Field-Portable Spectroscopy

Contact: World Agroforestry Centre (ICRAF), P.O. Box 30677-00100 Nairobi, Kenya. Tel: +254 020 722 4000. www.worldagroforestry.org

• 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

about the composition, structure, and concentration of substances in a sample.

• Obtaining spectra of end-members (e.g. green and dry vegetation, surface soils, bright and dark targets) for spectral unmixing and calibration of remote sensing imagery.

• 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.

• Portable

• Useful for field applications

• Little sample preparation

• Rapid acquisition of unprocessed

samples

• Low cost instrument

• Visible wavelength range gives

information on iron oxides in soil

• 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.

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.

White light dispersed by a prism into the colors of the optical spectrum.

• External white reference is

required

• No internal reference or

validation available

• Instrument needs annual service

at factory

• Reproducibility across

instruments and over time is

unknown

• Readings are sensitive to

movement of the fibre-optic

cable

Increasing Frequency

Increasing Wavelength

X-Ray Vis NIR MIR FIR, Microwave UV

200 nm 380 nm 780 nm 2,500 nm 25,000 nm

50,000 cm-1 12,820 cm-1 4,000 cm-1 400 cm-1

Figure 1: The electromagnetic spectrum 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.

Figure 2. Visible near infrared spectrum of a soil sample.

Sediment plume in Nyakach Bay

Oxisols Lake plain soils Vertisols