Pepperl+Fuchs Blog

What's the Difference Between Light ON and Dark ON Modes for Photoelectric Sensors?

Posted by Michael Turner on Tue, Jul 16, 2013

Because photoelectric sensors are available in such a wide variety of sensing modes (thru-beam, diffuse, retroreflective, etc.), the terms light ON and dark ON were introduced to better define what the sensor’s output is doing in the absence or presence of light. These terms apply only to photoelectric sensors.

Light ON (LO) means that the sensor’s output is only enabled or ON when it receives light. Otherwise, the output remains OFF. Dark ON (DO) means the opposite; when light is received, the output is OFF. The sensor’s output is only enabled or ON when it is dark or when it receives no light. See the figure below for further explanation.

Rather than using the standard convention for describing the outputs as normally open (NO) or normally closed (NC), most photoelectric sensor manufacturers specify the output behavior as being either light ON or dark ON. For most people, this method is more straightforward and easier to understand.

Photoelectric sensing modes

Still confused?

It’s okay to be unsure, especially if this is your first time being introduced to these terms.

If you know what sensing mode you need, just answer one simple question and you can be certain you have chosen a sensor with the correct output behavior.

What behavior do I want when an object breaks the light beam, Output ON or Output OFF?

Diffuse Mode Sensors

Output ON: light ON (most common configuration)
Output OFF: dark ON

Diffuse mode sensors

Retroreflective Mode Sensors

Output ON: dark ON (most common configuration)
Output OFF: light ON

Retroreflective mode sensors

Thru-beam Sensors

Output ON: dark ON (most common configuration)
Output OFF: light ON

Thru-beam sensors


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Topics: Terminology, Photoelectric Sensors

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