Electromagnetic radiation that passes through the earth’s atmosphere without being absorbed or scattered reaches the earth’s surface to interact in different ways with different materials constituting the surface.
There are three ways in which the total incident energy will interact with earth’s surface materials. These are
Absorption (A) occurs when radiation (energy) is absorbed into the target while transmission (T) occurs when radiation passes through a target. Reflection (R) occurs when radiation “bounces” off the target and is redirected.
How much of the energy is absorbed, transmitted or reflected by a material will depend upon:
• Wavelength of the energy
• Material constituting the surface, and
• Condition of the feature.
In remote sensing, we are most interested in measuring the radiation reflected from targets.
Reflection from surfaces occurs in two ways:
When the surface is smooth, we get a mirror-like or smooth reflection where all (or almost all) of the incident energy is reflected in one direction. This is called Specular Reflectionand gives rise to images.
When the surface is rough, the energy is reflected uniformly in almost all directions. This is called Diffuse Reflection and does not give rise to images.
Ultraviolet (UV) light falls in the range of the EM spectrum between visible light and X-rays. It has frequencies of about 8 × 1014 to 3 × 1016 cycles per second, or hertz (Hz), and wavelengths of about 380 nanometers (1.5 × 10−5 inches) to about 10 nm (4 × 10−7 inches). According to the U.S. Navy’s “Ultraviolet Radiation Guide,” UV is generally divided into three sub-bands:
UVA, or near UV (315–400 nm)UVB, or middle UV (280–315 nm)UVC, or far UV (180–280 nm)
The guide goes on to state, “Radiations with wavelengths from 10 nm to 180 nm are sometimes referred to as vacuum or extreme UV.” These wavelengths are blocked by air, and they only propagate in a vacuum.
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 700 nanometers. In terms of frequency, this corresponds to a band in the vicinity of 430–770 THz.
The spectrum does not contain all the colors that the human eyes and brain can distinguish. Unsaturated colors such as pink, or purple variations like magenta, for example, are absent because they can only be made from a mix of multiple wavelengths. Colors containing only one wavelength are also called pure colors or spectral colors.
Visible wavelengths pass largely un-attenuated through the Earth’s atmosphere via the “optical window” region of the electromagnetic spectrum. An example of this phenomenon is when clean air scatters blue light more than red light, and so the midday sky appears blue. The optical window is also referred to as the “visible window” because it overlaps the human visible response spectrum. The near infrared(NIR) window lies just out of the human vision, as well as the medium wavelength infrared (MWIR) window, and the long wavelength or far infrared (LWIR or FIR) window, although other animals may experience them.
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