Passive Terrestrial Systems

Passive imaging systems are attractive not only because they do not require an active illuminator, but also because they can produce images similar in appearance to optical photographs, without the deleterious effects of speckle and other coherent phenomena [1]. Because the thermal power grows linearly with bandwidth $b$, whereas the noise only grows as $b^{1/2}$, the minimum detectable thermal contrast for passive imaging becomes \begin{equation}\Delta T_{min}=T_n\left(\frac{B}{b}\right)^{(1/2)}\end{equation} Consequently, it is ordinarily advantageous to choose as broad a bandwidth $b$ as the electronics and optics of the system will allow, and a $B$ that is as small as is consistent with frame rates, etc. With $T_n=3000$ K, $b=10$ GHz and $B=100$ Hz, $\Delta T_{min}=0.3$ K.