• Thermal Properties• Thermal IR Atmospheric Windows• Thermal IR Environmental Considerations• Thermal Radiation Laws• Emissivity

• Reminder: Read rest of Chapter 8 for next class• Midterm Exam on Monday. Review sheet is posted!

Thermal IR

Selected Applications of Thermal Infrared

Remote Sensing

Selected Applications of Thermal Infrared

Remote Sensing

Nighttime Thermal Infrared Imagery of an AirportNighttime Thermal Infrared Imagery of an Airport

• Kinetic Temperature (Tkin) – true kinetic temperature• Radiant Temperature (Trad) – temperature calculated from radiant exitance (radiant flux)• Usually a pretty darn good correlation, but not always!!• Depends on the thermal emissivity of an object (and discussed later in this chapter).

Thermal Properties

Atmospheric Windows in the Electromagnetic SpectrumAtmospheric Windows in the Electromagnetic Spectrum

Where are the Thermal IR atmospheric windows?

• Thermal IR region of the EM Spectrum is from 3 to 14 µm• Three primary windows:

– 3 - 5 µm– 8 - 9.2 µm– 10.5 – 12.5 µm

Thermal IR Atmospheric Windows

Peak Period of Daily Outgoing Longwave

Radiation and the Diurnal Radiant Temperature of

Soils and Rocks, Vegetation, Water, Moist

Soil and Metal Objects

Peak Period of Daily Outgoing Longwave

Radiation and the Diurnal Radiant Temperature of

Soils and Rocks, Vegetation, Water, Moist

Soil and Metal Objects

When is/are the best time(s) of day to acquire thermal imagery? Why?

When is/are the worst time(s) of day to acquire thermal imagery? Why?

Thermal Infrared Radiation PrinciplesThermal Infrared Radiation Principles

• An analyst cannot interpret a thermal infrared image as if it were an aerial photograph or a normal image produced by a multispectral scanner or charge-coupled device.

• Rather, the image analyst must think thermally.

• The analyst must understand how energy from the Sun or from the Earth interacts with the various terrain components and how the detectors function as they record the terrain’s emitted thermal infrared electromagnetic radiation.

• An analyst cannot interpret a thermal infrared image as if it were an aerial photograph or a normal image produced by a multispectral scanner or charge-coupled device.

• Rather, the image analyst must think thermally.

• The analyst must understand how energy from the Sun or from the Earth interacts with the various terrain components and how the detectors function as they record the terrain’s emitted thermal infrared electromagnetic radiation.

Pre-dawn Thermal Infrared Image of Effluent Entering the Savannah River Swamp SystemPre-dawn Thermal Infrared Image of Effluent Entering the Savannah River Swamp System

March 31, 19814:28 am; 3 x 3 m

March 31, 19814:28 am; 3 x 3 m

2x reduction2x reduction

Savannah River Savannah

River

Pre-dawn Thermal Infrared Image of a Residential Subdivision in Forth Worth, Texas

Pre-dawn Thermal Infrared Image of a Residential Subdivision in Forth Worth, Texas

a b

c

d

e

f

h

g

a b

c

d

e

f

h

g

250 m AGL1 mrad IFOV

6:45 amJan 10, 19800.25 x 0.25 m

250 m AGL1 mrad IFOV

6:45 amJan 10, 19800.25 x 0.25 m

Daytime Optical and Nighttime Thermal

Infrared Imagery of the University of South

Carolina Campus

Daytime Optical and Nighttime Thermal

Infrared Imagery of the University of South

Carolina Campus

April 26, 19814:56 am 1 x 1 m

April 26, 19814:56 am 1 x 1 m

2x reduction

steam lines

steam plant

manhole cover

library

parking

soccer field

dorms

one-dimensional relief

displacement

Vertical Aerial Photograph

Pre-dawn Thermal Infrared Image

line-of-flight

science buildings

a.

b.

steam lines

steam plant

manhole cover

library

parking

soccer field

dorms

one-dimensional relief

displacement

Vertical Aerial Photograph

Pre-dawn Thermal Infrared Image

line-of-flight

science buildings

a.

b.

• Blackbody – Theoretical construct that absorbs and radiates energy at the maximum possible. • Wien’s Displacement Law – Dominant wavelength is inversely proportional to temperature. Thermal Radiation Laws

Blackbody Radiation Curves for Several

Objects including the Sun and Earth

Blackbody Radiation Curves for Several

Objects including the Sun and Earth

For example, the average temperature of the Earth is 300 K (80 ˚F).

We compute the Earth’s dominant wavelength as:

max = 2898 m K

T

max = 2898 m K = 9.67 m

300 K

For example, the average temperature of the Earth is 300 K (80 ˚F).

We compute the Earth’s dominant wavelength as:

max = 2898 m K

T

max = 2898 m K = 9.67 m

300 K

Wein’s Displacement LawWein’s Displacement Law

• The dominant wavelength provides valuable information about which part of the thermal spectrum we might want to sense in. For example, if we are looking for 800 K forest fires that have a dominant wavelength of approximately 3.62 m then the most appropriate remote sensing system might be a 3-5 m thermal infrared detector.

• If we are interested in soil, water, and rock with ambient temperatures on the earth’s surface of 300 K and a dominant wavelength of 9.66 m, then a thermal infrared detector operating in the 8 - 14 m region might be most appropriate.

• The dominant wavelength provides valuable information about which part of the thermal spectrum we might want to sense in. For example, if we are looking for 800 K forest fires that have a dominant wavelength of approximately 3.62 m then the most appropriate remote sensing system might be a 3-5 m thermal infrared detector.

