Field Spectroscopy: Part One

Part One – Mitigating the impact of sub optimal and changing weather conditions.

by | Apr 2, 2018 | Agriculture

James Caudery

Spatial/Data Analyst at GHD

 

James is a Spatial Analyst at GHD in Melbourne.

James worked for 2Excel Geo as a Geospatial Analyst from 2015-2018. James was a functional lead for Field Operations and was responsible for the planning and execution of multiple successful field trials.

 

What is Field Spectroscopy?

Field spectroscopy involves measuring the spectral reflectance of targets in the natural environment within which they exist. In order to achieve this, specialised spectrometers are used which have been adapted from a laboratory environment to be operated in the field. 2Excel geo use FieldSpec4 instruments from Analytical Spectral Devices, This measures spectral radiance from 350nm to 2500nm (visible to short wave infrared). Measurements can be made in direct contact with the target, using an artificial light source, or can be made a short distance away (0.5 – 3 metres) using natural light from the sun.

The choice of measurement technique depends upon the context. For example, in the context of plant science, the contact technique is appropriate for the measurement of individual leaves, whereas the short-standoff technique is appropriate to the measurement of plant canopies.

In either technique, measurements of spectral radiance are converted into absolute spectral reflectance via comparison to a standard (white) reference. This reference is calibrated in a laboratory, giving a known absolute reflectance. These are usually made from a material such as Spectralon or Zenith Polymer, which have a near-flat spectral reflectance across the visible-near infrared region of the spectrum, are chemically inert and have diffuse (near-Lambertian) reflectance. If maintained well, these provide a reliable reference for conversion to absolute reflectance.

Figure 1

FieldSpec4 spectrometer in action, keeping the optic fibre at a consistent distance from the target.

Figure 1

FieldSpec4 spectrometer in action, keeping the optic fibre at a consistent distance from the target.

Standard Field Spectroscopy Methods

Measurements are made under the illumination conditions present in the field, and therefore must be taken in a very strict order. A measurement of the white reference is made first, followed immediately by several measurements of the canopy, with an optional (but recommended) re-measurement of the white reference taken last. The initial measurement of the reference is used to standardise the reflectance of the canopy and assumes that the illumination was stable during the intervening period. The second measurement of the reference is used to provide a quality check. If the two measurements are different by more than a few percent, it can be concluded that the illumination conditions have changed. In this case, the canopy measurements may be unusable.

The apparent reflectance of a plant canopy depends on the direction of the observation and illumination conditions, specifically the proportion of diffuse radiance and the direction of direct radiance; we will address this issue in a future article! We collect our measurements looking straight down as this mimics the geometry of our airborne collection.

The useful interval between measurements of the reference target depends on the speed of change of the illumination conditions. More stable illumination allows a greater number of canopy measurement to be made. However, if conditions are changing quickly, then only a few reliable measurements can be taken. An experienced operator can judge the tempo of measurement; however, this is a highly subjective approach, since illumination changes can be imperceptible to the human eye, especially when high cirrus cloud is present.

Overall this approach, except in the very best conditions, results in delays, frustration, low numbers of samples and potentially variable quality spectra.

Figure 2

Dual Spectrometer System, simultaneously measuring the target and reference to mitigate the impact of changing light conditions.

Figure 2

Dual Spectrometer System, simultaneously measuring the target and reference to mitigate the impact of changing light conditions.

Modern Field Spectroscopy Methods

2Excel Geo’s wheat disease campaign demanded a solution to this issue in order to increase our capacity to collect high quality canopy reflectance spectra. We explored several options before committing to the new Dual Spectrometer System by Analytical Spectral Devices.

This system operates two field spectrometers synchronously, allowing for simultaneous measurements of the target and white reference. This eliminated the need to repeatedly measure the white reference, as in the standard approach. The impact of the illumination conditions is thus dramatically reduced, as every target measurement is corrected to reflectance using a white reference measurement made under near-identical illumination conditions.

This system, combined with our new data logging protocol, increased our ability to collect high quality canopy spectra by many orders of magnitude. For example, over the 2017 season, we achieved a collection rate of almost 12,000 samples per hour.

This reduced the commitment of valuable resources, enabled regular collection co-incident with crop development (less reliant on weather conditions) and produced a comprehensive spectral library for analysis.

At the time of writing, 2Excel Geo is the only operator of the Dual Spectrometer System in the UK, and has a team of trained, experience field spectroscopists, able to operate the system effectively to produce reliable results. The equipment and operator are available as a stand-alone service or as a component of a remote sensing project.

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