IEEE Geoscience and Remote Sensing Magazine - June 2019 - 73

exploitation because it provides information about the composition of the imaged surface. The
temperature of the surface, known as land surface temperature, is useful in environmental sciences.
Potential applications for LWIR HSI include
◗ mineral identification in terrestrial and planetary geologic studies to map surface materials
◗ detection of mining and drilling activity
◗ rapid detection and identification of hazardous deposits to aid first responders, military operations, and environmental monitoring and clean up
◗ near- or real-time identification and mapping of recently emplaced materials (this capability is
highly desired immediately after natural or man-made disasters)
◗ trafficability assessment and terrain categorization (e.g., moisture content, new construction,
and temporal changes in the terrain) for mission planning and intelligent battlefield preparation
in military operations
◗ identification of man-made solid-phase targets for military and civilian applications
◗ gas-phase target detection for defense, monitoring, and environmental applications.
The primary mission requirements [i.e., scene classification and target (solid-phase and gas-phase)
detection and identification] for all of these applications are based on extracting information from remotely sensed spectral emissivity data using unmixing, classification, and detection algorithms. Furthermore, identifying subtle thermal anomalies in a scene can indicate subsurface density changes;
therefore, emissivity retrieval is a crucial step in LWIR data processing. The retrieval of emissivity is
complicated because of its coupling with temperature and the atmosphere. Estimating the emissivity
from sensor data requires correction for the atmospheric effects, a process known as atmospheric compensation (AC), and estimation of temperature. Joint estimation of emissivity and temperature from
radiometrically calibrated sensor radiance measurements, known as temperature-emissivity separation
(TES), poses a problem because it is mathematically underdetermined and has no unique solution.
However, some practical and useful TES solutions have been developed with the aid of some assumptions or constraints on the structure of emissivity.
The main objective of this article is to introduce unique aspects of the LWIR region and discuss
their implications on LWIR HSI data exploitation. For convenience, we have divided the subject into
different areas. For each area, we provide a concise tutorial introduction to the basic problems, assess
the current state of the art, identify open problems, and point to areas for future research.
PRINCIPLES OF REMOTE LONGWAVE INFRARED SPECTROSCOPY
Surface emissivity is an intrinsic property of natural materials and can be derived from the emitted
radiance measured by airborne, satellite, and standoff sensors. It is therefore important to introduce
the basic principles of radiative transfer theory and use them to derive a signal model relating the atsensor measured radiance to the ground surface emissivity.
DEFINITION OF EMISSIVITY
The kinetic energy of the random motion of particles of matter is known as heat energy. As particles
move, they collide with each other, causing changes in vibrational, orbital, or rotational motions.
These changes result in the emission of EM radiation; thus, heat energy is changed to radiant energy.
A blackbody is a material that transforms heat energy to radiant energy at the maximum rate. The
spectral radiance of a blackbody at temperature T is determined by Planck's law, which is given by
B (m; T) =

C 1 m -5
,
exp (C 2 /mT) - 1

(1)

where C 1 = 1.19104 # 10 10 ^nW/cm 2 /sr/nm h - nm 5 and C 2 = 14387.7 nm - K. Materials are not
perfect blackbodies; rather, they emit radiation depending on their physiochemical makeup, thereby
absorbing some radiation through molecular interactions.
The emissivity of a material, e, at a given wavelength m and temperature T is defined as the ratio of the
radiance L (m; T) emitted by a body at temperature T and the radiance B (m; T) emitted by a blackbody at
the same temperature T, i.e.,
e (m) =

JUNE 2019

IEEE GEOSCIENCE AND REMOTE SENSING MAGAZINE

L (m; T)
,
B (m; T)

(2)
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IEEE Geoscience and Remote Sensing Magazine - June 2019

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