The U. S. Geological Survey, Digital Spectral Library: Version 1 (0.2 to 3.0µm)

Roger N. Clark¹, Gregg A. Swayze¹, Andrea J. Gallagher¹
Trude V.V. King¹, and Wendy M. Calvin²

U.S. Geological Survey, Open File Report 93-592

1993

1326 Pages
499 Figures
5 Tables

¹ U.S. Geological Survey, MS 964
Box 25046 Federal Center
Denver, CO 80225-0046

² U.S. Geological Survey
2255 N. Gemini Dr.
Flagstaff, AZ 86001

For further information, contact :

Roger N. Clark at (303) 236-1332 (office)
(303) 236-1425 (FAX)
rclark@speclab.cr.usgs.gov (Internet mail)

(Any use of trade names is for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey.)

TABLE OF CONTENTS

|CONTENTS |
|ABSTRACT |
|INTRODUCTION|
|WHAT IS A IDEAL SPECTRAL LIBRARY?|
|THE SPECTRAL LIBRARY|
|Sample Documentation|
|Sample Naming |
|The Digital Data File|
|Table 1: Spectral Library Entries|
|Table 2: Minerals by Group |
|Table 3: Spectral Entries by Type|
|Table 4: Sample Listing of Splib04a |
|Table 5: Specpr Format Digital Spectral Lib. |
|SPECTRAL PURITY |
|WAVELENGTH PRECISION |
|SPECTRAL PLOTS AND DATA PRECISION |
|INSTRUMENT SPECTRAL LIBRARIES |
|MINERAL MIXTURES |
|AVAILABILITY |
|ACKNOWLEDGEMENTS |
|FUTURE PLANS |
|REFERENCES |
|FIGURE CAPTION |
|FIGURE 1 |

Contents

========================================

Spectral Library Entries

ABSTRACT

We have developed a digital reflectance spectral library, with management and spectral analysis software. The library includes 498 spectra of 444 samples (some samples include a series of grain sizes) measured from approximately 0.2 to 3.0 µm . The spectral resolution (Full Width Half Maximum) of the reflectance data is <= 4 nm in the visible (0.2-0.8 µm) and <= 10 nm in the NIR (0.8-2.35 µm). All spectra were corrected to absolute reflectance using an NIST Halon standard. Library management software lets users search on parameters (e.g. chemical formulae, chemical analyses, purity of samples, mineral groups, etc.) as well as spectral features.

Minerals from borate, carbonate, chloride, element, halide, hydroxide, nitrate, oxide, phosphate, sulfate, sulfide, sulfosalt, and the silicate (cyclosilicate, inosilicate, nesosilicate, phyllosilicate, sorosilicate, and tectosilicate) classes are represented. X-Ray and chemical analyses are tabulated for many of the entries, and all samples have been evaluated for spectral purity. The library also contains end and intermediate members for the olivine, garnet, scapolite, montmorillonite, muscovite, jarosite, and alunite solid-solution series. We have included representative spectra of H2O ice, kerogen, ammonium-bearing minerals, rare-earth oxides, desert varnish coatings, kaolinite crystallinity series, kaolinite-smectite series, zeolite series, and an extensive evaporite series. Because of the importance of vegetation to climate-change studies we have include 17 spectra of tree leaves, bushes, and grasses.

The library and software are available as a series of U.S.G.S. Open File reports. PC user software is available to convert the binary data to ascii files (a separate U.S.G.S. open file report). Additionally, a binary data files are on line at the U.S.G.S. in Denver for anonymous ftp to users on the Internet. The library search software enables a user to search on documentation parameters as well as spectral features. The analysis system includes general spectral analysis routines, plotting packages, radiative transfer software for computing intimate mixtures, routines to derive optical constants from reflectance spectra, tools to analyze spectral features, and the capability to access imaging spectrometer data cubes for spectral analysis. Users may build customized libraries (at specific wavelengths and spectral resolution) for their own instruments using the library software.

We are currently extending spectral coverage to 150 µm. The libraries (original and convolved) will be made available in the future on a CD-ROM.

INTRODUCTION

Analysis of spectroscopic data from the laboratory, from aircraft, and from spacecraft requires a knowledge base. The spectral library discussed here forms a knowledge base for the spectroscopy of minerals and related materials of importance to a variety of research programs being conducted at the U. S. Geological Survey. Much of this library grew out of the need for spectra to support imaging spectroscopy studies of the Earth and Planets.

