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GMAT 9600GMAT 9600Principles of Remote SensingPrinciples of Remote Sensing

Topic 3: Spectral Reflectance and Topic 3: Spectral Reflectance and Atmospheric AttenuationAtmospheric Attenuation

Dr. Linlin Ge

GMAT 9600 Principles of Remote Sensing Session 1

Optical Remote SensingOptical Remote Sensing

(CRISP, 2007)


OutlineOutline3.13.1 Definition of terms (see lecture notes P42)Definition of terms (see lecture notes P42)3.2 3.2 ReflectanceReflectance3.3 3.3 Spectral signaturesSpectral signatures3.4 3.4 Atmospheric effectsAtmospheric effects3.5 3.5 Transmittance and scatteringTransmittance and scattering3.6 3.6 Path radiancePath radiance3.8 3.8 Global irradianceGlobal irradiance3.9 3.9 Target reflectanceTarget reflectance3.103.10 Atmospheric Calculations Atmospheric Calculations –– Landsat MSSLandsat MSS3.113.11 Approximate methods for calculating Approximate methods for calculating

reflectancereflectance


3.2 Reflectance3.2 ReflectanceThree fundamental energy interactions with Three fundamental energy interactions with the feature when electromagnetic energy is the feature when electromagnetic energy is incident on any given earth surface feature:incident on any given earth surface feature:

reflectionreflectionabsorption absorption transmissiontransmission


Energy balance equationEnergy balance equation

EEII((λλ) = ) = EERR((λλ) +) +EEAA((λλ) + ) + EETT((λλ))

EEII((λλ): incident energy): incident energyEERR((λλ): reflected energy): reflected energyEEAA((λλ): absorbed energy): absorbed energyEETT((λλ): transmitted energy): transmitted energy


3.2 Reflectance Cont.3.2 Reflectance Cont.The amount of reflected, absorbed and The amount of reflected, absorbed and transmitted energy will vary for different transmitted energy will vary for different materials and conditions. materials and conditions.

The variation is wavelength dependent.The variation is wavelength dependent.

These enable us to distinguish different These enable us to distinguish different features using different spectral ranges features using different spectral ranges (bands). (bands).

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3.2 Reflectance Cont.3.2 Reflectance Cont.For many remote sensing systems, the For many remote sensing systems, the reflectance properties of earth features are reflectance properties of earth features are very important. very important.

We can reWe can re--write earlier write earlier EqEq as:as:EERR((λλ) = ) = EEII((λλ) ) –– [[EEAA((λλ) + ) + EETT((λλ)])]


Specular Specular vsvs DiffuseDiffuseTwo types of reflectors: Two types of reflectors: SpecularSpecular and and DiffuseDiffuse..

Specular: Specular: Flat surfacesFlat surfacesMirrorMirror--like reflectionslike reflectionsAngle of reflection = angle of incidenceAngle of reflection = angle of incidence

DiffuseDiffuseAlso called Also called LambertianLambertian reflectorsreflectorsReflection is uniform in all directionsReflection is uniform in all directions


3.2 Reflectance Cont.3.2 Reflectance Cont.Most earth surfaces are not perfectly specular Most earth surfaces are not perfectly specular nor diffuse reflectors. It is more likely a nor diffuse reflectors. It is more likely a combination of the two. combination of the two.


RayleighRayleigh’’s Criterions CriterionThe theoretical boundary roughness between specular The theoretical boundary roughness between specular and diffuse reflection is given by Rayleighand diffuse reflection is given by Rayleigh’’s Criterion:s Criterion:

HHBB= = λλ / (8 / (8 coscosθθ))where where

θθ is the incident angleis the incident angle


3.2 Reflectance Cont.3.2 Reflectance Cont.HHBB is the boundary condition surface is the boundary condition surface roughness.roughness.The actual surface roughness is HThe actual surface roughness is HSS..

If HIf HSS < H< HB B , specular reflection., specular reflection.If HIf HSS > H> HBB , diffuse reflection., diffuse reflection.For HFor HSS is near or equal to His near or equal to HBB, reflection , reflection intermediate between diffuse and specular intermediate between diffuse and specular will occur. will occur.


