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Optical Proximity sensors
 

 A complete optical proximity sensor includes a light source, and a sensor that detects the light. The light source is supplied because it is usually critical that the light be "tailored" for the light sensor system. The light source generates light of a frequency that the light sensor is best able to detect, and that is not likely to be generated by other nearby sources. Infra-red light is used in most optical sensors. To make the light sensing system more foolproof, most optical proximity sensor light sources pulse the infra-red light on and off at a fixed frequency. The light sensor circuit is designed so that light that is not pulsing at this frequency is rejected.

Optical proximity generally used to detect all types of object. Advantage of optical sensor is they have long sensing range.

Optical sensors are divided into three main sub categories :-
    Diffuse type : Here infra- red light transmitter and receiver are at one end. When any object enters in its field, light is reflected back and detected by receiver giving output.
    Retro reflective sensor : Here also both transmitter and receiver are at one end. A reflector is placed in front of sensor. Reflector reflects light and any object that breaks light beam is detected and output is activated.
    Through beam : Here transmitter and receiver are at opposite end. Any object that breaks light beam is detected and output is activated.
    Laser optical sensors : These sensor uses laser light instead of infra-red light. These are used to detect small object, distance measurement etc.