• If we are interested in soil, water, and rock with ambient temperatures on the earth’s surface of 300 K and a dominant wavelength of 9.66 m, then a thermal infrared detector operating in the 8 - 14 m region might be most appropriate.

Wein’s Displacement LawWein’s Displacement Law

• The world is not composed of radiating blackbodies. Rather it is composed of selectively radiating bodies such as rocks, soil, and water that emit only a fraction of the energy emitted from a blackbody at the same temperature. Emissivity, , is the ratio between the radiant flux exiting a real-world selective radiating body (Fr) and a blackbody at the same temperature (Fb):

Fr

= ______

Fb

• The world is not composed of radiating blackbodies. Rather it is composed of selectively radiating bodies such as rocks, soil, and water that emit only a fraction of the energy emitted from a blackbody at the same temperature. Emissivity, , is the ratio between the radiant flux exiting a real-world selective radiating body (Fr) and a blackbody at the same temperature (Fb):

Fr

= ______

Fb

EmissivityEmissivity

• All selectively radiating bodies have emissivities ranging from 0 to <1 that fluctuate depending upon the wavelengths of energy being considered. A graybody outputs a constant emissivity that is less than one at all wavelengths.

• Some materials like distilled water have emissivities close to one (0.99) over the wavelength interval from 8 - 14 µm. Others such as polished aluminum (0.08) and stainless steel (0.16) have very low emissivities.

• All selectively radiating bodies have emissivities ranging from 0 to <1 that fluctuate depending upon the wavelengths of energy being considered. A graybody outputs a constant emissivity that is less than one at all wavelengths.

• Some materials like distilled water have emissivities close to one (0.99) over the wavelength interval from 8 - 14 µm. Others such as polished aluminum (0.08) and stainless steel (0.16) have very low emissivities.

EmissivityEmissivity

• No objects in the world are true blackbodies; rather, they are selectively radiating bodies.• Emissivity (є) is the ratio between the radiant flux exiting a real world selective radiating body (M r) and a blackbody at the same temperature (Mb).

• A graybody outputs a constant emissivity that is less than one at all wavelengths.

Emissivity

Spectral emissivity of a blackbody, a graybody,

and a hypothetical selective radiator

Spectral emissivity of a blackbody, a graybody,

and a hypothetical selective radiator

2x reduction2x reduction

1

0.5

0.1

0.1 1 10 100

0.1 100

100101

102

104

106

108

Wavelength, m

Wavelength, m

Spec

tral

Em

issi

vity

,

Spec

tral

Rad

iant

Exi

tanc

e

W m

-2

m-1

selective radiator

blackbody

6,000 ÞK blackbody = 1.0

graybody

6,000 ÞK graybody = 0.1

6,000 ÞK selective radiator

a.

b.

1

0.5

0.1

0.1 1 10 100

0.1 100

100101

102

104

106

108

Wavelength, m

Wavelength, m

Spec

tral

Em

issi

vity

,

Spec

tral

Rad

iant

Exi

tanc

e

W m

-2

m-1

selective radiator

blackbody

6,000 ÞK blackbody = 1.0

graybody

6,000 ÞK graybody = 0.1

6,000 ÞK selective radiator

a.

b.

Spectral radiant exitance distribution of

the blackbody, graybody, and

hypothetical selective radiator

Spectral radiant exitance distribution of

the blackbody, graybody, and

hypothetical selective radiator

Spec

tral

Em

issi

vity

, eSp

ectr

al R

adia

nt E

xita

nce

W m

-2 u

m-1

• What is the difference between thermal capacity, thermal conductivity, and thermal inertia?

• Thermal capacity (c) is the ability of a material to store heat. It is measured as the number of calories required to raise a gram of material (e.g., water) 1 ˚C (cal g-1 ˚C-1).

• Thermal conductivity (K) is the rate that heat will pass through a material and is measured as the number of calories that will pass through a 1-cm cube of material in 1 second when two opposite faces are maintained at 1 ˚C difference in temperature (cal cm-1 sec-1 ˚C).

• Thermal capacity (c) is the ability of a material to store heat. It is measured as the number of calories required to raise a gram of material (e.g., water) 1 ˚C (cal g-1 ˚C-1).

• Thermal conductivity (K) is the rate that heat will pass through a material and is measured as the number of calories that will pass through a 1-cm cube of material in 1 second when two opposite faces are maintained at 1 ˚C difference in temperature (cal cm-1 sec-1 ˚C).

Thermal Properties of TerrainThermal Properties of Terrain

• Thermal inertia (P) is a measurement of the thermal response of a material to temperature changes and is measured in calories per square centimeter per second square root per degree Celsius (cal cm-2

sec -1/2 ˚C-1). Thermal inertia is computed using the equation:

P = (K x p x c)1/2

where K is thermal conductivity, p is density (g cm-3), and c is thermal capacity. Density is the most important property in this equation because thermal inertia generally increases linearly with increasing material density.

• Thermal inertia (P) is a measurement of the thermal response of a material to temperature changes and is measured in calories per square centimeter per second square root per degree Celsius (cal cm-2

sec -1/2 ˚C-1). Thermal inertia is computed using the equation:

P = (K x p x c)1/2

where K is thermal conductivity, p is density (g cm-3), and c is thermal capacity. Density is the most important property in this equation because thermal inertia generally increases linearly with increasing material density.

Thermal InertiaThermal Inertia

There is an inverse relationship between having high spatial resolution and high radiometric resolution when collecting thermal infrared data.

There is an inverse relationship between having high spatial resolution and high radiometric resolution when collecting thermal infrared data.

Thermal Infrared Remote SensingThermal Infrared Remote Sensing

Forward Looking Infrared (FLIR)

Examples

Forward Looking Infrared (FLIR)

Examples