Imaging spectrometers, such as the Airborne Visible/Infra-Red Imaging Spectrometer (AVIRIS), have narrow band widths in many contiguous spectral channels that permit accurate definition of absorption features from a variety of materials. Identification of materials from such data requires a knowledge base: a comprehensive spectral library of minerals, vegetation, man-made materials, and other subjects in the scene.

Our research involves the use of the spectral library to identify the components in a spectrum of an unknown. Therefore, the quality of the library must be very good. However, the quality required in a spectral library to successfully perform an investigation depends on the scientific questions to be answered and the type of algorithms to be used. For example, to map a mineral using imaging spectroscopy and the Clark et al. (1990) mapping algorithm, one simply needs a diagnostic absorption band. Such a feature can be obtained from a spectrum of a sample containing large amounts of contaminants, including those that add other spectral features, as long as the shape of diagnostic feature of interest is not modified. If, however, the data are needed for radiative transfer models to derive mineral abundances from reflectance spectra, then completely uncontaminated spectra are required. This library contains spectra that span a range of quality, with purity indicators to flag spectra for (or against) particular uses.

Acquiring spectral measurements and sample characterizations for this library has taken about 8 years. This first release contains 498 spectra of 444 samples. Software to manage the library and provide scientific analysis capability is also provided (Clark, 1980, 1993, Gorelick and Clark, 1993). A personal computer (PC) reader for the library is also available (Livo et al., 1993).

This document describes the contents of the library and presents a paper copy of the spectra in the form of plots and written text showing the documentation. The intent of the paper copy is as a reference document. You can look up specific documentation, or examine and compare plots of various spectra. It is not intended to be completely cross referenced and easily searchable--that is what computers are for. This paper copy is a print-out of the digital data set. However, this is a digital spectral library; searches for spectra and information should be carried out with the aid of a computer and not on the paper copy. The section on availability describes the details for obtaining digital data.

WHAT IS AN IDEAL SPECTRAL LIBRARY?

In our view, an ideal spectral library consists of pure samples, covering a very wide range of materials, a large wavelength range with very high precision, and sample analysis and documentation to establish the quality of the spectra. Budgets, time, and available equipment limit what can be achieved.

Ideally, for minerals, the sample analysis would include X-ray diffraction (XRD), electron microprobe (EM) or X-ray Fluorescence (XRF), and petrographic microscopic analyses. For some minerals, like iron oxides, additional analyses, such as Mossbauer, would be helpful. We have found that to make the basic spectral measurements, provide XRD, EM or XRF, microscopic analysis, document the results, and complete an entry of one spectral library sample, takes about one person-week. Additional spectra of the same sample (e.g. a grain size series) increases the time, but usually not an additional week per spectrum, but more like 0.5 day per spectrum (mostly sample preparation). We had hoped as our experience increased this time would decrease, but it did not.

Thus an ideal spectral library with 498 spectra of 444 samples would take on the order of 444 person-weeks, or about 8.9 person-years. Our budgets and time commitment have not allowed this level of effort, so this release of the library does not have all samples completely characterized. We estimate, however, that this release represents about 7.5 person-years of effort. The characterization of samples will continue as our budgets allow, and results will be added in future releases of the database. Latest updates on the characterization (e.g. new XRD analyses and spectral purity revisions) will be kept online for anonymous ftp on the Internet.

The ideal spectral coverage depends on the desired research. This release covers the range 0.2 to 3.0 µm. Future releases will include coverage to 150 µm for many of the samples listed here.

THE SPECTRAL LIBRARY

The spectral library contains spectra of 423 minerals, 17 plants and some miscellaneous materials (Table 1, 2, and 3). In some cases, several spectra were measured to span a solid solution series and/or a grain size series. We tried to include spectra of all mineral classes, particularly those important to imaging spectroscopy remote sensing. In other cases, we have studied particular solid solution series because we are mapping them in the field with imaging spectroscopy or studying that mineral in detail. This explains, for example, why there are so many alunite, olivine, and topaz samples in the database. Future releases of the database will likely include additional spectra of solid solution series.

All spectra were run on a modified Beckman 5270 spectrometer (Clark et al., 1990b) from 0.2 to 3.0 µm and corrected to absolute reflectance. They were run with a signal-to-noise of at least 500 at a reference reflectance level of 1.0. A few minerals were also run over a slightly smaller wavelength range because of sample size limitations. For example, small sample quantities, necessary for purity, were measured using apertures in the beam to restrict the spot size of the spectrometer. This reduced light made integration times longer and the achievable range was sometimes reduced, typically to 0.3 to 2.7 µm. This also, in some cases, limited the signal to noise that was achievable. It is not possible with the current instrumentation to substantially improve the spectral data on small volume samples. The ice sample, measured at 77K, only includes the infrared range, 0.8 to 3.0 µm.

