Journal of Quantitative Spectroscopy and Radiative Transfer
Polarised bidirectional reflectance factor measurements from soil, stones, and snow
Introduction
Polarisation observations have been coming to land surface remote sensing at least 20 years [1], [2], [3]. Despite many promising results [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], practical applications have remained few. The biggest polarisation project has been the polarisation and directionality of the earth's reflectances (POLDER) sensor [14]. Although it is primarily designed for atmospheric observations [15], [16], some land surface applications have also been developed [17], [18]. The airborne version of POLDER (OSIRIS) is opening new possibilities [19]. Another planned atmospheric polarisation sensor is the aerosol polarimetry sensor (APS) [20] in the NASA Glory mission. It is assumed, that in near future polarisation missions more suitable for land surfaces may be introduced.
However, before new missions can be seriously planned and proposed, polarisation from various targets should be much better understood and modelled. Even, when observing primarily atmosphere, the land surface background reflectance and polarisation signals must be well known and removed from the data. Polarisation measurements performed so far have been few, and limited by spectral and directional coverage. Large number of more systematic measurements are needed, before useful invertible models can be developed, and new applications created.
Polarisation of electromagnetic radiation waves, such as light, is defined by the direction of the electric field. Natural light is a mixture of a very large number of various wave packets. Sunlight and many other light sources contain equal amount of all polarisation states, thus said to be unpolarised. Scattering, reflection, or transmission through asymmetric media often polarise light. The most significant polarisation mechanisms are the Rayleigh scattering (resulting in up to 90% polarisation of blue sky) and Fresnel reflection (explaining the up to 100% polarisation of water surfaces). However, all scattering mechanisms polarise to some extent, although the more complicated ones usually less so. One discussed subtlety is the small negative polarisation near backward scattering angles caused by multiple scattering coherence effects [21], [22]. The polarisation properties depend mostly on the chemical composition (refractive index) and wavelength and subwavelength scale structures of the scatterers, and amount of multiple scattering.Conversely, these properties could be studied using polarisation, as is successfully done in aerosol research and astronomy.
The most convenient method for measuring the reflective properties of surfaces is to determine the bidirectional reflectance factor (BRF for short, denoted as here), which is the ratio of the reflected light to an ideal Lambertian reflector under same incident irradiation. This factor is a function of four angles and wavelength, see Fig. 1. The reflected radiance can then be given as , where is the incident radiance (distribution) and integration goes over the hemisphere (, see Fig. 1). In description of the polarisation the BRF is pictured as a Muller matrix, and both reflected and incident light are described with the Stokes vector .
Earlier measurements have already shown that land surfaces polarise weaker than atmosphere, and mostly in forward direction. Vegetation is discussed more in a preceding paper [23]. Goloub et al. [18] analysed POLDER data about snow and clouds and observed that snow polarised a few per cent in the forward direction, but did not show the rainbow arc feature present in clouds. Perovich [7] measured polarised reflectance from many ice and snow samples, observing very strong forward polarisation from ice and rather weak polarisation from dry snow and half of that from melting snow. Bréon et al. [6] and Taixia and Yunsheng [10] measured polarisation from some bare soil samples. They too observed small negative polarisation in the backward directions and moderate positive polarisation in the forward direction that could be explained for the most part by Fresnel reflections.
In this paper we aim to provide spectrally- and directionally-resolved measurement data of selected land surface objects in order to quantify in detail the polarisation features and their usefulness. Puttonen et al. [24] have already reported some gonio-spectro-polarimetric reflection measurements from asphalt, sand, and concrete surfaces, and Suomalainen et al. [23] measured bidirectional polarisation factor from several pieces of vegetation. This paper provides more data from non-vegetated land surfaces.
Section snippets
Instruments and measurement practices
The measurements were taken using the Finnish geodetic institute field goniospectrometer (FIGIFIGO), see Fig. 2. The FIGIFIGO system consists of a motor-driven moving arm that tilts up to from the vertical, variable fore optics in the high end of the arm, and an ASD FieldSpec Pro FR 350–2500 nm spectroradiometer. Accurate angles are read with an inclinometer and an electronic compass. The position is determined with GPS. The system can be mounted on a light sledge for snow measurement, on a
Campaigns and targets
The snow measurements come from two campaigns in Sodankylä, in North Finland, and one in Masalasee Table 1. Year 2007 measurements were taken in a semi open area in sunlight under clear sky. The snow was melting rapidly, and the snow surface was rather rough. During the next campaign in 2008, the sky was overcast most of the time, and all but one measurements are performed during nights using artificial illumination in a tight spot inside sparse forest. In the beginning the snow was several
Results
The results are shown in Fig. 3, Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9, Fig. 10, Fig. 11. For comparison, the angle of incidence for the presented results is selected around for snow and for gravel and sand. For each target a six element figure of measurement results is given. An illustrative photograph is shown first, then in the left column the BRF cake in red channel () and the brightness spectrum in four angles in the principal plane. In the right column, the degree
Conclusions
We have measured the spectral and angular distribution of the linear polarisation () of reflected light from several bare-land surface types in more detail than has been done before. In total, our measurement data base now contains about 15 vegetation samples and 20 bare-land samples measured with polarisation, and BRF measurements without polarisation from over 200 samples. This study therefore confirmed the prior knowledge, that in general, land surfaces polarise weakly, and no dramatic
Author contribution
Jouni Peltoniemi supervised the work, analysed the data and wrote most of the paper, with contributions from everyone. Juha Suomalainen, Eetu Puttonen and Teemu Hakala performed the measurements. Teemu Hakala constructed the polarisation device with Juha Suomalainen.
