Thermal Remote Sensing Overview
Thermal remote sensing is based in the infrared portion of the spectrum, more specifically the longwave infrared portion. In remote sensing we examine the near-infrared from 0.7 – 1.3 μm, the shortwave infrared (SWIR) from 1.3– 3 μm and the longwave or thermal infrared from 3-14 μm. The near infrared and shortwave infrared sensors measure reflected infrared light.
Thermal remote sensing refers to measuring the energy that is being emitted from the Earth’s surfaces rather than measuring the reflected energy. Therefore it is important to remember that thermal infrared equates to emitted infrared energy rather than reflected. Thermal remote sensing is a type of passive remote sensing since it is measuring naturally emitted energy. Thermal infrared remote sensing is used to measure land and ocean temperatures, atmospheric temperatures and humidity and the Earth's radiation balance.
Incoming shortwave radiation from the Sun, which includes ultraviolet, visible, and a portion of infrared energy reaches the Earth. As learned in previous modules, some of this energy is reflected and absorbed by the atmosphere and some eventually reaches the surface. At the surface, portions of this energy are reflected and absorbed depending on the material types. The shortwave energy that is absorbed by the atmosphere and the surface is converted to kinetic energy and then emitted as longwave or thermal radiation. Most of the emitted longwave radiation warms the lower atmosphere, which in turn warms the Earth's surface. In thermal remote sensing there is often a need to compensate and correct for atmospheric interaction and emitted energy.
Thermal Atmospheric Windows
While Thermal IR region extends from 3-14 μm, only portions of the spectrum are suitable for remote sensing applications. There are several atmospheric windows in the thermal portion of the spectrum, but none of the windows transmits 100 % of the emitted radiation. Water vapor and carbon dioxide absorb some of the energy across the spectrum and ozone absorbs energy specifically in the 10.5-12.5 μm range. The gases and particles in the atmosphere also absorb incoming radiation and emit their own thermal energy. Most thermal sensing is performed in the 8-14 μm region of the spectrum not only because it includes an atmospheric window, but because it contains the peak energy emissions for most of Earth’s surface features.
Kinetic Temperature vs. Radiant Temperature
Thermal remote sensing measures what is known as the radiant temperature of an object. The radiant temperature is a measure of the emitted energy of an object. This also sometimes referred to as the brightness temperature or the "apparent temperature" of an object. All objects with a temperature greater than absolute zero (0 K) emit electromagnetic energy. Normally when we refer to temperate we are referring the kinetic temperature. Kinetic temperature or heat is generated by the vibration of molecules in all objects. Kinetic temperature is sometimes called “true temperature”. Kinetic temperatures can be measured using a thermometer and is measured using conventional temperature scales (°F,°C, K). There is a high correlation between the true kinetic temperature of an object and its radiant temperature. Therefore we can utilize remote sensing technology like radiometers to measure radiant temperature (from emitted energy) and correlate this to the true kinetic temperature. Often the radiant temperature temperature is often lower than we would expect based on the true kinetic temperature. This is due to a property known as emissivity.