Propagation Background and Noise

Noise from the detector and source is always considered in a system design study. However, as an optical field propagates, it acquires additional noise and background radiance from the path defined by the source and by the field of view of the detector. This is typically in the form of propagation path emission and background emission, and bulk scattering within the propagation medium and surface scattering at the propagation medium boundaries of hot-object radiation (e.g., the sun) into the sensor field of view. In many cases this severely limits system performance. Also, in a passive system when no source is present, the background radiance is the signal of interest. Path emission can be modeled by the radiation transfer equation given by Eq. 2.85a. The source function, ℘+(s), must now be given an explicit representation. Figure 11.1 illustrates the incremental emittance per incremental length and bandwidth along an optical path in thermal equilibrium.