A hidden camera is a still or video camera used to film people, usually 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 a wide range of applications including monitoring family / partners and household surveillance, they can be built into virtually any common household object such as pens, clock radios, motion detectors, smoke detectors, caps, plants, and cellphones. Hidden cameras are also used commercially and industrially for security purposes.