Optimization; Spectral Discontinuities - ASD FieldSpec 3 User Manual

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B.5

Optimization

Note:
Note:
B.6

Spectral Discontinuities

ASD.Document 600540 Rev. C
The spectrometer must be re-optimized for:
• Light changes.
• Any atmospheric changes or changes in temperature.
• Outdoor solar changes at least every 10 to 15 minutes.
• Indoor use every 30 minutes.
Conditions can change rapidly or slowly. It all depends on clouds, wind
(affecting temperature), instrument warm up time, etc.
The SWIR 2 dark current increases with temperature and dramatically
reduces the dynamic range of the spectrometer.
Optimization depends on the response to light in a particular spectral region.
A well-optimized instrument will display between 20,000 and 35,000 raw
digital numbers, if the source is within the gain and integration time abilities
of the system. The magnitude of the VNIR rawDN will be directly dependent
on the light intensity.
The Spectralon® blank should be used when optimizing and for taking a
white reference measurement.
It is important that the angle for the white reference measurement is the same
as the angle used for capturing data from the samples.
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.
Three separate detectors are used to collect data over the complete spectral
range; therefore spectral discontinuities should be expected at 1000 nm and
1830 nm when operating in raw DN mode. These discontinuities decrease in
the reflectance mode when the ratio of the raw DN file to the calibration file
(a white reference or a radiometric calibration response to a calibrated light
field) is calculated because both files will contain the same discontinuities in
the data.
Excessive discontinuities in radiometric spectra can be caused by not
allowing the instrument to warm up fully. Normal discontinuities can be
addressed by using the parabolic correction utility.
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Appendix B Reference Information
FieldSpec® 3 User Manual