SAMPLE DOCUMENTATION

Each sample has a text entry describing the mineral, its composition, its formula, sample description, and pointers to corresponding spectra in the digital data file. The text entry is coded by keywords for computer program searching. Each spectrum has a pointer to the documentation, so that all related data are properly cross-referenced and can be found by a computer program as well as manually.

The sample documentation is organized by keyword so that it is computer readable. The program spsearch (Gorelick and Clark, 1993) may be used to search entries in the database. With this software, it is possible to do searches such as: show me all the entries whose sample formulae have OH (hydroxyl), whose compositions contain from 20 to 30% SiO2, and have a 2.2-µm band in their spectra.

The sample documentation includes extensive analyses such as X-ray diffraction, electron microprobe or XRF, X-ray fluorescence, and petrographic microscope examination. Not all analyses have been completed for all samples, but most samples have at least one analysis.

SAMPLE NAMING

The mineral name for each sample occurs in 3 places: 1) the specpr title field for the spectrum, 2) the specpr title field for the description entry, and 3) after the "MINERAL:" keyword in the description. We have tried to use only proper mineral names as given in Fleischer 1980, and Klein and Hurlbut, 1985. Some users of the library may be unfamiliar with all the mineral names. For example, if you want to find all scapolites, you would have to know that Dipyre was a scapolite if you only looked at the specpr title fields. Because of the 40 character limit in a specpr title field, we could not include all common names there. However, use the "MINERAL:" keyword in the description for each sample. Here you could search for scapolite and you would find all entries in the "scapolite group" (Dipyre, Marialite, Meionite, and Mizzonite).

We have used specific mineral names except in a few cases where we still do not have sufficient data. For example, technically, there is no "hornblende," only ferro-hornblende and magnesio-hornblende. We have two samples where we can't make the distinction, so they are labeled hornblende.

THE DIGITAL DATA FILE

The digital spectral library data are all included in one file in "specpr" format (see Clark, 1993). This file, splib04a, has been assembled and managed using specpr and is 8.2 megabytes in size (3.9 megabytes when compressed with the Unix compress command). The data are in IEEE binary floating point format. The entire library is assembled, plotted and printed by command files consisting of 1300 Unix shell commands, which in turn generate an additional 100 Unix shell commands which generate about 44,000 specpr commands plus about 6000 specpr commands for each instrument convolved library.

Specpr runs on Unix workstations. If the binary file is read on an IBM-PC compatible machine, the floating point numbers need their bytes swapped (this is done in the Livo et al., 1993 program). An ascii version is 15.8 megabytes uncompressed, or about 5.4 megabytes when compressed with the unix compress command.

The organization of the binary data file, in the form of a specpr listing, is shown in Table 4. The listing shows the record number, title, length of the data set (number of channels for spectra; number of bytes for text), and the time and date of data acquisition. Record 6 contains the wavelength set for the spectra, and record 8 contains the the spectral resolution data set. The resolution is shown in Figure 1. Entries with the keyword "DESCRIPT" are sample description records, and contain all the sample documentation. After the DESCRIPT are (usually) two empty records (title ..) for future expansion of the description. Next comes the reflectance data, with the keyword "ABS REF." The identifier "W1R1Bx," which signifies the wavelength range, resolution, spectrometer, and spectral purity which is described below (the "x" is a lower case spectral purity letter code). This release of the spectral library has only one spectral region (W1), resolution (R1) and spectrometer (B). After the reflectance record is the "errors to previous data" record. These are the standard deviation of the mean for each reflectance value. The next record in the listing contains the feature analysis for the spectrum. This feature analysis was done using the specpr f45 special function and is described in Clark et al. (1987), Clark (1993).

In the spectral library, any value of -1.23x1034 is considered a deleted point. Because of the inherent floating point inaccuracies of single precision numbers on various computers, values in the range -1.23001x1034 to -1.22999x1034 should be considered deleted points.

