Commission VII , W. G. VII / 9
H.-J . BOEHNEL , W. FIS CH£ R,G. KNOLL,
Fraunhofer - Institut fur Physikal i sche Messtechnik
(former Inst itut fur Physikalische Weltraumforschung)
Freiburg i . Br ., Federal Republic of Germany
THE DEP ENDENCE OF TH E SP ECTRA L SIGNA TURE OF SUGAR BEETS
ON THE OBSERVA TI ON LEVEL AND THE RE FLE CTI ON GEOMETRY .
PART B: IN SITU MEASUREMENTS WITH SP ECTRORAD IOME TERS .
Abstract
A joint e x periment was carried out in September 19 79 in order
to determine the spectral r e flection characteristics of sugar
beets by (nearly) simultaneous measurements with a spectraradiometer and an airborne multispectral scanner in situ and
with a spectrophotomete r in the laboratory . The measurements
with the spectroradiometer were made in the spectral ranges
from 0 . 5 to 1 .1 p m and from 1. 5 to 2 . 4 pm at a height of about
15m and with different zenith angles (0° , 15° , 30°, 45°) and
two az i muth directions having 1 80 difference . The spectral
reflectance factors were calculated and compared with the cor responding values determined from the MSS data and the labora tory measurements of leaf reflectance (Part A and C of the
joint paper) .
Introduction
In the frame of a research project on the detection of vegeta tion stress by changed reflection characteristics diseased
sugar beets were investigated by different instruments and from
different observation levels :
2
- airborne multispectral scanner BENDIX M s flown at alti tudes of 300 m and 2000 m above ground
(see part A of the paper) ,
- ground - based field spectroradiometers ,
- laboratory spectral photometer ZEISS PMQ II/RA3
(see part C of the paper)
This paper reports on the field measurements with spectroradiometers of the type EG&G 555 and BARNES Spectral Master 12 - 550
operating in the wavelength ranges 0 . 5 to 1 .1 pm and 1 .4 to
2 . 5 pm, respectively . The radiometers were mounted at the top
of a turnable jib of 18m length looking at the same target
area and at different zenith angles ~ r and fi x ed azimuth direc tions ~ rl ~+ ± 1 80° by turning the jib around a horizontal axis
( 8) , see Flgures 1 (hatched plane) and 2. The target size had a
diameter of about 2 . 5 m for the EG &G 555 (10° FOV) and about
5 m for the BARNES SM (20° FOV) .
102.
Measurements
The field measurements were carried out with the BARNES Spectral
Master on September 1 8 , 19 7 9 and on the following day (day of
the scanner flight)
with the EG&G spectroradiometer . The meas uring program consisted of three cycles of observations made in
the late morning , at noon , and in the afternoon in order to
study the de p endence of R(A) on the posit i on of the sun (ex pressed by the coordinates ;J' i and Cf i , see Fig . 1) .
Each cycle started wi th a vertical measurement ( ~+ = 0) , fol l owed by the inc l ination of the jib to the left slde in three
steps (;;J'r = 1 5° , 30° and 45° resp . , cp r " 0°) , a furthe r verti cal measurement , three incl i ned positions ( ~r = 15° , 30° and
45° resp ., cp
0°) , and a final vertical observation . These
nine observations could be performed in about 75 minutes in the
case of measurements with the EG&G 555 , and in about 50 minutes
with the BARNES SM , respectively , including the reference meas urements with a reflection standard .
The ( ~i ,f i) - diagram in Figure 3 shows for wh i ch solar positions
the data with the two spectroradiometers were co l lected . Starting and end point of each cycle are marked by a symbol and
connected by a straight line . The (~r '~ i -fr) - diagram in the
same Figure indicates the observation geometry with respect to
the solar azimuth . The dark sectors of the diagram correspond
to more or less front l ighting ( [ cp . -<fr: ~ 4 5°) or back lighting
(! ~.-~ [ ~ 135°) , respectively . Tfie l ight fields symbolize side
ligfitifig conditions .
The output signals of the instruments were converted into values
of the object radiance LA
calculated in steps of 5 nm with a
spectral resolution of 1o ' fim in the case of measurements with
the EG&G 555 , and in steps of 10 nm with a spectral resolution
of about 40 nm from the Spectral Master data .
From L ~r and corresponding values of the radiance LA , r ; w of a
reflectlon standard the spectral reflectance factor
R(A)
L
=
L
A, r
"A , r ; w
has been calculated / 1 / , /2/ .
Results
The Figures 4 to 7 show the spectral reflectance factor versus
wavelength in the two spectral ranges covered by the two types
of spectroradiometers and determined in the first cycle . The
highest values of R(A) were obtained with front lighting and
~r Rt ,3'i (in this case with~r = 45°) . In the opposite viewing
direction with back l ighting the splitting of the curves for
different values of ~r is very small . Results of measurements
made at other sugar beet test sites indicate a clearer separation for a higher elevation of the sun with decreasing reflect ance factors for increasing values of :J' r /3/ .