3.2 Reflectance Cont.3.2 Reflectance Cont.A diffuse surface will reflect in accordance with A diffuse surface will reflect in accordance with LambertLambert ’’s cosine law of radiation.s cosine law of radiation.Definition:Definition:Flux per unit solid angle leaving a surface in any Flux per unit solid angle leaving a surface in any direction is proportional to the cosine of the angle direction is proportional to the cosine of the angle between that direction and the normal to the surface.between that direction and the normal to the surface.

Iθ = Io cosθ

Unit = W/Sr

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3.2 Reflectance Cont.3.2 Reflectance Cont.The radiance of diffuse reflectance at angle The radiance of diffuse reflectance at angle θθ to the to the surface normal is:surface normal is:

LLθθ = = IIθθ / (/ (∆∆A A coscosθθ) = I) = Io o coscosθθ / (/ (∆∆A A coscosθθ))= I= Io o / / ∆∆A A

Therefore, the radiance for a diffuse surface is Therefore, the radiance for a diffuse surface is independent of independent of θθ. .


3.2 Reflectance Cont.3.2 Reflectance Cont.Hemispherical reflectance, RHemispherical reflectance, RHH

RRHH of a surface is defined by the dimensionless ratio of of a surface is defined by the dimensionless ratio of the reflected exitance, M, of a surface, to the irradiance, the reflected exitance, M, of a surface, to the irradiance, E, on that surface.E, on that surface.

RRHH = M / E = M / E Definition:Definition:exitanceexitance -- radiant power emitted into a full sphere, i.e., 4 radiant power emitted into a full sphere, i.e., 4 srsr ((steradianssteradians), by a unit area of a source, watts per ), by a unit area of a source, watts per square meter (W / msquare meter (W / m22))

irradianceirradiance -- radiant power incident per unit area upon a radiant power incident per unit area upon a surface, watts per square meter (W / msurface, watts per square meter (W / m22))


3.2 Reflectance Cont.3.2 Reflectance Cont.Hemispherical reflectance, RHemispherical reflectance, RHH ,cont.,cont.

M is the sum of all exitance leaving the surface M is the sum of all exitance leaving the surface in any direction (out).in any direction (out).E is the sum of all irradiance reaching the E is the sum of all irradiance reaching the surface from any direction within a hemisphere surface from any direction within a hemisphere above the surface (in).above the surface (in).


3.2 Reflectance Cont.3.2 Reflectance Cont.The property of diffuse reflectance whose The property of diffuse reflectance whose radiance is independent of radiance is independent of θθ shown earlier shown earlier gives the hemispherical reflectance as:gives the hemispherical reflectance as:

RRHH = = ππLL / E / E where M = where M = ππLL

therefore, for particular therefore, for particular λλ: : RRHHλλ = = ππLLλλ / E/ Eλλ


3.2 Reflectance Cont.3.2 Reflectance Cont.Specular ReflectanceSpecular Reflectance

The angle of incidence is equal to the angle of The angle of incidence is equal to the angle of reflection.reflection.Radiance will vary with the angle of view and will Radiance will vary with the angle of view and will be greatest in the direction of the specular be greatest in the direction of the specular reflectance component.reflectance component.Radiance in any other direction will be less than Radiance in any other direction will be less than that calculated given that calculated given LambertianLambertian (diffuse) (diffuse) conditions.conditions.


3.2 Reflectance Cont.3.2 Reflectance Cont.Generally, reflectance of natural surfaces at high solarGenerally, reflectance of natural surfaces at high solar--elevation angles approximate those of diffuse reflectors elevation angles approximate those of diffuse reflectors = the radiance measured by an air= the radiance measured by an air-- or spaceor space--borne borne sensor will be independent of the look angle of the sensor will be independent of the look angle of the sensor system. sensor system.

ButBut……....Most natural surfaces will have a small component of Most natural surfaces will have a small component of specular reflection, e.g. water body and roads.specular reflection, e.g. water body and roads.The independence from the look angle does not The independence from the look angle does not account for variation due to the different path lengths account for variation due to the different path lengths through the atmosphere (more in 3.4).through the atmosphere (more in 3.4).

