the additional protection provided is generally offset by the increase in
18.104.22.168 Application of Volumetric Sensors. Volumetric sensors are
sometimes used for object protection. The types used for such
applications are generally restricted to those whose energy can be
focused or limited to coverage of the individual object(s). Passive
infrared sensors are used frequently. Ultrasonic sensors are used sometimes
for object protection within a confined area. Because of the difficulty of
limiting the energy, microwave sensors are used seldomly for this
application. However, several bistatic microwave or photoelectric sensors
may be used to establish an electronic "fence" around an individual object.
The effects of environmental conditions, the difficulty of restricting
sensor energy to the object(s) involved, and cost are important tradeoff
considerations in deciding to apply a volumetric sensor to object
protection. Capacitance sensors are designed for this purpose; volumetric
sensors are not. Often, the application is dictated by whether or not the
protected object is metal and can be effectively isolated from ground.