Table 1: Spectral Library Entries

===============================================================================

Nr.of Samples	Mineral	Nr.of Samples	Mineral	Nr. of Samples	Mineral
1	Acmite	1	Elbaite	2	Oligoclase
5	Actinolite	1	Endellite	12	Olivine
1	Adularia	1	Enstatite	2	Opal
3	Albite	2	Epidote	3	Orthoclase
1	Allanite	1	Epsomite	2	Palygorskite
6	Almandine	2	Erionite	1	Paragonite
6	Alunite	1	Eugsterite	1	Pectolite
1	Ammonioalunite	1	Europium	4	Phlogopite
1	Ammoniojarosite	1	Fassaite	1	Pigeonite
1	Ammonium_Chloride	1	Ferrihydrite	1	Pinnoite
1	Ammonium_Illite/Smectite	1	Ferro-Hornblende	1	Pitch
1	Ammonium_Smectite	2	Ferroan Clinochlore	1	Plumbojarosite
1	Amphibole	1	Fluorapatite	1	Polyhalite
1	Analcime	6	Galena	1	Praseodymium
1	Andalusite	1	Gaylussite	1	Prochlorite
1	Andesine	2	Gibbsite	1	Psilomelane
5	Andradite	1	Glauconite	5	Pyrite
1	Anhydrite	1	Glaucophane	1	Pyrope
2	Annite	4	Goethite	2	Pyrophyllite
3	Anorthite	5	Grossular	1	Pyrrhotite
1	Anthophyllite	2	Gypsum	4	Quartz
2	Antigorite	1	Halite	2	Rectorite
1	Arsenopyrite	5	Halloysite	2	Rhodochrosite
4	Augite	1	Hectorite	2	Rhodonite
1	Axinite	2	Hedenbergite	2	Richterite
1	Azurite	6	Hematite	2	Riebeckite
1	Barite	2	Heulandite	1	Rivadavite
1	Bassanite	1	Holmquistite	1	Roscoelite
2	Beryl	2	Hornblende	2	Rutile
1	Biotite	1	Howlite	1	Samarium
1	Bloedite	1	Hydrogrossular	2	Sanidine
1	Bronzite	1	Hydroxylapatite	1	Saponite
1	Brookite	2	Hypersthene	1	Sauconite
1	Brucite	1	Ice (water)	1	Scolecite
2	Buddingtonite	4	Illite	2	Sepiolite
1	Butlerite	1	Ilmenite	2	Serpentine
1	Bytownite	1	Jadeite	1	Siderite
3	Calcite	7	Jarosite	1	Siderophyllite
1	Carbon	1	Kainite	1	Sillimanite
2	Carnallite	8	Kaolinite	1	Smaragdite
1	Carphosiderite	5	Kaolinite/Smectite	1	Sodium
1	Cassiterite	2	Labradorite	4	Spessartine
1	Celestite	1	Laumontite	5	Sphalerite
1	Celsian	1	Lazurite	1	Spodumene
1	Chabazite	1	Lepidocrosite	1	Staurolite
1	Chalcedony	5	Lepidolite	2	Stilbite
2	Chalcopyrite	1	Limonite	1	Strontianite
1	Chert	1	Lizardite	1	Sulfur
1	Chlorapatite	1	Maghemite	1	Syngenite
2	Chlorite	1	Magnesio-Hornblende	4	Talc
1	Chromite	1	Magnesite	1	Teepleite
1	Chrysocolla	2	Magnetite	1	Tephroite
1	Chrysotile	1	Malachite	2	Thenardite
1	Cinnabar	1	Manganite	1	Thuringite
2	Clinochlore	1	Margarite	1	Tincalconite
2	Clinoptilolite	1	Marialite	1	Titanite
1	Clinozoisite	1	Mascagnite	18	Topaz
1	Clintonite	2	Meionite	1	Tourmaline
1	Cobaltite	1	Mesolite	2	Tremolite
1	Colemanite	1	Mg-Clinochlore	1	Trona
1	Cookeite	7	Microcline	2	Ulexite
1	Copiapite	1	Mirabilite	1	Uralite
1	Coquimbite	4	Mizzonite	1	Uvarovite
1	Cordierite	1	Monazite	3	Vermiculite
1	Corrensite	1	Monticellite	1	Vesuvianite
1	Corundum	9	Montmorillonite	1	Witherite
1	Covellite	1	Mordenite	1	Wollastonite
1	Cronstedtite	1	Mordenite+Clinoptilolite	1	Zincite
1	Cummingtonite	13	Muscovite	1	Zircon
1	Cuprite	1	Nacrite	1	Zoisite
2	Datolite	3	Natrolite
1	Diaspore	1	Neodymium
2	Dickite	1	Nepheline	Other:
3	Diopside	1	Nephrite	3	Desert_Varnish
1	Dipyre	1	Niter	1	Kerogen
2	Dolomite	3	Nontronite	17	Plants
1	Dumortierite

