Astronomer Federick W Herschel has discovered thermal portion of electromagnetic spectrum in the year 1800. Detector elements for thermal energy were discovered during the world war – II. This lead to the development of thermal infra red remote sensing.
Initially, lead salt photo detectors are used in thermal remote sensing, but now a days detectors are very improved and we are using lead antimonide and mercury doped germanium detectors instead. These germanium and lead detectors are very fast in detecting and reacting to the thermal energy.
First public satellite launched was Televison IR Operational Satellite (TIROS), it has a thermal band in it. The resolution of thermal data acquired using TIROS was very coarse, how ever it is used for mapping and monitoring clouds and frontal movement.
One of the first missions specifically related to thermal infrared remote sensing was launched by NASA in the year 1978. It is named as Heat Capacity Mapping Mission (HCCM). The data is acquired in between 10.5 – 12.6 micrometers and it has very coarse spatial resolution i.e., 600mx600m.
Basics of Thermal Remote Sensing:
Remote sensing can be divided into different categories based on wavelength of energy which is being sensed. Different types are Optical Remote Sensing, Reflective near infra red remote sensing, Thermal remote sensing and Microwave remote sensing. The types of measurements made in the above remote sensing are also different in different cases.
In case of optical and near infra red remote sensing, the wavelength of electromagnetic radiation used ranges from 0.4 to 2.3 micrometers. Here in this type, reflected component of the electromagnetic radiation is sensed.
In case of thermal infra red remote sensing, radiation from 3 to 34 micrometers wavelength is utilized.
However, the effectively used wavelength region in case of thermal remote sensing is from 3 to 14 micrometer only. Because, after 14 micrometer the radiation is predominantly absorbed by the atmospheric constituents.
This thermal infra red energy is emitted by all objects which are having a temperature above absolute zero i.e., 0 degree kelvin or -273 degree centigrade.
All objects above absolute zero emit infrared radiation both during the day and night times.
Thermal infra red remote sensing refers to the detection of remote objects by recording the amount of energy radiated in the infrared region by the objects.
Some portion of sun’s energy is absorbed by the objects. This energy is later released or radiated in the form of infra red radiation by the objects. This radiated energy is used in thermal infra red remote sensing.
Different Wavelength Regions Used in Thermal Remote Sensing:
Human’s sense thermal energy through the sense of touch and is perceived as heat. Human eyes are capable of sensing energy only between 0.4 and 0.7 micrometers, and beyond 0.7 micrometers special sensors required for observing the energy.
0.7 to 3 micrometers is termed as reflective near infra red energy and from 3 to 14 micrometers is thermal infra red energy.
In this 3-14 µm, 3-5µm is often called as MIR i.e., middle infra red, and 8-14µm as thermal infra red. These 2 regions are where atmosphere is mostly transparent to the radiation and is suitable for remote sensing. In other words, the said 2 regions are the atmospheric windows present in the spectrum.
Surface temperature of an object is what that determines this remote sensing. Hence, this thermal remote sensing can be performed during the night time also.
Using this thermal remote sensing it is possible to estimate the surface temperature of an object.
Thermal waves are absorbed by the clouds and hence remote sensing is not possible in cloudy conditions. But they are able to penetrate through smoke and haze.
For example, in case of forest fire there may be large amount of smoke which is spread over the forest area. In case of optical remote sensing, it is not possible to identify the location of fire. Where as in case of thermal remote sensing it is possible to identify the location of fire because thermal waves are capable of passing through smoke and haze.
Some Important Points Related to Thermal Infrared Remote Sensing:
It is very important to remember the following points related to thermal infra red remote sensing.
The energy which is released from the objects during the night time is normally at higher wavelengths i.e., in between 8 to 14 µm.
Where as during the day time, the infrared energy recorded consists of both reflected solar radiation and some amount of emitted radiation from the objects.
The longer waves i.e., from 8 to 14 µm consists more of emitted radiation from objects and less amount of solar radiation.
Hence compared to 3 to 5 µm, 8 – 14 µm is more useful for studying various phenomena about earth surface features in thermal remote sensing.
It is important to note that, optical remote sensing also provides very good information about surface objects, how ever in case of studies that are connected with temperature, thermal data sets (often coupled with optical data) provide more important information.