EM 1110-2-2907
1 October 2003
(b) The amount of each interaction will be a function of the incoming wave-
length, the composition of the material, and the smoothness of the surface.
(4) Reflectance of Radiation. Reflectance is simply a measurement of the percent-
age of incoming or incident energy that a surface reflects
Reflectance = Reflected energy/Incident energy
(2-7)
where incident energy is the amount of incoming radiant energy and reflected energy is
the amount of energy bouncing off the object. Or from equation 2-5:
EI = EA + ET +ER
Reflectance = ER/EI
(2-8)
Reflectance is a fixed characteristic of an object. Surface features can be distinguished
by comparing the reflectance of different objects at each wavelength. Reflectance com-
parisons rely on the unchanging proportion of reflected energy relative to the sum of in-
coming energy. This permits the distinction of objects regardless of the amount of inci-
dent energy. Unique objects reflect differently, while similar objects only reflect
differently if there has been a physical or chemical change. Note: reflectance is not the
same as reflection.
Specular and diffuse reflection
The nature of reflectance is controlled by the wavelength of the
radiation relative to the surface texture. Surface texture is defined by
the roughness or bumpiness of the surface relative to the wavelength.
Objects display a range of reflectance from diffuse to specular.
Specular reflectance is a mirror-like reflection, which occurs when an
object with a smooth surface reflects in one direction. The incoming
radiation will reflect off a surface at the same angle of incidence
(Figure 2-18). Diffuse or Lambertian reflectance reflects in all
directions owing to a rough surface. This type of reflectance gives the
most information about an object.
2-22
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