===============================================================================

Table 2: Minerals in the Spectral Library by Group

===================================================================

Nr.of Minerals	Mineral Group	Nr.of Minerals	Mineral Group
16	Alunite group	22	Garnet group
21	Amphibole group	8	Hematite group
3	Apatite group	23	Kaolinite-Serpentine group
2	Aragonite group	30	Mica group
1	Arsenopyrite group	17	Montmorillonite group
1	Axinite group	1	Nepheline group
2	Barite group	13	Olivine group
5	Calcite group	5	Pyrite group
2	Chalcopyrite group	16	Pyroxene group
9	Chlorite group	2	Rutile group
1	Cobaltite group	8	Scapolite group
1	Copiapite group	1	Sodalite group
2	Dolomite group	3	Spinel group
4	Epidote group	2	Tourmaline group
28	Feldspar group	16	Zeolite group

===================================================================

Table 3: Spectral Library Entries by Type

============================================================

Table 4: Sample Specpr Listing of the start of splib04a

===============================================================================

Size is the number of data channels for spectra and FEATANL results

and number of bytes for text records.

===============================================================================

Table 5: Specpr Format Digital Spectral Library Versions

splib04a master spectral library (Beckman range andresolution)

splib04b interpolated splib04a spectra to 950 channels for use in convolutions.

splib04c AVIRIS 1990 convolution 224 channels. (e.g. Cuprite)

splib04d AVIRIS 1990 convolution 208 channels

splib04e AVIRIS 1991 convolution 224 channels.

splib04f AVIRIS 1991 convolution 208 channels.

splib04g AVIRIS 1992 convolution 224 channels.

splib04h AVIRIS 1992 convolution 197 channels.

splib04i AVIRIS 1993 convolution 224 channels.

splib04j AVIRIS 1993 convolution 197 channels.

splib04k TM

splib04m Galileo NIMS

splib04n Cassini VIMS proposed

note: the letter l was skipped in the designation splib04_ to avoid confusion with the number 1.

===============================================================================

SPECTRAL PURITY

Each spectrum has a purity code in its header. In this version of the spectral library, the code is: W1R1Bx The "W" stands for wavelength region followed by the region measured. All spectra in this version cover the nominal range of 0.2 to 3.0 µm which is region 1. The digital data for the wavelength set are located in splib04a, record 6.

The "R" stands for resolution, followed by the resolution index. All spectra in this version of the library were measured using resolution set 1 in wavelength region 1. Figure 1 shows the resolution function, and the digital data for the resolution are located in splib04a, record 8.

The next letter signifies the instrument used. All spectra in this version of the library were measured on the USGS, Denver Spectroscopy Laboratory, Beckman 5270 spectrometer, and are designated by the letter "B".

Following the instrument letter is a lower case letter signifying the spectral purity of the spectrum for this wavelength range and resolution (the "x" in the above example is one of the following letters).

a: The spectrum and sample are pure based on significant supporting data available to the authors. The sample purity from other methods (e.g. XRD, microscopic examination) indicates essentially no other contaminants.
b: The spectrum appears spectrally pure. However, other sample analyses indicate the presence of other minerals that probably affect the absolute reflectance level to a small degree, but do not add any spectral features. The spectral features of the primary minerals may be slightly less intense, but the feature positions and shapes should be representative. For example, in this wavelength region (W1), quartz would tend to increase the reflectance level and decrease absorption band strength, but would not add any measurable features to the spectrum. Such a sample would rate a "b." In a few cases, where we have little support data, but the spectra for that mineral are well known, we assigned the spectral purity based on the spectra data along with a microscopic examination of the sample. There are a few "b" classes done this way.
c: The spectrum is spectrally pure except for some weak features with depths of a few percent or less caused by other contaminants. For example, some minerals may have some slight alteration that is apparent. Spectroscopic detection of alteration is easier for more transparent minerals. For example, some of the albite spectra show weak 2.2-µm features due to alteration. From the knowledge of the mineral formula, you can often tell which features do not belong to the mineral. Albite, for instance does not have OH in the formula, so water features (1.4, 1.9, 2.2 µm) are not due to albite. However, you could argue that incipient alteration due to weathering is common in minerals at the Earth's surface. Thus, spectral bands due to weathering are somewhat characteristic of many samples (e.g. feldspars), even if they are not a property of the pure mineral. Thus these alteration spectral features might be useful in some cases.
d: Significant spectral contamination. The spectrum is included in the library only because it is the best sample of its type currently available and the primary spectral features can still be recognized. However, the spectrum should be used with care. The sample description should be consulted as a guide to what features are a part of the actual mineral. This sample may be purged from the database in future releases as better samples become available.
e: There are insufficient analyses and/or knowledge of the spectral properties of this material to evaluate its spectral purity. In general we have included such samples because we believe their spectra to be representative. These are samples for which we are concentrating future analyses in order to resolve the purity issue. Updates to the spectral purity and sample documentation will be placed online for anonymous ftp as the information becomes available. (See the section below on availability on how to electronically access the data and obtain further information.)

