Sense and avoid systems for civilian unmanned air vehicles (UAVs) are essential in controlled airspace under visual flight rules (VFR). A prototype optical sensor accomplishes the task with attractive performance specifications. Key requirements include long-range detection (up to 10 km), wide field of view, discrimination of small threats against the background and tolerance of direct solar illumination. We demonstrate a prototype system based on a network of independent camera modules equipped with local processing. Availability of a fly-by-wire helicopter configured as a UAV emulator allows for realistic field tests with consumer components. Aspects of the design, implementation and evaluation of the prototype sensor are presented here, as are preliminary measurements to clarify the roles of platform motion, system optical point-spread, noise, direct sunlight and target highlighting.