Photoelectric sensors detect and measure physical objects or quantities by emitting a field or beam of electromagnetic radiation, an object is detected by measuring alterations in the return signal. There are 3 main types of photoelectric sensors which provide reliable detection, these are through-beam, retro-reflective and diffuse. The basic function of each is to detect the presence or absence of objects or measure the distance to the object.
Through-beam Sensors
Through-beam sensors (aka thru-beam) rely on two separate housings, one for the transmitter and one for the receiver, with the transmitter providing a continuous beam of light to the receiver. Each time an object passes through the beam this interrupts the signal between the transmitter and receiver, which in turn causes the receiver to send an electrical signal to the output. Through-beam sensors are suited to long-distance sensing and are capable of detecting almost any object, irrespective of colour or angular motion.
Retro-reflective Sensors
Retro-reflective photoelectric sensors (retroreflective) have both the transmitter and receiver contained within the same housing but require a reflector opposite to the sensor. The reflector bounces the light beam back to the transmitter until an object breaks the beam. Objects which are highly reflective such as aluminium require sensors with polarising filters. The filter allows the sensor to acknowledge that the reflected light from reflecting materials is different from the reflector.Diffuse Sensors
Diffuse sensors (proximity sensor) are easier to install as only one device has to be mounted. This is because the transmitter and receiver are contained within one housing. Diffuse sensors use the reflection from the target object within a predetermined sensing range. The transmitter sends out a continuous beam of light, once this hits the target it is diffused in all directions, a part of this light is reflected back and recognised by the receiver, the receiver then sends an electrical signal to the output.
A special type of diffused mode is background suppression which enables the user to precisely control the sensing range. A diffuse mode sensor with background suppression is less sensitive to the target colour. They can also use a visible red light or infrared light sources.