Commenting on the spectra in general, reflectance tends to decrease in the UV and beyond about 2.7 µm. Some of the spectra show minima in the UV. We have taken careful measurements of scattered light and believe all these features are real. Beyond 2.7 µm, even anhydrous minerals show absorption due to water adsorbed onto the surfaces of the mineral grains. Our experience has shown that these water absorptions are still present in dry nitrogen purged environments, although slightly weaker. Spectra of similar samples obtained at other facilities, like those in Hawaii or the east coast of the US. have shown us that the water absorptions in the spectra from relatively dry Colorado are really quite small in comparison. Placing the sample in a dry nitrogen atmosphere or a vacuum oven has little effect on the water absorption as water from the atmosphere will readsorb onto the sample by the time it reaches the spectrometer. Experiments by the senior author when he was at the University of Hawaii have also shown that most of the adsorbed water remains even under a strong vacuum at room temperatures. We decided in general not to heat our samples in order to avoid any temperature induced alteration.

The overall spectral purity is high for this library. Seventy-one percent of the spectra have a purity code of either a or b (36% a, and 35% b), while only 17% have c, and 2% have d. Ten percent are yet to be classified.

WAVELENGTH PRECISION

The wavelength precision of our custom-modified, computer-controlled Beckman spectrometer was checked using Holmium Oxide filters in the visible and the positions of known mineral bands in the near infrared. In particular, we developed pyrophyllite as a wavelength standard because of its many narrow absorption bands (Clark et al., 1990b). The positions of the absorption bands have been checked, using the same pyrophyllite standard, on two FTIR spectrometers. In general, the wavelength accuracy is on the order of 0.0005 *mm (0.5 nm) in the near-IR and 0.0002 *mm (0.2 nm) in the visible, always a small fraction of the spectral resolution.

SPECTRAL PLOTS AND DATA PRECISION

Plots of the spectra presented here are limited to one of seven vertical scales (0.0 to 1.03, 0.8, 0.6, 0.5, 0.4, 0.3, or 0.2) and the same horizontal range for easy comparison. The error bars are plotted only when they are above a threshold that allows them to be distinguished on the plot. Most error bars are too small to be distinguished. At the bottom of each plot is the specpr title, date and time of acquisition, file name and record number and the specpr plot options.

Each spectrum was run with a desired signal-to-noise of at least 500 relative to unity reflectance. In practice, it would take too long a time to obtain such a signal-to-noise in regions where the signal is low, so an upper limit to the integration time per channel was also specified. Thus, typically at the ends of the spectra, the precision drops slightly. Refer to the error bars for each spectrum to determine the precision at a given wavelength for any individual spectral channel. The error bars are located in the records labeled "errors to previous data" and represent one standard deviation of the mean.

Each spectrum was measured relative to Halon, and then corrected to absolute reflectance as described in Clark et al. (1990b). That paper also describes the details of the spectrometer, and the viewing geometry of the system.

INSTRUMENT SPECTRAL LIBRARIES

The intent of the spectral library is to serve as a knowledge base for spectral analysis. Comparison of spectral data is best done when the spectral resolutions of the knowledge base and the spectra undergoing analysis are identical. The specpr software has tools for convolving the spectral library to the resolution and sampling interval of any instrument. The native (laboratory) spectral library will also be convolved to AVIRIS and TM resolution and sampling for the terrestrial instruments, and to NIMS and VIMS for the planetary imaging spectrometer instruments. See Table 5 for a listing of instrument spectral libraries. These convolved data files, as well as specpr command files for convolving the database to your own instruments will be made available in the anonymous ftp directory (see below).

Each instrument convolved library requires about 0.8 megabyte of specpr-format disk space if the instrument has less than 256 channels. The proposed VIMS has 320 channels and its spectral library will be about 1.6 megabytes. The convolution to any other instrument (laboratory, or flight) is a simple matter of changing pointers in a command file to the custom resolution and sampling data sets and running the specpr command file. (Specpr runs on many Unix workstations; there is not a PC version at this time; a PC version might be difficult due to the large volume of code, over 60,000 lines.)

