  The conventional McCartney model simplifies fog as a scattering medium with
space-time invariance, as the time-variant nature of fog is a pure noise for
classical optical imaging. In this letter, an opposite finding to traditional
idea is reported. The time parameter is incorporated into the McCartney model
to account for photon number fluctuation introduced by time-variant fog. We
demonstrated that the randomness of ambient photons in the time domain results
in the absence of a stable correlation, while the scattering photons are the
opposite. This difference can be measured by photon number fluctuation
correlation when two conditions are met. A defogging image is reconstructed
from the target's information carried by scattering light. Thus, the noise
introduced by time-variant fog is eliminated by itself. Distinguishable images
can be obtained even when the target is indistinguishable by conventional
cameras, providing a prerequisite for subsequent high-level computer vision
tasks.
