Use of Spectralon as a diffuse reflectance standard for ...

STIEG MAN, BRUEGGE, and SPRINGSTEEN

from which Spectralon is manufactured, but can be easily removed by a vacuum bake-out procedure. A significant point is that, except for this volatile impurity, Spectralon is an otherwise clean material that will only become more contaminated by washing in water or, worse, organic solvents. Spectralon should always be handled with white cotton lint-free gloves in a clean environment (preferably a clean room). More specifically, it should not be exposed to volatile organics after the cleaning procedure. By vacuum-baking Spectralon (10- 6 Torr at 90°C) for a period of 48 h, all of the volatile organics are readily removed. This is the only cleaning procedure that we recommend. As mentioned previously, washing with water or solvents will only contaminate the material further. After the vacuum bake-out procedure, the Spectralon should again be handled with cotton gloves in a clean environment. Because the material will have a tendency to reabsorb volatile organics, it should be stored and transported only in clean air-tight containers, preferably glass or oil-free metal containers. Under no circumstances should it be placed in nonspace qualified plastic containers or bags, because plasticizers and organic antistatic agents can readily leach from those containers and recontaminate the material. If plastic containers must be used, we recommend Fluo-

roware@l H22 Series individual wafer shipping trays (Fluoroware Inc., Chaska, Minnesota) (used to ship semiconductor wafers), or 3M brand 2110E embossed recloseable static-shielding bags. The material should be left in the sealed container until integrated into the spacecraft.

Use of Spectralon as a diffuse reflectance standard for in-flight calibration of earth-orbiting sensors

Acknowledgments TRW UV exposure and hemispheric reflectance data were acquired by M. R. Gilmore. Chemical analysis testing was conducted by G. Plett of JPL. The research described in this paper was carried out by JPL, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Carol J. Bruegge, MEMBER SPIE Albert E. Stiegman, MEMBER SPIE Richard A. Ralnen Jet Propulsion laboratol)' California Institute of Technology 4800 Oak Grove Drive Pasadena, California 91109

References Arthur W. Springsteen, MEMBER SPIE Lapsphere Incorporated P.O. Box 70 North Sutton, New Hampshire 03264

I. M. R. Gilmore, "Ultraviolet degradations of Spectralon samples for the Jet Propulsion Laboratory," TRW Space Technology Group Internal Document LlZJ.Hig-90-16 (March 1990). 2. C. J. Bruegge, A. E. Stiegman, R. A. Rainen, and A. W, Springsteen, "Use of Spectralon as a diffuse reflectance standard for in-flight calibration of earth-orbiting sensors,~ Opt. Eng. 32(4), in this issue{ 1993).

Biographies and photographs of the authors appear with the paper "Use of Spectralon as a diffuse reflectance standard for in-flight calibration of earth-orbiting sensors" in this issue.

Abstract. For the Multiangle Imaging SpectroRadiometer (MISR), currently under development for the Earth Observing System, we plan to use deployable diffuse reflectance panels to provide a flight calibration of its nine cameras. Near-lambertian reflectance characteristics are desirable to facilitate flat-field camera intercomparisons and to allow each camera to be calibrated under the same illumination levels. Panel spatial and spectral uniformity and stability with time are also required. SpectralonTM, a commercially available diffuse reflectance material made from polytetrafluoroethylene (PTFE), has been baselined in the MISR design. To assess the suitability of this material, a series of environmental exposure tests were performed. No degradation of the optical properties was apparent following proton bombardment, and stability through UV illumination was satisfactory, provided simple cleaning and handling procedures were implemented. One surprise during the testing, however, was a buildup of several thousand volts of static charge, which developed while simulating a rare pass through an auroral storm. Such a potential for charge buildup is not unique to PTFE, but exists for many thermal control paints used to cover spacecraft exteriors. Further testing of the charged Spectralon failed to produce arcing to the metallic housing frame, and models indicate that charge neutralization will occur after passage through the storm. For these reasons we intend to fly Spectralon as per our original plan. Subject terms: Spectre/on; po/ytetrefluoroethylene: diffuse reflectance standards; calibration; flight qualification: space environment exposure testing; Multiangle Imaging SpectroRediometer; Esnh Observing System. OptrCsf Engineering 32(4), 805-814 (April I 993).

1 Introduction

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As part of the Earth Observing System (EOS) mission, NASA plans to launch a series of platforms beginning in mid-1998. The first of these will include instruments to measure geophysical parameters used in the study of the terrestrial surface, atmospheric composition, clouds, aerosols, and radiation balance. The Multiangle Imaging SpectroRadiometer (MISR) instrument 1 has been selected for flight on the initial platform, designated EOS-AM. Absolute radiometric calibration is to be maintained at 3% uncertainty throughout the 5-yr mission life. Due to this challenging requirement, an on-board calibration (OBC) subsystem has been designed as part of the instrument. Elements include a pair of near-lambertian panels that will be used at approximately monthly intervals to reflect solar irradiance into the MISR cameras. Similar panels have also

Paper 2!082 w:eived Aug. 26, 1992, accepted for publication Oct 24. 1992. This paper is a revision of a paper presented at the SPIE conference on Calibration of Passi•e Remote Observing Opt1cal and Micro\>ave Instrumentation, April 199!. Orlando. FL. The paper presented there appears (unrefereed) in SPIE Proceedings Vol. 1493. © 1993 Society of Photo-Optical instrumentation Engineers. 0091·32861931$2.00.

