Satellite detectors that are sensitive to radiation in the visible(0.4 to 0.7 µ m) to the 4µ m wavelengths are in a region where reflected solar radiation dominates or at least is significant. In this region, the reflective properties of the Earth and atmosphere are important to understand. Reflection is the process by which a fraction of incident radiation is immediately re-emitted, at the same wavelength, from a surface like a cloud top, a water body, or the Earth's surface, and is not absorbed or transmitted. The process involves reradiation of photons in unison by atoms or molecules in a layer approximately one-half wavelength deep. Reflection exhibits fundamental characteristics that are important in remote sensing. First, the incident radiation, the reflected radiation, and a vertical to the surface from which the angles of incidence and reflection are measured all lie in the same plane. Second, the angle of incidence and the angle of reflection are approximately equal for specular reflectors. The amount of reflection from an object depends on what percentage of the incident radiant flux, defined as the amount of radiant energy onto, originating from, or through a surface per unit time, is absorbed and/or transmitted versus how much is reflected by the object. A ratio of reflected radiant flux from a surface to the incident radiant flux is often used to describe the hemispherical reflectance or albedo of an object. The reflectance of surfaces can also change depending on the angle of the viewer with respect to the sun, a phenomenon known as bidirectional reflectance. The Bidirectional Reflectance Distribution Function (BRDF) is a mathematical description of how reflectance varies for all combinations of illumination and viewing angles at a given wavelength. Also, the BRDF can vary considerably with wavelength. Variations in bidirectional reflectance can significantly affect the appearance of objects in satellite imagery, causing them to appear brighter or darker as a result of the angular relationships among the sun, the object, and the sensor, without regard to any actual reflectance differences on the surface.
There exist various types of reflecting surfaces. Mirror-like surface such as calm water act as a specular reflector. Fresh snow reflects light in nearly every direction and is thus called an diffuse reflector. Typically, the size of surface objects compared to the wavelength of incident radiation determines the type of reflection by a surface. A paved parking lot is a diffuse reflector in visible light, but in the microwave portion of the electromagnetic spectrum, it is a specular reflector.
Here are some subjective comparisons of diffuse reflected energy in shortwave (3 to 4 µ m) and visible (0.4 to 0.7 µ m) channels.
| Table 1. Reflected radiation vs. wavelength | |||
| Surface | Reflectance | ||
|---|---|---|---|
| Vis | Shortwave IR | Longwave IR | |
| Snow | high | low | none |
| Ice | medium | low | none |
| Lake | low | low | none |
| Land Surface | low | medium | none |
| Water Cloud | high | high | none |
| Ice Cloud | low | medium | none |
| Dust | medium | medium | none |