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ABOUT THE IMPORTANCE OF THE DEFINITION OF REFLECTANCE QUANTITIES -
RESULTS OF CASE STUDIES
G. Schaepman-Strub * ”, T. Painter ^, S. Huber *, S. Dangel “, M. E. Schaepman “, J. Martonchik *, F. Berendse "
* RSL, Dept. of Geography, Univ. of Zurich, 8057 Zurich, Switzerland - gschaep@geo.unizh.ch
” Nature Conservation and Plant Ecology Group, WUR, 6708 PD Wageningen, The Netherlands
“ National Snow and Ice Data Center, University of Colorado Boulder, Boulder, CO, USA
“ Centre for Geo-Information, WUR, 6700 AA Wageningen, The Netherlands
* Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA, USA
KEY WORDS: Terminology, Comparison, Simulation, Accuracy, Vegetation, Snow, MISR
ABSTRACT:
In the remote sensing user community there is a lack of consistency in definitions and properties of reflectance quantities. On
one hand, more recent satellite programs such as NASA's MODIS and MISR sensors take into account the directional
dimension of the different reflectance products. On the other hand, many published studies still remain unspecific on the
reflectance quantities they are based on, or do not follow common definitions. One example is the term 'albedo' assigned to
significantly differing products. This fact makes it difficult and confusing to evaluate and compare published results.
Our contribution briefly summarizes basic reflectance nomenclature articles. The main aim is to quantify differences of
reflectance products to stress the importance of adequate usage of reflectance definitions and quantities. Results from the
comparison of directional-hemispherical reflectance versus bihemispherical reflectance and bidirectional reflectance factors
versus hemispherical-directional reflectance factors are shown. We exemplify differences of these quantities using modelling
results of a black spruce forest canopy and snow cover, as well as selected biome-specific MISR reflectance products of the
year 2001.
The presented case studies can only give an insight into the dimension of the problem. The actual differences in the
reflectance products of a remotely sensed surface depend on the atmospheric conditions, the surroundings, topography, and
the scattering properties of the surface itself. Never the less the presented results are urging the user community to be more
specific on the application and definition of reflectance quantities.
1. INTRODUCTION
The Earth-looking remote sensing community increasingly
understands the effects of solar illumination geometry and
sensor viewing geometry on airborne and satellite data due
to the anisotropic reflectance of the Earth's surface and the
atmosphere. Not only the direction of illumination and
observation influence the measured reflectance, but also
their opening angle. Different reflectance quantities have
been defined to describe the corresponding conditions of
the measurements (Nicodemus, 1977; Martonchik, 2000).
Nevertheless, these conditions are often partly or fully
neglected by the user community, and different reflectance
quantities are equated, which is especially true for the so-
called surface reflectance and albedo (e.g., Breuer, 2003).
The reflectance anisotropy of observed surfaces contains
unique information about its structure and the optical
properties of the scattering elements. The underlying
concept for the characterization of the anisotropy is the
bidirectional reflectance distribution function (BRDF). It
describes the radiance reflected by a surface as a function of
a parallel beam of incident light from a single direction into
another direction of the hemisphere. Under natural
conditions, i.e. for all field, airborne and spaceborne sensor
measurements, the assumption of a single direction of the
incident beam does not hold true. Natural light is composed
of a direct part, thus uncollided radiation, as well as a diffuse
component scattered by the atmosphere, and/ or the
surroundings of the observed target. The amount and
spectral character of the diffuse light irradiating the
observed surface is thus depending on the atmospheric
conditions, as well as on the topography and the scattering
properties of the surroundings. Previous studies have shown
the effects of different atmospheric conditions in simulated
361
and measured data. The aim of this study is to highlight the
differences in reflectance caused by different geometries of
the opening angle of the illumination, i.e., directional and
hemispherical extent. To get a better impression of the
influence of the diffuse component included in the
hemispherical extent, different direct to diffuse irradiance
scenarios are considered. This modelling approach is
performed for a black spruce forest canopy, and a snow
cover. Secondly, first results of a comparison between
directional and hemispherical reflectance products from the
Multi-angle Imaging SpectroRadiometer (MISR) are
presented for selected test sites.
This study gives an easy access to the basic concept of
reflectance quantities for the user community, by
summarizing the nomenclature articles of Nicodemus (1977)
and Martonchik (2000). It highlights the importance of a
proper usage of definitions through quantitative
comparison of different reflectance products.
2. DEFINITIONS
2.1 Radiance, reflectance, reflectance factors
Spectral radiance is the most important quantity to be
measured in spectroradiometry. In particular it is the
quantity required for quantitatively analyzing directional
effects. The surface leaving radiance is assumed to be
dependent on the incident radiation onto the surface, thus
the reflectance is defined as the ratio of reflected to incident
flux. Following the concept of energy conservation, its
values are in the inclusive interval O to 1. The reflectance
factor is the ratio of the radiant flux reflected by a surface to
that reflected into the same reflected-beam geometry by an
ideal (lossless) and diffuse (Lambertian) standard surface,
