Spectral Calibration

     These 120 channel lunar telescopic data have been calibrated to
directional hemispheric reflectance (DHR) using a measurement of Apollo
16 soil made by J. B. Adams (McCord et al., 1981 [MCCORDETAL1981];
Pieters, 1986 [PIETERS1986]). This procedure is described in Pieters and
Pratt, 2000 [PIETERSANDPRATT2000]. (See the file CATALOG/REF.CAT for
complete references.) When the telescopic data were acquired, this
spectral photometric calibration procedure using the properties of
returned lunar soil measured in the laboratory was more accurate than
using spectral measurements of stars.  Directional hemispheric spectra
of lunar samples were the form of laboratory data available.  All
telescopic data were corrected the same way, so the data could be
compared directly and also productively compared to similar spectra of
lunar samples.

     Nevertheless, all remotely acquired data are acquired as
bi-directional reflectance (BDR) data, with a given geometry of incident
radiation and reflected radiation. Since the brightness of a surface
varies as a function of geometry, a standard geometry is typically used
and photometric corrections are made to transform any measurement to the
standard geometry.  For example, Clementine data use the standard
geometry of angle of incidence = 30 degrees and angle of emergence = 0
degree.

     Variations between directional-hemispheric and bi-directional
reflectance data for the Apollo 16 reference soil 62231 is discussed in
Pieters, 1999 [PIETERS1999]. Corrections must be made to these 120
channel telescopic lunar data (calibrated to directional hemispheric
geometry) if they are to be compared directly to other remotely acquired
data (such as data from Clementine) which are bi-directional by nature.

     In addition, during the mid 1980's it was noticed that there was a
wavelength calibration offset for much of the directional hemispheric
data in use acquired on the J B Adams (JBA) machine (both for lunar
samples and for meteorites). To allow these JBA data to be compared with
more modern data, a wavelength correction must also be made to the
original soil spectrum used in calibration (a shift of 15 - 25 nm).

     Data for both types of correction (DHR to BDR, and wavelength
offset) are presented here.

     Shown in Figure 1 (file calfig1.gif) are spectra of the Apollo 16
soil used as a standard in lunar spectral calibration. The JBA original
(dashed line) as well as wavelength-corrected (solid) directional
hemispheric spectra are shown to be brighter and slightly steeper in
continuum slope than the bi-directional spectrum (an average of several
independent measurements measured using the RELAB facility).

     Shown in Figure 2 (file calfig2.gif) are correction factors
necessary to convert the 120 channel lunar telescopic spectra to
bi-directional spectra at i=30 degrees, e=0 degrees
(RELAB/JBA original). Note, since the telescopic spectra are scaled to
unity, this correction does not include corrections for brightness
differences.

     Data files are presented as tables with columns for wavelength and
reflectance as follows:

     JBADHR.TAB: The (original) Adams directional hemispheric Apollo 16
62231 data used to calibrate these telescopic data.  Shown as a dashed
line in Figure 1.

     JBADHRWC.TAB: Adams directional hemispheric Apollo 16 62231
(wavelength corrected) data.  Shown as a solid line in Figure 1.

     RELABBDR.TAB: The bi-directional reflectance data for Apollo 16 62231
used for Clementine calibrations.  They were obtained at i=30 degrees,
e=0 degrees and are described in Pieters, 1999 [PIETERS1999].  Shown as
a solid line with diamonds in Figure 1.

     CORRBDR.TAB: Correction factors shown in Figure 2 to correct the 120
channel lunar data to bi-directional reflectance.  These factors include
both the wavelength as well as geometry corrections.

     The above data files are found in the CALIB directory of this
archive. They are accompanied by PDS label files that describe their
format in detail.