acquisition of bidirectional reflectance factor data with field goniometers

Acquisition of Bidirectional Reflectance FactorData with Field Goniometers

Stefan R. Sandmeier*

With the launch of new spaceborne sensors capable of portant as a tool to (1) validate currently available bidirec-tional reflectance distribution function (BRDF) models;acquiring bidirectional reflectance distribution function

(BRDF) data of the earth surface, extensive ground (2) support the development of new, more accurateBRDF models; (3) investigate the physical mechanismsBRDF measurements will become necessary to provide

much-needed ground reference data for numerous remote of BRDF effects; (4) study the relationships betweenbiogeophysical parameters and BRDF effects; (5) cali-sensing applications. To obtain homologous BRDF data

from field goniometer devices, guidelines and recommen- brate large and small reflectance-reference panels; and(6) to validate satellite-inferred measurements of BRDF.dations are required that will allow a better comparison

between BRDF data from different goniometer systems, The BRDF is a theoretical concept that describes direc-tional reflectance phenomena by relating the incidentfrom air- and spaceborne sensors, as well as from theo-irradiance from one given direction to its contributionretical studies with physically based BRDF models. Into the reflected radiance in another specific directionthis study, two examples of recent goniometer systems are(Nicodemus et al., 1977).given, and the quality and limits of field goniometer sys-

Field goniometric radiometer devices, herein calledtems are discussed, along with detailed practical advice“field goniometers,” have been used for a number offor the acquisition, processing, and documentation ofyears to assess the BRDF of natural and man-made sur-BRDF data and ancillary information. Special emphasisfaces under natural illumination conditions (Shibayamais on the characterization and documentation of atmo-et al., 1986; Jackson et al., 1990; Kuusk, 1991; Ranson etspheric and measurement surface conditions. The studyal., 1991; Walter-Shea et al., 1993; Hosgood et al., 1999).also presents common shortcomings and limitations of fieldRecently, two field goniometer systems have becomeBRDF data, such as the influence of diffuse irradiance andavailable to the BRDF science community: the Swisschanges in illumination conditions, and gives recommenda-Field Goniometer System (FIGOS), designed and builttions for correcting these effects from an application-ori-by Willy Sandmeier at Fa. Lehner & Co. AG, Granichen,ented viewpoint. Elsevier Science Inc., 2000Switzerland, in joint operation with the Remote SensingLaboratories (RSL) of the Department of Geography atthe University of Zurich, Switzerland (Sandmeier and It-INTRODUCTIONten, 1999) and the Sandmeier Field Goniometer (SFG),

With the availability of multiangular remote sensing data constructed by the Systems Engineering Division atfrom spaceborne sensors, such as MISR, MODIS, and NASA Ames Research Center, Moffett Field, CA, underPOLDER (all to become available in year 2000), ground commission of the Commercial Remote Sensing Officebidirectional reflectance data will become increasingly im- at Stennis Space Center in South Mississippi (Turner,

1998). Both goniometers consist of three major parts: azenith arc and an azimuth rail, each of 2-m radius, and

* USRA/NASA Goddard Space Flight Center, Biospheric Sciences a sled where the sensor is mounted. In both goniometersBranch, Greenbelt, MDthe sensor is pointing to the same central target areaAddress correspondence to S. R. Sandmeier, USRA/NASA God-

dard Space Flight Center, Biospheric Sciences Branch, Code 923, within a 2-m radius hemisphere for all viewing positions.Greenbelt, MD 20771, USA. E-mail: [email protected] While the FIGOS is semiautomated, the SFG is fully

The use of company and brand names implies no approval of the controlled by a laptop computer, which allows full con-product and does not exclude others that may also be suitable.Received 6 August 1999; revised 18 January 2000. trol of positioning the spectroradiometer at any position

REMOTE SENS. ENVIRON. 73:257–269 (2000)Elsevier Science Inc., 2000 0034-4257/00/$–see front matter655 Avenue of the Americas, New York, NY 10010 PII S0034-4257(00)00102-4

258 Sandmeier

Figure 1. (A) The Swiss Field Goniometer System (FIGOS) in operation in a nonirrigated barley field during anaircraft campaign in Spain. (B) The Sandmeier Field Goniometer (SFG) during final testing at Stennis Space Cen-ter, Mississippi.

between 08 and 758 view zenith angle (VZA) and 08 and tem, detailed recommendations are given for the acquisi-tion, processing, and documentation of field BRDF data.3608 view azimuth angle (VAA). With the SFG a hemi-

spherical BRDF data set is acquired in less than 10 min- Much of the advice can be applied to BRDF instrumentswith rotating heads and also to nadir-looking radiometerutes with a resolution of 158 and 308 in zenith and azi-

muth directions, respectively. FIGOS and SFG are devices, but sections on system calibration and part ofthe diffuse irradiance correction methods are applicablecurrently operated with a PC-controlled GER-3700 spec-

troradiometer that covers the spectrum between nominal to field goniometers only.300 nm and 2,500 nm in 704 bands with a resolution of1.5 nm (300–1050 nm), 6.2 nm (1050–1840 nm), and 8.6 QUALITY ASSURANCE AND LIMITS OFnm (1950–2500 nm), respectively (GER, 1997). Figure 1 FIELD GONIOMETER DATAgives an overview of both goniometers.

