differential reflectance of natural and man-made materials at co_2 laser wavelengths

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Differential reflectance of natural and man-made materials at CO 2 laser wavelengths M. S. Shumate, S. Lundqvist, U. Persson, and S. T. Eng The differential reflectance of several naturally occurring and man-made materials at CO 2 laser wavelengths is determined. The computer-controlled measurement system has two CO 2 lasers and determines the dif- ferential reflectance of each material by measuring the ratio of the reflected signals at two wavelengths si- multaneously. These results can be used to improve the accuracy of air pollution measurement systems which derive their return signals from topographic targets. I. Introduction Over the past several years there has been an exten- sive effort to develop laser systems for the remote measurement of atmospheric trace constituents from aircraft and spacecraft altitudes. 1 - 3 These systems operate in a nadir-looking mode with the laser signals being scattered from the earth's surface. Typically two carbon dioxide lasers and two heterodyne receivers are all aimed at the same spot on the ground. Measure- ment of the differential transmission between the two different laser wavelengths is assumed to provide enough information to calculate the amount of the se- lected atmospheric species. When the laser beams strike the earth's surface, most of their energy is absorbed, and the rest is scattered back toward the sky. A very small fraction of the original energy actually returns to the detection system. The concentration of the atmospheric trace constituent of interest is determined from the differential attenuation of the two wavelengths due to molecular absorption. However, the different spectral reflectance of the vari- ous materials encountered along the flight path also causes a variation in the relative intensities of the re- ceived signals, thereby leading to uncertainty in the determination of the trace constituent concentra- tion. 1 ' 4 The authors are with Chalmers University of Technology, De- partment of Electrical Measurements, S-412 96 Gteborg, Sweden. Received 2 February 1982. 0003-6935/82/132386-04$01.00/0. ©1982 Optical Society of America. All minerals have characteristic molecular vibration bands which are exhibited in both reflection and emis- sion spectra. These features are referred to as rest- strahlen bands in reflection. Silicate rocks and minerals characteristically display spectral features in the 8- 12-,m wavelength region due to molecular vibration bands in the SiO 4 tetrahedron spectra. 5 In the 9-um spectral region, pure quartz has the strongest molecular vibration bands of any silicate or nonsilicate mineral. Using a low-resolution spectrometer, Logan et al. 6 showed that it is possible to separate silicate rocks into several different categories but that the emissivity is affected by particle size, packing, background temper- ature, and pressure. The classical concepts of radiometry and radiative transfer hold only in the limit of strict spatial incoher- ence. The degree of spatial coherence of a radiation source is crucial for determination of its radiant inten- sity. Baltes et al. 7 have studied the intensity distri- bution of radiation scattered by rough surfaces when illuminated with coherent light. Their results show a strong wavelength dependence of the reflectivity due to surface roughness for surface plane correlation lengths equal to or less than the wavelength of the laser radiation. However, for typical particle sizes encoun- tered on the earth's surface, the spectral emissivity at CO 2 laser wavelengths closely follows the behavior of the bulk material. Several groups of investigators have reported results of methods for passive remote detection of minerals from aircraft-borne instruments. 8 - 10 The Nimbus earth satellite IR interferometer-spectrometer has been used for the remote sensing of the surface emissivity at 9 m over the entire globe." 1 Features in the 8-12-um region were observed over arid and semiarid land areas, which revealed emissivity characteristics due to quartz. Weisemann et al. 12 reported detection of a quartz- containing clay using an active airborne laser system. 2386 APPLIED OPTICS / Vol. 21, No. 13 / 1 July 1982

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Differential reflectance of natural and man-made materials atCO2 laser wavelengths

M. S. Shumate, S. Lundqvist, U. Persson, and S. T. Eng

The differential reflectance of several naturally occurring and man-made materials at CO2laser wavelengthsis determined. The computer-controlled measurement system has two CO2 lasers and determines the dif-ferential reflectance of each material by measuring the ratio of the reflected signals at two wavelengths si-multaneously. These results can be used to improve the accuracy of air pollution measurement systemswhich derive their return signals from topographic targets.

I. Introduction

Over the past several years there has been an exten-sive effort to develop laser systems for the remotemeasurement of atmospheric trace constituents fromaircraft and spacecraft altitudes.1-3 These systemsoperate in a nadir-looking mode with the laser signalsbeing scattered from the earth's surface. Typically twocarbon dioxide lasers and two heterodyne receivers areall aimed at the same spot on the ground. Measure-ment of the differential transmission between the twodifferent laser wavelengths is assumed to provideenough information to calculate the amount of the se-lected atmospheric species.

