simulator for remote sensing and its application to soil moisture measurements

Simulator for remote sensing and its application to soilmoisture measurements

Hidesaburo Genda and Hiroshi Okayama

It is of great significance to experiment with a simulator for remote sensing to confirm the properties andmeaning of remote-sensed information and to forecast certain phenomena. This paper describes a simula-tor for remote sensing. The simulator, suitable for the measurement of soil moisture, consists of an opticalsource, a polarimeter, orbital guides for them, and a sample stage. SiC and MgO were used as soil models.The moisture in beach sand was also estimated. The degree of polarization increases with the moisture con-tent and particle size of the sample.polarimeter.

1. Introduction

In recent years techniques (photographic, digital,and analog processing) for image analyses on remotesensing have developed rapidly. They are utilized toanalyze not only the image information transmittedfrom the LANDSAT but also the earth image obtainedby aircraft. An enormous amount of work in thesefields has been reported.

Application areas of remote sensing are divided intothree categories: water, land and vegetation, and at-mosphere. The problem of water contains water re-sources inventory and management for agricultural use,storage of water in the snow and ice cover, water quality,sea-ice and salinity distribution, sea pollution, etc. Soilmoisture is an especially important factor in agriculture.The amount of moisture in the surface of the ground isvery useful for selecting the kind of plant, establishingthe planting time, planning the initial deployment offertilizers before planting, deciding the amount ofsprinkling water, etc., as well as for investigating theterrain. Besides, soil moisture is an important pa-rameter for a flood forecast.

The problems of land and vegetation are land use, soilclassification, mineral inventory, control of plant dis-eases, land pollution, etc. In the atmosphere there areproblems involving global weather mapping, horizontaland vertical temperature and water vapor distribution,

The authors are with Chiba University, Institute of Color Tech-nology, Department of Remote Sensing Image Processing, 1-33Yayoi-cho, Chiba-shi 280, Japan.

Received 16 July 1977.0003-6935/78/0301-0807$0.50/0.© 1978 Optical Society of America.

The field capacity point of each sample is determined by means of the

atmospheric pollution, etc. Thus, remote sensingtechniques are applied to the problems of the socialenvironment of human beings.'

Remote sensing techniques are also used for thepurpose of making image analysis of information ob-tained from sensors and in developing useful sensors.2On the other hand, computer simulations uses mathe-matical models to forecast a phenomenon and to con-firm properties and meanings of remotely sensed in-formation. Furthermore, it is significant for thesepurposes to use a remote sensing simulator in the lab-oratory. Therefore, the authors have designed a sim-ulator which can be used in the above-mentioned threecategories of application of remote sensing and mea-sured with it the degree of polarization of the lightscattered from the surface of the soil to estimate themoisture contained in soil. The results are describedin this paper.

II. Design and Characteristics of the Simulator

From a design viewpoint, a simulator for remotesensing should have a relatively large space for thepurpose of simulating the phenomena of atmosphereand also have a sample stage to set water and soil on forthe purpose of simulating their phenomena. An opticalsource and its guide are needed to simulate the sun andits orbit. Consideration should be given to avoid theshadow of the device being cast on the object. A sim-ulator for remote sensing which we have made in ac-cordance with this design viewpoint is shown in Fig. 1.It consists of an optical source, corresponding to the sunand its orbital guide, a sample stage, and a detector andits orbital guide.

In our experiment this simulator has been applied tothe measurement of moisture in soil by determining the

1 March 1978 / Vol. 17, No. 5 / APPLIED OPTICS 807

Fig. 1. Schematic construction of the simulator for remotesensing.

FIELD LIMITING

ROTATINGOLARIZING

PRISM

Fig. 2. Schematic construction of the polarimeter.

5 0 r

-'40

Z 30

5 20u)zX 10

n-

Frank-Ritter Prism

300 500 700- -WAVE LENGTH ( nm )

900

Fig. 3. Transmittance characteristics of the polarizing Frank-Ritterprism.

power supply for this halogen lamp was designed. Themaximum output current of the power supply is 5 A.The ripple ratio at load when the lamp is lighted is 3.2X 10-2%. No lens or mirror is included in the opticalsystem of the source to avoid the polarization whichmight be introduced.

The degree of polarization of the optical source fromthe dc voltage regulated power supply was examined bythe polarimeter in the simulator, the polarizationcharacteristics of which had been known beforehand,and the value obtained was P = 0.16. This value, whichis peculiar to the optical source, is not so meaningful inthe measurement of the degree of polarization of lightscattered from the soil.

