Itstrukturelis
egelung _ in
Ökologische
lultispektral-
Mitteilungen
1986, Graz.
ung Band 1
eitung von
ungsbericht,
Stuttgart.
IN SITU BRDF MEASUREMENTS OF SELECTED SURFACE MATERIALS TO IMPROVE ANALYSIS OF
REMOTELY SENSED MULTISPECTRAL IMAGERY
Gerhard Meister, Rafael Wiemker, Johann Bienlein, Hartwig Spitzer
Il. Institut für Experimentalphysik
Universität Hamburg
Germany
Mail: G. Meister / KOGS, Vogt-Kölln-Str. 30, 22527 Hamburg, FRG
WWW-Page: http://kogs25.informatik.uni-hamburg.de/projects/censis/remotesens.html
E-mail: meister@kogs.informatik.uni-hamburg.de
Commission VII, Working Group 1
KEY WORDS: Radiometry, Multispectral, Infrared, Surface, Classification, Radiometric Measurement, BRDF Measurement,
Spectralon BRDF
ABSTRACT
The spectral bidirectional reflectance distribution function (BRDF) of selected artificial surfaces (e. g. roof materials) was
measured under natural illumination. Data were obtained with a spectrometer in the wavelength range 600 nm — 900 nm with
a spectral resolution of 5 nm, using 61 spectral bands. The samples were placed on a goniometer, which allowed to set all
the desired angles of incidence and reflection. A Spectralon panel (made by Labsphere Inc., Boulder, Colorado) was used as a
reference target for determining the incoming irradiance. The BRDF of this Spectralon panel was measured in the laboratory.
The BRDF of the Spectralon panel is not Lambertian, however it obeys Helmholtz's theorem of reciprocity. For the in situ
measurements, an additional measurement was made for each combination of angles with the samples being shadowed, in order
to determine the contribution of sky light. The samples could be fitted to a function which was developed by [Walthall, 1985]
and modified by [Liang, 1994]. In this work, the function was extended by a specular peak, increasing the number of free
parameters to 7. The diffuse part of the BRDF of the samples does not vary in a systematic way. The intensity of the specular
peak grows with increasing zenith angles.
KURZFASSUNG
Dieser Beitrag beschàftigt sich mit der Messung der winkelabhàngigen, spektralen Reflexionsfunktion (BRDF) von einigen
ausgewáhlten künstlichen Oberfláchen (z. B. Dachbedeckungen)bei natürlicher Beleuchtung. Als MeBgerát diente ein Spek-
trometer, welches im Bereich 600 nm — 900 nm Messungen in 61 Kanälen mit einer spektralen Auflösung von 5 nm ermöglicht.
Die Proben wurden auf einem GoniometermeBtisch plaziert, so daß alle gewünschten Ein- und Ausfallswinkel eingestellt wer-
den konnten. Zur Bestimmung der einfallenden Strahlungsflußdichte diente ein Reflektanznormal (Spectralon, hergestellt duch
Labsphere Inc., Boulder, Colorado) als Referenzfläche, dessen BRDF zuvor im Labor bestimmt wurde. Es ergibt sich, daß die
BRDF des Spectralons nicht dem Lambertstrahler entspricht, wohl aber dem Helmholtzschen Reziprozitätstheorem genügt.
Bei den Feldmessungen wurde zusätzlich für jede Winkelkombination eine Messung bei beschatteter Probe durchgeführt, um
den Einfluß des Himmelslichts zu bestimmen. An die Proben konnte eine Funktion angepaßt werden, die von [Walthall, 1985]
entwickelt, von [Liang, 1994] modifiziert und hier um einen Term zur Modellierung des gespiegelten Anteils erweitert wurde,
so daß diese Funktion insgesamt 7 freie Parameter besitzt. Während die diffuse Komponente der BRDF der Proben keine Sys-
tematik erkennen läßt, konnte bei der gespiegelten Komponente ein Anstieg der Intensität mit großen Zenitwinkeln festgestellt
werden.
1 INTRODUCTION parameters, and thus finally improving classification of re-
"OD ; motely sensed multispectral imagery.
The Bidirectional Reflectance Distribution Function (BRDF)
as defined by [Nicodemus, 1970] describes the radiance emit- In the line of our research we are interested in the use of
ted by a reflecting target into the sensor direction, depending ~~ Multispectral imagery of high spatial resolution for purposes
on the incoming radiation, the angles of incidence and reflec- — of urban planning and treaty verification in the framework
tion (0;, ®i, 0, ®,) (see fig.1) and wavelength A. of Open Skies. Most BRDF related efforts undertaken so far
The kno v Mii have been directed at natural surfaces such as vegetation and
nowledge of the BRDF of surface materials is certainly soils. In contrast to that we have chosen samples of artificial
crucial for proper analysis and evaluation of remotely sensed materials found in urban areas and man made structures
multispectral imagery. Spectral signatures change with view- such as tiles, bricks, asphalt, metall, roof materials etc., and
in illuminati : . : : à uf
í S ud Illumination geometry, and the BRDF is thus needed „m agsyred their reflective behavior under natural illumintion.
or correct classification and change detection.
However, so far most multispectral image processing and clas-
sification is based on the simplifying assumption of Lamber- 2 EXPERIMENTAL SETUP
tian reflection, and thus prone to error. One of the rea-
sons for this may be that only relatively few BRDF data are — The measurements were done with the spectrometer OVID
available, due to the tremendous effort necessary for its mea- (Optical Visible and Infrared Detector) in cooperation with
surement. The work described here is aimed at measuring Max-Planck-Institut für Meteorologie, Hamburg, and Institut
relevant BRDF data, finding proper BRDF models with few für Meteorologie, Universität Hamburg. A description of the
493
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B7. Vienna 1996
