Sometimes the things that seem the simple can be the most confusing. In the world of the photo sensor, the principle of “light-on” vs. “dark-on” is one of these things. What am I talking about you might ask? Let’s see if I can help define these concepts.
Light-on or dark-on is a method of detection behavior a photo sensor will exhibit. I say behavior as it does not necessarily represent the sensors physical output behavior or status. This behavior also differs depending on the type of sensor. Specifically, both through-beam and retro-reflective photo sensors behave basically the same in this regard, but diffuse photo sensors will behave exactly the opposite.
For light-on operation, this means that if the receiver in the photo sensor see’s its emitting light source, it will indicate an active or detected indication, typically via a feedback LED on the sensor. So in the case of a through-beam or retro-reflective sensor, if the emitter and receiver, or the reflector (respectively) are properly aligned, the presence of the emitting light source will activate the sensor. But in the case of a diffuse sensor, because the emitting light source is not being reflected by an object in its field of view, the sensor is not active because its light source is being shown out into open space. Now if an object comes between the emitter and receiver or the reflector of a through-beam or retro-reflective sensor respectively, the sensor will no longer show active indicating a blocked beam. In the case of a diffuse sensor, because the object should reflect the emitting light source back to the receiver now, it will show an active state, again, the opposite behavior of the through-beam/retro-reflective types.
For dark-on operation, the behavior is essentially reverse of light-on. If the through-beam or retro-reflective sensors see there emitting light source, the sensor will show inactive. A diffuse sensor on the other hand will show active because it does not see its emitting light source. Sometimes this mode is referred to as negative-logic for you PLC folks. On the other hand, if a through-beam or retro-reflective are blocked and can no longer see their emitting light source, they will show active. In the case of a diffuse type, it will show in-active when the emitting light source is reflected by an object back to the receiver.
The light-on/dark-on concept can seem confusing, but it can offer much freedom in the use of the logical behavior of a photo sensor. For example, using light-on with a through-beam sensor can indicate proper operation when objects are not present and actively indicate an objects presence. Dark-on on the other hand can be used to directly trigger a solenoid valve or gate device when an object “breaks” the beam.
For examples of photo sensors, click here.