Field goniometers are a special case of ground-based Geometrical Accuracy of a GoniometerBRDF instruments that allow one to focus on the same The mechanical accuracy of a BRDF measurement sys-target area throughout a measurement series. They facili- tem is an important quality feature because it defines thetate rigorous control of the experiment design and of an- repeatability and stability of the measurement geometry.gular and instrumental factors. Typical surfaces to be as- It can be assessed with the help of almost any commer-sessed are short-growing agricultural surfaces, grasslands, cially available laser pointer that can be mounted on thesoils, rock, and man-made surfaces. Another category of sensor sled in such a way that the laser points to theinstruments used to acquire BRDF data is based on a goniometer center at zenith position. The intersectionrotating head that points a radiometer device in any de- of the laser ray with the goniometer origin plane indi-sirable direction from a fixed pivot point. Examples are cates the projected center of the sensor field of view.the PARABOLA instrument (Deering and Leone, 1986; Ideally, the center of the sensor’s field of view alwaysDeering, 1989) and the German Aerospace’s rotating points to the center of the goniometer hemisphere forCCD-line camera device (Radke et al., 1999). Contrary all measurement positions. Thus, the trace of the laserto goniometer devices, these instruments are suitable for spot resulting from moving the laser along the zenith arcacquiring BRDF data from large-structured natural sur- implies the mechanical quality of the zenith arc. Simi-faces such as patchy landscapes or woodlands. However, larly, the geometrical accuracy of the azimuth arc can bethey assume homogeneous target reflectance characteris- evaluated with a laser by rotating the zenith arc alongtics within the sampling area of the instrument and show the azimuth direction while monitoring the laser trace.different measurement distances for different angular Scatter plot diagrams showing the coordinates of a laserpositions as the sensor rotates around its pivot point. spot on the goniometer origin plane for systematic angu-

Regardless of the measurement system, BRDF field lar positions allow qualitative and quantitative analysis ofcampaigns have to be carefully planned and performed the goniometer’s mechanical accuracy. Examples on howunder well-known measurement conditions to achieve the laser testing has been applied are given in Solheimthe full benefit of ground BRDF data. In this study, ba- et al. (1996) for the European Goniometric Facilitysic principles are developed for the acquisition of BRDF (EGO) at the Joint Research Center in Ispra, Italy anddata with field goniometers. After a brief overview of the in Sandmeier and Itten (1999) for FIGOS. Results from

testing FIGOS showed that the roundness of the zenithmechanical accuracy and limits of a field goniometer sys-

Data Acquisition with Field Goniometers 259

and azimuth arcs of the goniometer is nearly perfect. 108 and 708 VZA, respectively (source zenith angle508,k5450 nm).The spot of a laser moving between 2608 and 1608 on

the zenith arc showed deviation from the goniometer Still, most panel manufacturers specify the spectralreflectance characteristics for a single illumination-view-center of less than 61 cm. On the azimuth arc, a spot

from a laser pointing vertically from the center of the ing geometry only and assume Lambertian reflectancecharacteristics for the reference material. Since the re-zenith arc moved less than 61 cm on the ground when

the zenith arc was rotated. flectance in field and laboratory campaigns in many casesis determined by reference to a reflectance-referenceThe positioning accuracy of the goniometer can be

tested by repeatedly positioning the sensor sled at a spe- panel, the BRDF characteristics (Rref) of panels have tobe carefully determined by goniometric reflectance mea-cific zenith and azimuth position and comparing the

position with a specified range of angular deviations. surements before they can be used as an adequate re-flectance reference. Rref is then used as a correction fac-For FIGOS, the positioning precision on the zenith arc

is 60.28. tor for the nonideal panel reflectance properties in theTests of the geometrical accuracy should be per- calculation of surface reflectance (Jackson et al., 1987).

formed with the goniometer inclined at various angles to A useful discussion of this aspect is given in Kimes andinvestigate the influence of stress on the goniometer Kirchner (1982) and later in this work.structure. Both positioning accuracy and the mechanical Another possibility is to calculate reflectances by col-quality of the goniometer should be reassessed over lecting incident irradiance with a hemispherical cosine-time, preferably before and after major field campaigns. corrected lens rather than determine reflectance factors

using a reflectance-reference panel. However, reflectanceSpectroradiometer Calibration factor measurements generally achieve a higher precision

(repeatability) than reflectance measurements since theyRadiometric calibration is the process of converting rawallow one to use the same radiometer with unchanged lensdigital numbers (DN) observed by a sensor into physicalconfiguration for irradiance and target measurements,units. For percentage reflectance measurements a rela-leaving the beam-defining apertures for both source andtive calibration is sufficient as long as the irradiance con-detector beams undisturbed (Nicodemus et al., 1977).ditions during data acquisition are well known. Relative

calibration includes the measurement of signal-to-noise,Sensor Field of View and Sensor Shadownoise equivalent signal, dark current, band center wave-

length, spectral response function, nonlinearity, direc- The term “bidirectional” in BRDF implies that direc-tional and positional effects, spectral scattering, size and tional measurements should be taken in infinitesimallyshape of field of view, polarization, size-of-source effect, small solid angles and that illumination from a single di-and the temperature and time dependence (repeatabil- rection would be required. A directional field of view,ity) of the radiometric device (Kostkowski, 1997; Schaep- however, technically is not feasible and would cause dif-man, 1998). ficulties in obtaining representative BRDF data sets.

