THERMAL INFRARED IMAGES AND THEIR APPLICATIONS

 

Thermal infrared radiation is the part of electromagnetic spectrum which has a wavelength of between 3.0 and 20 micrometers. Most remote sensing applications make use of the 8 to 13 micrometer range. The main difference between thermal infrared and the infrared (color infrared - CIR) is that thermal infrared is emitted energy that is sensed digitally, whereas the near infrared (also called the photographic infrared) is reflected energy.  Absorption by water and other gases in the atmosphere restricts sensors to record thermal images in two wavelength windows - 3 to 5 µm and 8 to 15 µm.  For this reason, thermal IR imagery is difficult to interpret and process because there is absorption by moisture in the atmosphere. 

All objects, both natural and manmade, emit infrared energy as heat. By detecting very subtle temperature differences of everything in view, infrared (or thermal imaging) technology reveals what would otherwise not be visible to the naked eye. Even in complete darkness and challenging weather conditions, thermal imaging gives users the ability to see the unseen.

First developed for military purposes, thermal imaging has since been adopted by law enforcement, fire and rescue teams, security professionals, maintenance operations, and more. This technology can be used to detect approaching people or vehicles, to track the footsteps of a fugitive, or to learn why a fire resists extinguishment.

Applications in Agriculture

Thermal imaging has been growing fast and playing an important role in various fields of agriculture starting from nursery monitoring, irrigation scheduling, soil salinity stress detection, plants disease detection, yield estimation, maturity evaluation and bruise detection of fruits and vegetables. Thermal Imaging has gained popularity in agriculture due to its higher temporal and spatial resolution. However, intensive researches need to be conducted for its potential application in other fields of agriculture (e.g. Yield forecasting) that are not yet investigated. In spite of the fact that it could be used in many agriculture operations during pre-harvest and post-harvest period, as a noncontact, non-destructive technique, it has some drawbacks viz., it is more expensive and thermal measurements depend on environmental and weather conditions.  Thus it may not be possible to develop a universal methodology for its application in agricultural operations since thermal behaviors of crops vary with climatic conditions.

Other areas of application of thermal infrared imaging are detection of water stress in crops and evapotranspiration in crops and river basins, which are significant inputs in the management of agricultural practices and integrated watershed management.

Applications in Forestry

Thermal infrared imaging is used in forestry to map and monitor forest cover in terms of vegetation stress and evapotranspiration which is important in environmental management since trees and other plants help cool the environment, making vegetation a simple and effective way to reduce urban heat islands.

Quantitative information about forest canopy structure, biomass, age, and physiological condition have been extracts from thermal infrared data. Basically, a change in surface temperature can be measured by an airborne thermal infrared sensor (e.g., TIMS or ATLAS) by repeatedly flying over the same area a few times. Usually a separation of about 30 minutes results in a measurable change in surface temperature caused by the change in incoming solar radiation. Average surface net radiation is measured in situ for the study area and is used to integrate the effects of the non-radiating fluxes. The change in surface temperature from time period t1 to t2 (i.e., t) is the value that reveals how those non-radiative fluxes arc reacting to radiant energy inputs. The ratio of these two parameters is used to compute a surface proper defined as a Thermal Response Number (TRN).

Terrains containing mostly soil and bare rock have the lowest TRN values, while forests have the highest. The TRN is a site-specific property that may be used to discriminate among various types of coniferous forest stands and some of their biophysical characteristics.

Applications in Water Resources

Detection of water stress and evapotranspiration retrieval are key applications for water management purposes. Thermal infrared remote sensing has been recognized for a long time one of the most feasible means to detect and evaluate water stress and to quantify evapotranspiration over large areas and in a spatially distributed manner. 

Water stress is considered to be a major environmental factor limiting plant productivity world-wide. Water stress develops in plants as evaporative losses cannot be sustained by the extraction of water from the soil by the roots. Evapotranspiration (ET) is a term used to describe the loss of water from the Earth’s surface to the atmosphere by the combined processes of evaporation from surface and transpiration from vegetation. Evapotranspiration depends on the presence of water and is regulated by the availability of energy, needed to convert liquid water to water vapor, and to transport vapor from the land surface to the atmosphere. Physiological regulations also occur in plants through mechanisms controlling water extraction by the roots, water transport in plant tissue, and water release to the atmosphere via the stomata at the leaf surface (in direct relation with the mechanisms of CO2 assimilation and photosynthesis).