804 I OPTICAL ENGINEERING I April1993 I VoL 32 No.4

been proposed for the moderate-resolution imaging spectrometer (MODIS), sea-viewing wide-field-of-view sensor (SeaWiFS), medium resolution imaging spectrometer (MERIS), and other flight instruments imaging in the visible. An extensive investigation has been conducted to define an appropriate panel material. This paper reports on the progress we have made in evaluating a reflectance standard sold under the trade name of Spectralon™ SRS-99 (Labsphere, North Sutton, New Hampshire). Results are applicable to all polytetrafluoroethylene (PTFE) compounds of this form, including those referred to as Sintered Halon. UV exposure under vacuum conditions, proton bombardment tests, and static charge tests have been conducted. In addition, extensive chemical analyses have been performed to gain a better understanding of the test results, and to define the process control and handling procedures needed to produce a product that meets MISR calibration specifications. Details of the contamination analyses and recommended handling procedures are given in a companion paper (Stiegman, Bruegge, and Springsteen2). This paper concludes with a summary of the test plan to be conducted to qualify Spectralon for flight use. OPTICAL ENGINEERING I April1993 I Vol. 32 No.4 I 805

USE OF SPECTRALON AS A DIFFUSE REFLECTANCE STANDARD

BRUEGGE et al.

2

Table 1 Diffuse panel requirements.

MISR

Instrument The MISR instrument will provide a unique opportunity for studying the Earth's ecology and climate through the acquisition of systematic, global multiangle imagery in reflected sunlight. It will be designed and built by the Jet Propulsion Laboratory (JPL). MISR employs nine cameras; A nadir camera and two banks of four cameras each will point forward and aft along the spacecraft ground track to image at the Earth's surface with view angles of 0, ± 26.1, ± 45.6, ± 60.0, and ± 70.5 deg. Radiometrically calibrated images at each angle will be obtained at bands centered at 0.443, 0.555, 0.670, and 0.865 ~m, with bandwidths between 20 and 40 nm. MISR will acquire images in each of its 36 channels (9 cameras, each with 4 spectral bands) with spatial sampling ranging from 275 m to 2.2 km, depending on the on-board averaging mode used prior to transmission of the data. The instrument is capable of global coverage every 9 d, and flies in a 705-km descending polar orbit. MISR radiometric calibration goals include ± 3% absolute uncertainty at the maximum signal level for a uniform scene observed by the instrument, ± I% camera-to-camera relative uncertainty at a given wavelength, and ± 0.5% relative radiometric uncertainty pixel to pixel within a given camera. Although achieving a highly accurate postlaunch radiometric calibration is inherently difficult, redundant techniques will be employed to reduce systematic errors. A key hardware component of the MISR OBC is a pair of deployable diffuse panels made from a material that has a high, near-lambertian reflectance. Figure 1 shows the layout of the nine MISR cameras, with the aft-camera panel deployed for calibration. While not in use, the panels are stowed and protected by a labyrinth seal. At approximately monthly time intervals, the panels are deployed for calibration. Over the North Pole, one of the plates will swing aft to reflect diffuse sunlight into the fields of view of the aft-looking cameras. Over the South Pole, the other plate will swing forward for calibration of the forward-looking cameras. The nadir camera will view both panels, providing a link between the two sets of observations. The cumulative space exposure time (deploy time) for each panel is expected to be no more than 100 h per mission life. The diffuse calibration targets will be monitored by two types of diodes: radiation-resistant p-i-n photodiodes and high quantum efficiency (HQE) diodes in a trapped configuration. Thus, the calibration is enabled by a direct measure of panel radiant exitance, rather than a model of exoatmospheric irradiance and assumption of panel reflectance. The radiation-resistant photodiodes will be packaged in clusters of four, with each diode in the cluster filtered to a MISR spectral band. Five such clusters will be used, one of which is mechanized on a goniometric arm to monitor the angular reflectance properties of the panels. The HQEs consist of three silicon photodiodes arranged so that light reflected from one diode is absorbed by another diode. The output of each diode is summed in parallel, resulting in nearly 100% quantum efficiency. Four nadir-viewing HQEs, in four separate packages, will be used. Each HQE is filtered to a different MISR wavelength. Calibrations independent of the OBC will be performed through the use of semiannual 2.1

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I 8061 OPTICAL ENGINEERING I April1993/ Vol. 32 No.4

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Fig. 1 MISR camera layout. Cylinders at top of figure represent the 9 M!SR cameras. Two diffuse panels are to be used in flight. Shown is the attcamera panel in the stow position, and forecamera panel in both a stow and unstowed position. Distance unit is in inches.

ground calibrations, using targets such as White Sands, New Mexico, and a transfer calibration from an orbital instrument whose field of view falls within a homogeneous region of the target. These ground validation field campaigns are key to the long-term maintenance of MISR radiometric calibration.

