Video

1. Lecture 3: Remote Sensing Spectral signatures, VNIR/SWIR, MWIR/LWIR Radiation models

2. Video http://www.met.sjsu.edu/metr112-videos/MET%20112%20Video%20Library-MP4/energy%20balance/ • Solar Balance.mp4 Jin: We failed to show this one on class, you can access it from the link above

3. Spectral signature • Much of the previous discussion centered around the • selection of the specific spectral bands for a given theme • In the solar reflective part of the spectrum (350-2500 nm), the shape of the • spectral reflectance of a material of interest drives the band selection • Recall the spectral reflectance of vegetation • Select bands based on an absorbing or reflecting feature in the material • In the TIR it will be the emissivity that is studied • The key will be that • different materials have • different spectral reflectances • As an example, consider • the spectral reflectance • curves of three different • materials shown in the graph

4. These divisions are not precise and can vary depending on the publication • Visible-Near IR (0.4 - 2.5); • Mid-IR (3 - 5); • Thermal IR (8 - 14); • 4) Microwave (1 - 30 centimeters) VNIR - visible and near-infrared ~0.4 and 1.4 micrometer (µm) Near-infrared (NIR, IR-A DIN): 0.75-1.4 µm in wavelength, defined by the water absorption Short-wavelength infrared (SWIR, IR-B DIN): 1.4-3 µm, water absorption increases significantly at 1,450 nm. The 1,530 to 1,560 nm range is the dominant spectral region for long-distance telecommunications. Mid-wavelength infrared (MWIR, IR-C DIN) also called intermediate infrared (IIR): 3-8 µm Long-wavelength infrared (LWIR, IR-C DIN): 8–15 µm Far infrared (FIR): 15-1,000 µm

5. Spectral Signature • Spectral signature is the idea that a given material has a • spectral reflectance/emissivity which distinguishes it from • other materials • Spectral reflectance is the efficiency by which a material reflects energy as a function of wavelength • The success of our differentiation depends heavily on the sensor we use and the materials we are distinguishing • Unfortunately, the problem is not as simple as it may appear since other factors beside the sensor play a role, such as • Solar angle • View angle • Surface wetness • Background and surrounding material • Also have to deal with the fact that often the energy measured by the sensor will be from a mixture of many different materials • This discussion will focus on the solar reflective for the time being

6. Spectral Signature - geologic • Minerals and rocks can have distinctive spectral shapes based • on their chemical makeup and water content • For example, chemically bound water can cause a similar feature to show up in several diverse sample types • However, the specific spectral location of the features and their shape depends on the actual sample 1

7. Spectral signature - Vegetation • Samples shown here are for a variety of vegetation types • All samples are of the leaves only • That is, no effects due to the branches and stems is included

8. Vegetation spectral reflectance • Note that many of the themes for Landsat TM were based on • the spectral reflectance of vegetation • Show a typical vegetation spectra - KNOW THIS CURVE • Also show the spectral bands of TM in the VNIR and SWIR as well as some of the basic physical process in each part of the spectrum

9. Spectral signature - Atmosphere • Recall the graph presented earlier showing the transmittance • of the atmosphere • Can see that there are absorption features in the atmosphere that could be • used for atmospheric remote sensing • Also clues us in to portions of the spectrum to avoid so that the ground is • visible

10. A signature is not enough • Have to keep in mind that a spectral signature is not always enough • Signature of a water absorption feature in vegetation may not indicate the • desired parameter • Vegetation stress and health • Vegetation amount • Signatures are typically derived in the laboratory • Field measurements can verify the laboratory data • Laboratory measurements may not simulate what the satellite sensor • would see • Good example is the difficult nature of measuring the relationship between • water content and plant health • Once the plant material is removed from the plant to allow measurement • it begins to dry out • Using field-based measurements only is limited by the quality of the • sensors • The next question then becomes how many samples are needed to • determine what signatures allow for a thematic measurement

