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7.1
Illumination
Note:
7.2
Characteristics of Natural Illumination
ASD.Document 600540 Rev. C
Issues such as the timing of the data collection, spatial scale of the field
measurement, target viewing and illumination geometry, and the collection of
ancillary data sets must be considered in light of the objectives of the study.
The lack of appropriate ancillary data sets often makes previously collected
data sets unusable for a new application.
Frequently, the experimental design must be modified to account for the
characteristics of the available instrumentation.
For example, vegetation canopy spectra collected using a slow scanning
instrument will sometimes have small wind-induced "absorption" features in
those portions of the spectra when the instrument was viewing more shadow.
In order to determine the reflectance or transmittance of a material, two
measurements are required:
•
The spectral response of a reference sample.
•
The spectral response of the target material.
The reflectance or transmittance spectrum is then computed by dividing the
spectral response of the target material by that of a reference sample.
Using this method, all parameters which are multiplicative in nature and
present in both the spectral response of a reference sample and the target
material, are ratio-ed out, such as:
•
The spectral irradiance of the illumination source.
•
The optical throughput of the field spectrometer.
An inherent assumption when determining the reflectance or transmittance of
a material in the field is that the characteristics of the illumination are the
same for the reference and target materials. Variability of the illumination
characteristics between the time the reference and target materials are
measured will result in errors in the resultant spectra.
Field spectrometry typically involves the ambient solar illumination. As such,
the target can be illuminated by three or more sources (see
with its own spectral characteristics (Curtiss and Ustin, 1988). Unless the
target is in a shadow, the direct solar illumination is the dominant source of
illumination.
54
Chapter 7 Field Measurements
Figure
FieldSpec® 3 User Manual
7-1), each
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