Sensing plays a critical role for human biology, as well as for technological systems. As distributed sensing, mass surveillance, and body-worn sensors become widespread, there is a need for a clear metric of sensing, to enable design and regulation of sensory devices in line with privacy requirements. In this article, sensing is quantified using veillance flux, to create a metric and visualization of the capacity-to-sense. Tracking the capacity-to-sense “emitted” from cameras, and other sensors, is conceptualized in terms of physics, with applications in transportation vehicles and industrial control, in journalism and scientific discovery, as well as in how human sensing might be protected, shielded, and enhanced, in the face of false information in our increasingly networked world.