When the laser beams strike the earth's surface, mostof their energy is absorbed, and the rest is scattered backtoward the sky. A very small fraction of the originalenergy actually returns to the detection system. Theconcentration of the atmospheric trace constituent ofinterest is determined from the differential attenuationof the two wavelengths due to molecular absorption.However, the different spectral reflectance of the vari-ous materials encountered along the flight path alsocauses a variation in the relative intensities of the re-ceived signals, thereby leading to uncertainty in thedetermination of the trace constituent concentra-tion.1 '4

The authors are with Chalmers University of Technology, De-partment of Electrical Measurements, S-412 96 Gteborg, Sweden.

Received 2 February 1982.0003-6935/82/132386-04$01.00/0.© 1982 Optical Society of America.

All minerals have characteristic molecular vibrationbands which are exhibited in both reflection and emis-sion spectra. These features are referred to as rest-strahlen bands in reflection. Silicate rocks and mineralscharacteristically display spectral features in the 8-12-,m wavelength region due to molecular vibrationbands in the SiO4 tetrahedron spectra.5 In the 9-umspectral region, pure quartz has the strongest molecularvibration bands of any silicate or nonsilicate mineral.Using a low-resolution spectrometer, Logan et al. 6

showed that it is possible to separate silicate rocks intoseveral different categories but that the emissivity isaffected by particle size, packing, background temper-ature, and pressure.

The classical concepts of radiometry and radiativetransfer hold only in the limit of strict spatial incoher-ence. The degree of spatial coherence of a radiationsource is crucial for determination of its radiant inten-sity. Baltes et al. 7 have studied the intensity distri-bution of radiation scattered by rough surfaces whenilluminated with coherent light. Their results show astrong wavelength dependence of the reflectivity dueto surface roughness for surface plane correlationlengths equal to or less than the wavelength of the laserradiation. However, for typical particle sizes encoun-tered on the earth's surface, the spectral emissivity atCO2 laser wavelengths closely follows the behavior ofthe bulk material.

Several groups of investigators have reported resultsof methods for passive remote detection of mineralsfrom aircraft-borne instruments.8-10 The Nimbusearth satellite IR interferometer-spectrometer has beenused for the remote sensing of the surface emissivity at9 m over the entire globe."1 Features in the 8-12-umregion were observed over arid and semiarid land areas,which revealed emissivity characteristics due to quartz.Weisemann et al.1 2 reported detection of a quartz-containing clay using an active airborne laser system.

2386 APPLIED OPTICS / Vol. 21, No. 13 / 1 July 1982

The purpose of the work presented in this paper wasto perform laboratory measurements of the differentialreflectance of materials of the type encountered duringaircraft flights with a CO2 laser system. The results ofthese measurements can be used to improve the accu-racy of airborne pollution measurements and to givesome indications of application in the measurement ofsurface geology. We have chosen to determine thedifferential reflectance of a rough surfaced sample bymeasuring the ratio of the reflected signals at twowavelengths simultaneously. The absolute value of thereflectance is, therefore, not measured. Our methoddiffers from the recent work of Boscher and Lehmann,13

who performed reflectance measurements on quartz andseveral other materials at single carbon dioxide laserwavelengths. We have utilized a laboratory laser sys-tem that had previously been used for measurement oftrace gases in the atmosphere. 1 4 The apparatus has twocarbon dioxide lasers with selectable wavelengths andwas modified to permit the beams to be directed at testsamples made from several different geologic, living, andman-made materials. The differential reflectance isreported for pairs of laser wavelengths in the P branchof the 9.4-grm band of carbon dioxide. In addition,some measurements are presented which cover both the9.4- and 10.4-gm bands.

II. Experimental

Our instrument was originally developed for operationover long horizontal paths to a cooperative retroreflec-tor14 and for the studies presented here was modifiedslightly to permit the transmitted beam to be aimed ata diffusely scattering test sample located nearby. Asimplified schematic of the apparatus is shown in Fig.1. The beams from two line-selectable CO2 lasers arecombined by the scanner driven mirror into a singlespatially filtered beam, which serves as the transmittedsignal after part of it is split away to the reference de-tector DT. The scanning mirror multiplexes the twolaser signals into a sequentially pulsed optical signal,which repeats every 16 msec. The transmitted signalis reflected by a large mirror so that it can strike asample target which is mounted on a motor-driventurntable. A portion of the scattered signal is collectedby the receiving optics onto the receiving detector DR.The received signal is then amplified by two separatelock-in amplifiers whose phase controls are set inquadrature to demultiplex the signals from the twoseparate lasers. The transmitted and received signalsare all sampled by analog-to-digital converters andstored by the computer.

