1. METR155 Remote Sensing Lecture 4: Thermal Radiation, Spectral Signature

3. Question: Earth Surface Equilibrium temperature (ET)
The energy that is absorbed by the Earth will be one that reaches the Earth from the Sun then subtracted from that which is reflected from the top of the atmosphere. The fraction of light reflected from the top of the atmosphere called the albedo (A). The albedo of Earth is 0.3 (approximately 30%). What would be the ET?

4. Answer the total power absorbed by the Earth is given by equation
Pabsorbed= S.(1-A).RT2
The power emitted by Earth is given by
Pemitted = 4πRT2.σeT4
To be in radiative equilibrium the condition
Pabsorbed = Pemitted
Question?
But the actual average temperature
of Earth is 288 kelvin (15ºC) and
not 255 kelvin. How is this possible?
This can be rearranged to produce
Answer: Due to greenhouse effect.
T=255 K

7. This diagram for remote sensing

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

9. There will be three components of greatest interest in the
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

10. Much of the previous discussion centered around the
Spectral signature
Much of the previous discussion centered around the
selection of the specific spectral bands for a given theme
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

11. Spectral Signature
For any given material, the amount of solar radiation that it reflects, absorbs, transmits,
or emits varies with wavelength
a general example of a
reflectance plot for
some (unspecified) vegetation type
(bio-organic material)

12. Spectral Signature
For example, at some wavelengths, sand reflects more energy than green vegetation but at other wavelengths it absorbs more (reflects less) than does the vegetation. In principle, we can recognize various kinds of surface materials and distinguish them from each other by these differences in reflectance.

13. 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

14. 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

15. 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

16. Recall the graph presented earlier showing the transmittance
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

17. 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
Challenges:
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

18. Have to keep in mind that a spectral signature is not always enough
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

19. 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

20. Band selection and resolution for spectral signatures should
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

21. Spectral Signature is important property of matter makes it possible to identify different substances or classes and to separate them by their individual spectral signatures, as shown in the figure below.

22. Vegetation: NDVI NDVI - Normalized Difference Vegetation Index Video
Negative values of NDVI (values approaching -1) correspond to water. Values close to zero (-0.1 to 0.1) generally correspond to barren areas of rock, sand, or snow. Lastly, low, positive values represent shrub and grassland (approximately 0.2 to 0.4), while high values indicate temperate and tropical rainforests (values approaching 1).

23. Vegetation Spectral Signature
where RED and NIR stand for the spectral reflectance measurements acquired in the red and near-infrared regions, respectively.
These spectral reflectances are themselves ratios of the reflected over the incoming radiation in each spectral band individually,
hence they take on values between 0.0 and 1.0. By design, the NDVI itself thus varies between -1.0 and +1.0.
The pigment in plant leaves, chlorophyll, strongly absorbs visible light
(from 0.4 to 0.7 µm) for use in photosynthesis.
The cell structure of the leaves, on the other hand, strongly reflects
near-infrared light (from 0.7 to 1.1 µm).
The more leaves a plant has, the more these wavelengths of light are affected, respectively.

24. Class Participation
Calculate NDVI in these two trees.

25. Spectral signatures, image display, data systems
Remote Sensing Models
Lecture 3
Spectral signatures, image display, data systems

26. 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

27. 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

28. 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

29. 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

30. 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

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

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

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

34. having complete absorption
At longer wavelengths, absorption plays a stronger role with some spectral regions
having complete absorption
Three absorption bands, at µm, µm, and µm

35. Absorption

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

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

38. Infrared
Infrared: 0.7 to 300 µm wavelength. This region is further divided into the following bands:
Near Infrared (NIR): 0.7 to 1.5 µm.
Short Wavelength Infrared (SWIR): 1.5 to 3 µm.
Mid Wavelength Infrared (MWIR): 3 to 8 µm.
Long Wanelength Infrared (LWIR): 8 to 15 µm.
Far Infrared (FIR): longer than 15 µm.

39. Visible Light
Visible Light: This narrow band of electromagnetic radiation extends from about 400 nm (violet) to about 700 nm (red). The various colour components of the visible spectrum fall roughly within the following wavelength regions:
Red: nm
Orange: nm
Yellow: nm
Green: nm
Blue: nm
Indigo: nm
Violet: nm

40. UV
Ultraviolet: 3 to 400 nm