For radiance measurements in absolute units Most radiometers exhibit field of views between 18 and(W·m2·sr21), an absolute calibration is required to relate 158. Therefore, measured reflectance factors are some-the instrument’s response to a radiometric standard using times referred to as “biconical” reflectance factors ratheran integrating sphere that is calibrated with NIST stan- than “bidirectional” reflectance factors. If the sensor fielddards. This step involves modeling the characteristics of of view is sufficiently small (e.g., 28) and field measure-the optics and electronics of the sensor in addition to the ments are taken under clear sky conditions with predom-procedures performed for relative calibration. Calibra- inantly direct radiation, the measured values closely ap-tion procedures should be repeated on a regular sched- proximate bidirectional reflectance factors (Robinson andule or before and after major measurement campaigns. Biehl, 1979).

To allow data collection without shadowing effects inBRDF Characteristics of a the solar principal plane, the zenith arc of a goniometerReflectance-Reference Panel is mounted eccentrically on the azimuth rail. Still, due to

practical problems, measurements in the hot spot direc-Many laboratory and field studies have demonstratedthat reflectance-reference panel materials such as BaSO4, tion are hampered by the sensor shadow. To reduce the

impact, it is recommended to place the sensor case andhalon, or Spectralont, a polytetrafluorethylene-basedmaterial (Labsphere Inc., North Sutton, NH, USA), are any obstructing construction elements out of the measure-

ment position and to keep only the lens entry point offar from perfect isotropic scatterers (e.g., Kimes andKirchner, 1982; Jackson et al., 1992; Sandmeier et al., the spectroradiometer in the measurement direction.

Figure 1B shows an example of a custom foreoptic right-1998a). Sandmeier et al. (1998a), for instance, reporteddeviations from a perfect Lambertian (i.e., isotropic) sur- angle lens connected to the spectroradiometer used for

the SFG that effectively reduces the shadowing effects.face of 6% and 8% for a Spectralon panel observed at

260 Sandmeier

Table 1. Advantages and Disadvantages of Field Measurements Compared to Laboratory Measurement Conditions

Advantages Disadvantages

Replicate measurements from various parts of target surface are Wind and other environmental parameters affect measurements.possible.

Irradiance is spatially homogeneous and quasicollimated. Moving sun constantly changes illumination conditions.Source of illumination is identical for both ground and remotely Variable atmospheric conditions affect irradiance conditions during

sensed data. data acquisition.Excellent signal-to-noise ratio under clear sky conditions Varying amount of diffuse irradiance according to the atmo-

spheric aerosol optical thickness and solar zenith angle.Targets can be measured in their natural habitat and are not Constantly moving sun forces the acquisition time to be kept as short

exposed to artificial environmental conditions. as possible and compromises the angular sampling resolution.

Another possible solution might be the use of fiber op- spectroradiometer, the sensor requirements for far-fieldtics to transmit the surface reflectance signal to a spec- conditions and small solid angles are sufficiently well ap-troradiometer that is placed outside the solar principal proximated, and the detector-averaging area is practicalplane, but the limitations of fiber attenuation and intra- for a majority of man-made materials as well as fine-modal dispersion have to be taken into account carefully. structure natural surfaces such as soils, grasslands, crops,

and other agricultural canopies.Advantages and Disadvantages of Field Ideally, BRDF data sets are acquired under constantBRDF Campaigns illumination conditions, cover the complete hemisphere

with maximum angular sampling resolution, are consis-Acquisition of BRDF data in the field has nearly comple-tent for replicate measurements of different areas of thementary advantages and drawbacks to laboratory experi-target surface, and show maximum data dynamics. Inments data. In general, laboratory data are less affectedpractice, the number of measurements taken for a spe-by environmental influences such as atmospheric condi-cific source zenith angle is limited in the field by thetions and wind than field measurements, and they allowmovement of the sun and speed of angular measure-the operator to accurately control the position of thements. Sensitivity analysis in the laboratory showed thatlight source. In contrast, during field campaigns, vegeta-the light source should be kept within 618 source zenithtion targets are measured in their natural habitat and un-angle during data acquisition to obtain homologousder natural illumination conditions. Table 1 lists the mostBRDF data (Sandmeier et al., 1998a). Depending on theimportant advantages and disadvantages of field experi-geographical latitude, day of year, and time of day, a 28ments. Recommendations on how to minimize the draw-change in the sun’s zenith angle position corresponds tobacks of field conditions are given in the Experimental

Design and BRDF Data Processing sections. a different time range. For example, at 458N on 21 Juneat 2 p.m. local time, the sun moves by 28 and 48 in zenithand azimuth directions, respectively, in approximatelyEXPERIMENT DESIGN13 minutes.

Sampling Design Laboratory analyses also suggested that an angularsampling interval of 158 and 308 in zenith and azimuthThe sensor field of view, angular sampling resolution,directions, respectively, is adequate to capture the gen-and the measurement distance must be carefully choseneral BRDF characteristics of most natural and man-madein accordance with the scale of the target structure. Gen-surfaces. But in the hot spot area, where illuminationerally, the sensor needs to be in the far field and exhibitand viewing positions coincide, a much higher samplingsmall solid angles. Far field is the region where the an-resolution is required to represent the rapid change ofgular field distribution is essentially independent of dis-reflectance intensities. The same is true for the speculartance from the source. If the source has a maximumpeak in the forward scattering direction of metallic oroverall dimension D that is large compared to the wave-glossy surfaces that are not specifically dealt with herein.length, the far-field region is commonly taken to exist at