Acknowledgements
The work has been supported by the Academy of Finland, the European Commission Interreg program project Norsen, and the Finnish Ministry of Agriculture and Forestry. Some measurements were made during the Snortex campaign. The authors would like to thank many of our Norsen and Snortex partners.
The authors declare that, at the time of submission, they have not been anyhow involved in the afore mentioned nor competing satellite missions.
References (30)
Spectropolarised ray-tracing simulations in densely packed particulate medium
J Quant Spectrosc Radiat Transf
(2007)Photometry and polarization in remote sensing
(1985)- et al.
Remote sensing using partially polarized light
Int J Remote Sensing
(1986) - et al.
Specular, diffuse, and polarised light scattered by two wheat canopies
Appl Opt
(1985) - et al.
Polarisation de la lumiere par les couverts végétaux: possibilités d’applications agronomiques
Can J Remote Sensing
(1990) - et al.
Polarization of light by vegetation
- et al.
Polarized reflectance of bare soils and vegetation: measurements and models
IEEE Trans Geosci Remote Sensing
(1995) Observation of polarisation of light from sea ice
J Geophys Res
(1998)Polarization optics of random media
(2003)- Elias TG, Cairns B, Chowdhary J. Surface optical properties measured by the airborne research scanning polarimeter...
The bidirectional polarized reflectance model of soil
IEEE Trans Geosci Remote Sensing
Estimating the leaf inclination angle of wheat canopies using reflected polarized light
Plant Prod Sci
Testing polarisation measurements with adjusted view zenith angles in varying illumination conditions for detecting leaf orientation in wheat canopy
Plant Prod Sci
Remote sensing signatures: measurements, modelling and applications
The POLDER mission: instrument characteristics and scientific objectives
IEEE Trans Geosci Remote Sensing
Cited by (84)
Non-Lambertian snow surface reflection models for simulated top-of-the-atmosphere radiances in the NIR and SWIR wavelengths
2024, Journal of Quantitative Spectroscopy and Radiative TransferExtension of the Hapke model to the spectral domain to characterize soil physical properties
2022, Remote Sensing of EnvironmentCitation Excerpt :Reportedly, soil reflectance generally decreases from the backscattering to the forward-scattering directions and typically shows a reflectance peak in the backward direction when the incidence and viewing directions coincide (i.e., the hotspot effect) (Hapke, 1981, 2002, 2012; Liang and Mishchenko, 1997). Soil anisotropy is very essential in remote sensing scenes, and its effect is even more important when data are collected at different solar-view angles (Peltoniemi et al., 2009). Additionally, soil is a complex medium, and many factors, such as the soil moisture content (SMC), affect its directional and spectral characteristics under natural conditions (Liang and Townshend, 1996a, 1996b; Jacquemoud, 1992).
Direct reflectance transformation methodology for drone-based hyperspectral imaging
2021, Remote Sensing of EnvironmentSpectral degree of linear polarization and neutral points of polarization in snow and ice surfaces
2021, Journal of Quantitative Spectroscopy and Radiative TransferOn the negative polarization's effects at top of atmosphere
2021, Atmospheric EnvironmentLight scattering from volcanic-sand particles in deposited and aerosol form
2019, Atmospheric EnvironmentCitation Excerpt :As such, our experiment reproduces both of measurement scenarios relevant for remote-sensing observations of volcanic sand, i.e., in the atmosphere or deposited on an ice/snow surface. Previous related measurements mainly focus on either single-particles or deposited-particles (Muñoz et al., 2004, Dabrowska et al., 2015; Hadamcik, 2002; Sun et al., 2014; Peltoniemi et al., 2009, Wilkman et al., 2016). There are only few examples where both light-scattering scenarios are simultaneously studied (e.g., Shkuratov et al., 2004, Shkuratov et al., 2006; Mirvatte et al., 2011) and Icelandic volcanic sand is not encompassed in that work.