CAPP (canopy pasture probe)

The CAPP consists of an inverted rounded black plastic bin, painted inside with a flat black paint; it is 45 cm high and has a diameter of 26 cm (Figure 2.1). Mounted in the top is a 50 Watt ASD Pro-lamp with a tungsten-quartz-halogen bulb. The light was placed in the top centre of the instrument at zenith, and it is powered by 12 volt battery. To insert the spectroradiometer fibre optic, a grip was initially fixed on the top of the probe beside

the lamp, i.e. 45 cm from the target and pointing at approximately 11 degrees; later another grip was fixed on the side of the CAPP, at 40 cm height and 18 degrees from zenith. The field-of-view (FOV) of the spectroradiometer used is 25 degrees and the potential area viewed by the instrument (area measured) is very close to a circle with a diameter of approximately 20 cm. For the reflectance standard a square, matt white ceramic tile, measuring 295x295 mm was used (Sanches et al. 2009, Chapter 3).

Figure 2.1. (a) CAPP with the spectroradiometer ASD FieldSpec® Pro FR; (b) ASD light source mounted on the top and fibre optic cable grips on the top and side of the CAPP; (c) acquiring reflectance spectra from sheep-grazed pasture.

2.2.1.1. The frame

The CAPP was made of a resistant plastic suitable for field work conditions. Although the plastic used was already in black colour, the interior of the bin was painted with a flat black paint to minimize any reflectance from this surface. The paint used (Dulux Quick DryTM _SparykoteTM_{) is spectrally non selective for the range analysed (350-2500 nm)}

and has very low reflectance (around 0.06) (the paint‟s reflectance was measured using the ASD FieldSpec® _{Pro FR attached to an ASD plant probe and a spectralon}® _{panel as}

reflectance standard). The CAPP‟s frame is light enough to be easily carried along with the backpack-mounted spectroradiometer; and most importantly, it blocks any interference from the outside environment during the measurements, such as secondary sources of illumination and wind.

2.2.1.2. Fiber optic input position

There are two types of reflectance from Earth-surface materials, specular and diffuse (Lambertian). For specular, the angle of incidence and reflection of the energy are equal and no scattering occurs at the surface. For diffuse reflectance, the radiance incident upon the surface is backscattered in all upward directions (Mather 1999). The diffuse component of reflectance from plant leaves comes mainly from inside the leaf, whereas the specular component arises primarily from the leaf surface (Grant 1987, Brakke 1994). The spectral variation of the diffuse reflection depends on leaf biochemistry and so can be used to predict the amount of leaf constituents (Bousquet et al. 2005). In most natural materials,

like in vegetation targets, the diffuse reflectance is more abundant and causes fewer measurement problems (Hatchell 1999). Nevertheless, because the specular reflection depends on surface biophysical properties it can be useful for studies of refractive index and roughness of the epidermis/cuticle layer (Bousquet et al. 2005). Since the CAPP was

developed to assist studies for the prediction of pasture biochemistry, our interest was to measure the diffuse reflectance and avoid the specular reflectance.

One way to avoid measuring specular reflectance is to set the spectroradiometer sensor input far away from the specular plane, for example by placing the sensor input at a right angle to the illumination source (Hatchell 1999). Since this set up of right angle was not compatible with the physical structure of the CAPP, two other set ups were tested. The choice for the set ups was guided by the objective to measure a circular area of at least 15 cm diameter (considering the FOV of the spectroradiometer of 25 degrees), with a probe that was practical for field work. Two different mounting positions for the FieldSpec® _Pro

FR fibre optic grip on the CAPP were tested. For position 1, the grip was fixed on the top of the probe (45 cm height) at 11 degrees to the light source (Figures 2.2a and 2.3a). For position 2, the cable grip was moved from the top to the side of the probe; it was placed at 40 cm height and 18 degrees from zenith (Figures 2.2b and 2.3b). The intention was not to

have a probe with two different configurations, but to test if at least one of the probes would suit our needs.

Figure 2.2. Diagram of pasture specular and diffuse reflection being captured by the CAPP with the ASD FieldSpec® Pro FR fibre optic input placed: a) on the top of the CAPP, b) on the side of the CAPP.

Figure 2.3. Calculating area measured within the FOV of the sensor for measurements acquired by the CAPP with the ASD FieldSpec® Pro FR fibre optic input placed: a) on the top of the CAPP, b) on the side of the CAPP.

The area measured by the Spectro-CAPP, as demonstrated in Figure 2.3, is slightly different for each set-up, but the potential ground area viewed by both set-ups corresponds closely to a circle with a diameter of approximately 20 cm. However, for both set ups the actual area measured will depend on the pasture sward height (PSH), since the distance between sensor input and target will decrease with any increase of the pasture height. The change in measured area by the Spectro-CAPP (top and side-grips) for a pasture sward with different heights is illustrated in Figure 2.4. One factor to be considered is that comparing two pasture plots of equal size, one with a sward higher than the other, the plot with higher sward might need more spectral measurements to be taken (measurements posterior averaged) in order to cover the same area (c.f. fig 2.4 PSH = 15 cm and PSH = 5 cm); especially for the side-grip set-up, where the area measured is slightly smaller than with the top-grip set-up.

Figure 2.4. The area inside the white circles illustrates the approximate area measured by the Specto-CAPP using the fibre optic input placed on the top of the probe and on the side of the probe, for different pasture sward heights (PSH).