This suggests the adoption of a nested sampling de-distances greater than 2D2/k from the source, with k be-sign (Fig. 2). First, data should be taken in the solaring the wavelength (National Communications Systems,principal plane (SPP) with a resolution of 158 view zenith1996). At the same time, the averaging area of the detec-angle (VZA). In the solar principal plane the sensor, thetor must be large enough to filter reflectance singularit-measurement surface’s normal, and the sun are alignedies and to keep the number of samples required to coverin one plane. In this constellation BRDF effects are mostthe hemisphere manageably small. The current configu-pronounced. Then the hot spot area should be reevalu-ration used for FIGOS and SFG proved to be suitableated with additional measurements of a higher angularfor data acquisition in the field. With a constant mea-

surement distance of 2 m and a 28 field of view of the resolution (e.g., 28) along the SPP. According to Qin and


Measurements with Obscured SunOften BRDF data are intended to be acquired under di-rectional irradiance conditions (see Atmospheric Condi-tions). In field experiments, however, atmospheric scatter-ing processes always introduce some diffuse irradiance.Even under clear sky and low aerosol optical thicknessconditions, diffuse irradiance is present and complicatesthe measured BRDF effect of the target surfaces. Thehigher the aerosol optical thickness of the atmosphereand the greater the solar zenith angle, the stronger theinfluence. One way to assess and correct the impact ofdiffuse irradiance is to acquire two hemispherical datasets from the same target, one obtained under total solarirradiance and one with the sun occluded with an opaquecircular disk placed in front of the sun (Epema, 1991).The occulting disk needs to be large enough to shade

Figure 2. Recommended sampling design for fieldthe complete sampling area of the sensor. In addition togoniometers. Each dot represents a measurementtarget surface measurements, reflectance-reference mea-position. The box and the sun symbol represent ad-

ditional measurements taken in the hot spot area. surements are taken for both data series. The two dataFor metallic or glossy surfaces, additional measure- sets should be acquired quasisimultaneously to keep illu-ments were needed in the forward scattering direc- mination and atmospheric conditions as constant as pos-tion to capture the specular peak (not shown).

sible. Then, the impact of diffuse irradiance on BRDFfield data acquisition can be eliminated to a large degree

Goel (1995), an angular range of 6108 centered to the by taking the difference between the two data series (seehot spot is sufficient to capture the hot spot zenithal the Correction of Diffuse Irradiance Effects section).width of most vegetation canopies. If desirable, addi- Obscuring the sun with an occulting disk allows onetional measurements in the hot spot area can be taken to separate the direct irradiance from total irradiance. Atin two additional azimuth profiles with 7.58 angular dis- the same time, the diffuse irradiance is greatly reducedtance from the SPP to improve the characterization of too, since its distribution peaks in the solar direction. Tothe hot spot shape. Then, all other azimuth planes are limit the reduction of diffuse irradiance, the occultingconsecutively measured with a resolution of 158 and 308 disk should be positioned as far away from the target sur-in zenith and azimuth directions, respectively. Reference face as possible. For the configuration of FIGOS andpanel measurement should be taken from nadir position

SFG, a distance of 3 m between the occulting disk andon a regular basis (e.g., in the middle of each azimuththe target surface is feasible. To shade the completeseries of goniometric measurements). Another approachsampling area of the GER-3700 spectroradiometer foris to use a second intercalibrated spectroradiometer thatmeasurement positions of up to 758 VZA, an occultingcontinuously takes reflectance-reference measurementsdisk of 0.25-m radius is sufficient (Sandmeier and Itten,during BRDF data acquisition.1999, Fig. 5). The total sky solid angle obstructed by the

Diurnal and Seasonal Effects occulting disk is 0.02 sr, which is equivalent to 0.4% ofBRDF effects vary greatly for different solar zenith angles. the sky hemisphere. To compensate for the portion ofTo capture diurnal BRDF effects, measurements should the sky blocked by the occulting disk, Che et al. (1985)be taken continuously throughout the day at regular in- and Walter-Shea et al. (1993) suggested keeping thecrements of solar zenith angles (e.g., every 108). Prefera- shading of the disk next to the target surface during totalbly extreme solar zenith angles at sun culmination, soon (unshaded) irradiance measurements. However, Epemaafter dawn, and before sunset are included in the mea-

(1991) concluded from atmospheric modeling that thissurement series. Due to very rapid change of the suntechnique is unreliable, especially with the occulting diskposition and due to atmospheric refractive effects andat a distance of less than 3 m, due to the nonuniformvery high diffuse irradiance at sunrise and sunset, solardistribution of diffuse irradiance. It therefore is recom-zenith angles of 758 to 808 are suggested to be used asmended to follow an approach by Robinson and Biehlthe maximum solar zenith positions. Depending on the(1979), who assumed that the diffuse irradiance coveredsensitivity of the radiometer device, the maximun solarby the solar disk can be considered as a directional com-zenith angle for collecting reflectance factor data mightponent and therefore does not need to be compensated.be further limited. To monitor seasonal changes ofSubsequent data processing steps are outlined in theBRDF characteristics, multiple measurement campaigns

should be performed throughout the year. Correction of Diffuse Irradiance Effects section.

