Johns Hopkins University Spectral Library
This text is a general introduction to the library, with an overview of Measurment techniques, which do differ for different materials. There is a separate introductory text for each kind of material (rocks, minerals, lunar soils, terrestrial soils, man made materials, meteorites, vegetation, snow&ice, etc.) that contains more specific information pertinent to that material.
Any questions concerning the Johns Hopkins Library can be e-mailed to Jack Salisbury at salisburys@worldnet.att.net.
MEASUREMENT TECHNIQUE
Two different kinds of spectral data are resident in this library. Spectra of minerals and meteorites were measured in bidirectional (actually biconical reflectance (see two Salisbury et al., 1991 references below for details).
These spectra, recorded from 2.08-25 micrometers, cannot be used to quantitatively predict emissivity because only hemispherical reeflectance can be used in this way. However, when recorded properly, as described in the meteorite paper, curve shape is accurate enough for remote sensing applications.
All other spectral data, with the exception of portions of generic snow and vegetation spectra (see the introductory text for each type of material), were measured in directional hemispherical reflectance. Under most conditions, the infrared portion of these data can be used to calculate emissivity using Kirchhoff's Law (E=1-R), which has been verified by both laboratory and field measurements (Salisbury et al., 1994; Korb et al., 1996). The unusual circumstances (e.g., the lunar environment) where thermal gradients may cause significant departure from Kirchhoffian behavior are discussed in Salisbury et al., 1994.
The apparently seamless reflectance spectra from 0.4 to 14 micrometers of Rocks and soils were generated using two different instruments, both equipped with integrating spheres for measurement of directional hemispherical reflectance, with source radiation impinging on the sample from a center line angle 10 degrees from the vertical.
Unless specified otherwise (see relevant introductory texts for generic snow and vegetation spectra, and spectra of manmade materials), all visible/near-infrared (VNIR) spectra were recorded using a Beckman Instruments model UV 5240 dual-beam, grating spectrophotometer at the U.S. Geological Survey, Reston, VA. The data were obtained digitally and corrected for both instrument function and the reflectance of the Halon reference using standards traceable to the U. S. National Institute of Science and Technology. Measurements of such standards indicate an absolute reflectance accuracy of plus or minus 3 percent. Wavelength accuracy was checked using a holmium oxide reference filter and is reproducible and accurate to within plus or minus 0.004 micrometers, or 4 nm (one digitization step). Spectral resolution is variable because the Beckman uses an automatic slit program to keep the energy on the detector constant. The result is a spectral bandwidth typically less than 0.008 micrometers over the 0.4 to 2.5 micrometers spectral range measured, but slightly larger at the two extremes of the range of the lead sulfide detector (0.8-0.9 micrometers and 2.4- 2.5 micrometers). This instrument has a grating change at 0.8 micrometers, which sometimes results in a spectral artifact (either a small, sharp absorption band, or a slight offset of the spectral curve) at that wavelength.
Two similar instruments were used to record reflectance in the infrared range (2.08 to 15 micrometers). Briefly, both are Nicolet FTIR spectrophotometers and both have a reproducibility and absolute accuracy better than plus or minus 1 percent over most of the spectral range. Early measurements of igneous rocks with an older detector were noisy in the 13.5-14 micrometers range and do not quite meet this standard in that region. Because FTIR instruments record spectral data in frequency space, both wavelength accuracy and spectral resolution are given in wave numbers (reciprocal centimeters). Wavelength accuracy of an interferometer type of instrument is limited by the spectral resolution, which yields a data point every 2 wavenumbers for these measurements. The X-axis was changed from wavenumbers to micrometers for all of these data before the infrared segment was joined to the VNIR data from the Beckman.
Spectra from the Beckman and the FTIR instruments were compared in the overlap range of 2.08-2.5 micrometers. If the difference was greater than 3 percent, measurements were repeated. Typically, however, the agreement was within the 3 percent limit. In view of the greater accuracy of the FTIR measurements, any small discrepancy between the two spectral segments was resolved by adjusting the Beckman data to fit the reflectance level of the segment measured by the FTIR instruments.
Recent publications describing and interpreting spectral data in the library, as well as some spectral data not yet included, are listed below.