The sample is placed in a glass Petri dish in the centerof a turntable which rotates continuously about a ver-tical axis at -21 rpm. The transmitted beam is -10mm in diameter and strikes the sample at near-normalincidence at a point -20 mm from the center of rotation.The motion of the sample surface past the transmittedbeam permits averaging of the coherent speckle effectsand signal fluctuations due to variations in the samplesurface structure. The received signal processing sys-tem averages the signals for 8 sec. To minimize scat-

D = L: XRORECEIVING OPTICS III

SAMPLE 1ITURNTABLE (

Fig. 1. Block diagram of the apparatus used for measurement ofdifferential reflectance. The HP 21 computer controls the entire

measurement process.

tering or coherence effects, the size distribution of themineral samples was analyzed. The California silicasample, for example, had <0.3% of particles of <75 min size, which is much greater than the wavelength of theradiation used.

The spectral response of the measuring system wasnot constant with wavelength due to variation in thesensitivity of the IR detectors used. It was, therefore,necessary to calibrate the system response against areflectance standard. The material chosen as a refer-ence for these measurements was sulfur flowers.Kronstein et al. 15 have performed extensive measure-ments upon sulfur in the IR and suggested that its re-flectance properties are repeatable enough so that it canserve as a reflectance standard. Over the range of thespectrum where our measurements were made, the re-flectance of sulfur flowers shows little variation withwavelength.

The measurement procedure used took advantage ofthe computer to control much of the process. After thesystem was turned on and allowed an adequate warm-up, the optical alignment was checked, both lasers wereadjusted to the same wavelength, and measurement ofthe reflectance ratio was performed. Variations of thisratio from unity served as an indication of misalignmentof the system, whose correct alignment was repeatedlychecked. The reflectance ratio at the selected wave-lengths was then measured repeatedly in a computer-controlled measurement sequence. The data werestored for later processing. Each data point is the av-eraged result of 500 measurement samples, the mea-surement sequence being repeated several times tocheck repeatability.

III. Results

Most of the measurements of differential reflectancewere made in the 9.4-gtm band of C0 2, especially at thelaser wavelengths of interest in the measurement ofozone in the troposphere.' One CO2 laser line P(22) (X= 9.569 gim) was used as a reference against which the

1 July 1982 / Vol. 21, No. 13 / APPLIED OPTICS 2387

Table . Laser Measurement System Response to Sulfur Flowers in the9.4-pm Band of CO2

Reference wavelength = P(22) line of the 9.4-pim band of CO 2(9.569 pm)

Laser Wavelength Differential Standardline (pm) reflectance deviation

P(14) 9.504 1.144 0.027P(16) 9.520 1.102 0.019P(18) 9.536 1.070 0.021P(20) 9.552 1.040 0.022

differential reflectance at the other wavelengths wasmeasured.

Table I presents results of measurements performedon the sulfur flowers reflectance standard. Three dif-ferent samples were prepared, and a total of 75 mea-surement sequences was performed. Results show theaveraged values, since no differences among the threesamples could be discerned. The standard deviationof the measurements is also presented in Table I. Thedeviations of the measured differential reflectances arecaused by slight variations in the control system's abilityto reset the CO2 lasers each time a measurement se-quence was repeated. The change as a function ofwavelength in the ratio of reflectance of sulfur flowersis a measure of the spectral response characteristic ofthe system. This response characteristic is used tonormalize the data taken on other test samples.

Table II presents results of measurements performedin the P branch of the 9.4-gum band of CO2. These re-sults are placed into three groups: natural geologicmaterials; man-made materials; and living plant ma-terials. The identification of many of these materialsis not too specific to save space in the table. The originof each material is not so important as is the fact thatdifferences in reflectance do exist. Measurements ofsilica sand and asphalt were performed over the P andR branches of both the 9.4- and 10.4-gtm bands of CO2and are presented in Fig. 2. The repeatability of themeasurements can be expressed by the standard de-viation of the measurements, which is 0.04 for the worstcases.

Two sources of variation which we observed arechanges in the sample reflectance with position (e.g., aflat surface such as a crystal face which produces aglint-type reflection) and laser output fluctuationscaused by slight grating misalignments. A minor sourceof variation is that some of the samples were not ho-mogeneous materials and variations could occur frommeasurement to measurement due to the laser beamstriking different spots on the sample. This effect wasminimized by moving the sample and using long aver-aging times.