As shown in Fig. 1, since the polarimeter of the de-tector is on a guide 100 cm distant from the center of thesample stage, it can rotate with the guide even in anazimuth direction ( = 0 360°). The polarimeter isshown in Fig. 2. A very narrow field of view angle of thepolarimeter, i.e., 3, was set up for the simulation of afield experiment under natural light. The polarizingprism used is a Frank-Ritter prism made by ShimadzuSeisakusho Ltd.4 The transmittance in the visible re-gion is 42%, and the degree of polarization is 99.99%.The characteristics of the prism are shown in Fig. 3.The polarizing Frank-Ritter prism is continuously ro-tated at a rate of 9 sec per rotation by use of a synchro-nous motor, D-5N (Nippon Servo Co.). A linearly po-larized beam from the prism arrives at the photomul-tiplier, R-889 (Hamamatsu T.V. Co.), through theWratten filter no. 25 and a diffuser. Here a red filterof Wratten no. 25 was selected for the purpose of mea-suring the moisture in the soil, and the diffuser was usedto decrease the polarizing effect of the photomultiplier.The photomultiplier R-889 is an S-20 type of highsensitivity. As the size is as small as 1.3 cm (1/2 in.) indiameter, the polarimeter could be designed as a com-pact detector.

The degree of polarization peculiar to the polarimeterof about 0.035 was determined by measuring the po-larization using an integrating sphere. The light fromthe lamp having entered through a small hole on theintegrating sphere emits from another small hole on thesphere and reaches the polarimeter.

As shown in Fig. 4, the wavelength region measurablewith this polarimeter would be between about 580-850nm, if the energy emitted by the halogen lamp is takeninto consideration. The degree of polarization P of thebeam of light is represented by

degree of polarization of the scattered light. A polar-imeter has been used as a detector. As shown in Fig. 1,the optical source is on the quadrant orbital guide 180cm distant from the center of the sample stage to giveuniform light to the sample. Though a great variety oflamps are used as optical sources at present, in this ex-periment a halogen lamp (Osram, 12 V, 50 W) wasadopted as it is free from dissipation of the filament andvariation in color temperature of brightness until theend of the life of the lamp. A dc voltage regulated

PO~ III ax - in (P 01)11I+I Ix + Imi

When the polarizing prism of the polarimeter rotates,the light beam is separated into two vertical compo-nents. Imax is the maximum intensity of the beam, andImin is the minimum. One instance of the output fromthe polarimeter recorded on the X-Y recorder is shownin Fig. 5. Then the degree of polarization of the lightbeam scattered from the object is obtained by use of theabove equation.

808 APPLIED OPTICS / Vol. 17, No. 5 / 1 March 1978

--

Ill. Experiments and Results

The degree of polarization is one of the importantparameters characterizing the radiation properties ofan object. It is the distribution of the electric field inthe plane normal to the propagation direction.Blackbody radiation is completely unpolarized, but theemission of light from many natural features showspronounced polarization effects, which can be useful for

101WRATTEN 25

'R889 l

I~~~~~~~I

Uo z

z -

10' 10'400 500 600 700 800 900 1000

WAVELENGTH (nm)

Fig. 4. Transmittance of Wratten filter no. 25 and the relative sen-sitivity of the photomultiplier R-889.

identifying the nature of the feature. Therefore, oneway to learn the components and characteristics of amaterial surface is to measure the degree of polarizationof the light scattered from the object.5 6 Polarizationas well as wavelength and intensity are fundamental andimportant aspects of light. Stockhoff and Frost re-ported that soil moisture could be estimated by mea-suring the degree of polarization of light scattered fromsoil.7' 8 The authors have estimated soil moisture bymeasuring the degree of polarization of the light scat-tered from the surface of soil, using a simulatorequipped with a polarimeter as a measurement de-vice.