11. This is a black spruce forest in the BOREAS experimental region in Canada. Left: backscattering (sun behind observer), note the bright region (hotspot) where all shadows are hidden. Right: forwardscattering (sun opposite observer), note the shadowed centers of trees and transmission of light through the edges of the canopies. Photograph by Don Deering. http://www-modis.bu.edu/brdf/brdfexpl.html

12. A soybean field. Left: backscattering (sun behind observer). Right: forwardscattering (sun opposite observer), note the specular reflection of the leaves. Photograph by Don Deering. http://www-modis.bu.edu/brdf/brdfexpl.html

13. Signature and resolution The next thing to be concerned about is the fact that we will not fully sample the entire spectrum but rather use fewer bands In this case, all four bands will allow us to differentiate clay and grass Using bands 1, 3, and 4 would also be sufficient to do this Even using just bands 3 and 4 would allow us to separate clay and grass

14. Signature and resolution • Band selection and resolution for spectral signatures should • be chosen first based on the shapes of the spectra • That is, it is not recommended to rely on the absolute difference between • two reflectance spectra for discrimination • Numerous factors can alter the brightness of the sample while not • impacting the spectral shape • Shadow effects and illumination conditions • Absolute calibration • Sample purity • Bands showngive • Gypsum • - Low, high, lower • Montmorillonite • - High, high, low • Quartz • - high, high, • not so high

17. Quantifying radiation • It is necessary to understand the energy quantities that are typically • used in remote sensing • Radiant energy (Q in joules) is a measure of the capacity of an EM wave to do work by moving an object, heating, or changing its state. • Radiant flux (Φ in watts) is the time rate (flow) of energy passing through a certain location. • Radiant flux density (watts/m2) is the flux intercepted by a planar • surface of unit area. • Irradiance (E) is flux density incident upon a surface. • Exitance (M) or emittance is flux density leaving a surface. • The solid angle (Ω in steradians) subtended by an area A on a spherical surface of radius r is A/r2 • Radiant intensity (I in watts/sr) is the flux per unit solid angle in a given direction. • Radiance (L in watts/m2/sr) is the intensity per unit projected area. • Radiance from source to object is conserved

18. Radiometric Definitions/Relationships Radiant flux, irradiance (radiant exitance), radiance The three major energy quantities are related to each other logically by examining their units In this course, we will deal with the special case Object of interest is located far from the sensor (factor of five) Change in radiance from object is small over the view of the sensor Then Φdetector = L object × Areacollector × ΩGIFOV Φdetector = E object × Areacollector E detector = L object × ΩGIFOV ΩGIFOV= AreaGIFOV/H2 ΩGIFOV= Areadetector/f2

19. Electromagnetic Spectrum: Transmittance, Absorptance, and Reflectance

20. Radiometric Definitions/Relationships Emissivity, absorptance, and reflectance All three of these quantities are unitless ratios of energy quanities Emissivity, ε, is the ratio of the amount of energy emitted by an object to the maximum that could possibly emitted at that temperature Absorptance, α, is the ratio of the amount of energy absorbed by an object to the amount that is incident on it Reflectance, ρ, is the ratio of the amount of energy reflected by an object to the is incident on it All three can be written in terms of the emitted, reflected, incident, and absorbed radiance, irradiance, radiant exitance, or radiant flux (but since above three quantities are unitless, numerator and denominator must be identical units) In terms of radiant flux we would have

21. Radiometric Laws - Cosine Law Cosine Law - Irradiance on surface is proportional to cosine of the angle between normal to the surface and incident radiance E = E0cosθ In figures below, if E0 (or L0 converted to irradiance using the solid angle) is normal to the surface, we have a maximum incident irradiance For E0 that is tangent to surface, the incident irradiance is zero

22. Cosine effect example Graph on this page shows the downwelling total irradiance as a function of time for a single day as measured from a pyranometer