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3.2 Reflectance Cont.3.2 Reflectance Cont.SummarySummaryIf diffuse natural surface approximation is used:If diffuse natural surface approximation is used:

As As λλ increases or solar elevation become less (solar increases or solar elevation become less (solar zenith angle increases), there is an increased probability zenith angle increases), there is an increased probability that specular component will increase. that specular component will increase.

For a constant solar elevation angle, the measured For a constant solar elevation angle, the measured radiance and calculated reflectance should be constant radiance and calculated reflectance should be constant for the same surface. for the same surface.


3.3 Spectral Signatures3.3 Spectral SignaturesThe spectral reflectance of an object as a The spectral reflectance of an object as a function of wavelength can be represented as a function of wavelength can be represented as a spectral reflectance curvespectral reflectance curve. .

This is the spectral characteristics of an object This is the spectral characteristics of an object and has a strong influence on the choice of and has a strong influence on the choice of λλregion in which remote sensing data are region in which remote sensing data are acquired for a particular application. acquired for a particular application.


3.3 Spectral Signatures3.3 Spectral SignaturesExample:Example:

Selecting an airborne Selecting an airborne sensor system to assist sensor system to assist in preparing a map of a in preparing a map of a forested area forested area differentiating differentiating deciduous versus deciduous versus coniferous trees.coniferous trees.


3.3 Spectral Signatures3.3 Spectral SignaturesNote:Note:

The curve for each of the species is plotted as a The curve for each of the species is plotted as a envelope of values. The spectral envelope of values. The spectral reflectancesreflectancesvary somewhat within a given material class. vary somewhat within a given material class. The spectral reflectance of the same object will The spectral reflectance of the same object will not be exactly equal. not be exactly equal. The spectral reflectance curves for each tree The spectral reflectance curves for each tree type are very close to each other in the visible type are very close to each other in the visible portion of the spectrum.portion of the spectrum.But they are distinguishable in NIR region of the But they are distinguishable in NIR region of the spectrum. spectrum.


3.3 Spectral Signatures3.3 Spectral SignaturesThe spectral signature of the same object but The spectral signature of the same object but under different weather conditions:under different weather conditions:Coastal Colour:Coastal Colour:

Image © Department of Physical Oceanography, Royal Netherlands Institutefor Sea ResearchGMAT 9600 Principles of Remote Sensing Session 1

Vegetation HealthVegetation Health

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3.3 Spectral Signatures3.3 Spectral SignaturesSpectral Reflectance of Vegetation, Soil and WaterSpectral Reflectance of Vegetation, Soil and Water

The lines represent average reflectance curves.Note that spectral reflectance for healthy green vegetation shows the valleys at about 0.45 and 0.67 µm. This is due to strong energy absorption by chlorophyll in plant leaves. The very high absorption of blue and red energy make the leaves “green”.


3.3 Spectral Signatures3.3 Spectral SignaturesIn the near IR region of the spectrum, the In the near IR region of the spectrum, the reflectance of healthy vegetation increases reflectance of healthy vegetation increases dramatically.dramatically.From 0.7~1.3 From 0.7~1.3 µµm, a plant leaf reflects 40~50 % m, a plant leaf reflects 40~50 % energy. Most of the remaining energy is energy. Most of the remaining energy is transmitted and < 5% is absorbed. transmitted and < 5% is absorbed. The reflectance is caused by the internal The reflectance is caused by the internal structure of plant leaves that is highly variable structure of plant leaves that is highly variable between plant species.between plant species.Reflectance measurements in NIR range often Reflectance measurements in NIR range often permit us to discriminate between species even permit us to discriminate between species even if they look the same in visible wavelengths. if they look the same in visible wavelengths.


3.3 Spectral Signatures3.3 Spectral SignaturesBeyond 1.3 Beyond 1.3 µµm energy incident upon vegetation m energy incident upon vegetation is essentially absorbed or reflected.is essentially absorbed or reflected.

Energy is absorbed at 1.4, 1.9 and 2.7Energy is absorbed at 1.4, 1.9 and 2.7µµm by the m by the water in the leaves. water in the leaves.

