ASD FieldSpec 3, Руководство пользователя

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ASD Document 600540 Rev. J 58 FieldSpec® 3 User Manual
www.asdi.com Chapter 7 Field Measurements
7.3 Characteristics of Artificial Illumination
The use of artificial illumination allows:
• More control over illumination and viewing geometry.
• More control over sample geometry.
• Measurements during non-optimal conditions (e.g. cloud cover or at
night).
• Measurement of reflectance and transmittance in the deep atmospheric
absorption bands.
Several problems with using artificial illumination include:
• Difficulty in maintaining a constant distance between the sample or
reference and the light source when measuring samples with irregular
geometry.
• “Cooking” vegetation samples under the lights (water loss, chlorophyll
degradation).
In a typical lamp configuration for indoor use, the sample is viewed with the
collecting optics of the spectrometer nadir to the sample. Use one or two 200
to 500 Watt quartz-halogen cycle tungsten filament lamps (~3400°K color
temperature) in housings with aluminum reflectors about one meter above the
surface being measured.
Alternatively, the light source can be either incorporated into the field
spectrometer (often precluding the use of solar illumination) or can be
provided in the form of an optional accessory that mounts to the light
collecting optics of the instrument.
7.4 Atmospheric Characteristics
Absorbing molecules in the atmosphere strongly modify the incoming solar
irradiance. All of the absorption features will increase in intensity as the
atmospheric path length of the incoming solar radiation increases (e.g. with
changing solar elevation angle).
By far, water vapor is the strongest modifier of the incoming solar spectrum.
Water vapor has absorption features spanning the solar reflected region of the
spectrum (see
Figure 7-2 ), and varies both spatially and temporally.