Thermal Imaging Thermal Imaging

Deepali GadkariDeepali Gadkari

Thermal EnergyThermal Energy

All bodies having All bodies having temperature more than temperature more than 00°°k or -273k or -273°°C (absolute C (absolute zero), emit heat.zero), emit heat.

Thermal Infra Red is this Thermal Infra Red is this emitted energy.emitted energy.

IntroductionIntroductionMany multispectral (MSS) systems Many multispectral (MSS) systems

sense radiation in the thermal sense radiation in the thermal infrared as well as the visible and infrared as well as the visible and reflected infrared portions of the reflected infrared portions of the spectrum. spectrum.

However, remote sensing of energy However, remote sensing of energy emitted from the Earth's surface in emitted from the Earth's surface in the thermal infrared (3 μm to 15 μm) the thermal infrared (3 μm to 15 μm) is different than the sensing of is different than the sensing of reflected energy. reflected energy.

Spectral RangeSpectral RangeThere are two windows:There are two windows:33µµm to5 m to5 µµmm8 8 µµm to 14 m to 14 µµ m m

3µm to 5 3µm to 5 µµm image is more m image is more useful for interpretation. More useful for interpretation. More the emittance, the brighter the emittance, the brighter the image is. the image is.

Thermal Energy and SensorsThermal Energy and Sensors

Thermal sensorsThermal sensors use use photo detectors sensitive photo detectors sensitive to the direct contact of to the direct contact of photons on their surface, photons on their surface, to detect emitted thermal to detect emitted thermal radiation. radiation.

Thermal SensorsThermal SensorsThe detectors are cooled to The detectors are cooled to

temperatures close to temperatures close to absolute zero in order to limit absolute zero in order to limit their own thermal emissions. their own thermal emissions. Thermal sensors essentially Thermal sensors essentially measure the surface measure the surface temperature and thermal temperature and thermal properties of targets. properties of targets.

Thermal ImagersThermal Imagers

Thermal imagersThermal imagers are are typically across-track typically across-track scanners scanners that detect emitted that detect emitted radiation in only the thermal radiation in only the thermal portion of the spectrumportion of the spectrum. .

Thermal SensorsThermal Sensors

Thermal sensors employ Thermal sensors employ one or more one or more internal internal temperature references for temperature references for comparison with the comparison with the detected radiation,detected radiation, so they so they can be related to absolute can be related to absolute radiant temperature. radiant temperature.

Recording MediumRecording Medium

The data are generally The data are generally recorded on film and/or recorded on film and/or magnetic tape and the magnetic tape and the temperature resolution of temperature resolution of current sensors can reach current sensors can reach

0.1 °C. 0.1 °C.

ThermogramThermogramFor analysis, an image of For analysis, an image of

relative radiant temperatures relative radiant temperatures ((a thermograma thermogram) is depicted ) is depicted in grey levels, with warmer in grey levels, with warmer temperatures shown in light temperatures shown in light tones, and cooler tones, and cooler temperatures in dark tones. temperatures in dark tones.

Meteorological PurposesMeteorological Purposes

For meteorological For meteorological purposes, this convention purposes, this convention is typically reversed so is typically reversed so that clouds (cooler than that clouds (cooler than the earth’s surface) appear the earth’s surface) appear light toned. light toned.

Thermal ImageryThermal Imagery

Imagery which portrays Imagery which portrays relative temperature relative temperature differences in their differences in their relative spatial locations relative spatial locations are sufficient for most are sufficient for most applications. applications.

Absolute Absolute temperature temperature measurements may be measurements may be calculated but require calculated but require accurate calibration and accurate calibration and measurement of the measurement of the temperature references and temperature references and detailed knowledge of the detailed knowledge of the thermal properties of the thermal properties of the target, geometric distortions, target, geometric distortions, and radiometric effects. and radiometric effects.

Interactions with AtmosphereInteractions with Atmosphere

Because of the relatively long Because of the relatively long wavelength of thermal radiation wavelength of thermal radiation (compared to visible radiation), (compared to visible radiation), atmospheric scattering is atmospheric scattering is minimal. minimal.

However, absorption by However, absorption by atmospheric gases normally atmospheric gases normally restricts thermal sensing to two restricts thermal sensing to two specific regions - 3 to 5 μm and specific regions - 3 to 5 μm and 8 to 14 μm. 8 to 14 μm.

IFOV of Thermal SensorsIFOV of Thermal Sensors

Because energy decreases Because energy decreases as the wavelength increases, as the wavelength increases, thermal sensors generally thermal sensors generally have large IFOVs to ensure have large IFOVs to ensure that enough energy reaches that enough energy reaches the detector in order to make the detector in order to make a reliable measurement. a reliable measurement.

Spatial Resolution of Thermal Spatial Resolution of Thermal SensorsSensors

Therefore the spatial Therefore the spatial resolution of thermal resolution of thermal sensors is usually fairly sensors is usually fairly coarse, relative to the coarse, relative to the spatial resolution possible spatial resolution possible in the visible and reflected in the visible and reflected infrared. infrared.

All-Time Sensing All-Time Sensing Thermal imagery can be Thermal imagery can be

acquired during the day or night acquired during the day or night (because the radiation is emitted (because the radiation is emitted not reflected) and is used for a not reflected) and is used for a variety of applications such as variety of applications such as military reconnaissance, disaster military reconnaissance, disaster management (forest fire management (forest fire mapping), and heat loss mapping), and heat loss monitoring. monitoring.

Most thermal scanning Most thermal scanning operations, such as geologic and operations, such as geologic and soil mapping are qualitative in soil mapping are qualitative in nature. In these cases, it is not nature. In these cases, it is not usually necessary to know usually necessary to know absolute ground temperatue and absolute ground temperatue and emissivities, but simply to study emissivities, but simply to study relative differences in the radiant relative differences in the radiant temperatures within a scene.temperatures within a scene.

Some thermal scanning Some thermal scanning operations require operations require quantitative data analysis quantitative data analysis for determining absolute for determining absolute temperatures – e.g. thermal temperatures – e.g. thermal scanning for monitoring scanning for monitoring surface water temperature surface water temperature of the effluent from a of the effluent from a nuclear power plant. nuclear power plant.

Fields of applicationFields of applicationDetermining rock type and Determining rock type and

structurestructureLocating geological faultLocating geological faultMapping soil type and soil Mapping soil type and soil

moisturemoistureLocating irrigation canal Locating irrigation canal

leaksleaksDetermining the thermal Determining the thermal

characteristics of volcanoescharacteristics of volcanoes

Fields of applicationFields of applicationStudying evapotranspiration Studying evapotranspiration

from vegetationfrom vegetationLocating cold-water springLocating cold-water springLocating hot-water springs and Locating hot-water springs and

geysersgeysersDetermining the extent and Determining the extent and

characteristics of thermal plumescharacteristics of thermal plumes

How would thermal imagery be useful in an urban environment?

Q.1

Detecting and monitoring heat loss from buildings in urban areas is an excellent application of thermal remote sensing. Heating costs, particularly in northern countries such as Canada, can be very expensive.

Extra ShotsExtra Shots

Thermal imaging in both residential and commercial areas allows us to identify specific buildings, or parts of buildings, where heat is escaping. If the amount of heat is significant, these areas can be targeted for repair and re-insulation to reduce costs and conserve energy.