Spectral Reflect Ance

8/2/2019 Spectral Reflect Ance

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Welcome to the worldof Geoinformatics



Created by:Anubhav Srivastava

Spectral Reflectance



Interaction of Electromagnetic Energywith the Earth’s Surface:-

The electromagnetic energy which comesinto contact with the matter is calledincident radiation.

When electromagnetic energy interactswith the earth’s surface, then followingfive types of phenomenon takes place:

Reflection(for smooth surface)

Transmission(passed through thesubstance)



Reflected

Absorption(Energy is given up- the matteris heated)

Scattering(deflected in all directions-caused by surfaces with roughness, relief,etc.)

Emission(transmitted by matter).

Absorbe

d

Transmitted

ScatteredEmitted

Incident

Smooth Surface

Rough Surface



The vibration of these energies enablesthe interpreter to distinguish the features.

The properties of incident radiation thatcan change are intensity, direction,wavelength, phase and polarization.

Emission, scattering and reflection are

called surface phenomenon. These aredetermined primarily by the properties of the surface.

Transmission and absorption are calledvolume phenomenon. These aredetermined by internal characteristics of



the matter.

These interactions are recorded on remote

sensing images, from which thecharacteristics of the matter can beinterpreted.

Energy balance equation at a particularwavelength is given by

EI = ER + EA + ET

where, EI= Incidence Energy

ER= Reflected EnergyEA= Absorbed Energy

ET= Transmitted Energy



The proportions of the energy reflected,absorbed and transmitted will vary for

different earth features. These differencesenable us to distinguish different featureson an image.

It should be noted that two features may

be indistinguishable in one spectral rangeand may appear different in anotherwavelength band.

It is because even within a given feature

type, the proportions of reflected,absorbed and transmitted energy will varyat different wavelength.



The above equation can be rearranged as

ER= EI – (EA + ET)

ER /EI = 1-((EA /EI) + (ET /EI))ρ = 1 – (α+β)

where, ρ= Reflectance, α= Absorbance &

β= Transmittance

Since almost all the earth surfaces arequite opaque, the transmittance can be

neglected.



According to Kirchhoff's Law, theabsorbance is taken as emissivity (ξ).

ρ= 1 - ξ

This equation is the fundamental equationfor the conceptual design of the remote

sensing technology.♯ For ξ=0;ρ=1:Total incident energy is

reflected by the object and recorded bythe sensor and the object appear white.

♯ For ξ=1;ρ=0:Total incident energy isabsorbed by the object and object appearas black.



Since most of the remote sensing systemsoperate in the wavelength regions in

which reflected energy predominates, thereflectance properties are of primeimportance.

The reflected energy is primarily afunction of the surface roughness. Flatsurfaces manifest mirror-like reflection(the angle of incidence is equal to angle of

incidence) and are called specularreflectors.



Rough surfaces reflect energy uniformly inall directions and are called diffuse or

Lambertian reflectors. Most earth surfaces are neither perfectly

specular nor perfectly Lambertian andtheir characteristics lie between these twoextremes.

Ideal Specular Reflector Near Ideal Specular Reflector



The diffuse reflectance property of terrainfeatures is of great interest since itcontains spectral information on thecolour of the reflecting surface, whereas

specular reflectors do not.

Ideal Diffuse Reflector Near Ideal Diffuse Reflector



Due to this reason in remote sensing, thediffuse reflectance properties of terrain

features are measured. To quantified thereflectance characteristics of the earth’ssurface features, the reflected portion andthe incidence energy is measured. The

reflected energy measured as a functionof wavelength is called spectralreflectance.

ρλ= ER(λ)/ EI(λ) * 100

where, ρλ= Spectral ReflectanceER(λ)= Energy of wavelengthreflected



EI(λ)= Energy of wavelength

incident

A graph of the spectral reflectance of anobject as a function of the wavelength iscalled spectral reflectance curve. It can beprepared in laboratory using aninstrument called spectrometer.

The configuration of this curve gives aninsight into the special characteristics of

an object. Several such curves fordifferent objects may be prepared and



used for comparing and discriminating

various features in the images.

Spectral Reflect Ance

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