:1.03.
The relative change of R(A) with respect to the variation of~
for five selected wavelengths \ k and the three measuring cycle~
is illustrated in ::i0ure 8 . As shown in the two diagrams for
the first cycle front and back lighting conditions produce a
remarkable difference in the spectral response between wavelength ranges which correspond to processes determining the
reflection properties of plants (pigment-absorbing region 0 . 4
to 0 . 7 pm, internal - leaf - structure region 0 . 7 to 1 . 3 pm, and
leaf - moisture-content reg ion 1 . 3 to 2. 5 pm , I 4 I) . Viewed with
side lighting the sugar beet c an opy shows a reflection behavioL
approximately like a Lambertian surface .
Due to technical limitations these measurements could only be
performed in two opposite azimuth directions 'f r = -30° and Cf r
150° (relative to South). In order to study the influenc e of
the azimuth ~ 0 = 13° of the sugar beet rows the relative change
of R( A) by variation of the difference between solar and row
azimuth has be e n calculated for the same wavelengths as above
and vertical observation of the target . As shown in Figure 9
there is no evidence for an influence of the row azimuth (the
rows were visible but without any strong structure) . The unexpected increased values in the right part of the diagram for
the afternoon measurements could have several causes . Firstly a
small slope of the terrain r e sulted in a diminution of~i in
th e afternoon against the values given in the diagram for an
ideal horizontal surface . From other measurements on sugar beets
and f ee ding turnips it is known that R( \ ) increases with a
higher solar elevation 131 , 151 . Secondly the sugar beets were
stressed by a water deficiency (caused b y a gravel ground layer
and also by nematodes) leading to slacking leaves after some
hours of sola~ irradiation so that the leaf-arrangement changed
during the day . And finally also the wind which blowed strongly
with 5 to 9 r>1/ s from South-\Jest could have influenced the re-·
flection properties of the canopy.
As a consequence of the disease the sugar beets did not fully
cover the ground . About 20 % of the surface within the field of
view of the instrument was uncovered soil which contributed to
the reflected radiance . The influence of the soil refl e ctance
on the spectral signatures will be discussed in part C of the
joint paper by Mr Sanwald.
The assistance of Mr A . Kadro of the University of Freiburg
during the field measurements and of Dr . C . Munther in the
preparation of computer programs is gratefully acknowledged .
Financial support was given by the Deutsche Forschungsgemeinschaft .
References
111 Kriebel, K.T .: Reflection Terminology for Remote Sensing
Applications. Proc . Int . Symp. on Remote Sensing for Observation and Inventory of Earth Resources and the Endangered
Environment, Freiburg, July 1978, Vol. I, pp . 539-541 .
/2/ Boehnel , H.- J ., W. Fischer , and G. Knoll : Spectral Field
Measurements for the Determination of Reflectance Characteristics of Vegetated Surfaces . Proc . Int . Symp . on Remote
Sensing for Observation and Inventory of Earth Resources
and the Endangered Environment , Freiburg , July 1978 , Vol . I ,
pp . 579 - 589 .
/3/ Boehnel , H.-J ., W. Fischer , G. Knoll , and A. Kadro :
Differences in the Spectral Characteristics between Healthy
and Diseased Crops Determined for Sugar Beets and Winter
Barley . ISP Congress Hamburg 1980 , Commission VII/2 .
/4/ Plessey Radar : Multispectral Scanning Systems and their
Potential Application to Earth Resou r ces . Reports to ESRO
under Contract No . 1673/72EL , Vol . 2 , Dec . 1972 .
/5/ Fischer , W.: Determination of Spectral Signatures of Vege tated surfaces by radiometric ground measurements . Proc .
12th Int . Symp . on Remote Sensing of Environment , Manila ,
April 1978 , Vol . III , pp . 2333 - 2342 .
105.
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3
o 2nd
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3rd
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measurements wi th EG
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BARNES SM
(Sept. 19, 1979)
(Sept. 18, 1979)
.. · ..
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Sept . 19 , 1979
t-
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~
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a:::
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700
SUGAR BEETS
Fig. 4
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.
500
DISEASED
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ARMSHEIM
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I.JAVELENGTH I NM
190979
Sep\.19,1979
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SUGAR BEETS, DISeASeD
ARMSHEIM I SEPT. 18, 1979
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SEPT18. 1979
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SEPT.19, 1979
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a).=675nm
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SEPT.19, 1979
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2.0
SEPT.19.1979
1.8
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1.6
1.6
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1.4
1.4
1. 4
1.2
1.2
1.2
1.0
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0.5
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0.8
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0° 15
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3-r
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41
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0° 15
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FIG.
8
4
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45
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0 ° 15
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'Pj - 'Pr
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R( 'Pi-'Po =ct)
b.
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1.4
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1.2
1.1
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-30
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9
:1. :1. :1. .
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