PrintChapter 2 of Campbell (2007) delves into the scientific principles of electromagnetic radiation that are fundamental to remote sensing. If you have studied an engineering or physical science discipline, much of this may be familiar to you. You will see a few equations in this chapter, and while you won't need to memorize or make computations with these equations, it is important to gain a conceptual understanding of the physical relationships represented.
Electromagnetic energy is described in terms of
- wavelength (the distance between successive wave crests),
- frequency (the number of wave crests passing a fixed point in a given period of time), and
- amplitude (the height of each wave peak).
These important terms are further explained in the course textbook. The visible and infrared portions of the electromagnetic spectrum are the most important for the type of remote sensing discussed in this course. Figure 1.01 below illustrates the relationship between named colors and wavelength/frequency bands; it will be a useful reference.
| Color | Angstrom (A) | Nanometer (nm) | Micrometer (µm) | Frequency(hz x 1014) |
|---|---|---|---|---|
| Ultraviolet, sw | 2,537 | 254 | 0.254 | 11.82 |
| Ultraviolet, lw | 3,660 | 366 | 0.366 | 8.19 |
| Violet (limit) | 4,000 | 400 | 0.40 | 7.50 |
| Blue | 4,500 | 450 | 0.45 | 6.66 |
| Green | 5,000 | 500 | 0.50 | 6.00 |
| Green | 5,500 | 550 | 0.55 | 5.45 |
| Yellow | 5,800 | 580 | 0.58 | 5.17 |
| Orange | 6,000 | 600 | 0.60 | 5.00 |
| Red | 6,500 | 650 | 0.65 | 4.62 |
| Red (limit) | 7,000 | 700 | 0.70 | 4.29 |
| Infrared, near | 10,000 | 1,000 | 1.00 | 3.00 |
| Infrared, far | 300,000 | 30,000 | 30.00 | 0.10 |
Understanding the interactions of electromagnetic energy with the atmosphere and the Earth's surface is critical to the interpretation and analysis of remotely sensed imagery. Radiation is scattered, refracted, and absorbed by the atmosphere, and these effects must be accounted for and corrected in order to determine what is happening on the ground. The Earth's surface can reflect, absorb, transmit, and emit electromagnetic energy and, in fact, is doing all of these at the same time, in varying fractions across the entire spectrum, as a function of wavelength. The spectral signature that recorded for each pixel in a remotely sensed image is unique, based on the characteristics of the target surface and the effects of the intervening atmosphere. In remote sensing analysis, similarities and differences among the spectral signatures of individual pixels are used to establish a set of more general classes that describe the landscape or help identify objects of particular interest in a scene.