An optical signature is the manifestation of the radiometric characteristics of an object. The signature is formed by self-emitted flux, transmitted flux, and flux reflected from the object's surface. The magnitude of the different signature components depends on the state of the object itself (e.g., internal temperature) as well as the state of its environment (e.g., incident sunlight). The environment can also affect the long-term signature properties such as an increase of the object's temperature resulting from solar irradiance. Some objects' signatures may also depend on the internal state of the object (e.g., aircraft engine setting). Optical signatures also have three-dimensional spatial properties.
The object's intensity varies with view angle around the object. The calculation or measurement of the optical signature from one view is not always indicative of its signature from another view. Figure 8.1 shows three-dimensional spherical plots of calculated contrast intensity signatures. The models used in these calculations are physically accurate models validated by measurement at selected view angles (Appendix B).
A conceptual description of the main contributors to the apparent radiance from a small, nominally uniform, semi-transparent Lambertian surface with uniform surface temperature in open sunshine is shown in Equation (8.1), and Figure 8.2. Signatures for more-complex objects can be constructed as collections of signatures from such small areas.
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