Soil which exists in nature varies in quality, particlesize, etc. We used silicon carbides (SiC), widely usedas a polishing agent, as a model of the soil. The degreeof polarization of the light scattered from SiC whichcontained a known amount of moisture was measured.As shown in Fig. 6, the measurement directions wereazimuth and zenith. In the direction of the azimuthfour angles were selected, o = 00, 450, 90°, and 135°; andthe incident angles were 0O = 100, 450, and 60°; and thedetecting angles (angle of view) were 0 = 100, 200, 300,400, 500, and 600. Therefore, seventy-two combina-tions were possible. The tray to hold the sample waspainted black to avoid a reflection from the bottom ofthe tray. The depth of the sample was 5 cm. Resultsof the measurements are shown in Figs. 7-19. Figure7 shows the degree of polarization of dry silicon carbideand magnesium oxide (MgO) as a function of mesh sizeand the angle of view 0 in the principal plane (00 = 600,(p = 0°). The smaller the mesh number, i.e., the largerthe particle size, the higher the degree of polarization.For magnesium oxide the degree of polarization is al-most invariable between 0 = 100 and 600. The value isroughly the same as the degree of polarization of thepolarimeter measured by use of an integrating sphere,as described in the previous section. This fact indicatesthat when a polarimeter is used as a sensor in remotesensing, MgO can be one of the materials to be used asa reference object. The polarizations shown in Fig. 7

1Or

Fig. 5. One instance of recording on an X- Y recorder of the outputfrom the polarimeter.

RS

* RE~~P(A)

& PR

(B)

Fig. 6. Measurement direction: (a) an azimuth direction; (b) anincidence direction and an angle of view.

. L

zI-Nr00.5

LL0LLLu

(9Ld0

0'

SiC and MgODryQ,= 60°9' =0

# 320

/#500.

4 800# 1000.

4 2000

Mg o

10 20 30 40 50 60°ANGLE OF VIEW (0)

Fig. 7. Degree of polarization of SiC and MgO in a dry state.


I v

1.0

Q

z

I-Nc:_j0.5

0a.m

L

LuIx(DLin

sicDry

E= 6d'= 0'

9= 50

I 1 0 0 3 0 5

_0 10 20 30 40 50PARTICLE SIZE (m)

Fig. 8. Degree of polarization vs particle size of SiC.

1.0

Qz

0-

0Ix0ILLu

Lef

01

are plotted as a function of particle size when 0 = 50 inFig. 8. SiC in a dry state shows a rapid increase in thedegree of polarization when the particle size is largerthan 20 ,um.

The influence of the incident angle 00 and azimuth pon the degree of polarization is shown in Fig. 9. Dry SiC(320, 42 jim) was used. With p = 00, the degree of po-larization is shown in Fig. 9(a) as a function of angle ofview 0 when the incident angle 00 = 600, 450, and 100.Results when the azimuthal angle sp = 450, 900, and 1350are shown in Figs. 9(b), 9(c), and 9(d), respectively.Figure 9(c) suggests that the degree of polarization isalmost invariable regardless of the angle of view whenthe azimuthal angle = 900 and that it decreases as theangle of view increases when the incident angle 00 = 10°.And in backscattering of so = 1350, as is shown in Fig.9(d), the degree of polarization decreases as the angle

1.0rSiC #320

Dry9-o'

60= 60' I-

Nix

-JM

CL0'0LLL0u

Luj

CDLu0

0s/ e, '=10

SiC #320Dry9'=45

0o=6d'

0.5p

0'10 20 30 40 50ANGLE OF VIEW ( )

(a)

k9- A---- A-- o.--00=10A---- G~~~.

10 20 30 40 50ANGLE OF VIEW ( )

(b)

60r

1OrCL

z0N-

L

Lu0

1.OrSiC 320

Dry9' =90

0L

z0

N-

<100.50..L0

LuLu

LuC

A_-A - A ' -45

10 20 30 40 50 60°ANGLE OF VIEW (0 )

(C)

SiC #320Dry9' =135'

F

0O

,o=60'

6-- 0010- "" __,e=45~ A 0 O_

10 20 30 40 50 60ANGLE OF VIEW (6)

(d)

Fig. 9. Degree of polarization of SiC 320 in a dry state. (a), (b), (c),and (d) show that azimuth angles ( are 0, 450, 90°, and 1350,

respectively.


i60

1.0SiC 320

Dry

Z sE)=60'0 To~~~~~~~~~~~Fo

ix 9/-45'<~~~~~~~~~00.5 AIL0

LL]O_ _e t_~~~~~~~9 90°Lu Ai

Lii~~ -.-. S 5

0 10 20 30 40 50 60'ANGLE OF VIEW(6)

Fig. 10. When the incident angle 0 is 60°, degree of polarizationvs angle of view (0) for SiC 320 in a dry state.

