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The specific regions or subdivisions of the electromagnetic spectrum are named and plotted. Mechanisms for generation of electromagnetic radiation are reviewed. The idea of incoming solar radiation or irradiance and its interaction (relative absorption) with the atmosphere is reviewed, and the notion of multispectral remote sensing over different spectral intervals is illustrated with a set of astronomical examples.

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Electromagnetic Spectrum: Distribution of Radiant Energies

As noted on the previous page, 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.

I-9: Given that 1 nanometer (NM) = 10^-9 m, 1 micrometers = 10^-6 m and 1 Angstrom (A) = 10^-10 m, how many nanometers in a micrometer; how many Angstrom units in a micrometer? ANSWER

Referring to the Phenomenology diagram (fourth illustration above): That chart indicates many of the atomic or molecular mechanisms for forming these different types of radiation; it also depicts the spectral ranges covered by many of the detector systems in common use. This diagram indicates that electromagnetic radiation is produced in a variety of ways. Most involve actions within the electronic structure of atoms or in movements of atoms within molecular structures (as affected by the type of bonding). One common mechanism is to excite an atom by heating or by electron bombardment which causes electrons in specific orbital shells to momentarily move to higher energy levels; upon dropping back to the original shell the energy gained is emitted as radiation of discrete wavelengths. At high energies even the atom itself can be dissociated, releasing photons of short wavelengths. And photons themselves, in an irradiation mode, are capable of causing atomic or molecular responses in target materials that generate emitted photons (in the reflected light process, the incoming photons that produce the response are not necessarily the same photons that leave the target).

Most remote sensing is conducted above the Earth either within or above the atmosphere. The gases in the atmosphere interact with solar irradiation and with radiation from the Earth's surface. The atmosphere itself is excited by EMR so as to become another source of released photons. Here is a generalized diagram showing relative atmospheric radiation transmission of different wavelengths.

Blue zones (absorption bands) mark minimal passage of incoming and/or outgoing radiation, whereas, white areas (transmission peaks) denote atmospheric windows, in which the radiation doesn't interact much with air molecules and hence, isn't absorbed. This next plot, made with the AVIRIS hyperspectral spectrometer (see page page 13-9), gives more a more detailed spectrum, made in the field looking up into the atmosphere, for the interval 0.4 to 2.5 µm (converted in the diagram to 400-2500 nanometers).

Most remote sensing instruments on air or space platforms operate in one or more of these windows by making their measurements with detectors tuned to specific frequencies (wavelengths) that pass through the atmosphere. However, some sensors, especially those on meteorological satellites, directly measure absorption phenomena, such as those associated with carbon dioxide, CO₂ and other gaseous molecules. Note in the second diagram above that the atmosphere is nearly opaque to EM radiation in part of the mid-IR and almost all of the far-IR region (20 to 1000 µm). In the microwave region, by contrast, most of this radiation moves through unimpeded, so radar waves reach the surface (although raindrops cause backscattering that allows them to be detected). Fortunately, absorption and other interactions occur over many of the shorter wavelength regions, so that only a fraction of the incoming radiation reaches the surface; thus harmful cosmic rays and ultraviolet (UV) radiation that could inhibit or destroy certain life forms are largely prevented from hitting surface environments.

I-10: From the first atmospheric absorption figure, list the four principal windows (by wavelength interval) open to effective remote sensing from above the atmosphere. ANSWER

Backscattering (scattering of photons in all directions above the target in the hemisphere that lies on the source side) is a major phenomenon in the atmosphere. Mie scattering refers to reflection and refraction of radiation by atmospheric constituents (e.g., smoke) whose dimensions are of the order of the radiation wavelengths. Rayleigh scattering results from constituents (e.g., molecular gases [O₂, N₂ {and other nitrogen compounds}, and CO₂], and water vapor) that are much smaller than the radiation wavelengths. Rayleigh scattering increases with decreasing (shorter) wavelengths, causing the preferential scattering of blue light (blue sky effect); however, the red sky tones at sunset and sunrise result from significant absorption of shorter wavelength visible light owing to greater "depth" of the atmospheric path as the Sun is near the horizon. Particles much larger than the irradiation wavelengths give rise to nonselective (wavelength-independent) scattering. Atmospheric backscatter can, under certain conditions, account for 80 to 90% of the radiant flux observed by a spacecraft sensor.

Remote sensing of the Earth traditionally has used reflected energy in the visible and infrared and emitted energy in the thermal infrared and microwave regions to gather radiation that can be analyzed numerically or used to generate images whose tonal variations represent different intensities of photons associated with a range of wavelengths that are received at the sensor. This sampling of a (continuous or discontinuous) range(s) of wavelengths is the essence of what is usually termed multispectral remote sensing.

Images made from the varying wavelength/intensity signals coming from different parts of a scene will show variations in gray tones in black and white versions or colors (in terms of hue, saturation, and intensity in colored versions). Pictorial (image) representation of target objects and features in different spectral regions, usually using different sensors (commonly with bandpass filters) each tuned to accept and process the wave frequencies (wavelengths) that characterize a given region, will normally show significant differences in the distribution (patterns) of color or gray tones. It is this variation which gives rise to an image or picture. Each spectral band will produce an image which has a range of tones or colors characteristic of the spectral responses of the various objects in the scene; images made from different spectral bands show different tones or colors.

This point - that each spectral band image is unique and characteristic of its spectral makeup - can be dramatically illustrated with views of astronomical bodies viewed through telescopes (some on space platforms) equipped with different multispectral sensing devices. Below are four views of the nearby Crab Nebula, which is now in a state of chaotic expansion after a supernova explosion first sighted in 1054 A.D. by Chinese astronomers (see Section 20 - Cosmology - for other examples). The upper left illustration shows the Nebula as sensed in the high energy x-ray region; the upper right is a visual image; the lower left was acquired from the infrared region; and the lower right is a long wavelength radio telescope image.

By sampling the radiation coming from any material or class under observation over a range of continuous (or intermittent, in bands) spectral interval, and measuring the intensity of reflectance or emittance for the different wavelengths involve, a plot of this variation forms what is referred to as a spectral signature, the subject of the next page's discussion.

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