The effect of the surface roughness of the sample onthe measurements was minimized by using materialswhose particle size distribution was known. Some ofthe samples were quite coarse, with large lumps, andothers were fine grained. However, the smallest grainswere several times larger than the wavelength used inthese measurements. We have not made a detailedstudy of all aspects of scattering from rough surfaces

Table 11. Differential Reflectance Measurements for Various Materials atCO2 Laser Wavelengths a

Reference CO 2 laser line-P(22) (X = 9.569 pm)

CO2 laser lineP(14) P(16) P(18) P(20)(9.504 (9.520 (9.536 (9.552

Material pm) pm) pm) pm)

Silica, Calif. 1.189 1.131 1.089 1.050Silica, Sweden 1.227 1.140 1.118 1.052Loam, Irwindale 1.014 1.005 1.017 1.012Loam, Altadena 1.041 1.027 1.013 1.013Loam, Santa Barbara 1.067 1.044 1.023 1.026Actinolite 1.089 1.038 1.016 1.028Kaolinite 1.185 1.127 1.076 1.034Granodiorite, Calif. 0.974 0.982 0.979 0.982Asphalt 1.010 1.001 0.993 1.002Concrete 1.020 0.981 1.027 1.009Aluminum, sandblasted 1.061 1.022 1.022 0.995Emery cloth, No. 80 A120 3 1.066 1.041 1.020 1.024Grass (unknown species) 1.013 1.001 1.021 1.004Pine (Pinus silvestris) 1.083 1.062 1.041 1.010Norway Spruce (Picea abies) 1.075 1.043 1.048 1.004Bamboo (Sinarundinaria 0.961 0.948 0.979 0.998

murielae)

a These numbers are adjusted for the spectral reflectance of sulfurflowers.'5 The standard deviation of these measurements does notexceed 0.04.

4. 0

43

CI

a

431

C'-

3. 0 +

2. 0 -1.0O +

.

9.0 10.0 11. 0

Wav 1 n9Lh (jjr.)

Fig. 2. Differential reflectivity of sand and asphalt in the 9.4- and10.4-pm bands of the CO2 laser. P(22) in the 9.4-pm band (X = 9.569

pm) is the reference wavelength.

and make no distinction in our results between thesurface scattering and reflection properties of bulkmaterials. With each sample, the surface was arrangedso that there was a minimum of intensity variation asthe sample surface moved past the incident laserbeam.

Results in Tables I and II are for rough surfaces withthe radiation impinging at near-normal incidence. Thedifferential reflectance may vary with angle of inci-dence. However, in the applications for which theseresults could be used the surfaces are usually flat andat low angles of repose, so we have not attempted tomeasure effects as a function of incidence angle.

Information on three materials, silica, kaolinite, andasphalt, has been previously published. Boscher and

2388 APPLIED OPTICS / Vol. 21, No. 13 / 1 July 1982

% U Sand

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Lehmann13 reported measurements on silica at carbondioxide laser wavelengths. Interpolation of their datayields a value of 1.28 for the reflectance ratio at P(14)/P(22). Our value for California silica is 1.19 and forSwedish silica 1.23. Kahle and Rowan9 presented somelow-resolution data on kaolinite, which indicate a de-creasing reflectance ratio with wavelength and agreewith our result. Results of measurements on asphaltby Suits16 show a very uniformly flat reflectance withwavelengths in the 9-11-gim spectral region, in agree-ment with our measurements.

Results from concrete and asphalt studies should beconsidered only semiquantitatively, since the samplesused do not necessarily represent typical road-con-struction materials. Furthermore, it is suspected thatthe nature of a road surface may be altered significantlyafter use due to buildup of hydrocarbon residues frommotor vehicles.

The living plant samples used were small quantitiesof leaves measured within 2 h of picking. To what ex-tent they have characteristics typical of leaves that arestill on the plant is unknown. There does, however,.seem to be a difference between the differential reflec-tance of most living plants and many of the naturalgeologic materials which we have measured. Note-worthy are the differential reflective properties of thepine material.

Our results can be used to provide a correction fordifferential reflectance when using an airborne CO2laser system to monitor ozone concentration.' Forexample, if a flight path covers areas of pine trees fol-lowed by a region of asphalt, concrete, and grass, therewould be a reflectance difference of -0.07 using theP(14) and P(22) laser lines. If not taken into account,the resulting error in ozone burden measurement wouldbe 31 ppb when measuring from an altitude of 1 km.Our results support the conclusions of Boscher et al. 13who have discussed those results of their extensive lab-oratory measurement program relevant to measurementerrors with airborne laser systems.

Measurements performed on sandblasted aluminumand emery cloth indicate that these materials have aspectral response which is wavelength dependent.These materials would, therefore, not be suitable for useas a reference surface.