1.Or

0L

z0I-

N

0o

'i0.IL0LJLuJc]CDLu0

0.5-

0

SiC Moisture 20 0/.

e.= 60°SO =oO . 8320

Its 500A

/A/

(0)S : 800

a n/_ # 000A- -~~ ---- A#10

10 20 30 40 50ANGLE OF VIEW (6)

60'

Fig. 11. Degree of polarization of SiC with 20% moisture.

1.Or

0E

0I-

N-r

00.5a-LL0LuLuCDLuC

0'

SiC #80090= 60 27 l

°(

E)//

/ 2 0 %"_ 10 % I ~

O/ .. ..........~../,,.- ._ % -H

10 20 30 40 50ANGLE OF VIEW ()

bU

Fig. 12. Degree of polarization by moisture for SiC 800.

1.0

z0I-

Ncr-j0 0.501LJ0Lu

LuiCDLu0

0'

sic l1000e =60'

o= 0'

3D 'Il

10 20 30 40 50 60'ANGLE OF VIEW ()

Fig. 13. Degree of polarization by moisture for SiC 1000.

of view increases; but in 00 = 100 it increases. Here,when 00 = 100, the measurement in the angle of view 0= 100 was not made because the shadow of the simula-tor was cast on the sample. In Fig. 9 we see that thedegree of polarization is large whenever the incidentangle 00 is 60°. The degree of polarization is shown inFig. 10 as a function of angle of view when the incidentangle 00 = 600 and the azimuthal angle fo = 00, 450, 900,and 135°. When an azimuthal angle and an incidentangle are (p = 00 and 00 = 600, respectively, a high degreeof polarization is detected. So the amount of moisturein soil is estimated by fixing the incident angle 0O = 600and the azimuthal angle p = 00. The degree of polar-ization of SiC containing 20% moisture is shown in Fig.11 with regard to each mesh number of the particles andas a function of the angle of view. It indicates that thedegree of polarization increases as the particle size be-comes larger, as was seen in Fig. 6, but that the degreeof polarization of the light scattered from the SiC con-taining the 20% moisture is about 19% larger than thatin a dry state.

Next the difference of the degree of polarization bymoisture was measured with SiC 800 (22 Aim) and SiC1000 (12 jim). The results are shown in Figs. 12 and 13.The 27% and 30% moisture contained in SiC 800 andSiC 1000, respectively, are the maximum water thesesubstances can contain. Figures 12 and 13 indicate thatthe degree of polarization increases as the moisturecontent of these SiC samples becomes higher. Whenthe degrees of polarization based on these results areplotted with regard to each angle of view 0 as a functionof percent moisture, the curves in Figs. 14 and 15 areobtained. SiC 800 shows a sudden increase in the de-gree of polarization at the point of 20% moisture as toeach angle of view. As for SiC 1000, the point is about15%. In general there is a point of sudden increase inthe degree of polarization at a certain percent moisture,which we call a field capacity point of the soil. This


i

20%

(D lo./. -- 0,,, :�'_ - ''-)( 0 %

"=; V5:�

1 .OrSiC #800

8O= 60. = .

a-

z0r-Nrr

011-IL0

CDcr I

LQ

'I=20'

. o wo°

10 20PERCENT MOISTURE

Si C 1 000

= 6d'S =0°

0=36<//ni~=26

0 10 20PERCENT MOISTURE

Fig. 14. Degree of polarization vs percent moisture for SiC 800.

1.0

Sand of the Shirako Beacha- m O=60'0I-

18'? o

< 0.5

IL -o

Lu

CDW0/

0 10 20 30 40 50°ANGLE OF VIEW(0)

Fig. 16. Degree of polarization of the Shirako Beach sand in severaldegrees of moisture content.

point will give a value peculiar to each object with regardto the moisture. It has been noted from this experimentthat the field capacity of SiC of small particle size issmaller than that of large particles.

Next we made experiments using natural soil. Thesoil was collected from the Shirako Beach on the Ku-jukuri Coast and from the Katsuura Coast on the PacificCoast of the Japan Islands. The results of the mea-surements are shown in Figs. 16-19. The maximumwater holding capacity of the sand of the Shirako Beachwas 18% and that of the sand of the Katsuura Beach was15%. In these experiments, when water was added tothe sample, the maximum water holding capacity of thesample was determined as a state where the sample can

Fig. 15. Degree of polarization vs percent moisture for SiC 1000.