262 Sandmeier

Replicate Measurements tance factor measurements (e.g., in the middleof each azimuth series of goniometric measure-To ensure high data quality and to reduce the influence

of target surface heterogeneities, BRDF data are prefer- ments or continuously with the help of a secondintercalibrated spectroradiometer).ably taken at different areas of the target surface. Ideally,

replicate BRDF data remain constant. But even under The availability of spectral aerosol optical thicknessidentical illumination conditions, multiple BRDF data data allows us to quantify the amount of diffuse irradianceseries may vary due to natural variations of the target and to monitor the stability of the atmospheric conditions.surface. Therefore it is crucial that measurement plots Spectral aerosol optical thickness is a crucial parameterrepresent overall target surface characteristics. for correcting the influence of diffuse irradiance on field

Time constraints and practical limitations may not al- BRDF data (see the Correction of Diffuse Irradiance Ef-ways allow one to produce multiple BRDF data sets for fects section). Monitoring irradiance fluctuations alsoa given time and day. In some cases, it might be possible helps to account for changing illumination conditions be-to move the goniometer by a short distance and produce tween two reflectance-reference panel measurementsreplicate measurement series under almost identical so- (Epema, 1991).lar zenith angles. One should also consider taking mea-surements for multiple plots under identical solar zenith Environmental Conditionsangles before and after solar culmination and on consec-

Influences of the natural environment are mainly due toutive days.wind, which may alter the structure and orientation ofMan-made and in particular vegetated surfacesvegetation surfaces such as tall grasslands and crop fieldsshould be measured periodically throughout the year to(Lord et al., 1985). Thus, it would be ideal to monitorcapture target alterations due to aging and environmen-wind conditions so that BRDF data can be acquired dur-tal influences.ing calm or steady wind conditions. Air temperature andrelative humidity are additional parameters that might al-Atmospheric Conditionster the target BRDF and therefore should be monitored

The influence of atmospheric conditions must be consid- as well.ered for accurate ground BRDF data interpretation A major impact on BRDF data acquisition can be(Deering and Eck, 1987). Ideally, field measurements introduced when the measurement team or surroundingare taken under stable (low or high) aerosol conditions objects either block portions of the sky or reflect addi-on a cloudless (clear sky) day. Even though low aerosol tional radiation to the measurement target (Kimes et al.,conditions are normally preferred for field measure- 1983). To minimize these effects, BRDF data should bements, BRDF data acquired under high aerosol loading acquired in the open field, and transport and equipmentcan prove to be very useful in validating BRDF models, materials that are not immediately used for the measure-which can simulate the effect of varying diffuse fraction ments should be kept far away from the measurementon surface anisotropy. In practice, the condition of the surface. In addition, operators should preferably wearatmosphere is constantly changing. To reduce the effect dark radiation-absorbing clothing and keep the targetof change in illumination conditions on field BRDF data, surface undisturbed (Milton, 1987).the following recommendations should be taken intoaccount.

EXPERIMENT DOCUMENTATION• The fractional sky coverage of visible clouds

To allow accurate interpretation of ground BRDF data,must be minimal and continuously monitoreda field experiment has to be sufficiently documented. Asalong with the cloud type (e.g., cumulus, cirrus,a minimum, the following parameters should be reportedetc.).and documented for each measurement experiment:• No measurements should be taken under unsta-

ble hazy conditions or in the presence of cirrus • type of goniometer deployed and its serialclouds (Deering and Eck, 1987). number

• Spectral aerosol optical thickness (and precipita- • type of spectroradiometer sensor used and its se-ble water) must be monitored during BRDF rial numberdata acquisition, preferably over the same wave- • calibration parameters of spectroradiometer (e.g.,length range as the BRDF measurements [e.g., spectral response functions and date of cali-with sun photometers such as Cimel (Holben et bration)al., 1998) and Reagan instruments (Ehsani et al., • calibration coefficients of reflectance-reference1998), or with shadow-band radiometers (Har- panel (deviation from ideal loss-less Lambertianrison et al., 1994). reflector and date of calibration)

• Reflectance-reference measurements must be • viewing geometry (e.g., measurement distance,sensor field of view)taken frequently in the course of surface reflec-


• sampling resolution (angular increments in zenith of leaf angles (Lang, 1973; Welles and Norman, 1991).Optical properties of leaves and other plant tissuesand azimuth viewing angles, additional measure-

ments in the hot spot direction) should be measured in vivo using an integrating sphere(Walter-Shea et al., 1991). Multiple samples should be• slope and azimuth of target surface plane

• date of data acquisition taken for each parameter to get representative estimates.Samples must be taken from an adjacent area to keep• target surface location (e.g., geographical coordi-the BRDF measurement surface intact.nates of the test site, nearby town)

For sparse canopies it is crucial to obtain reflectance• altitude of target surfaceproperties of the substrate layer (soil and/or litter layer)• type of target surface (e.g., species composition,by acquiring BRDF data from an adjacent plot with thesoil type, leaf litter presence, surface material)canopy layer removed (Walter-Shea et al., 1992; Sand-• target surface characteristics (see next section)meier et al., 1999). Soil moisture data might be helpful• atmospheric conditions during data acquisitionto understand variations in BRDF.(see the previous Atmospheric Conditions

section)Photographic Documentation• azimuth position of solar principal plane at start

and end of each hemispherical series Photographs or video shots taken from the goniometer’s• time point of each measurement in UT (if time sensor platform from various view angle positions over

is not automatically recorded with each spectro- the harvested and the original measurement plots are ex-radiometer measurement, at least the start and tremely helpful for interpreting and documenting BRDFend time of each hemispherical series must be data. These images are preferably taken in the solar prin-recorded) cipal plane and also in the plane perpendicular. Figure

3 shows an example of images taken in the principalTarget Surface Characteristics plane over a perennial ryegrass surface.

