The nitrogen purge was turned off during the water reflectance tions with and without the cap in the specular exclusion port measurements.) We observe as much as a 30% change in some [32]. A sphere wall reflectance value of approximately ρ 0.98 regions of the infrared between our measured DHR curve and best replicates the measured signal ratio. the reflectance curve calculated using the indices of refraction Again, we are not familiar with how the Bruker sphere’s sur- from the paper by Downing and Williams [48]. They do not face is prepared, but it appears both visually, and from the re- provide estimates of their random or systematic sources of sults in Table 3, that the Bruker matte gold is not as porous as uncertainty in their n and k values that were fit to several the LabSphere Infragold. The less Lambertian (more specular) absorption and reflection data sets. Because our measurement nature of the Bruker sphere’s surface (D of 0.909 versus 0.990) of the more convenient NIST D25C02 standard, which has a has consequences on the performance of the sphere. Koehl and similar average reflectance to that of water, gives agreement Forcht [28] pointed out that essentially all IR spheres have within 0.002 reflectance units (6%) of the NIST reference gold-coated surfaces of some sort. As a sphere surface material spectrum, we will use this as a reference for low reflectance becomes more porous, it becomes more Lambertian; conse- values until the water reflectance/absorbance spectrum in the quently, the reflectivity, ρ, generally drops, particularly in infrared is better understood. the longwave IR. Increasing the smoothness of the surface Though the PNNL measurements of the five NIST stan- generally increases ρ, but, as Hanssen [49] demonstrated, the dards compare well with the NIST measurements, there are increased specularity of a non-Lambertian surface leads to ar- clearly some significant differences in the appearance of the tifacts in the reflectance values, particularly for dark samples PNNL and NIST data. The variability in the percent changes whose reflectivities differ greatly from those of the reference between the PNNL and NIST measurements of the DHR and material. Reflectance measurements are thus not only sample diffuse reflectances of the different standards show that a simple Research Article Vol. 57, No. 3 / January 20 2018 / Applied Optics 443 scaling factor cannot account for the differences. There are systematic differences in the way PNNL and NIST make their measurements. NIST uses the absolute reflectance method developed by Hanssen [24,33,39] and a sphere that is symmet- rical with respect to the geometry of the sample and reference measurements. The NIST sphere does not use an internal mirror to steer light to the ports; rather light from the interferometer is steered external to the sphere. In contrast, the sample and reference measurement geometries of the Bruker sphere are not symmetric, and the sphere has an internal steering mirror that is a minor source of light loss. When making reflectance measurements with a sphere, one is ratioing results from two different geometries, and open ports can lead to loss of hemispherically distributed light from the sample and the reference point on the wall in the case of the Bruker sphere. The amount of light lost Fig. 11. Light loss plot for the Bruker sphere measurement reference through the ports for the sample and reference measurements is geometry is shown. The BRDF for a typical diffuse gold interior sphere affected by differences in measurement geometries and any dif- wall at 10.6 μm is shown, and regions of light loss are shown with solid ference in the BRDF of the sample and reference [33,39]. A vertical lines. The horizontal axis is incidence angle with respect to the direct example of the effect of this is the apparent crossover reference’s (sphere wall) surface normal. The angles associated with each of the DHR and diffuse curves in Fig. 2. This is at least partially port loss are estimated from the schematic shown in Fig. 1. Below the ∕π due to the fact that D25A02 is likely more Lambertian (flat horizontal line (1 ) light loss is from diffusely reflected light, and above the horizontal line light loss is from specularly reflected light. BRDF) than that of the integrating sphere wall. The wave- This plot represents a 2D cross-section of the actual 3D case. length dependence is what one would expect: at longer wavelengths, a diffuse surface begins to appear more specular. To obtain the diffuse reflectance, the specularly reflected light needs to be removed. In the case of the Bruker sphere, entrance port and the input mirror, which will reflect sample this is done by removing the cap in the specular exclusion port. scattered light back through the entrance port. In the case of an To obtain a reference measurement without changing the ideal sphere coating, and a sample that has perfectly specular sphere