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B.6.2
White Reference
B.6.3
Other Tips on Discontinuities
B.7
Spectrum Averaging (or Sample Count)
ASD Document 600540 Rev. J
differing reflectance values at the crossover wavelengths. Averaging several
measurements of slightly different areas on the ground will minimize the
steps.
White references should be collected at least every 10 minutes under typical
atmospheric conditions. The frequency of white reference collect should be
increased for less stable atmospheric conditions. The "flatness" of the
reflectance of the reference panel when observed as a sample can be used to
judge changes in the atmospheric conditions from when the white reference
was originally collected.
As the instrument warms up, the sensitivity at the red end of the VNIR array
will increase while that at the blue end of SWIR2 will decrease, leading to
discontinuities. The SWIR 2 dark current increases with temperature and
dramatically reduces the dynamic range of the spectrometer.
The Spectralon blank should be used when optimizing and for taking a white
reference measurement. It is not necessary to re-optimize the instrument prior
to the recollection of the white reference.
When saving reflectance data, point the probe at the Spectralon once every
few measurements for a minute or two with the same viewing geometry. If the
relative reflectance of the Spectralon is less than or greater than one, a new
white reference may be needed. If the relative reflectance of the Spectralon is
greater than one, re-optimization is recommended.
Outdoor spectra typically display a great deal of noise in the water bands,
particularly in the 1900 nm region. Indefinable reflectance values due to
atmospheric water absorption bands appear as two zones of very noisy data
(the first zone located approximately between 1350-1400 nm and the other
between 1810-1940 nm). If you look at a typical outdoor spectral irradiance
file, you will see the energy in these bands drop to (or very nearly to) zero.
If signal levels are low, the only way to increase S/N is by reducing noise
through spectrum averaging. However, spectrum averaging takes more time
per spectrum.
The drawback to this is that the resultant data can be compromised by
introducing low-frequency noise factors, such as varying cloud conditions or
sudden gusts of wind changing the color of the tree canopy.
93
Appendix B Reference Information
FieldSpec® 3 User Manual
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