REFERENCES AND RECENT PUBLICATIONS
Clark, B. E., Fanale, F. P., and Salisbury, J. W., 1992, Meteorite-asteroid Spectral comparison: The effects of comminution, melting, and recrystallization: Icarus, v. 97, p. 288-297.
Korb, A. R., Dybwad, P., Wadsworth, W., and Salisbury, J. W., 1996, 1996, Portable FTIR spectrometer for field measurements of radiance and emissivity: Applied Optics, v. 35, p. 1679-1692.
Nash, D. B. and Salisbury, J. W., 1991, Infrared reflectance spectra of plagioclase feldspars: Geophysical Research Letters, V 18, p. 1151-1154.
Rowan, L. C., Salisbury, J. W., Kingston, M. J., Vergo, N.S. and Bostick, N. H., 1991, Evaluation of visible, near-infrared and thermal-infrared reflectance spectra for studying thermal alteration of Pierre shale, Wolcott, Colorado: Journal of Geophysical Research V96, p. 18,047-18,057.
Salisbury, J. W., D'Aria, D. M., and Jarosevich, E., 1991a, Midinfrared (2.5-13.5 micrometers) reflectance spectra of powdered stony meteorites: Icarus, v. 92, p. 280-297.
Salisbury, J. W., Walter, L. S., Vergo, N., and D'Aria, D. M., 1991b, Infrared (2.1- 25 micrometers) Spectra of Minerals: Johns Hopkins University Press, 294 pp.
Salisbury, J. W. and Wald A. E., 1992, The role of volume scattering in Reducing spectral contrast of reststrahlen bands in spectra of powdered minerals: Icarus, v. 96, p. 121-128.
Salisbury, J. W. and D'Aria, D. M., 1992, Infrared (8-14 µm) remote sensing of soil particle size: Remote Sensing of Environment, v. 42, p. 157-165.
Salisbury, J. W. and D'Aria, D. M., 1992, Emissivity of terrestrial materials in the 8-14 µm atmospheric window: Remote Sensing of Environment, v. 42, p. 83-106.
Salisbury, J. W., 1993, Mid-infrared spectroscopy laboratory data: Chapter 4 in Remote Geochemical Analysis, C. M. Pieters and P. A. J. Englert eds., Cambridge University Press, New York, p. 79-98.
Salisbury, J. W., D'Aria, D. M. and Sabins, F. F., 1993, Thermal infrared Remote sensing of crude oil slicks: Remote Sensing of Environment, v. 45, p. 225-231.
Salisbury, J. W., and D'Aria, D. M., 1994, Emissivity of terrestrial materials in the 3-5 µm atmospheric window: Remote Sensing of Environment, v. 47, p. 345-361.
Salisbury, J. W., D'Aria, D. M., and Wald, A., 1994, Measurements of thermal infrared spectral reflectance of frost, snow, and ice: Jour. of Geophys. Res., v. 99, p. 24,235-24,240.
Salisbury, J. W., Wald, A., and D'Aria, D. M., 1994, Thermal-infrared remote sensing and Kirchhoff's law 1. Laboratory measurements: Jour. of Geophysical Research, v. 99, p. 11,897-11,911.
Salisbury J. W., Murcray, D. G., Williams, W. J., and Blatherwick, R. D., 1995, Thermal infrared spectra of the Moon: Icarus, v. 115, p. 181-190.
Salisbury, J. W., Basu, A., and Fischer, E. M., 1997, Thermal infrared spectra of lunar soils: submitted to Icarus, March, 1997.
Thompson, J. L., and Salisbury, J. W., 1993, The mid-infrared reflectance of mineral mixtures (7-14 µm): Remote Sensing of Environment, v. 45, p. 1-13.
Wald, A. É. and Salisbury J. W., 1992, Angular dependence of spectral Emissivity of quartz and basalt: (Extended abstract) Twenty-third Annual Lunar and Planetary Science Conference, p. 1485-1486.
Wald, A. E., and Salisbury, J. W., 1995, The thermal infrared directional emissivity of powdered quartz: Jour. of Geophys. Res., v. 100, p. 24,665-24675.