IV. Conclusions

This paper reports measurements of the differentialreflectance of several natural and man-made materialsperformed at CO2 laser wavelengths. The instrumentused is described, and the need for a reflectance stan-dard to serve as a means of calibration is discussed.The results have direct application to the measurementof atmospheric ozone with CO2 laser differential ab-sorption systems.

The results support previous conjectures",4 thatchanges in differential reflectance are a source of ozonemeasurement error, which may be assessed using datareported here. The data also show interesting differ-ences among the natural materials which have beenmeasured, suggesting that it may be possible to develop

a remote sensing system which could be used to detectcertain geologic structures.1 4 "17 Further work must beperformed over a wider wavelength range to assess thispossibility.

The authors wish to thank Bo Olsson for his assis-tance with the computer system, Henrik van Ginhovenfor help with the electronics, and Inger Eriksson ofGbteborgs Botaniska Trddgard for providing manyplant specimens. Furthermore, we would like to ac-knowledge support and technical assistance from R. T.Menzies and W. B. Grant at the Jet Propulsion Labo-ratory (JPL), California Institute of Technology. Weappreciate the effort of C. R. Webster for help withediting the manuscript. One of the authors, M. S.Shumate, was on sabbatical leave from JPL when thisresearch was performed.

References1. M. S. Shumate, R. T. Menzies, W. B. Grant, and D. S. McDougal,

Appl. Opt. 20, 545 (1981).2. M. S. Shumate and R. T. Menzies, "The Airborne Laser Ab-

sorption Spectrometer: A New Instrument for Remote Mea-surement of Atmospheric Trace Gases," in Proceedings, FourthJoint Conference on Sensing of Environmental Pollutants(American Chemical Society, Washington, D.C., 1978), p. 420.

3. W. Wiesemann, "Weiterentwicklung des DIALEX-Gerates zumfunktionstilchtigen Ingenieurmodell und Flugerprobung imDFVLR-Fluzeug Typ DO 28," Report BF-R-63.545-1 (BattelleInstitute e.V., Frankfurt, W. Germany, 1979).

4. J. Boscher, W. Englisch, and W. Wiesemann, in Digest of TopicalMeeting on Coherent Laser Radar for Atmospheric Sensing(Optical Society of America, Washington, D.C., 1980), paperThC2.

5. R. J. P. Lyon, "Evaluation of Infrared Spectrophotometry forCompositional Analysis of Lunar and Planetary Soils," NASARep. NASA Tech. Note 1871 (1963).

6. L. M. Logan, G. R. Hunt, J. W. Salisbury, and S. R. Balsamo, J.Geophys. Res. 78, 4983 (1973).

7. H. P. Baltes, B. Steinle, E. Jakeman, and B. Hoenders, InfraredPhys. 19, 461 (1979).

8. R. J. P. Lyon and J. W. Patterson, "Airborne Geological MappingUsing Infrared Emission Spectra," in Proceedings, Sixth Sym-posium on Remote Sensing of the Environment (U. Michigan,Ann Arbor, 1969), Vol. 1, p. 527.

9. A. B. Kahle and L. C. Rowan, Geology 8, 234 (1980).10. A. B. Kahle, D. P. Madura, and J. M. Soha, Appl. Opt. 19, 2279

(1980).11. C. Prabhakara and G. Dalu, J. Geophys. Res. 81, 3719 (1976).12. W. Wiesemann, R. Beck, W. English, and K. Gurs, Appl. Phys.

15, 257 (1978).13. J. Boscher and F. Lehmann, "Experimentelle Untersuchungen

der physikalischen Grundlagen zur Fernmessung von Boden- undVegetationsfeuchte durch aktive Infrarot-Reflextionsspektros-kopie mit Hilfe der CO2 Lasertechnik," Report BF-R-64.028-2(Battelle Institut e.V., Frankfurt, W. Germany, 1980).

14. B. Marthinsson, J. Johansson, and S. T. Eng, Opt. QuantumElectron. 12, 327 (1980).

15. M. Kronstein, R. J. Kraushaar, and R. E. Deacle, J. Opt. Soc. Am.53, 458 (1963).

16. G. H. Suits, "Natural Sources," in The Infrared Handbook, W.L. Wolfe and G. J. Zissis, Eds. (Office of Naval Research, U. S.Department of the Navy, Washington, D.C., 1978), Chap. 3, pp.3-100.

17. A. F. H. Goetz and L. C. Rowan, Science 211, 781 (1981).

1 July 1982 / Vol. 21, No. 13 APPLIED OPTICS 2389