Remote Sensing-Electromagnetic Spectrum: Distribution of Radiant Energies

Electromagnetic radiation (EMR) extends over a wide range of energies and wavelengths (frequencies). A narrow range of EMR extending from 0.4 to 0.7 µm, the interval detected by the human eye, is known as the visible region (also referred to as light but physicists often use that term to include radiation beyond the visible). White light contains a mix of all wavelengths in the visible region. It was Sir Isaac Newton who first in 1666 carried out an experiment that showed visible light to be a continuous sequence of wavelengths that represented the different color the eye can see. He passed white light through a glass prism and got this result:

The principle supporting this result is that as radiation passes from one medium to another, it is bent according to a number called the index of refraction. This index is dependent on wavelength, so that the angle of bending varies systematically from red (longer wavelength; lower frequency) to blue (shorter wavelength; higher frequency). The process of separating the constituent colors in white light is known as dispersion. These phenomena also apply to radiation of wavelengths outside the visible (e.g., a crystal's atomic lattice serves as a diffraction device that bends x-rays in different directions).

The distribution of the continuum of all radiant energies can be plotted either as a function of wavelength or of frequency in a chart known as the electromagnetic (EM) spectrum. Using spectroscopes and other radiation detection instruments, over the years scientists have arbitrarily divided the EM spectrum into regions or intervals and applied descriptive names to them.The EM spectrum, plotted here in terms of wavelengths, is shown here.

Beneath is a composite illustration taken from the Landsat Tutorial Workbook (credited there to Lintz and Simonett, Remote Sensing of the Environment, who identify it as a modification of an earlier diagram by Robt. Colwell) that shows in its upper diagram the named spectral regions in terms of wavelength and frequency and in the lower diagram the physical phenomena that give rise to these radiation types and the instruments (sensors) used to detect the radiation. (Although the width of this second diagram scales closely to the width of the spectrum chart above it, the writer experienced difficulty in centering this second diagram on the present page; it needs some leftward offset so that the narrow pair of vertical lines coincides with the visible range in the upper diagram.)

Although it is somewhat redundant, we reproduce here still another plot of the EM Spectrum, with added items that are self-explanatory:

Colors in visible light are familiar to most, but the wavelength limits for each major color are probably not known to most readers. Here is a diagram that specifies these limits (the purple on the far left is in the non-visible ultraviolet; the deep red on the far right is the beginning of the infrared). The human eye is said to be able to distinguish thousands of slightly different colors (one estimate placed this at distinguishable 20000 color tints).

Different names for (wave)length units within intervals (those specified by types) that subdivide the EM spectrum, and based on the metric system, have been adopted by physicists as shown in this table:

(Both in this Tutorial and in other texts, just which units are chosen can be somewhat arbitrary, i.e., the authors may elect to use micrometers or nanometers for a spectral location in the visible. Thus, as an example, 5000 Angstroms, 500 nanometers, and 0.5 micrometers all refer to the same specific wavelength; see next paragraph.)

At the very energetic (high frequency and short wavelength) end are gamma rays and x-rays (whose wavelengths are normally measured in angstroms [Å], which in the metric scale are in units of 10^-8 cm). Radiation in the ultraviolet extends from about 300 Å to about 4000 Å. It is convenient to measure the mid-regions of the spectrum in one of two units: micrometers (µm), which are multiples of 10^-6 m or nanometers (nm), based on 10^-9 m. The visible region occupies the range between 0.4 and 0.7 µm, or its equivalents of 4000 to 7000 Å or 400 to 700 NM The infrared region, spanning between 0.7 and 1000 µm (or 1 mm), has four subintervals of special interest: (1) reflected IR (0.7 - 3.0 µm), and (2) its film responsive subset, the photographic IR (0.7 - 0.9 µm); (3) and (4) thermal bands at (3 - 5 µm) and (8 - 14 µm). We measure longer wavelength intervals in units ranging from mm to cm. to meters. The microwave region spreads across 0.1 to 100 cm, which includes all of the interval used by radar systems. These systems generate their own active radiation and direct it towards targets of interest. The lowest frequency-longest wavelength region beyond 100 cm is the realm of radio bands, from VHF (very high frequency) to ELF (extremely low frequency); units applied to this region is often stated as frequencies in units of Hertz (1 Hz = 1 cycle per second; KHz, MHz and GHz are kilo-, mega-, and giga- Hertz respectively). Within any region, a collection of continuous wavelengths can be partioned into discrete intervals called bands.

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