Through the range beyond 1.3 Through the range beyond 1.3 µµm, leaf m, leaf reflectance is approximately inversely related to reflectance is approximately inversely related to the total water present in a leaf = both the the total water present in a leaf = both the moisture content and the thickness of a leaf. moisture content and the thickness of a leaf.



3.3 Spectral Signatures3.3 Spectral SignaturesThe soil curve shows considerably less peakThe soil curve shows considerably less peak--andand--valley variation in reflectance. valley variation in reflectance. Factors affecting soil reflectance: moisture Factors affecting soil reflectance: moisture content, soil texture (portion of sand, silt and content, soil texture (portion of sand, silt and clay), surface roughness and organic matter clay), surface roughness and organic matter content, etc.content, etc.Thus, the reflectance properties of a soil are Thus, the reflectance properties of a soil are consistent only within particular ranges of consistent only within particular ranges of conditions. conditions.


3.3 Spectral Signatures3.3 Spectral SignaturesThe most distinctive characteristic of clear water The most distinctive characteristic of clear water is the energy absorption at nearis the energy absorption at near-- IR and beyond. IR and beyond. In the visible region, the reflectance of water In the visible region, the reflectance of water varies with the contents in the water.varies with the contents in the water.

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3.3 Spectral Signatures3.3 Spectral SignaturesThe broad feature types are normally spectrally The broad feature types are normally spectrally separable.separable.The degree of separation between types is a The degree of separation between types is a function of function of ““where we lookwhere we look”” spectrally. spectrally. Keep in mind that in real world, the spectral Keep in mind that in real world, the spectral response patterns measured by remote sensors response patterns measured by remote sensors may be quantitative but not absolute.may be quantitative but not absolute.They may be distinctive but not necessarily They may be distinctive but not necessarily unique. unique.


3.3 Spectral Signatures3.3 Spectral Signatures

Spectral reflectance of representative leaves of four agricultural crops .


3.3 Spectral Signatures3.3 Spectral SignaturesSpectral response patterns or signatures Spectral response patterns or signatures can be affected bycan be affected by

temporal effectstemporal effects and and spatial effectsspatial effects . .


Temporal EffectTemporal Effect

Temporal effects: any factors that change the Temporal effects: any factors that change the spectral characteristics of a feature over time.spectral characteristics of a feature over time.E.g. the spectral characteristics of many species E.g. the spectral characteristics of many species of vegetation are in a nearly continual state of of vegetation are in a nearly continual state of change throughout a growing season. These change throughout a growing season. These changes often influence when we might collect changes often influence when we might collect sensor data for a particular application.sensor data for a particular application.



Changes in the spectral reflectance of oak leaves during the growing season June-November


Spatial EffectSpatial Effect

The factors that cause the same types of The factors that cause the same types of features at a given point in time to have different features at a given point in time to have different characteristics at different geographic locations.characteristics at different geographic locations.

This may be negligible in smallThis may be negligible in small--area (metres area (metres apart) analysis.apart) analysis.

When locations may be hundreds of kilometres When locations may be hundreds of kilometres apart (e.g. satellite data), the difference of soils apart (e.g. satellite data), the difference of soils and climates might exist.and climates might exist.

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Influence of the moisture content of maize leaves on the spectral reflectance.


3.3 Spectral Signatures3.3 Spectral SignaturesMeasurement of temporal effects lead to Measurement of temporal effects lead to the application of change detection. the application of change detection.

E.g. the detection of the change in E.g. the detection of the change in suburban development near a suburban development near a metropolitan area by using data obtained metropolitan area by using data obtained on two different dates. on two different dates.


(a) Normal colour (b) Colour IR

(a) the normal colour composite, bands 1 (blue), 2 (green) and 3 (red).

(b) the colour IR composite, bands 2 (green), 3 (red) and 5 (near-IR).

Note the colour differences for:• Vegetation• Water-body (river)


(a) Normal colour

(b) Colour IR

Image © Lillesand and Kiefer (2000)


(a) Normal colour

(b) Colour IR

Orange - IR energy emitted from the flowing lava.

Pink - living vegetation.

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