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USGS Digital Spectral Library Wavelength Precision

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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 micron (0.5 nm) in the near-IR and 0.0002 micron (0.2 nm) in the visible, always a small fraction of the spectral resolution.

We have found that for the samples we have measured, pyrophyllite absorptions in the near-infrared are stable, and located at the following positions. The PYS1A sample is in the spectral library. High spectral resolution spectra are published in (Clark et al., 1990b).

May 1984 measurements:

                      Pyrophyllite Standard PYS1A
--------------------------------------------------------------------------------
Band Position  Position   FWHM   Spect. Resolution   Band
(micrometers)  Error +/-          (micrometers)      Depth  Comments
--------------------------------------------------------------------------------
   0.952         0.002    0.007      0.006           0.07
   1.234         0.002    0.006      0.0030          0.05
   1.335         0.002    0.011      0.0030          0.04   assymmetric band
   1.362         0.002    0.006      0.0034          0.05
   1.394         0.001    0.009      0.0037          0.59   very strong & sharp
   1.41          0.005    -----      0.0036          ----   shoulder only
   1.430         0.002    0.011      0.0034          0.03
   2.060         0.005    -----      0.009           ----   shoulder only
   2.085         0.002    0.024      0.009           0.19
   2.167         0.002    0.023      0.009           0.53
   2.316         0.005    0.036      0.020           0.19   complex shape
   2.504         0.004    0.042      0.021           0.15
--------------------------------------------------------------------------------
Notes: see Hunt (1979, Geophysics, vol 44, pp1974-1986) for bandpositions on
the same instrument when it was new.  Hunt (1979) cites a FWHM = 0.006 micron
for the 1.394 band using a spectrometer resolution of 0.0006 micron.
--------------------------------------------------------------------------------

Data are from the USGS Denver using a Beckman 5270 spectrometer with an integrating sphere and a horizontal particulate sample. Incident beam= normal to sample surface, temperature ~20 C.


From Januray 18, 1990:

                      Pyrophyllite Standard PYS1A
--------------------------------------------------------------------------------
Band Position  Position   FWHM   Spect. Resolution   Band
(micrometers)  Error +/-          (micrometers)      Depth  Comments
--------------------------------------------------------------------------------
B  0.952         0.002    0.007      0.006           0.07
B  1.234         0.002    0.006      0.0030          0.05
B  1.335         0.002    0.011      0.0030          0.04   assymmetric band
B  1.362         0.002    0.006      0.0034          0.05
B  1.394         0.001    0.009      0.0037          0.59   very strong & sharp
N  1.3932        0.0006   0.0056    ~0.0008          0.453  very strong & sharp
B  1.41          0.005    -----      0.0036          ----   shoulder only
B  1.430         0.002    0.011      0.0034          0.03
B  2.060         0.005    -----      0.009           ----   shoulder only
B  2.085         0.002    0.024      0.009           0.19
N  2.0844        0.001    0.        ~0.001           0.19
B  2.167         0.002    0.023      0.009           0.53
N  2.1664        0.001    0.        ~0.001           0.53
B  2.316         0.005    0.036      0.020           0.19   complex shape
B  2.504         0.004    0.042      0.021           0.15
--------------------------------------------------------------------------------
B Data are from the USGS Denver using a Beckman 5270 spectrometer with an
  integrating sphere and a horizontal particulate sample.
  Incident beam= normal to sample surface, temperature ~20 C.

N Data are from the Nicolet FTIR 740, spectrum in SPD0075 r 377 wav 389

December 29, 1992 measurements:

PYS1A 4x beckman spectrum in splib001, recs 230, 234.
Parameters derived from specpr interactive band analysis 12/29/92

                       observed observed  spectrometr   true
                       wave      FWHM       resol       FWHM
Pyrophyllite PYS1A 1|  1.2328   .0066       .00125      .0065
Pyrophyllite PYS1A 1|  1.3350   .0109       .00125      .0108
Pyrophyllite PYS1A 1|  1.3616   .0067       .00125      .0066
Pyrophyllite PYS1A 1|  1.3934   .0077       .00125      .0076
Pyrophyllite PSY1A 2|  2.0866   .0243       .0025       .024
Pyrophyllite PSY1A 2|  2.1669   .0225       .0025       .022
Pyrophyllite PSY1A 2|  2.2205   .0135       .0025       .013
Pyrophyllite PSY1A 2|  2.3194   .0153       .0025       .015

It is our experience that spectrometers are often out of wavelength calibration. Of those that we have checked (USGS-owned instruments, as well as those of others when we have obtained spectra of certain minerals), the majority have had wavelength calibration errors! Occasionally, we have found the errors to vary from spectrum to spectrum for a spectrometer. FTIR spectrometers are excellent; we have never seen one that had a wavelength error. Our Beckman has been very stable and has never been out of calibration. Our field spectrometers have gone out of calibration. Thus we constantly check our calibration on known materials. The tables shown above are only a few of the checks we have done.

A common mineral you might also use to check wavelengths is well crystallized kaolinite. We have found the 2.2-micron doublet to be reasonably stable from sample to sample, with the sharpest absorption occuring from 2.2063 to 2.2077 microns (Clark et al., 1990b).


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


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This page is maintained by: Dr. Roger N. Clark rclark@usgs.gov
Last modified November 13, 1998.