EM 1110-2-2907
1 October 2003
Figure 2-18. Specular reflection or mirror-like reflection (left) and diffuse reflection (right).
(5) Spectral Radiance. As reflected energy radiates away from an object, it moves
in a hemi-spherical path. The sensor measures only a small portion of the reflected radia-
tion--the portion along the path between the object and the sensor (Figure 2-19). This
measured radiance is known as the spectral radiance (Equation 2-9).
I = Reflected radiance + Emitted radiance
2-9
where I = radiant intensity in watts per steradian (W sr1). (Steradian is the unit of cone
angle, abbreviated sr, 1 sr equals 4π. See the following for more details on steradian.)
Figure 2-19. Diffuse reflection of radiation from a single target point.
Radiation moves outward in a hemispherical path. Notice the sensor
only samples radiation from a single vector. Modified after
2-23
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