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Fundamentals of Photoelectric Sensing

A photoelectric switch is an electrical device that responds to a change in the intensity of the (transmitted) light falling on it.

It has two main parts - a transmitter of light (most often a Light Emitting Diode or LED) and a receiver of light (generally a photodiode or a phototransistor). The LED is generally pulse modulated at relatively high speeds and the receiver is "tuned" to this frequency of modulation. The receiver thus responds mainly to the transmitted light and is thus relatively unaffected by ambient factory lighting.

Since the transmitter and receiver may be either housed together or separately, three different modes of photoelectric sensing, each with their own unique application capabilities, are possible. These are discussed below.

Through Beam (or Opposed Beam) Sensing


Here the transmitter and receiver are housed in separate enclosures. These are positioned opposite each other, across the detection area, and aligned so that the transmitted light is fully available to the receiver (see figure below). When the target object interrupts the effective beam, detection occurs.

This mode of sensing is thus best suited for detection of opaque objects. It should also be considered for the detection of relatively small targets. Lastly this sensing mode is the most reliable in adverse environments where dust, dirt, moisture, smoke etc could seriously degrade the performance of the other modes.

The separate mounting arrangements for the transmitter and receiver notwithstanding, the opposed mode will always be the most reliable of the three modes for the detection of opaque objects.

For more application information on this mode please see the Application Note "How to select a Photoelectric Switch Type".

Retroreflective Sensing


Here the transmitter and receiver are housed together in one enclosure with a prismatic reflector positioned across the detection area and aligned with the sensor (see figure below). This reflector returns nearly all the transmitted light that falls on it back to the receiver (with the returned light beam being parallel to the transmitted beam) thus enabling relatively large sensing distances. When an opaque object interrupts the effective beam of the switch (or atleast reduces its intensity to below a threshold level), detection occurs.

Given the comparatively large size of the effective beam, this mode is not suited for the detection of small objects. It is also not recommended for the detection of shiny objects (as the light beam returned by such objects is parallel to the transmitted beam, the sensor may confuse them for the reflector and thus may not detect them reliably). It however finds wide application in the detection of relatively larger objects in conveyor applications. Where space does not permit the installation of two enclosures (as required by the through beam mode), the retroreflective mode should be the mode of choice.

For more application information on this mode please see the Application Note "How to select a Photoelectric Switch Type".

Diffuse Reflection (or Proximity) Sensing


Here too the transmitter and receiver are housed together in one enclosure. If the target object returns an adequate amount of light to the receiver, detection occurs (see figure below).

This sensing mode offers comparatively lower distances than the above two modes. It is particularly useful in applications where mounting space exists only on one side of the detection path (for instance overhead detection of objects moving along a conveyor).

For more application information on this mode please see the Application Note "How to select a Photoelectric Switch Type".