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THE SPECTRAL REFLECTANCE OF COMMON ARTIFICIAL PSEUDO INVARIANT MATERIALS

Pfitzner, K., Staben, G and Bartolo, B. Supervising Scientist Division,

Department of the Environment, Water, Heritage and the Arts (DEWHA) GPO Box 361 Darwin 0801

+61 8 89201141 (ph) +61 8 89201199 (fax) kirrilly.pfitzner@environment.gov.au

Abstract Pseudo “invariant” features (PIFs), whether natural or artificial, are used for both calibration and validation of remotely sensed data. Where invariant features are not present in the landscape, artificial targets may be placed in the field prior to image capture. The ideal target should have a known reflectance, be spectrally invariant with environmental condition, span the spectral range of interest in the scene and be affordable, readily transportable, and preferably be at least four times the size of an image pixel. We asked members of the remote sensing community to make suggestions for materials suitable as artificial targets. Potential candidate materials included “shade cloth” in various colours, polyethylene plastic of varying grades and colours, cotton of different textures and colours, and building materials such as sislation sheeting and Tyvek®. Each of these materials was spectrally characterised in a controlled laboratory environment using an Analytical Spectral Devices (ASD) FieldSpecPro-FR spectrometer, covering 350-2500 nm. The 350-2500 nm spectra can be resampled to match any optical sensor. A contact probe fore optic was used to minimise BRDF effects from shiny surfaces. The laboratory measurements identified several suitable artificial materials spanning the range of, dark targets, moderately-bright targets and bright targets. Further in-field testing showed that white Tyvek, white and silver tarpaulin material, black canvas and white and black shadecloth were the best PIF materials from the range tested. Whilst polyethylene plastic was a potentially suitable target the shiny surface makes it susceptible to the effects of BRDF in the field.

Introduction Raw remotely sensed data are compromised to varying extents by “noise” introduced by variable conditions in the atmosphere and on the earth’s surface. The influence of the atmospheric component must be removed prior to quantitative analysis of surface reflectance. This conversion of raw data to actual ground reflectance should be a standard process that is applied to not only hyperspectral data, but also to other multispectral data (such as World-View 2 data) as these types of data are able to be easily converted from relative radiance to absolute radiance. Correction of raw image data for atmospheric effects can be carried out in essentially three ways: physically based methods; normalisation; and using calibration targets of known reflectance.

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Physically based methods rely on measurements of the properties of the atmosphere (such as water vapour, smoke, haze and the distribution of aerosols). These data may be obtained through in situ collection of irradiance (e.g. the use of a remote cosine receptor), or by using models of atmospheric radiation transfer that infer the atmospheric component of radiance data. Techniques that rely on the use of a radiation transfer code (such as MODTRAN) require a radiometrically calibrated spectral radiance image (µW/cm2 * nm* sr) and image bands of high spectral resolution. Normalisation techniques can be useful to convert image data to relative reflectance. Where deep water targets are present in the scene, dark pixel subtraction can be used to apply atmospheric scattering corrections to the image data. Flat field normalisation can be used where spectrally featureless endmembers occur in the image scene. To produce an estimate of the true surface reflectance using targets of known spectra, the empirical line calibration method can be used to force spectral data to match selected field reflectance spectra using a linear regression for each band to equate Digital Number (DN) and reflectance. This is equivalent to removing the contributions of solar irradiance and the atmospheric path radiance. Targets of known reflectance can be artificial or naturally occurring. The advantage of artificial materials is that they may be placed anywhere, preferably on a flat and open area, in the field prior to the image capture. PIFs are also useful for validation of the effectiveness of correction methods. Targets need to satisfy a number of criteria. Their reflectance must be known relatively accurately, and within the same spectral bands as used by the imager; the range of reflectance represented by the targets must span the range of interest in the scene and each target should cover an area of at least several pixels. The targets should also be well distributed over the entire scene, so that possible variations of atmospheric conditions across the scene can be addressed if required. Ideally the spectral response of PIFs should be measured at the same viewing angle and time as the scene capture. The goal of this research was to identify a suitable set of PIFs that they could be reused for all data captures of optical imagery. Members of the remote sensing community in CSIRO, University of Queensland, DSTO, University of Adelaide and RMIT were asked for their suggestions on suitable PIF materials. Suggestions for bright targets included: blue plastic woven tarps (ground sheets for camping); white woven polypropylene material; Tyvek (building material used for insulation); painted canvas panels; white canvas (VNIR only); white vinyl; and double thickness light calico. Suggestions for moderately bright spectral targets included: pink plastic and orange heavy duty building plastic. Dark target suggestions were: shade cloth; shade cloth layered on top of blue sheeting; black plastic; and, double thickness black calico. Most respondents said that painted surfaces tend to fade and change through time. Plastics are more stable (dust deposition aside), but both painted surfaces and plastics are susceptible to glint and BRDF effects and are not invariant with viewing and illumination geometry changes.