The convolution routine used to create the instrument spectral libraries is that in specpr. The specpr routine uses a trapezoidal integration over each bandpass. To make this integration more accurate, the original spectral library is resampled to 950 channels (splib04b) and the convolutions done on these spectra. The convolutions appear to be excellent, and tests with AVIRIS data have shown that very subtle spectral differences can be distinguished, like that between the 2.2-*mm doublet in kaolinite and halloysite.

MINERAL MIXTURES

This spectral library is largely a pure material library (except for a few cases where minerals tend to exist with other minerals). For computing intimate mineral mixtures (e.g. rocks or soils), radiative transfer algorithms using the Hapke reflectance model (Hapke, 1981) are part of the specpr package. To compute mixture or pure end-member spectra, a set of optical constants are required as a function of wavelength. The algorithms use the model at the optical constant level so spectra can be calculated as a function of grain size, abundance in the mixture, and viewing geometry. Reflectance spectra of grain size distributions can also be simulated by computing a mixture of the same mineral (or even several minerals) at several grain sizes. A future release of the library will include optical constants for the spectra in the library. Optical constant libraries will also be computed for the same flight instrument spectral resolutions and wavelengths shown in Table 5.

We included one mixture of hematite and quartz because we have found it useful in mapping hematite with imaging spectroscopy data. Usually, a pure hematite spectrum has bands that are too strong and saturated compared to that typically found in spectra of the Earth's surface. The mixture of hematite plus quartz simulates to some degree cases closer to those encountered in field data.

Table 1: AVAILABILITY

The software and spectral libraries are published as a series of USGS Open File Reports. The hardcopy spectral library, and users manuals are available from:

USGS/Dept. of the Interior
Books and Open-File Reports Section
U.S. Geological Survey
Box 25425, Federal Center
Denver, CO 80225

USGS Books, Open File Reports and Maps:
Phone number: (303) 236-7476

The relevant Open-File documents are:

93-592 Spectral Library paper version (this document), 1326 pages.

93-595 Specpr users manual, approximately 210 pages. (Clark, 1993)

93-594 Spsearch users manual, approximately 30 pages. (Gorelick and Clark, 1993)

93-593 Spview manual, approximately 15 pages. (Includes the digital spectral library and spectral library reader software on 3.5-inch floppy disks for IBM-PC compatible computers.) (Livo et al., 1993)

------------------------------------------------------------ ------------------------------------------------------------

The digital data for the spectral library, software, and above manuals are available via anonymous ftp on the internet: 1. ftp speclab.cr.usgs.gov 2. login as anonymous 3. password is your userid@machine 4. cd pub/spectral.library 5. get README

Follow instructions in the README file for obtaining the data. The pub/spectral.library directory will contain all the different versions listed in Table 5, as well as additional ones as they become available (again see the README file for details). Similarly, obtain the specpr and spsearch software in the pub/specpr and pub/spsearch directories. The specpr distribution also includes an independent Fortran program, spprint, that reads a specpr format file and prints titles. For independent subroutines in C that read a specpr file, see the README file.

After you have retrieved the library, please send mail to rclark@speclab.cr.usgs.gov with your name, address, phone number and email address. We will put you on a mailing list for future announcements and updates.

Alternatively, contact any of the authors at their address, or send electronic (internet) mail if you have questions to :

rclark@speclab.cr.usgs.gov

A CD-ROM version will be available in the future.

ACKNOWLEDGEMENTS

A successful spectral library has extensive sample documentation. We are indebted to J. S. Huebner, and Judy Konnert of the USGS for their support in analyzing the X-ray diffraction data on minerals for the last couple of years. We thank the late Norma Vergo for many of the earlier X-ray analyses. Norma's attention to detail has certainly made this spectral library a quality product, and we miss her. Without these dedicated people providing superb analysis and feedback, this library would not have been possible.

Of course, a spectral library needs quality samples. We are indebted to Jim Crowley, Jim Post, Fred Kruse, and Jack Salisbury for donating excellent samples. Thanks to the British Museum and the National Museum of Natural History for mineral samples.

Several additional people worked on entering documentation for this database; a task that didn't seem to have an end. We thank Barry Middlebrook for completing some of the documentation on samples in the early stages, and Shelly Moore helped considerable entering data in the later stages of the project. Melissa Cowoski helped with the optical examination of the samples.