1.0

Sand of the ShirakoQ BeachZ G==60I- 0=50°z ,~~~~~~~~~dN

<0.5 E=40d

Go .0=30

Lu _

0u 10 20

0

PERCENT MOI STUREFig. 17. Degree of polarization vs percent moisture for the Shirako

Beach sand.

hold no more water, i.e., a state just before the water inthe sample begins to drop from the bottom of thetray.

It is shown in Figs. 17 and 19 that the field capacitypoint of the sand of the Shirako Beach is about 12% andthat of the sand of the Katsuura Beach about 10%. Theparticle size of the sand of the Katsuura Beach and thatof the Shirako Beach is about 454 jim and 366 jim indiameter on an average, respectively. The former wasin greater variety than the latter. Since, in actuality,the natural sand is composed of various sizes of particlesand various kinds of components, it will have suchcurves of the degree of polarization as are shown in Fig.18.


AL

z0-

N

_JO.5a-IL0

IxICD-LuC

30

l o

IV. Conclusion

A polarimeter, an optical source, their orbital guides,and a dc voltage regulated power supply were set up asa simulator for remote sensing. The degree of polar-ization of the polarimeter is P = 0.035. This polarim-eter performs one measurement in 9 sec. As it is pos-sible to use this simulator in various application areasof remote sensing, it would be very useful for confirmingproperties and meanings of information obtained byremote sensing and forecasting certain phenomena.

1.0

0

Nix

10.5

IL0

LuLL

0cr

Sand of the Katsuura Beach

8= = 60= '

The degrees of polarization of the light scattered fromSiC, MgO, and beach sand have been measured, and themoisture contained in them has been estimated. It hasbeen noted that the degree of polarization increases withparticle size in the experimental region of 0 = 10-60°.In this experiment the degree of polarization has in-creased as the moisture becomes higher. The existenceof the field capacity point peculiar to each sample hasbeen recognized. It has been demonstrated that soilmoisture is sensitively detected by measuring the degreeof polarization of the light scattered from the soil.

When a polarimeter is used as a sensor for remotesensing, magnesium oxide (MgO) will be used as one ofthe materials for a reference object. As the natural soiland sand contain awide variety of components andparticle sizes, the estimation of the moisture using thedegree of polarization is a very difficult problem. Agreat accumulation of experimental data obtained bysimulation will be needed to cope with this problem.

20 30 40 50ANGLE OF VIEW ( )

60

Fig. 18. Degree of polarization of the Katsuura Beach sand in severaldegrees of moisture content.

1.0 Sand of the Katuura

n BeachZ - E.= 60° * =40'Z d=0 oE=500

-J 0.5 ~ ~ ~ A0=0

0~~~~~~~

IL0L u A

Lu

CD

(O) 10 20PERCENT MOISTURE

Fig. 19. Degree of polarization vs percent moisture for the KatsuuraBeach sand.

The authors wish to thank Josef Cihlar of the CanadaCentre for Remote Sensing and A. R. Mack of the De-partment of Agriculture Resarch Branch of the CanadaSoil Research Institute for useful suggestions for thisreport, and A. Futamura of our institute for his help inthis work.

References1. E. Schanda et al., Remote Sensing for Environmental Sciences

(Springer-Verlag, Berlin, 1976).2. H. Genda and H. Okayama, Appl. Opt. 16, 601 (1977).3. A. Strogryn, IEEE Trans. Antennas Propag. AP-IS, 278 (1967).4. A. Matsui, Oyo Butsuri 35, 55 (1966).5. M. Kerker and M. I. Hampton, J. Opt. Soc. Am. 43, 370 (1953).6. H. H. Blau, Jr., E. L. Gray, and G. M. B. Bouricius, Appl. Opt. 6,

1899 (1967).7. E. H. Stockhoff and R. T. Frost, in Proceedings of the Seventh

International Symposium on Remote Sensing of Environment(Univ. Michigan Press, Ann Arbor, May, 1971), Vol. 1.

8. E. H. Stockhoff and R. T. Frost, in Proceedings of the Ninth In-ternational Symposium on Remote Sensing of Environment(Univ. Michigan Press, Ann Arbor, Apr. 1974), Vol. 1.

1 March 1978 / Vol. 17, No. 5 / APPLIED OPTICS

O 10

simulator for remote sensing and its application to soil moisture measurements

Documents