The Ocular Halo is a device typically worn by the Foundation's Military Police (A division of the Crimson Blade): a video-recording device that functions by recieving light in a 360 degree array around the individual wearing it. This allows the halo not only to record everything the wearer sees, but also everything behind and outside their FOV.
The system works based on the Foundation's light-capture sensors deployed in a variety of scenarios. Essentially it appears as a narrow band, but is receiving light from all directions, giving it a much better recording range than a traditional lens-based camera system. The recorder data can be fed into a computer and create a complete 3D simulation of any area the wearer has passed through.