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Method An Analytical Spectral Devices (ASD) FieldSpecPro-FR spectrometer (covering 350-2500 nm with a spectral sampling interval of each channel at 2nm or less and a spectral resolution [FWHM] of approximately 3 nm at around 700 nm varying between 10–12 nm [SWIR] was used to measure the spectral response of selected PIF materials. Measurements were made with a standard laboratory setup (Pfitzner et al. 2006) using the high intensity contact probe at 30° of nadir. The probe uses an internal light source (100W halogen reflectorised lamp) and provides a 3.14cm2 (ASD Inc, 2004). The contact probe is an alternative method for collection of laboratory spectra because SWIR spectra collected are found to exhibit improved S:N ratios, lower standard deviation and less intrinsic uncertainty than spectra collected using more traditional laboratory techniques (Mac Arthur, 2007). The spectrometer and contact probe were warmed prior to use for 90 minutes and 30 minutes, respectively. Optimisation was undertaken against a Spectralon surface. A white reference measurement was undertaken prior to each material being sampled. Spectral measurements were made using 30 averages. The following materials were characterised in the laboratory:

1. Polyethylene shade cloth in eight colours. An architectural shade cloth range named “Synthesis AF-350™” was acquired. The material is a lock-stitch knitted shade cloth of UV stabilised high density polyethylene available in the following colours: natural (white/ivory), olive, aquatic blue, charcoal, desert sand (cream), terracotta, Brunswick green and chocolate. The material has a UV block of up to 96.4%.

2. Tarpaulin - Tarpee in 12 different colours/textures. Tarpee (made by Halifax Vogel Group or HVG) is quality woven Polyethylene (PE) sheeting consisting of High Density Polyethylene (HDPE) woven base cloth with Low Density Polyethylene (LDPE) lamination. Three different grades were measured (Tarpee, Tarpee Xtra and Tarpee Hydroliner). Eight colours of Tarpee (scrim 14 x 14 1300D, 14 x 14 square inch 50/mc coating) were measured: beige, blue, green, green with a silver back, white, white with a silver back, ice blue with a white back and blue with a green back. Three colours of Tarpee Xtra (3034 denier 3036 denier, 8 x 8 per square inch), including green, silver/white and white/silver were measured. Tarpee hydroliner was measured in black/green colour (scrim construction 2000D x 2000D, 15 x 15 square inch 75/65 mc coating).

3. Tarpaulin - Polyturf (grey coloured water proof tarp). Waterproof polyethylene top layer containing Sunshield-UVI® (ultraviolet light inhibitor) 1.8 m x 2.4 m (available from www.mayohardware.com.au).

4. Tarpaulin - Polytarp (blue coloured general purpose tarp) 2.3 m x 2.9 m.

5. Tarpaulin – Polytuf (black poly tarp). 6. Tourneau (black Tourneau and silver underside). 7. Sislation material (silver and blue side).

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8. Tyvek (single and double layered).

9. White vinyl (shiny and less shiny side). 10. White cotton with UV lining underside (1 pass lining, 2 pass lining and

three pass lining, each with single and double layering. The polyester lining of the three pass lining was also measured).

11. 100% white cotton of different thickness and layering (natural calico, twill and drill).

12. Black vinyl. 13. Black Polyester “canvas”(100% synthetic and waterproof).

Four of the above potential PIFs were identified as suitable targets based on not only spectral characteristics, but also cost, ease of acquisition, transportation and deployment. These were Tyvek (brightest PIF), silver tarp (moderate PIF) white tarp (moderate) and black polyester “canvas”(dark PIF). Each of these PIFs were laid in the field and measured with the FieldSpecPro-FR using a 25° FOV and at a height of 1m from the ground. Spectral measurements were made using 25 average measurements for each spectrum and each PIF was characterised by obtaining a minimum of 10 averaged measurements for each target. Optimisation and a white reference to a Spectralon panel 80cm off the ground was undertaken prior to each target measurement. Weather conditions and photographs were recorded as metadata. As part of an earlier project, measurements of 50% white and black shadecloth has been measured during a CASI overpass, and these spectra are also presented.

Results The laboratory measured spectra for the dark targets, moderately bright targets and bright targets are provided in Plates 1, 2 and 3, respectively. Plate 4 shows the spectral response of targets deployed in the field including black and white shadecloth, black polyester “canvas”, TyVek and white and grey/silver tarps. The spectra presented here cover the 350-2500 nm range, yet the suitability of artificial targets as PIFs will also depend on the wavelength range of the sensor being used. Figure 1 illustrates the spectral response of these PIFs resampled to the WorldView2 wavelength range. Plate 5 compares the laboratory and field measured spectra for selected PIFs.