Water resources may be monitored and managed through detection of water stress in crops and forests, detection of and quantification of evapotranspiration in crops, river basins and continents.

Applications in Forest Fires

Forest fires are a major cause of degradation of India's forests. While statistical data on fire loss are weak, it is estimated that the proportion of forest areas prone to forest fires annually ranges from 33% in some states to over 90% in other. About 90% of the forest fires in India are created by humans. The normal fire season in India is from the month of February to mid June. India witnessed the most severe forest fires in the recent time during the summer of 1995 in the hills of Uttar Pradesh & Himachal Pradesh. The fires were very severe and attracted the attention of whole nation. An area of 677,700 ha was affected by fires.

Forest fires are characterized by their plumes, their temperature, and their luminosity.  Most in-situ daytime fire sightings result from the observation of smoke generated by fuel combustion, while most nighttime sightings result from high and unusual luminosity of the burning areas. The high temperature of the burning areas make the fires detectable from satellite through thermal infrared imaging.

Clouds consist of tiny particles of ice or water that have the ame temperature as the surrounding air. Images acquired from aircraft or satellites above cloud banks record the radiant temperature of the clouds. Energy from the earth's surface does not penetrate the clouds but is absorbed and reradiated. Smoke plumes however, consist of ash panicles and other combustion products so fine that they are readily penetrated by the relatively long wavelengths of thermal IR radiation.

In visible and thermal IR images acquired over forest fires even during daytime, it is observed that the smoke plume completely conceals the ground in the visible image, but terrain features are clearly visible in the IR image and the burning front has a bright signature. The US Forest Service uses aircraft equipped with IR scanners that produce image copies in flight, which are dropped to fire fighters on the ground. These images provide information about the fire location that cannot be obtained by visual observation through the smoke plumes. IR images are also acquired after fires are extinguished in order to detect hot spots that could reignite.  IR images are also useful in estimating the burnt area.

Applications in Volcanic Eruptions

Volcanic eruptions pose serious hazards to sensitive ecosystems, transportation and communication networks, and to populated regions. Knowing the mineralogy of a rock or alluvial surface is critically important to a geologist trying to interpret the geologic, climatic, or volcanic history of the surface.  The utility of TIR remote sensing for geology and mineralogy has become clear in the past decades and numerous air- and space-based instruments have become available.

Thermal infrared imaging helps scientists track potentially deadly patterns of heat in and around some of the world's 1,500 active volcanoes.  Thermal infrared data processed to highlight hotspots can alert volcanologists to volcanic activity before it becomes dangerous, and may one day help them better forecast eruptions.  Assessing volcanic hazards is an issue since ten percent of the global population lives underneath active volcanoes. 

In high resolution thermal IR images, active volcanoes stand out as bright spots.  They become brighter in a time series images as they ramp up for an eruption, and the speed with which they cool down can tell scientists much about their geological composition, which in turn helps them predict whether the volcanoes will erupt violently.

Scientists already know that volcanoes erupt because of density and pressure. Magma is less dense than rock and rises to the surface at weak points in the earth's crust.  As the magma rises, water and gases dissolved in it expand rapidly, often causing violent explosions – or volcanic eruptions. Volcanoes with a high silica content are of particular interest, because they tend to produce more viscous lava, which traps gas bubbles. As the pressure from the bubbles builds inside the volcano, so does the potential for a powerful and dangerous eruption.

Through thermal infrared images, it is possible to monitor and map eruption clouds, tropospheric Plumes, hot spots and active lava flows.  Post eruptive studies may also make use of thermal infrared imagery.

Epilogue

Temperature and emissivity are powerful biophysical variables critical to many investigations In the near future thermal infrared remote sensing will become even more important.  We now have very sensitive linear- and area-array thermal infrared detectors that can function in broad thermal bands or in hyperspectral configurations.  Very soon unmanned aerial vehicles (UAV) carrying miniature thermal infrared sensors would be seen being used by the military, scintists and even ordinary people.

Notes & Handouts

The Himalayas

Kumaon Himalayas

Askot Basemetals

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