Overview pictures about the test site document envi-Accurate characterization of the target surface structureronmental conditions and help in understanding their po-with ancillary information is essential for two reasons. Ittential influence on the BRDF data. Close-up photographsimproves the interpretation of surface BRDF data andfrom the target surface provide insight in the canopy ar-supports the verification of physiologically based BRDFchitecture and the ground cover of vegetation surfacesmodels. For nonvegetated surfaces such as bare soils,and are especially useful when detailed measurementssnow surfaces, and man-made objects, surface texture,about the surface architecture are not possible (Sand-structure, and roughness should be characterized. Grainmeier et al., 1999).size of particles, moisture content, temperature, and per-

centage of material components are additional parame-ters to be considered. BRDF DATA PROCESSING

For vegetation canopies, a variety of parametersData Preprocessingshould be assessed:Spectroradiometer data are recorded in digital numbers• species composition and phenological conditions(DN). In order to transform DN to a physically meaning-• leaf area index (LAI)ful unit, they are converted to radiance (L) in W·m22·sr21

• leaf angle distribution (LAD)using band specific calibration coefficients c0 and c1 [see• canopy height distributionEq. (1)]:• leaf length and shape

• optical properties of each plant tissue (e.g., leaf L5c01DN·c1 (1)transmittance and reflectance)

Coefficients c0 and c1 are defined by absolute calibration• optical properties of substrate layer (soil and leafof the spectroradiometer using NIST standards (see thelitter)Spectroradiometer Calibration section).

For deriving bidirectional reflectance factors R andLAI can be obtained from destructive (direct) sam-pling or from optical (indirect) measurements [e.g., with BRDF, absolute calibration of a spectroradiometer is not

required. As long as the same sensor or two intercali-a Li-Cor LAI-2000 Plant Canopy Analyzer (PCA)](Welles and Norman, 1991; Welles and Cohen, 1996). brated spectroradiometers are used for the target and the

reflectance-reference measurements, R and BRDF valuesIndirect measurements with optical instruments shouldbe taken under diffuse irradiance conditions, either for a specific illumination-viewing geometry can be de-

rived from DN values of the target surface and a reflec-shortly before sunrise or after sunset or during overcastcloud conditions. Most PCA instruments also give an es- tance-reference surface with known BRDF characteristics,

assuming that the radiometric responses of the sensors aretimate for the mean leaf inclination angle. More detailedLAD information is obtained from direct measurements linear and stable between the two measurements.

264 Sandmeier

Figure 3. Bidirectional reflectance effect on a soccer grass lawn (perennial ryegrass; Lolium perenne L.), observed underdifferent viewing angles in the solar principal plane from a FIGOS mounted photographic camera. Solar zenith angle is358, indicated by dashed arrows. The view directions are shown as solid lines. The camera settings are k516, t51/15,f5135 mm, d52 m. For details on the data see Sandmeier and Itten (1999).

To reduce the impact of changes in atmospheric azimuth angles of the measurement direction, respec-tively. k is wavelength. Procedures to determine Rdir

ref areconditions and sun position on bidirectional reflectancefactor data, R and BRDF should be derived from time- given in Jackson et al. (1987), Walther-Shea et al. (1993),

and Sandmeier et al. (1998a), along with examples forspecific reflectance-reference measurements (compare toSampling Design section). If irradiance fluctuations be- common reflectance-reference materials.

The diffuse sky radiance dLdif is hard to obtain fromtween the reflectance-reference measurements need tobe compensated, sun photometer or shadow-band radi- panel measurements since the correction of the nonideal

reference panel characteristics for diffuse irradiance isometer data may be used to interpolate between the in-complicated by the nonuniform angular distribution ofdividual reflectance-reference measurements, assumingnatural diffuse irradiance and thus depends on the ef-that clouds were not the source of the irradiance fluctua-fects of varying aerosol loading and solar zenith angletions since even clouds that do not obscure the sun can(Kimes and Kirchner, 1982). For simplicity, one may as-have significant impact on spectral irradiance (Robinsonsume that the total diffuse irradiance consists of a direc-and Biehl, 1979).tional component that is blocked by an occulting disk,and a uniformly distributed sky radiance componentCorrection of Diffuse Irradiance Effects(Robinson and Biehl, 1979). Equation (2) can then be

Measurement Approach rewritten as Eq. (3).The influence of diffuse irradiance on field BRDF datacan be corrected by consecutive acquisition of two Rdir(hi, ui; hr, uri; k)5

dLtotr (hi, ui; hr, ur; k)2dLdif

r (hi, ui; hr, ur; k)

dLtotref(hi, ui; hr, ur; k)2

dLdifref(2p; hr, ur; k)

qref(2p; hr, ur; k)BRDF data series. The first measurement series is ob-tained under total solar irradiance, while for the seconddata set the sun is obscured with an occulting disk. As- 3Rdir

ref(hi, ui; hr, ur; k) (3)suming that the atmospheric conditions and the sun posi-

where dLdifref is the radiance from the reflectance-refer-tion do not change significantly between the two data se-

ence panel in the measurement direction (e.g., nadir) un-ries, the bidirectional reflectance factors Rdir for directder uniform diffuse irradiance, and qref is the hemispheri-irradiance can be obtained by Eq. (2):cal-directional reflectance of the reference panel [seeEq. (4)].Rdir(hi, ui; hr, ur, k)5