throughput, the internal mirror is rotated to the refer- and diffuse components, the entire specular component of ence position, and the two spectra are ratioed. The effects of the the sample is removed, and the fractional loss of the diffusely sphere geometry under these circumstances can be understood scattered light is the same for both sample and sphere wall refer- from Figs. 10 and 11. In the Bruker sphere, reflected light is ence. In the case of a realistic sample BRDF and a realistic directly lost in three locations: the specular exclusion port, the sphere coating BRDF, the situation is more complex, as demonstrated in the figures. For the DHR measurement, light is only lost directly through the entrance port and mirror. In the ideal case for sample and wall BRDF, the specular and diffuse components are measured together, and the reference provides the proper throughput correction. However, with realistic BRDFs as shown, the relative correction may be different, resulting in an error that may show up as an effective negative specular component, when subtracting the diffuse reflectance from the DHR. It is important to note that, because the specular exclusion port of the Bruker sphere subtends a larger solid angle (23.5 deg full apex angle, 0.132 sr) than the specular exclusion port of the NIST sphere does (12.7 deg full apex angle; 0.0385 sr), the diffuse reflectance spectrum of a sample, as measured by the Bruker sphere, will always be equal to or smaller than that measured by the NIST sphere, depending on the BRDF of the sam- Fig. 10. Light loss plot for the Bruker sphere measurement sample ple. For a perfectly Lambertian sample, the two spheres should geometry is shown. The bidirectional reflection distribution function give, after the aforementioned corrections are made, equal dif- μ (BRDF) of a typical diffuse sample at 10.6 m is shown, and regions of fuse reflectance values. But, as the sample becomes more specu- light loss are shown with solid vertical lines. The horizontal axis lar, the Bruker sphere will let more specular light out and will represents the angle of reflected light relative to the sample’s surface normal. The range of angles associated with regions of light loss is consequently give a smaller diffuse reflectance spectrum (and estimated from the schematic shown in Fig. 1. The horizontal line hence smaller D value) than that measured by the NIST sphere. and vertical shaded region represent ideal diffuse and specular compo- Whenever comparing diffuse reflectance results from two nents, respectively. This plot is a 2D cross-section of the actual different integrating spheres, the geometry of the specular 3D case. exclusion port must be taken into account. 444 Vol. 57, No. 3 / January 20 2018 / Applied Optics Research Article

These concepts are illustrated in Figs. 10–12. In Fig. 10 we and diffuse reflectance cases. The reference measurement show a plot of one slice of the BRDF (log y axis) of a typical geometry is unchanged in both cases. See [33] for further de- diffuse sample at 10.6 μm. The horizontal line represents tails. In NIST’s case, the “specular reflectance” is determined by an idealized Lambertian reflector. The x axis is the angle with the difference in light lost from the two measurements, which respect to sample surface normal, and the angular extents of the both use the sample and hence the same—or quite similar— various ports are marked off as horizontal lines on the plot. The BRDFs at the 0 deg incidence versus 8 deg incidence measure- positions of these lines have been estimated from the schematic ment geometries. For the diffuse measurement, an 8 deg wedge in Fig. 1. The cap in the specular exclusion port, in this exam- is used between the sample and the sample port edge that re- ple, has been removed. Figure 10 represents the sample mea- sults in normal incidence on the sample. In NIST’s method, the surement geometry; Fig. 11 represents the reference geometry two results are subtracted to obtain the specular component, where the BRDF of a typical diffuse gold sphere lining has been with minor corrections [39]. Reference measurements are per- plotted. The differences in the specularly reflected light in the formed for both 0 deg and 8 deg angle of incidence measure- two plots would have to be accounted for to properly calculate ments and are ratioed with the sample measurements to obtain the diffuse reflectance spectrum, or in the case where the cap is reflectance values. However, the reference measurement geom- in place, the DHR spectrum. etry does not change and hence does not play any role in the The NIST absolute reflectance measurement geometries are specular/diffuse separation. shown in Fig. 12 [24,33,39]. The sample diffuse measurement geometry with regions of light loss are shown with solid lines, 5. CONCLUSION and the sample DHR geometry light loss is shown with dashed lines and an italic label. These are overlaid