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Plate 1. The spectral response of dark targets.

Black Tourneau Black vinyl (shiny side)

Black vinyl (less shiny side) Tarpaulin, black polytarp (Polytuf)

Grey shade cloth (Synthesis AF-350, charcoal) Brown shadecloth (Synthesis AF-350, chocolate)

Tarpee range Hydrolyser black (green underside) Black canvas (100% cotton)

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Plate 2a. The spectral response of bright targets.

Tyvek, single layer Tyvek, double layer

White vinyl, shiny side White vinyl, less shiny side

Natural shade cloth (Synthesis AF-350, white/ivory ) Tarpee range, white (white underside)

Tarpee range, white (silver underside) Tarpee range, ice blue (white underside)

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Plate 2b. The spectral response of bright targets (continued).

Tarpee range Xtra, white (silver underside) Polyester lining, 3 pass lining

White cotton, 1 pass lining, single layer White cotton, 1 pass lining, double layer

White cotton, 2 pass lining, single layer White cotton, 2 pass lining, double layer

White cotton, 3 pass lining, single layer White cotton, 3 pass lining, double layer

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Plate 2c. The spectral response of bright targets (continued).

100% cotton, natural calico, single layer 100% cotton, natural calico, double layer

100% cotton, twill, single layer 100% cotton, twill, double layer

100% cotton, drill, single layer 100% cotton, drill, double layer

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Plate 3a. The spectral response of moderately bright targets.

Tourneau, silver underside Grey/silver side of tarpaulin (black polytarp, Polytuf)

Sislation, silver side Sislation, blue side

Blue polytarp, single layer Blue polytarp, double layer

Light green shadecloth (Synthesis AF-350, Olive) Blue shadecloth (Synthesis AF-350, aquatic blue)

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Plate 3b. The spectral response of moderately bright targets (continued).

Cream shadecloth (Synthesis AF-350, desert sand) Red-brown shadecloth (Synthesis AF-350, terracotta)

Dark green shadecloth (Synthesis AF-350, �Brunswick green)

Tarpee range blue (blue underside)

Tarpee range blue (green underside) Tarpee range green (green underside)

Tarpee range green (silver underside) Tarpee range beige

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Plate 3c. The spectral response of moderately bright targets (continued).

Tarpee range silver (black underside) Tarpee range Xtra - green (green underside)

Tarpee range Xtra - silver (white underside)

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Plate 4. The spectral response of PIFs in the field

Black shadecloth White shadecloth

Black canvas Tyvek

Grey tarp White tarp

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Plate 5. The spectral response of selected PIFs - field compared to lab measurements

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(a) (b)

Figure 1. The spectral reflectance of artificial PIFS measured in the field (a) 350-2500 nm and (b) resampled to the Worldveiw2 wavelength range.

Discussion The spectral response of dark PIFs (Plate 1) shows that all measured potential targets have a suitable near-flat, low near-zero reflectance across the full wavelength range when measured in the laboratory. However, a number of the materials would be susceptible to glint and BRDF effects, including black tourneau and black vinyl (both also expensive) and black tarps. The shadecloth would also have some specular effect and the remote sensing community made the suggestion to layer this material to reduce the reflectance contribution of the underlying ground cover. In the laboratory, the black polyester “canvas” appeared to be a suitable dark coloured PIF. Variable response was observed for the spectral response of bright targets (Plates 2a-c). None of the materials had an ideal flat and high spectral response across the VNIR-SWIR region. Many of the materials had a relatively flat and high spectral magnitude in the VNIR and thus would be suitable for applications of sensors in the VNIR only (e.g. CASI, Worldview2, Quickbird). Tyvek has a high reflectance magnitude across much of the 350-2500nm range and showed the brightest response across the spectrum, particularly when layered. The advantage of Tyvek is that it has a semi-matt finish and is not hydrophilic. Compared with Tyvek all other materials showed a drop off in response in the near infrared and SWIR. The spectrum with the least absorption was the polyester lining of the white cotton. The layered lined white cotton also showed promise. However, any material with polyester thermal lining would be near impossible to work with in the field as the material sticks together when the polyester surfaces touch. Potentially two layers of the material could be used with the polyester sides touching, and the cotton sides facing up and towards the ground. Cotton twill and drill, particularly when layered, would be attractive for the VNIR, but these targets would be very difficult to keep clean. Natural-fibre based materials are also hydrophilic. White vinyl would be susceptible to the effects of BRDF and is expensive. While white poly tarps are susceptible to BRDF, they are a cheap option and show a relatively high reflectance, particularly in the VNIR. Natural

