A hidden camera is a still or video camera used to film people without their knowledge. The camera is "hidden" because it is either not visible to the subject being filmed, or is disguised as another object. Hidden cameras have become popular for household surveillance, and can be built into common household objects such as smoke detectors, clock radios, motion detectors, ball caps, plants, and mobile phones. Hidden cameras may also be used commercially or industrially as security cameras.A hidden spy camera can be wired or wireless. The former will be connected to a TV, VCR, or digital video recorder (DVR), whereas a wireless hidden camera can be used to transmit a video signal to a receiver within a small radius (up to a few hundred feet). Consequent to the wide proliferation and lower costs of electronics devices, hidden cameras are increasingly finding wider applications.Hidden cameras secretly installed within a common household object to monitor and record the activities of caregivers are commonly referred to as "nanny cams." Legality of using hidden cameras are usually a subject of controversy. For example, a case involving a nanny that was allegedly caught violently shaking a baby was thrown out as worthless evidence (but due to issues regarding video quality, not legality).