23. Radiometric Laws - 1/R2 Distance Squared Lawor 1/R2 states that the irradiance from a point source is inversely proportional to the square of the distance from the source Only true for a point source, but for cases when the distance from the source is large relative to the size of the source (factor of five gives accuracy of 1%) Sun can be considered a point source at the earth Satellite in terrestrial orbit does not see the earth as a point source Can understand how this law works by remembering that irradiance has a 1/area unit and looking at the cases below In both cases, the radiant flux through the entire circle is same Area of larger sphere is 4 times that of the smaller sphere and irradiance for a point on the sphere is ¼ that of the smaller sphere

24. Radiometric Laws - Lambertian Surface • Lambertian surface is one for which the surface-leaving radiance is constant with angle • It is the angle leaving the surface for which the radiance is invariant • Lambertian surface says nothing about the dependence of the surface- • leaving radiance on the angle of incidence • In fact, from the cosine law, we know that the incident irradiance will • decrease with sun angle • If the incident irradiance decreases, the reflected radiance decreases • as well • The radiance can decrease, as long as it does so in all directions • equally2

25. Radiometric Laws - Lambertian Surface Using the integral form of the relationship between radiance and irradiance we can show that Elambertian=¶Llambertian To obtain the irradiance we have to consider the radiance through an entire Hemisphere Because of the large range of angles, we cannot simply use E=LΩ

26. Radiometric Laws - Planck’s Law States that the spectral radiant exitance from a blackbody depends only on wavelength and the temperature of the blackbody A blackbody is an object that absorbs all energy incident on it, α=1 Corrollary is that a blackbody emits the maximum of energy possible for an object a given temperature and wavelength

27. Radiometric Laws - Planck’s Law Once you are given the temperature and wavelength you can develop a Planck curve Planck curves never cross Curves of warmer bodies are above those of cooler bodies

28. Radiometric Laws - Wien’s Law Peaks of Planck Curves get lower and move to longer wavelengths as temperature decreases Maximum wavelength of emission is defined by Wien’s Law λmax=2898/T [μm]

29. Solar Radiation Sun is the primary source of energy in the VNIR and SWIR Peak of solar curve at approximately 0.45 μm Distance to sun varies from 0.983 to 1.0167 AU Irradiance (not spectral irradiance) at the top of the earth’s atmosphere for normal incidence is 1367 W/m2 at 1 AU

30. Terrestrial Radiation Energy radiated by the earth peaks in the TIR Effective temperature of the earth-atmosphere system is 255 K Planck curves below relate to typical terrestrial temperatures

31. Solar-Terrestrial Comparison When taking into account the earth-sun distance it can be shown that solar energy dominates in VNIR/SWIR and emitted terrestrial dominates in the TIR Sun emits more energy than the earth at ALL wavelengths It is a geometry effect that allows us to treat the wavelength regions separately

32. Solar-Terrestrial Comparison Plots here show the energy from the sun at the sun and at the top of the earth’s atmosphere Also show the emitted energy from the earth

33. Vertical Profile of the Atmosphere • Understanding the vertical • structure of the atmosphere • allows one to understand better • the effects of the atmosphere • Atmosphere is divided into layers • based on the change in • temperature with height in that • layer • Troposphere is nearest the • surface with temperature • decreasing with height • Stratosphere is next layer and • temperature increases with height • Mesosphere has decreasing • temperatures

34. Atmospheric composition • Atmosphere is composed of dust and molecules which vary spatially • and in concentration • Dust also referred to as aerosols • Also applies to liquid water, particulate matter, airplanes, etc. • Primary source of aerosols is the earth's surface • Size of most aerosols is between 0.2 and 5.0 micrometers • Larger aerosols fall out due to gravity • Smaller aerosols coagulate with other aerosols to make larger particles • Both aerosols and molecules scatter light more efficiently at short wavelengths • Molecules scatter very strongly with wavelength (blue sky) • Molecular scattering is proportional to 1/(wavelength)4 • Aerosols typically scatter with 1/(wavelength) • Both aerosols and molecules absorb • Molecular (or gaseous absorption is more wavelength dependent • Depends on concentration of material

35. Absorption MODTRAN3 output for US Standard Atmosphere, 2.54 cm column water vapor, default ozone 60-degree zenith angle and no scattering

