Wavelength Regions of Electromagnetic Spectrum used in Remote Sensing
Electromagnetic radiation reveals its presence by the observable effects it produces when it interacts with matter. The sun radiates electro-magnetic energy with a peak wavelength of 0.5 μm. Remote sensing data obtained in the visible and reflective infrared regions mainly depends on the reflectance of objects on the ground surface. Therefore, information about objects can be obtained from the spectral reflectance. Optical remote sensing devices operate in the visible, near infrared, middle infrared and short wave infrared portion of the electromagnetic spectrum. These devices are sensitive to the wavelengths ranging from 300 nm to 3000 nm. Most sensors record the EMR in this range, e.g., bands of IRS P6 LISS IV sensor are in optical range of EMR.
The electromagnetic spectrum ranges from the very short wavelengths of the gamma-ray region (measured in fractions of nanometers) to the long wavelengths of the radio region (measured in hundreds of meters). This is divided on the basis of wavelength into regions that are described in Table 1. Fig 1 shows the various regions. It may be noticed that the visible region (0.4 to 0.7 Ám wavelengths) occupies only a small portion of the spectrum. Energy reflected from the earth during daytime may be recorded as a function of wavelength. The maximum amount of energy is reflected at 0.5 Ám wavelength, which corresponds to the green band of the visible region, and is called the reflected energy peak. The earth also radiates energy both day and night, with the maximum energy radiating at 9.7 Ám wavelength. This radiant energy peak occurs in the thermal band of the IR region.
Fig 1. Wavelength regions of the electromagnetic spectrum.
The earth's atmosphere absorbs energy at less than 0.3 Ám, which includes the entire γ-ray and X-ray regions and part of the UV region. These regions of the electromagnetic spectrum are therefore not used for remote sensing. However, some earth surface materials fluoresce or emit visible light when illuminated by longer wave UV radiation. Wavelength regions used for remote sensing therefore include the visible & near infrared, reflected infrared, thermal infrared and microwave regions.
The Ultraviolet Spectrum:
Ultraviolet radiation can be split into the shorter wavelength far ultraviolet and the longer wavelength near ultraviolet (the boundary between the two being at approximately 200nm). The extreme ultraviolet range overlaps with the far ultraviolet at wavelengths of between 1 and 100 nm). Ultraviolet radiation is absorbed by Ozone at an altitude of between 20 and 40 km.
The Visible Spectrum:
Part of the electromagnetic spectrum that our eyes can detect is the visible spectrum. Notice how small the visible portion is relative to the rest of the spectrum. There is a lot of radiation around us which is "invisible" to our eyes, but can be detected and measured by sensors and used to our advantage. The visible wavelengths cover a range from approximately 0.4 to 0.7 Ám. The longest visible wavelength is red and the shortest is violet. Common wavelengths of what we perceive as particular colours from the visible portion of the spectrum are listed below. The visible spectrum includes the reflected energy peak of the earth at 0.5 Ám, and can be used for imaging with film and photodetectors.
Blue,green, and red are the primary colours or wavelengths of the visible spectrum. They are defined as such because no single primary colour can be created from the other two, but all other colours can be formed by combining blue, green, and red in various proportions.
The Infrared Spectrum:
Another portion of the electromagnetic spectrum which is of interest in remote sensing is the infrared (IR) region. It covers the wavelength range from approximately 0.7 Ám to 100 Ám - more than 100 times as wide as the visible portion! Interaction with matter varies with wavelength. Atmospheric transmission windows are separated by absorption bands. The infrared region can be divided into two categories based on their radiation properties - the reflected IR, and the emitted or thermal IR. Radiation in the reflected IR region covers wavelengths from approximately 0.7 Ám to 3.0 Ám, and is used for remote sensing purposes in ways very similar to radiation in the visible portion. (The region from 0.7 to 0.9 Ám is detectable with film and is called the photographic IR band). The thermal IR region, covring a range of 3.0 Ám to 100 Ám is quite different than the visible and reflected IR portions, as this energy is essentially the radiation that is emitted from the Earth's surface in the form of heat. Principal atmospheric windows occur in the thermal region. Images at these wavelengths are acquired by optical-mechanical scanners and special vidicon systems but not by film.
The Microwave Spectrum:
The portion of the spectrum of more recent interest to remote sensing is the microwave region from about 1 mm to 1 m. This covers the longest wavelengths used for remote sensing. The shorter wavelengths have properties similar to the thermal infrared region while the longer wavelengths approach the wavelengths used for radio broadcasts. This portion of the electromagnetic spectrum is used for active remote sensing. Radar images are acquired at various wavelength bands. Longer wavelengths can penetrate clouds, fog, and rain. Images may be acquired in the active or passive mode.
Longest wavelength portion of electromagnetic spectrum - the radio waves - having wavelengths from 1 m to 100 km are mot used for remote sensing, except for some classified radars with very long wavelength which operate in this region.
This website is hosted by
Department of Geology
Aligarh Muslim University, Aligarh - 202 002 (India)