dLtotr (hi, ui; hr, ur; k)2dLdif

r (hi, ui; hr, ur; k)dLtot

ref(hi, ui; hr, ur; k)2dLdif(hi, ui; hr, ur; k)

qref(2p; hr, ur; k)51p#2p

Rdirref(hi, ui; hr, ur; k)dVi (4)3Rdir

ref(hi, ui; hr, ur; k) (2)

where dLtotr and dLdif

r are radiance measurements fromApplying Eq. (3), the directional component of thethe target surface acquired under total and diffuse irradi-

diffuse irradiance is added to the directional irradianceance conditions, respectively. dLtotref is the radiance from

while the uniform diffuse sky radiance is eliminated. Athe reflectance-reference panel obtained under total irra-similar approach is described by Dickinson et al. (1990).diance conditions, dLdif is the diffuse radiance from the

sky, and Rrefdir is the reference panel correction factor for Modeling Approach

If the sky radiance (not blocked by an occulting disk) isdirect irradiance. hi and ui are zenith and azimuth anglesof the irradiance direction, and hr and ur are zenith and largely nonuniform, Eq. (3) becomes invalid, and a mod-


Figure 4. (A) Polar coordinate system used for presenting BRDF data in three-dimensional plots. (B)Example of three-dimensional plot showing bidirectional reflectance factor data of a grass lawn sur-face (Lolium perenne) acquired with FIGOS (see Fig. 3) at 675 nm under 358 source zenith angle(Sandmeier and Itten, 1999).

eling approach is required that represents the scattering made at a single solar zenith angle. Of course thismethod assumes that the BRDF model correctly param-processes in the atmosphere under different types of dif-

fuse irradiance and as a function of solar zenith angle. eterizes the combined effects of canopy shadowing,transmittance, spectral optical properties of multiple can-Also the correction of diffuse irradiance effects with the

measurement approach is not always feasible in a field opy elements, etc. in allowing for BRDF prediction atdiffering solar zenith angles and varying diffuse irradi-campaign due to time and efficiency constraints as well

as practical limitations. Lyapustin and Privette (1999) re- ance conditions. To confine the multitude of possible so-lutions, parametric BRDF models are selected from thecently developed a rigorous method for correcting dif-class of reciprocal and rotationally symmetric functions,fuse irradiance effects in BRDF data acquired in thesuch as the Rahman-Pinty-Verstraete model (Rahman etfield. This method retrieves a composite bidirectional re-al., 1993).flectance function, which consists of measured BRDF

values for the directions of measurements and values ofData Quality Assessmentthe best-fit parametric model for all other directions.

Thus, BRDF characteristics captured in the measure- The quality of BRDF data acquired in the field can bements are preserved while the gaps in the measurement assessed by replicate measurements taken under approxi-series are filled with modeled values. mately identical target surface and illumination condi-

To accommodate different measurement strategies, tions. Standard deviations between multiple data seriestwo different algorithms of BRDF retrieval were sug- quantify the accuracy of BRDF data obtained in thegested. The first one relies on measurements of the inci- field. For a single hemispherical data series, the differ-dent (direct and diffuse) and reflected radiance fields ences between nadir reflectance values are a good indi-and does not require radiative transfer calculations. In cation of the data quality. In an ideal case, all nadir re-this case, BRDF is retrieved by inverting the Fredholm flectances are identical. Changes in the illumination andequation of second kind. The second algorithm uses atmospheric conditions are represented and correctedmeasurements only for reflected radiance and relies on with the panel measurements taken for each azimuthconcurrent sun photometer data for atmospheric aerosol plane and with the aerosol optical thickness data ob-and, if necessary, water vapor to determine irradiance tained with the sun photometer.conditions based on radiative transfer equations with an Outlying data points resulting from sensor problems

can be detected qualitatively by plotting the BRDF dataanisotropic boundary condition. Both algorithms employin a given coordinate system and quantitatively by com-an effective iterative minimization that generally con-paring BRDF values of adjacent measurement positions.verges in only three to six iterations.

Unlike previously developed algorithms, the ap-Data Presentationproach of Lyapustin and Privette (1999) can be applied

even if observations made only at a single solar angle are A common way to plot BRDF data is by three-dimen-available. While the standard way of solving the Fred- sional diagrams based on a polar coordinate system (Fig.holm equation requires data from multiple solar zenith 4). Since BRDF effects are quasisymmetrical to the SPP,angles, this approach utilizes a priori information on the the coordinate system used for presenting BRDF data isgeneral BRDF shape in form of an analytical model, normally referenced to the SPP rather than to geographi-

cal north.whose parameters can be found from measurements

266 Sandmeier

Figure 5. BRDF data of a perennial ryegrass acquired with FIGOS (see Fig. 3) under 358 source zenithangle in the principal plane: (A) reflectance factor values, (B) nadir-normalized reflectance values (Sand-meier and Itten, 1999).