We thank Jim Crowley and Eric Livo for excellent reviews; they certainly helped improve the final version.

This project has been funded by the USGS DAT program, and the NASA HIRIS, Cassini VIMS, Mars Observer TES, and the canceled Mars Observer VIMS, and CRAF VIMS teams.

FUTURE PLANS

The senior author (RNC) is a team member on the EOS HIRIS flight investigation team and is developing spectral libraries for the team. He is also a team member on Mars Observer Thermal Emission spectrometer, and Cassini (mission to Saturn) VIMS teams. To satisfy the requirements of all these missions, the mineral spectral library will be extended to cover the spectral range 0.2 to 150 *mm and include many more minerals. For the HIRIS team, spectral libraries will be developed for all disciplines represented by team members.

REFERENCES

Clark, R.N., 1980. A Large Scale Interactive One Dimensional Array Processing System, Pub. Astron. Soc. Pac., 92, 221-224.

Clark, R.N., T.V.V. King, and N.S. Gorelick, 1987. Automatic Continuum Analysis of Reflectance Spectra: Proceedings of the Third Airborne Imaging Spectrometer Data Analysis Workshop, JPL Publication 87-30, 138-142.

Clark, R.N., A.J. Gallagher, and G.A. Swayze: 1990a. Material Absorption Band Depth Mapping of Imaging Spectrometer Data Using a Complete Band Shape Least-Squares Fit with Library Reference Spectra, .I Proceedings of the Second Airborne Visible/Infrared Imaging Spectrometer (AVIRIS) Workshop. JPL Publication 90-54, 176-186.

Clark, R.N., T.V.V. King, M. Klejwa, G. Swayze, and N. Vergo, 1990b. High Spectral Resolution Reflectance Spectroscopy of Minerals: J. Geophys Res. 95, 12653-12680.

Clark, R.N.: 1993. SPECtrum Processing Routines User's Manual Version 3 (program SPECPR): U.S. Geological Survey, Open File Report 93-595, 210 pages.

Fleischer, M.: 1980. Glossary of Mineral Species: Mineralogical Record, Tucson, 192pp.

Gorelick, N, and Clark, R.N.: 1993. Spsearch: a program for relational database queries of the digital spectral library: U.S. Geological Survey, Open File Report 93-594, (software complete, documentation in preparation).

Hapke, B., 1981. Bidirectional reflectance spectroscopy 1. Theory, J. Geophys. Res. 86, 3039-3054.

Klein, C. and Hurlbut, Jr., C.S. Manual of Mineralogy: John Wiley and Sons, 596pp, 1985.

Livo et al. 1993: PC Reader for the U.S. Geological Survey Digital Spectral Library: U.S. Geological Survey, Open File Report 93-593, .

FIGURE CAPTION

Figure 1. The spectral resolution of the spectra in this release of the spectral library. The resolution is expressed as the Full Width at Half Maximum (FWHM) in micrometers. The spectral sampling is equal to the FHWM spacing. The sampling is one half Nyquist, such that if the response functions of the spectrometer were plotted for each wavelength, the half-maximum points would overlap the half-maximum points of the adjacent spectral channels. The FWHM rises where there is relatively low signal from the detector near the ends of the detector range. The spectral resolution is better than AVIRIS 1992 at all wavelengths except for a few channels near 0.87 µm and beyond 2.39 µm . The difference is so slight at 0.87 µm(12 nm versus 9 nm for AVIRIS) that for the spectral features encountered in this spectral library, there are no practical differences in AVIRIS convolved spectra. Beyond 2.39 µm, the Beckman rises to 22 nm at 2.42 µm and 32 nm at 2.50 µm compared to AVIRIS at a resolution of 14.6 nm in the 2.3 to 2.49-µm region. However, AVIRIS data are strongly affected by atmospheric absorption beyond 2.43 µm, so this difference is small in practice. Also, our future spectral library covering 2 to 150 µm will have resolution better than 2.5 nm in the 2 to 2.5-µm wavelength region.

The spectral resolution plotted here is our "standard 1x" resolution. Spectra are often obtained at higher resolution (1.5x, 2x, 3x, 4x, and 8x the standard). Some of these results have been reported in Clark et al. (1990b).

FIGURE 1

U.S. Geological Survey, a bureau of the U.S. Department of the Interior

This page URL= http://speclab.cr.usgs.gov/spectral.lib04/spectral_lib.html
This page is maintained by: Richard Wise and Roger N. Clark rclark@speclab.cr.usgs.gov
Last modified August 23, 1999.