![]() Another unit of EMR intensity commonly used in remote sensing is therefore something more closely related to what is actually measured by instruments, it is called radiance and is defined as the radiant flux density per unit of projected source area, in a particular direction defined by a solid angle (Figure 14). In other words, to measure exitance for a 1 m 2 part of the Earth’s surface, we would need to place a hemispherical sensor over the area in question, thus measuring the outgoing radiation in all upward directions (if measuring sunlight reflected from the surface, shading by the instrument would obviously make the measurement useless). While irradiance and exitance are useful concepts, we cannot measure them with instruments on flying platforms because they encompass radiation incident upon or leaving a surface in any direction. The arrows of different colour indicate that these two radiant flux densities are measured for all wavelengths present in the radiation field. On the right, exitance is the radiant flux leaving a surface per unit area. On the left, irradiance is the radiant flux incident on a surface per unit area. For practical use, the term irradiance is used to describe radiant flux density incident upon a surface, and exitance is used to describe radiant flux density leaving a surface (Figure 13).ġ3: Irradiance and Exitance. This is called the radiant flux density, and is measured as radiant flux per unit area, for example W m -2. one represented by a pixel in a satellite image). ![]() In remote sensing, we are almost always interested in measuring the intensity of radiation over a finite area (e.g. This is called radiant flux, and is measured in Joules per second, or Watts (W). Because radiation moves as the EMR waves propagate, an important measure of the intensity of an EMR field is the amount of energy moving onto, through or from a surface or volume per unit time. The fundamental SI unit of energy is the Joule (J). Several other properties of EMR fields relate to their intensity – the amount of energy contained in them.
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