Photoelectric Sensors, Light Operate/Dark Operate

Light Operate:

Light operate means that you'll get an output when the sensor detects its own light. Most diffuse mode photo eyes use light operate. In other words, they emit light, and they depend on the target to direct light back. Say you're counting boxes on a conveyor line. When no box is present, there is nothing to direct light back to the photo eye's receiver lens, so you get no count. When a box comes along, it bounces light back into the receiver lens, and you get a count. That's light operate. You get an output when the sensor detects its own light.

Dark Operate:

Dark operate means that you'll get an output when the sensor cannot detect its own light. This is usually true of opposed mode photo eyes. In normal condition, the emitter directs light straight across to the receiver lens. When a target comes along and blocks the effective beam, the receiver can no longer detect light. It's "dark", and you get an output.