Three-dimensional graphs are highly illustrative and sphere, these values must be interpolated from adjacentvisualize the general BRDF characteristics of a surface. measurements. Interpolation may be based on DelaunayFor quantitative analysis, however, two-dimensional plots triangulation techniques (Sandmeier et al., 1998a) or onare more suitable since they facilitate reading of bidirec- a BRDF model (see next section).tional reflectance factor values. This is illustrated in Fig-ure 5A for a perennial ryegrass surface acquired with Use of BRDF ModelsFIGOS under 358 source zenith angle in the principal A comprehensive description of surface BRDF charac-plane. teristics requires an infinite number of measurements for

As a method of investigating the spectral variability an infinite number of illumination conditions. Thus,of BRDF effects, BRDF data can be normalized with BRDF data acquisitions are bound to be incomplete.nadir reflectance (Jackson et al., 1990; Sandmeier et al., Models, on the other hand, are capable of providing1998b). This separates BRDF effects from the underly- BRDF information for arbitrary illumination viewing ge-ing target reflectance magnitude and emphasizes their ometry, and they allow us to perform various sensitivityspectral variability. Figure 5B shows nadir-normalized analyses. Thus, to obtain complete BRDF data sets, it isreflectance data for the same target and viewing illumi- ideal to combine accurate field or laboratory BRDF datanation geometry as depicted in Figure 5A. The nadir- with a suitable BRDF model. However, models must benormalized data demonstrate that BRDF effects of vege-

verified before they can be used, and even physicallytation surfaces are spectrally highly variable. They arebased modeling approaches require measured BRDFstrong in the red and blue chlorophyll absorption bands.data to fine-tune the model parameters. Models also mayIn the green and particularly in the near-infrared bands,have not accounted properly for all optically significanthowever, BRDF effects are rather limited due to multi-elements in a vegetation canopy, such as soil or litterple scattering effects in the vegetation canopy, which areBRDF, and comparisons to measurements may bringcontrolled by the optical properties of the canopy tissuesthis to light. A comprehensive overview of BRDF models(Sandmeier et al., 1998b; Sandmeier et al., 1999). Withand their applications and limitations is given in Goelnonnormalized reflectance data such as that shown in(1988), Myneni et al. (1989), and Strahler (1997). An ex-Figure 5A, the spectral variability of BRDF effects isample of using a BRDF model to complete a BRDFmuch less obvious since the spectral BRDF characteris-measurement series is demonstrated in Lyapustin andtics of the canopy are obscured with the spectral reflec-Privette (1999) for the semiempirical Rahman-Pinty-Ver-tance magnitude.straete model (Rahman et al., 1993) (see Modeling Ap-proach section).Data Interpolation

With field goniometers, BRDF data in the hot spot di-Processing Levels and Data Archivingrection are obscured by the sensor shadow, and the ac-BRDF data and ancillary information should be storedquisition of data at extreme view zenith angles (e.g.,in a relational database that has controlled access (e.g.,.758) is limited (Note: For the SFG, the sensor and thethrough the Internet). Target surfaces types and the illu-sensor sled are largely placed outside the measurementmination viewing geometry must be searchable itemsplane to reduce the effect of shadow in the hot spot di-along with spectral wavelength bands and surface struc-rection; see Fig. 1B). For the calculation of hemispheri-

cal albedo and for representing the complete hemi- ture parameters such as the leaf area index.


Four different processing levels for BRDF data be accurately documented, including illumination, atmo-should be made available: spheric, and environmental conditions during data acqui-

sition. The characterization of the measurement surface• level 1: band-specific raw bidirectional reflec-and environmental conditions are crucial for remotetance factor datasensing applications and modeling studies. For vegeta-• level 2: band-specific bidirectional reflectance fac-tion canopies, species composition, canopy architecture,tor data corrected for diffuse irradiance effectsheight distribution, leaf characteristics, and background• level 3: band-specific parameters of selectedreflectance of soil and litter accumulation should be pro-BRDF modelsvided. For measurements over bare soil, documentation• level 4: band-specific hemispherical reflectanceof surface microtopography and soil constituents is rec-corrected for diffuse irradiance effectsommended. To reduce the impact of diffuse irradiance

Level 1 data are derived from the ratio between surface on field BRDF data, aerosol optical thickness and illumi-measurements and corresponding reflectance-reference nation conditions must be continuously monitored anddata. Interpolated values for the hot spot and at view ze- taken into account in the processing of the measure-nith angles outside the measurement directions must be ment data.identified with a data flag. Level 2 data are bidirectional With the availability of spaceborne multiangular re-reflectance measurements corrected for diffuse irradi- mote sensing devices such as MISR, MODIS, and POL-ance effects using either the measurement or the model- DER in year 2000 and beyond, systematic acquisitioning approach (see the Corrections of Diffuse Irradiance and archiving of ground BRDF and corresponding ancil-Effects section). Level 3 specifies spectral BRDF model lary data will provide much-needed ground reference in-parameter derived from fitting selected BRDF models to formation for multiangular remote sensing and BRDFground BRDF data. The quality of the match between modeling experiments. Knowledge gained from thesemodel and measurement data should be specified with studies will ultimately improve our understanding of theroot mean square errors. Level 4 gives the spectral hemi- reflectance characteristics of the earth surface and theirspherical reflectance (i.e., the integration of bidirectional relationships with ecological and climatological parameters.reflectance factor data over the hemisphere).

Funding for this research was provided through the Verificationand Validation Effort of the NASA Commercial Remote SensingCONCLUSIONSProgram at Stennis Space Center, Mississippi. A special thanks

Field goniometers allow us to acquire ground BRDF is tendered to Jeff Jenner, NASA SSC, and Mark Turner, NASAAmes, who made this study possible, and to three anonymousdata under natural illumination conditions that can bereviewers for their most helpful comments and suggestions.used as ground reference for remote sensing applications

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acquisition of bidirectional reflectance factor data with field goniometers

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