Energy Sources

Electromagnetic waves are radiated through space from some source. When the energy encounters an object, even a very tiny one like a molecule of air, one of three reactions occurs. The radiation will be (1) reflected off the object, (2) absorbed by the object, or (3)transmitted through the object. The total amount of radiation that strikes an object is referred to as the incident radiation, and is equal to:

In remote sensing, we are largely concerned with REFLECTED RADIATION. This is the radiation that causes our eyes to see colors, causes infrared film to record vegetation, and allows radar images of the earth to be created. The source of a vast majority of this reflected radiation is the sun.

While the sun is the most obvious source of the electromagnetic energy measured in terrestrial remote sensing, it is not the only energy source one might encounter. This is because all matter at temperatures greater than absolute zero (0 Kelvin) continuously emits electromagnetic radiation. Generally, the hotter an object is, the more it radiates, but all objects with even the slightest sub-molecular motion radiate some energy. More on this idea follows.

Remote Sensing uses electromagnetic energy from both natural and man-made sources. Those energy sources which occur naturally are often referred to as passive energy sources. Remote sensing based on electromagnetic energy deriving from man-made sources is usually referred to as active. Solar energy (including infrared, visible, and ultraviolet light, as well as x-rays and gamma rays) and radiant heat (detectable as the far-infrared) are examples of passive energy sources. Radar and laser profilers are examples of active energy sources.

Electromagnetic energy (radiation) is one of many forms of energy (such as chemical, electrical, kinetic, magnetic, nuclear, or thermal). There are a number of transformation mechanisms that convert the forms of energy listed above into electromagnetic radiation. Some of these are summarized in the following table: