The camera may be "hidden" because it is not visible to the subject being filmed, or is disguised as another object. Such a camera may not be visible to the subject, for example, because it is fitted with a long-focus lens and located beyond the view of the subject, or located, say, behind a two-way mirror. Hidden cameras can be built into commonly used objects such as television sets, smoke detectors, alarm clocks, motion detectors, ink pen caps, plants, and mobile phones. Hidden cameras may be used for household surveillance devices and may also be used commercially or industrially as spying. The proliferation and lower costs of video recording devices have led to an increase in the use of hidden cameras for legitimate surveillance need, as well as for entertainment and other purposes.