The metal detectors covered by the standards are the metal detectors seen in airports and courthouses. They operate by generating a magnetic field that interacts with nearby objects. It is the interaction of those objects with the generated magnetic field that is the basis of detection. The standards are technical documents consisting of detection and system performance requirements and a description of the detection performance test methods. Performance standards are necessary to ensure that a particular model of a device performs as expected and that each device of that model behaves similarly. The first requirements and specifications in the standards are for safety, which includes electrical, mechanical, and exposure safety. The performance specifications involve detection sensitivity, speed, repeatability, discrimination, and throughput rate. Certain controls necessary to operate the handheld or walk-through metal detector need to be accessible to the operator only. There are four types of interferences addressed in the metal detector standards: electromagnetic, mechanical, body, and multiple metal objects. Handheld and walk-through metal detectors typically are used in a variety of environmental conditions and can be used both indoors and outdoors. They have weight requirements to reduce fatigue during long-term use and ease of relocation. A handheld metal detector is subject to a variety of abuses such as dropping, severe bumping, and liquid spills. The walk-through is required to have some means of storing its programmed operating parameters so that the walk-through can still operate normally after a power outage or interruption. The standards require that both audible and visual alarms be provided and that they can be easily heard and/or seen. Any given model of handheld or walk-through is required to have interchangeable parts and components to facilitate maintenance. Walk-through detectors are required to be designed for ease of maintenance and the electronics must be of modular design to provide ease of repair. Test methods that are unique to measuring the detection performance of detectors are described in NIJ-Std. Section 3.