with an example We have demonstrated that a commercial integrating sphere BRDF and compared to ideal diffuse and specular BRDFs. and Fourier transform spectrometer can, with the proper align- In the case of the NIST sphere, there is no specular port or ment and measurement procedures, be used to record DHR and diffuse reflectance spectra in the infrared and give quanti- internal mirror. (There is an empty reference port, but there tative results when compared with calibration standards. The is a baffle, which prevents sample reflected light being lost particular sphere used here allows for measurement of powder through it.) [39] The only lost light is through the entrance samples in a horizontal port in the bottom of the sphere port. The diffuse reflectance is obtained by using a compensat- [52,53]. Expanded uncertainties have also been determined ing wedge between the sample and sample port, which results for the measurements with coverage factor k 2 [37]. in normal incidence on the sample and consequently results in Comparing measured results for a set of NIST and common the loss of the specularly reflected light. The DHR measure- infrared reflectance reference materials shows that we can ob- ment is made without the wedge and has 8 deg incident light. tain good DHR values compared with reference spectra with- Figure 12 shows the sample measurement case for the DHR out corrections. For our measurement of the DHR of the four NIST reflectance standards with ρ > 0.18, our values for those standards are about 4% relative to the NIST DHR values, and for the one standard with ρ < 0.18 our value is about 7% relative to the NIST DHR data for that one standard. For our measurement of the diffuse reflectance of the five NIST reflectance standards, the percent change for the diffuse standards is 5% compared with the NIST diffuse reflectance values, and for the specular standards the diffuse reflectances, ρd , (all ≤ 0.01 reflectance units) are about an order of magnitude larger than the NIST calibration spectra for these standards. Discrepancies between the PNNL and NIST results have been discussed in terms of the differences in measurement geometries, BRDFs of the sample and reference surfaces, and light lost through ports in the sphere. Ray tracing of Bruker sphere has been conducted to determine absolute uncertainties for the PNNL reflectance measurements. Fig. 12. Light loss for the NIST absolute reflectance measurement Using the reflectance values and their uncertainties for the geometries is shown. For the sample geometry, regions of light loss are NIST standards given in Table 2, we can examine how well the shown with solid lines; for the reference geometry, regions of light loss PNNL and NIST DHR and diffuse reflectance fall within their are shown with dashed lines and an italic label. These are overlaid with combined uncertainty ranges. By taking the difference between an example sample BRDF and compared to ideal diffuse and specular the PNNL and NIST reflectance value for each standard, BRDFs. Here the “specular component” is the difference in light lost and adding their uncertainties, we can see that most of the from the two geometries, which both use the sample (and hence the same—or quite similar—BRDFs—0 deg incidence versus 8 deg inci- differences fall within their combined uncertainty. The excep- dence). The reference measurements are made in the same way for tions are the S25A02 diffuse reflectance measurement and the both 0 deg and 8 deg incidence measurements and are only used S25B02 DHR and diffuse reflectance measurements. to correct for any extremely small changes in throughput or drift A scaling method was described to allow direct comparison in going from the 0 deg and 8 deg incidence measurements. of the reflectance spectra of other materials with one of the Research Article Vol. 57, No. 3 / January 20 2018 / Applied Optics 445

NIST standards. The scaled DHR spectra of Infragold and 15. A. L. Miller, P. L. Drake, N. C. Murphy, J. D. Nollb, and J. C. Volkwein, “ 99% reflecting Spectralon as measured using our sphere and Evaluating portable infrared spectrometers for measuring the silica content of coal dust,” J. Environ. Monit. 14,48–55 (2012). method gives an agreement within 2% of the calibration 16. T. A. Blake, J. F. Kelly, N. B. Gallagher, P. L. Gassman, and T. J. spectra provided by LabSphere. Our DHR results for sulfur Johnson, “Passive standoff detection of RDX residues on metal sur- powder and water also agree well with literature values, though faces via infrared hyperspectral imaging,” Anal. Bioanal. Chem. 395, the provenance of these literature values makes it difficult to 337–348 (2009). 17. A. Plaza, J. A. Benediktsson, J. W. Boardman, J. Brazile, L. Bruzzone, make quantitative comparisons. G. Camps-Valls, J. Chanussot, M. Fauvel, P. Gamba, A. Gualtieri, M. Marconcini, J. C. Tilton, and G. 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