Basic Principles of Electromagnetic Energy, Part 1: The Electromagnetic Spectrum
Electromagnetic energy is the means by which information is transmitted from an object (target) to a sensor. While most of us are familiar with the visible form of electromagnetic energy, it exists in many forms besides visible light. Among these are:
Each of these describes the energy in a specific region of the electromagnetic spectrum. The electromagnetic spectrum represents the continuum of electromagnetic energy from extremely short wavelengths (cosmic and gamma rays) to extremely long wavelengths (radio and television waves).
So that we can discuss sections of the electromagnetic spectrum conveniently, names have been assigned to regions of the spectrum in which adjacent wavelengths behave similarly (or are generated by similar mechanisms). However, the division between "ultraviolet" and "visible", or "microwave" and "thermal infrared" is not hard and fast. The regions blur into one another. The chart below outlines generally accepted regions of the electromagnetic spectrum and gives their spectral ranges.
|
Regions of the Electromagnetic Spectrum |
|
| gamma ray | <0.03 nanometers |
| X-ray | 0.03 - 3 nanometers |
| ultraviolet | 3 nanometers - 0.4 micrometers |
| visible | 0.4 - 0.7 micrometers |
| near infrared | 0.7 - 1.3 micrometers |
| mid-infrared | 1.3 - 3.0 micrometers |
| thermal (far) infrared | 3.0 - 5.0 micrometers AND 8 - 14 micrometers |
| microwave | 0.3 - 300 centimeters |
The basic unit in which wavelengths are measured in the meter (m). In remote sensing, most energy in the visible and infrared portions of the electromagnetic spectrum is measured in micrometers (10-6 m). However, some wavelengths (such as radio and microwaves) are too long for the micrometer to be a convenient unit of measure. For example, while the wavelength of blue light is approximately 0.4-0.5 micrometers, a radio wave is in the neighborhood of 100,000,000 micrometers long (100 m)! You should be aware that visible wavelengths (including ultraviolet, visible, and near infrared) are frequently referred to in units other than the micrometer. Astronomers use a unit called angstrom (10-10 m) to measure these wavelengths. One micrometer equals 10,000 angstroms. Occasionally you may run across this unit when reading satellite documentation from NASA, although most of the information they have for remote sensing audiences uses micrometers. Also, some of the older literature in remote sensing refers to micrometers as microns, and many of the biological sciences still use "micron". One micron equals one micrometer.