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calico has a moderately bright response, but the attenuation in the blue region may be problematic for most sensors. Of the moderately bright targets (Plates 3a-c), grey/silver tarps and silver sislation showed the most spectrally flat response in the laboratory. However, both these materials would be susceptible to BRDF effects in the field. As expected, coloured materials, such as blue sislation, coloured tarps and coloured shadecloth were all characterised by strong absorption features. Some targets, such as shade cloth should be layered to reduce interference from the ground cover influencing the spectral response. Layering of materials influences the spectral response, and in the case of thin materials such as Tyvek, the reflected response is magnified when a double layer is used. Unfortunately a perfect suite of dark, moderately bright and bright PIFs does not exist. Most surfaces are shiny/specular, hydrophilic, and/or expensive and a trade-off needs to be made when choosing artificial materials to use as calibration targets. Considering spectral response and cost of purchasing large areas of material: suitable dark targets included black shadecloth (layered) and black polyester “canvas”; and moderate and light PIFS included Tyvek, and white and grey/silver tarps. The shiny surfaces of the tarps mean that these are susceptible to specular effects in the field and natural fibre-based targets such as canvas are susceptible to the effects of moisture from the ground or air. Plate 4 shows the spectral response of these PIFs as measured in the field. Tyvek, grey tarp and white tarp show the effects of water vapour in the atmosphere at the time of spectral acquisition. The black shadecloth and black canvas remain spectrally flat across the spectrum. The shadecloth response could probably be improved by layering the material. The grey tarp shows a relatively spectrally flat signal across the spectrum as a moderately bright target. The white materials show comparatively flat features in the VNIR and absorptions in the SWIR. Tyvek is a suitable bright target for use as a PIF. The goal of this research was to determine a suitable set of PIFs to be used for calibrating remotely sensed data, with the investment in a set of PIFs being worthwhile especially if these could be reused for many data captures. In reality, the spectral response of the PIFs need to be resampled and their individual suitability determined on a sensor by sensor basis. Figure 1 presents the spectral reflectance of the artificial PIFS measured in the field across the 350-2500 nm range and shown resampled to the WorldView2 wavelength range. The selected PIFs provide a good range of spectral magnitude. Figure 1 demonstrates that by resampling the selected spectra to match the WorldView2 sensor wavelength range, the selected PIFs are effectively spectrally flat. The high resolution laboratory-measured spectra can be resampled and tested for compatibility for a variety of sensor arrays. Caution must be used if laboratory measurements alone are used as the basis for PIF selection as the viewing and illumination geometry is different to that experienced in the field. While the field measurements of selected PIFs used in this study were taken at nadir only, a comparison of field and laboratory spectra are useful to appreciate the impact of solar illumination on shiny surfaces. Plate 5 compares Tyvek, silver tarp, white tarp and black polyester “canvas”

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measurements in the laboratory with that in the field. Whilst the overall spectral response is similar in the laboratory and field, the spectral response of the PIFs would still need to be made at the time of image acquisition, preferably with a similar viewing angle, to account for the absolute differences in signal response.

Conclusion Artificial “perfect” PIFs for the full 350-2500 nm range across a variety of reflectance magnitudes do not exist. Most surfaces are shiny/specular, hydrophilic, and/or expensive. Black shadecloth, particularly when layered, and black polyester “canvas” are suitable as dark PIFs across the full 350-2500 nm range. Tyvek is the spectrally brightest material measured. Despite the shininess of tarps, they are useful targets for moderately bright regions of the spectrum. However, the full spectral response of these PIFs still needs to be measured at the time of image acquisition to be able to account for the effects of environmental variables on the PIF spectral signature.

Acknowledgements: Thank you to the following remote sensing community members who made suggestions for potential PIF materials: Tom Cudahy, Paul Daniel, Stuart Phinn, Phil Bierwirth, Gavin Fowler, Megan Lewis and Simon Jones.

References ASD Inc., 2004, High intensity contact and plant probe. ASD doc ID 600017, Contact-Plant Probe Instructions (04/07/2004), http://support.asdi.com/Document/Documents.aspx, (accessed 6 April 2010). Mac Arthur, A., 2007, Field Guide for the ASD FieldSpec Pro-White Reference Mode. Version 2. Natural Environment Research Council Field Spectroscopy Facility. Available online at http://fsf.nerc.ac.uk/resources/guides/pdf_guides/asd_guide_v2_wr.pdf , (accessed July 2010). Pfitzner, K., Bollhöfer, A., & Carr, G., 2006. A standard design for collecting vegetation reference spectra: Implementation and implications for data sharing. Journal of Spatial Sciences, 52 , pp. 79–92.