Optical sensors are used in a wide variety of industrial applications and also have many domestic uses. Many are available as self-contained units, which not only consist of a light emitter and receiver but also perform all the necessary electronic functions such as modulation/demodulation, amplification and output switching. They are available in different formats, with many having very compact dimensions. Pulsed infra-red or a laser light source is usually employed and they generally fall into three main types: through-beam, auto-reflective or diffuse reflective. The through-beam (or opposed) configuration usually offers the highest degree of accuracy. The auto-reflective type has the emitter and receiver mounted together, utilising an opposing reflector. The diffuse-reflector (proximity sensor) relies on reflected light from a subject entering its operational field. They are widely used to indicate the presence or absence of a substance or object, as well as for measurement. Some employ more than one beam. Many specialised types such as displacement sensors, printed mark readers or colour sensors are available for specific tasks. A wide range of accessories is available, including amplifiers, mounting brackets and cabling. For hostile environments it may be preferable to use remote sensor heads and fibre optic cabling. Some applications may even require cameras.