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Satellite Imaging and Sensing
Published in John G. Webster, Halit Eren, Measurement, Instrumentation, and Sensors Handbook, 2017
All objects give off radiation at all wavelengths, but the emitted energy varies with the wavelength and with the temperature of the object. A “blackbody” is an ideal object that absorbs and reemits all incident energy, without reflecting any. If one assumes that the Sun and the Earth behave like blackbodies, then according to the Stefan–Boltzmann law, their total radiant exitance is proportional to the fourth power of their temperature. The maximum of this exitance, called dominant wavelength, can be computed by Wien’s displacement law (see Refs. [1–4] for more details on these two laws). These dominant wavelengths are 9.7 μm for the Earth (in the IR portion of the spectrum) and 0.5 μm for the Sun (in the green visible portion of the spectrum). It implies that the energy emitted by the Earth is best observed by sensors that operate in the thermal-IR and microwave portions of the electromagnetic spectrum, while Sun energy that has been reflected by the Earth predominates in the visible, near-IR, and mid-IR portions of the spectrum. Most passive satellite sensing systems operate in the visible, IR, or microwave portions of the spectrum. Since electromagnetic energy follows the rules of particle theory, it can be shown that the longer the wavelength, the lower the energy content of the radiation. Thus, if a given sensing system is trying to capture long wavelength energy (such as microwave), it must view large areas of the Earth to obtain detectable signals. This obviously is easier to achieve at very high altitudes, thus the utility of spaceborne remote sensing systems.
Thermal radiation and the blackbody problem
Published in Seán M. Stewart, R. Barry Johnson, Blackbody Radiation, 2016
Seán M. Stewart, R. Barry Johnson
In 1893 the recently licensed docent at the Universität zu Berlin, Wilhelm Carl Werner Otto Fritz Franz Wien (1864–1928), was able to extend the work of Boltzmann in an important way. The universal function of Kirchhoff’s, for a given wavelength λ, can be identified with what is today known as the spectral radiant exitance. The spectral radiant exitance will be discussed in greater detail in Chapter 2, where the sub- and superscripts appearing here will be clearly explained. Physically, the spectral radiant exitance corresponds to the amount of energy emitted by a body into a hemispherical envelope in space per unit time per unit area within the unit wavelength interval λ to λ + dλ. Using thermodynamic arguments together with a principle related to the change in wavelength a wave experiences as it moves relative to a source known as the Doppler effect, Wien was able to deduce theoretically [684] the important result equivalent10 to
Radiometry
Published in Antoni Rogalski, Infrared and Terahertz Detectors, 2019
Irradiance and radiant exitance have the same units but have different interpretations. Irradiance is the amount of power with respect to the unit area that falls on a surface, while radiant exitance is the amount of power per unit area that leaves a surface. Exitance thus characterizes a self-luminous source that is producing energy, while irradiance characterizes a passive receiver surface.
Theoretical basics of radiant heat transfer – practical examples of calculation for the infrared (IR) used in infrared thermography measurements
Published in Quantitative InfraRed Thermography Journal, 2021
In this chapter, no strict differentiation is made between radiant exitance (emittance) and radiant intensity. The author, as an electrician, came closer to the term radiant intensity, which meant that it was used throughout the chapter also in the sense of radiant exitance (emittance).