2 Interaction of EM energy with atmosphere and terrestrial objects Ө1Ө2Ө1> Ө2Solar irradiation arrives at Earth at wavelengths determined by the photospheric temperatureSome objects transmit the light through without significant diminutionSome materials absorb light (and in part re-emit at longer wavelengths)Or, the light is reflected at the same angle as it formed on approach.Or an object's surface roughness may cause scattering in all directions.TransmissionAbsorptionӨ1Ө2Ө1= Ө2As solar rays arrive at the Earth, the atmosphere reflects, absorbs or scatters a fraction of them and transmits the remainder.ScatteringreflectionUpon striking the terrestrial surface the solar irradiance partitions into 3 modes of energy-interaction response:Transmittance - Part of the radiation penetrates into certain surface materials (e.g., water) and if the material is transparent and thin in one dimension, passes through, generally with some diminution.Absorption - Some radiation is absorbed through electron or molecular reactions - a portion of this energy is then re-emitted, usually at longer wavelengths, and some of it remains and heats the target.Reflectance - some radiation reflects (moves away from the target) at specific angles and/or scatters away from the target at various angles, depending on the surface roughness and the angle of incidence of the rays. .
3 ReflectanceReflectance is the process whereby radiation “bounces off” an object like a cloud or the terrain. Actually, the process is more complicated, involving re-radiation of photons in unison by atoms or molecules in a layer one-half wavelength deep.Reflection exhibits fundamental characteristics that are important in remote sensing.First, the incident radiation, the reflected radiation, and a vertical to the surface from which the angles of incidence and reflection are measured all lie in the same plane.Second, the angle of incidence and the angle of reflection are equal.Ө1Ө2Ө1= Ө2
4 Reflectance• When specular reflection occurs, the surface from which the radiation is reflected is essentially smooth (i.e. the average surface profile is several times smaller than the wavelength of radiation striking the surface).• If the surface is rough, the reflected rays go in many directions, depending on the orientation of the smaller reflecting surfaces. This diffuse reflection does not yield a mirror image, but instead produces diffused radiation. White paper, white powders and other materials reflect visible light in this diffuse manner.• If the surface is so rough that there are no individual reflecting surfaces, then scattering may occur. Lambert defined a perfectly diffuse surface; hence the commonly designated Lambertian surface is one for which the radiant flux leaving the surface is constant for any angle of reflectance to the surface normal.Specular reflectionLambertian reflection
5 ReflectanceThere are no perfect specular and Lambertian surfaces in nature – most bodies approximate these two end members.Diffuse reflections contain spectral information about the “color” of reflecting surfaces, while specular reflections generally lack this information.Hence in remote sensing, we are mostly interested in measuring the diffuse reflection properties of terrestrial objects.
6 Radiation budget equation The total amount of radiant flux in a specific wavelength (l) incident to the terrain ( ) must be accounted for by evaluating the amount of radiant flux reflected from the surface ( ), the amount of radiant flux absorbed by the surface ( ), and the amount of radiant flux transmitted through the surface ( ):Or,
7 More terminology : Hemispherical reflectance, absorption, and transmittance The Hemispherical reflectance (rl) is defined as the dimensionless ratio of the radiant flux reflected from a surface to the radiant flux incident to it:Hemispherical transmittance (tl) is defined as the dimensionless ratio of the radiant flux transmitted through a surface to the radiant flux incident to it:Hemispherical absorption (al) is defined by the dimensionless relationship:
8 Percent reflectance, absorption, and transmittance These radiometric quantities are useful for producing general statements about the spectral reflectance, absorption, and transmittance characteristics of terrain features.If we take the simple hemispherical reflectance (or absorption/transmittance) equation and multiply it by 100, we obtain an expression for percent reflectance ( ):
9 Spectral reflectance curves Spectral reflectance curve is a graph of the spectral reflectance of an object as a function of wavelength.Configuration of the spectral reflectance curves gives us insights into the spectral characteristics of an object.Spectral reflectance curves guide us in selecting wavelengths region(s) in which remote sensing data should be acquired for the given science goal.
12 Spectral reflectance curves B -1, G -2, R - 3Bigleaf MapleDouglas Firnm Band 1nm Band 2nm Band 3nm Band 4nm Band 5nm Band 7LANDSAT ETM+BANDSConiferous – darkDeciduous - brightB -2, G -3, R - 4Dr. Nicholas Short, Remote Sensing Tutorial
13 Spectral reflectance curves: Soil a: Organic dominatedb: Fresh soil (unaltered)c: iron alteredd: organic affectede: Iron rich
14 Spectral reflectance curves: Water a: Ocean waterb: Turbid waterc: Water in leaf
15 Spectral reflectance curves B -1, G -2, R - 4Red shades : vegetation (forest, valley, crops) Medium greyish-browns - barren soil + dry grass Deep blues/black - ocean – (lighter where sediments are present) Blue and white patterns – breakersBluish tones + streets - Towns
16 Spectral signatures and response Spectral responses measured by remote sensors is often diagnostic of the type and/or condition of objects – hence these are called spectral signaturesHowever, a preferable term could be spectral response patterns, because signature is a pattern that is absolute and unique.Spectral response patterns are influenced by:Atmospheric effectsTemporal effectsSpatial effectsGeometric effectsPixel purity