36. Absorption Same curve as previous page but includes molecular scatter

37. Angular effect • Changing the angle of the path through the atmosphere effectively changes the concentration • More material, lower transmittance • Longer path, lower transmittance

38. Absorption At longer wavelengths, absorption plays a stronger role with some spectral regions having complete absorption

39. Absorption

40. Absorption The MWIR is dominated by water vapor and carbon dioxide absorption

41. Absorption In the TIR there is the “atmospheric window” from 8-12 μm with a strong ozone band to consider

42. Radiative Transfer Easier to consider the specific problem of the radiance at a sensor at the top of the atmosphere viewing the surface

43. Radiation components • There will be three components of greatest interest in the • solar reflective part of the spectrum • Unscattered, surface • reflected radiation Lλsu • Down scattered, • surface reflected Lλsd • skylight • Up scattered path Lλsp • radiance • Radiance at the sensor is the sum of these three

44. Radiative Transfer • Radiative transfer is basis for understanding how sunlight • and emitted surface radiation interact with the atmosphere • For the atmospheric scientist, radiative transfer is critical for • understanding the atmosphere itself • For everyone else, it is what atmospheric scientists use to allow others • to get rid of atmospheric effects • Discussion here will be to understand the effects the atmosphere will • have on remote sensing data • Start with some definitions • Zenith Angle • Elevation Angle • Nadir Angle • Airmass is 1/cos(zenith) • Azimuth angle describes • the angle about the vertical • similar to cardinal directions

45. Optical Depth • Optical depth describes the attenuation along a path in the atmosphere • Depends on the amount of material in the atmosphere and the type of • material and wavelength of interest • Soot is a stronger absorber (higher optical depth) than salt • Molecules scatter better (higher optical depth) at shorter wavelengths • Aerosol optical depth is typically higher in Los Angeles than Tucson • Total optical depth is less on Mt. Lemmon than Tucson due to fewer • molecules and lower aerosol loading • Optical depth can be divided into absorption and scattering components • which sum together to give the total optical depth • δtotal= δ scatter+ δabsorption • Scattering optical depth can be broken into molecular and aerosol • δscatter= δmolec+ δaerosol • Absorption can be written as sum of individual gaseous components • δabsorption= δ H2O+ δO3 + δCO2 + .........

46. Optical Depth and Beer’s Law • Beer’s Law relates optical depth to transmittance • Increase in optical depth means decrease in transmittance • Assuming that optical depth does not vary horizontally in the • atmosphere allows us to write Beer’s Law in terms of the vertical • optical depth • 1/cosθ=m for airmass is valid up to about θ=60 (at larger values must include refractive corrections) • Recalling that optical depth is the sum of component optical depths • Beer’s Law also relates an incident energy to the transmitted energy

47. Directly-transmitted solar term First consider the directly transmitted solar beam, reflected from the ground, and transmitted to the sensor - the unscattered surface-reflected radiation, Lλsu

48. Solar irradiance at the ground • Can also write the transmittance as an exponential in terms of optical dept • Beer’s law • Need to account for the path length of the sun due to solar zenith • angle of the sun in computing transmittance • Account for the cosine incident term to get the irradiance on the • surface • Recall m=1/cosθsolar • Eλground, solar is • the solar irradiance at • the bottom of the • atmosphere normal • to the ground surface • (shown here to • be horizontal) • Requires a 1/r2 to account • for earth-sun distance

49. Incident solar irradiance • The surface topography will play a critical role in • determining the incident irradiance • Two effects to consider • Slope of the surface • Lower optical depth because of higher elevation • Good example of the usefulness of a digital elevation model (DEM) and • assumption of a vertical atmospheric model

50. Example: Shaded Relief • Surface elevation • model can be used to predict • energy at sensor • Given • Solar elevation angle • local topography • (slope, aspect) from DEM • Simulate incident angle • effect on irradiance • Calculate incident • angle for every pixel • Determine cos[θ(x,y)] • Creates a “shaded- • relief” image TM: Landsat thematic mapper