2.2 Panel Specifications Table I summarizes the requirements of the diffuse panels and of the stray light that illuminates the panels. The strictest requirement is for uniformity in the reflected radiance field. This is necessary to accurately measure pixel-to-pixel differences in responsivity and to allow radiances measured by the diodes to be used in the camera calibration algorithm (the diodes and cameras View distinct portions of the panels). The lambert ian characteristics of the panel facilitate camerato-camera relative calibrations. Although the 3% calibration uncertainty must be maintained with time, slow drifts in panel reflectance are tolerable. By monitoring the plates with diode assemblies, and by performing comparison calibration using ground field sites, slow changes in panel reflectance can be determined. The potential for stray light (from reflections or shadowing off the spacecraft structure or instrument underside) prohibits utilization of a known solar irradiance spectrum as an in-flight standard source. If the stray-light level were below I%, it would be lost in the noise of other radiometric uncertainties, and using a known spectrum would be possible. If stray light were to exceed 1%, we could characterize its level using our photodiodes, provided that the sum of direct sunlight and stray light was spatially uniform across the plate (to 50 deg, while the less pigmented materials behave in a manner more like the unpigmented SRS-99.

3 Spectralon Characterization 3.1

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The terms Halon, Sintered Halon, PTFE, and Spectra/on are often used interchangeably to describe a specific diffuse

0.5% per year 1 MeV) particles through the rear of the panel while it was in the stowed position. These particles scatter and decrease in energy, with some resulting low-energy protons penetrating to the optically sensitive front surface. This impact mechanism is assumed in defining the bombardment test plan, thus, fluence levels of the high-energy levels are utilized as well as energies as low as I 00 ke V. Fewer particles will have the opportunity to directly impact the panel's front surface during the calibration exercise (unstowed), but here front-surface sputtering is possible. Once a test plan was developed, the California Institute of Technology was contracted to do the tests. The California Institute of Technology facility consists of a 6 million volt Model EN Tandem Van de Graff accelerator, operated as part of its applied physics and materials science research program. Built in 1960 by the High Voltage Engineering Corporation, it is capable of producing proton and helium beams from 1 to 11 MeV. Maximum beam currents are of the order of I J.LA. Additionally, a Model JN I million volt single-ended accelerator can extend the low-energy proton capability to 100 keV. Samples exposed at this facility included a Spectralon target, a water-cleaned Spectralon sample, and a NISTprovided PTFE sample. The samples were mounted to a rotating plate, which allowed omnidirectional exposure in vacuum conditions. The beam size was approximately 4 in, thus, only a few samples were exposed at a time. Exposure times were of the order of minutes, and indicative of the particle count environment in the EOS orbit. (Typically this facility uses much higher exposure times in the simulation

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814 I OPTICAL ENGINEERING I April1993 I Vol. 32 No.4

Abstract. The hit-or-miss operator is used as the building block of optimal binary restoration filters. Filter design methodologies are given for general-, maximum-, and minimum-noise environments, the latter two producing optimal thinning and thickening filters, respectively, and for iterative filters. The approach is based on the expression of translationinvariant filters as unions of hit-or-miss transforms. Unions of hit-or-miss transforms are expressed as canonical logical sums of products, and the final hit-or-miss templates are obtained by logic reduction. The net effect is a morphological representation and estimation of the conditional expectation, which is the overall optimal mean-absolute-error filter.

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is based on the Matheron representation theorem, 4 which states that every such filter can be represented as a union of erosions. Specifically, design of an optimal increasing translation-invariant filter reduces to the selection of a basis B = {BJ, 82, ... , Bn} of structuring elements for which the filter defined by 'l'(S') = U;S'SB;

(2)

yields minimal MAE, where

S'SB; = {z:B; + zCS'}

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is the erosion of S' by B;, and B;+z={b+z:b E B;}. As stated, estimation of the optimal basis involves considering realizations of SandS' and then comparing 'I'(S') and S for all possible bases of structuring elements. Except for smaJl windows, such an approach is computationally intractable and, therefore, a number of methods have been developed to derive suboptimal solutions. 5-8 In the present paper we consider optimal unions of hitor-miss operators. If we utilize a finite window and do not limit the number of hit-or-miss operators in the union, then the resulting filter is a morphological representation of the conditional expectation relative to the observations in the window (or, since we are actually estimating optimality, an estimate of the conditional expectation); if we limit the number of hit-or-miss operators, then the resulting union is an approximation of the conditional expectation.

where z is an arbitrary pixel, which for convenience we take to be 0. In the paradigm of Ref. 2, such optimization

2 Morphological Representation of the Conditional Expectation

Paper 05092 received Sep. 2. 1992, I