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Field-Portable Spectroscopy
Introduction
• Field-portable spectroscopy has traditionally been used in remote
sensing applications.
• The technology can be used for on site analysis of soils and agricultural
inputs and products.
• Field-portable spectroscopy utilizes diffuse reflectance of light in the
visible (350-750 nm) and near infrared (750-2,500 nm) wavelength
regions of the electromagnetic spectrum (Fig. 1).
• The measured spectrum (Fig. 2) contains interpretable information
White light dispersed by a prism into the colors of the optical spectrum.
about the composition, structure, and concentration of substances in a
sample.
Applications
• Obtaining spectra of end-members (e.g. green and dry vegetation,
Increasing Frequency
50,000 cm-1
X-Ray
UV
12,820 cm-1
Vis
4,000 cm-1
NIR
surface soils, bright and dark targets) for spectral unmixing and
calibration of remote sensing imagery.
400 cm-1
FIR, Microwave
MIR
• Rapid soil and organic resource characterization at remote or difficult
field locations where transport of samples is difficult.
• Mobile laboratories for soil and plant testing.
200 nm
380 nm
780 nm
2,500 nm
Oxisols
Lake plain soils
Vertisols
25,000 nm
Increasing Wavelength
Figure 1: The electromagnetic spectrum
0.35
Absorption (Log1/R)
0.3
0.25
Sediment
plume in
Nyakach
Bay
0.2
0.15
0.1
0.05
0
350
850
1350
1850
2350
Wavelength (nm)
Figure 2. Visible near infrared spectrum of a soil sample.
Spectral unmixing of visible near infrared spectra of three contrasting soil types
measured in the field for a Landsat image over Lake Victoria region in western
Kenya. The green colour of the sediment plume emanating from the River Nyando
in Nyakach Bay indicates the principal source is alluvial soils of the lake plain.
Instrumentation
• Field-portable instruments can run on battery power.
• Reflected light is gathered through a fibre-optic cable. An integrating
sphere with an external light source can be used.
Key Advantages/Limitations
• Portable
• External white reference is
required
• Useful for field applications
• Little sample preparation
• Rapid acquisition of unprocessed
samples
• Low cost instrument
• Visible wavelength range gives
• No internal reference or
validation available
• Instrument needs annual service
at factory
• Reproducibility across
instruments and over time is
information on iron oxides in soil
unknown
• Readings are sensitive to
movement of the fibre-optic
Field-portable spectrometer fitted with an integrating sphere containing
an integrated light source. In this example a soil sample contained in a
Petri dish is being scanned.
Contact: World Agroforestry Centre (ICRAF), P.O. Box 30677-00100 Nairobi, Kenya. Tel: +254 020 722 4000. www.worldagroforestry.org
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