METR155 Remote SensingLecture 4: Thermal Radiation,

Spectral Signature

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?

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

This can be rearranged to produce

T=255 K

But the actual average temperature of Earth is 288 kelvin (15ºC) and not 255 kelvin. How is this possible?

Question?

Answer: Due to greenhouse effect.

This diagram for remote sensing

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

Radiation components There will be three components of greatest interest in the

solar reflective part of the spectrum

Unscattered, surface reflected radiation Lλ

su

Down scattered, surface reflected Lλ

sd

skylight Up scattered path Lλ

sp

radiance Radiance at the sensor is the sum of these three

Spectral signature Much of the previous discussion centered around the

selection of the specific spectral bands for a given themeThe 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

Spectral SignatureFor 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)

Spectral Signature

Spectral Signature - geologicMinerals and rocks can have distinctive spectral shapes basedon 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

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

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

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 beused for atmospheric remote sensing

Also clues us in to portions of the spectrum to avoid so that the ground isvisible

Spectral SignatureSpectral signature is the idea that a given material has aspectral reflectance/emissivity which distinguishes it fromother 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 materialsThis discussion will focus on the solar reflective for the time being

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 thedesired 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 betweenwater content and plant health

Once the plant material is removed from the plant to allow measurementit begins to dry out Using field-based measurements only is limited by the quality of thesensors

The next question then becomes how many samples are needed todetermine what signatures allow for a thematic measurement

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 fourbands will allow us todifferentiate clay andgrass

Using bands 1, 3, and4 would also besufficient to do this

Even using just bands 3 and 4 would allow us to separate clay and grass

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 betweentwo reflectance spectra for discrimination

Numerous factors can alter the brightness of the sample while notimpacting the spectral shape

Shadow effects and illumination conditionsAbsolute calibrationSample purity

Bands showngive Gypsum - Low, high, lower Montmorillonite - High, high, low Quartz - high, high, not so high

• 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.

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).

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.

Class Participation

Calculate NDVI in these two trees.

Remote Sensing ModelsLecture 3

Spectral signatures, image display, data systems

Terrestrial RadiationEnergy radiated by the earth peaks in the TIR

Effective temperature of the earth-atmosphere system is 255 KPlanck curves below relate to typical terrestrial temperatures

Solar-Terrestrial ComparisonWhen taking into account the earth-sun distance it can be shown that solar energy

dominates in VNIR/SWIR and emitted terrestrial dominates in the TIRSun emits moreenergy than the earthat ALL wavelengthsIt is a geometry effectthat allows us to treatthe wavelengthregions separately

Solar-Terrestrial ComparisonPlots 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

Vertical Profile of the Atmosphere

Understanding the verticalstructure of the atmosphere allows one to understand better the effects of the atmosphere

Atmosphere is divided into layersbased on the change intemperature with height in thatlayer Troposphere is nearest thesurface with temperaturedecreasing with height Stratosphere is next layer andtemperature increases with height Mesosphere has decreasingtemperatures

Atmospheric compositionAtmosphere 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

AbsorptionMODTRAN3 output for US Standard Atmosphere, 2.54 cm column water vapor, default ozone 60-degree zenith angle and no scattering

AbsorptionSame curve as previous page but includes molecular scatter

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

AbsorptionAt longer wavelengths, absorption plays a stronger role with some spectral regions having complete absorption

Three absorption bands, at 1.3 - 1.5 µm, 1.8 - 2.0 µm, and 2.5 - 3.0 µm

Absorption

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

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

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.

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: 610 - 700 nm • Orange: 590 - 610 nm • Yellow: 570 - 590 nm • Green: 500 - 570 nm • Blue: 450 - 500 nm • Indigo: 430 - 450 nm • Violet: 400 - 430 nm

UV

• Ultraviolet: 3 to 400 nm