A key step in choosing an industrial sensor is to understand the operating mode of the sensor. With digital photoelectric sensors, the input and the output are characterized by one of two sensing terms: Light Operate or Dark Operate. Light Operate (LO) describes a condition in which a photoelectric sensor’s output energizes its load when the sensor “sees” a sufficient amount of light; in other words, no target is present or detected by the sensor. Dark Operate (DO), the complement of LO, is a condition in which the sensor output energizes its load when the target is present.
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For additional details on choosing a sensor type refer to:
How do I choose the right sensor?
Industrial Sensing Fundamentals – Back to the Basics: NPN vs PNP
Thanks, that explains it perfectly.