There are 3 main types of photo electric sensor used for detecting an object;

• Diffuse  (sometimes also called proximity mode or direct detection). There is also a variation of diffuse called diffuse with background suppression (diffuse with BGS).
• Retro reflective  (sometimes also called reflex). There is also a variation of reflex called polarised reflex (or polarised retro reflective).
• Through beam (sometimes also called thru' beam).

Determining normally open (N/O) or normally closed (N/C) operation?

Both the retro reflective and through beam sensors rely on the object 'breaking' the beam in order for it to be detected. Whereas the diffuse system requires the object itself to reflect the light beam.
Because of this fundamental difference in the way the sensors detect an object, retro reflective and through beam sensor outputs are the inverse of diffuse sensors. This leads to a lot of confusion.

Historically

Within the photo electric industry, to overcome the confusion of referring to N/O or N/C outputs, the term 'light switching' and 'dark switching' was introduced;

Light switching;

A sensor that is light switching switches it's output 'on' when the light beam is returned to the receiver. i.e. If N/O operation is required, a light switching diffuse sensor could be used.

Dark switching;

A sensor that is dark switching switches it's output 'on' when the light beam blocked from returning the beam to the receiver. i.e. If N/O operation is required, a dark switching retro reflective or through beam sensor could be used.

Recent developments

To simplify product selection, Schneider Electric (under the Telemecanique brand) pioneered the concept (launched as Osiconcept) of 'object detection' with it's Osiris multimode range of sensors. With these products, irrespective of whether the sensor is configured for retro reflective, through beam or diffuse the output will default to N/O. Another feature of these multimode sensors is the ability to invert the output to achieve a N/C mode of operation.

Released for:Schneider Electric Ireland