Photoelectric sensors are used in a wide variety of applications that you encounter every day. They are offered in numerous housing styles that provide long distance non-contact detection of many different types of objects or targets. Being used in such a variety of applications, there are several outputs offered to make integration to control systems easy and depending on the sensing mode when the output is activated in the presence of the target.
Diffuse sensors depend on the amount of light reflected back to the receiver to actuate the output. Therefore, Light-on (normally open) operate refers to the switching of the output when the amount of light striking the receiver is sufficient, object is present. Likewise, Dark-on (normally closed) operate would refer to the target being absent or no light being reflected back to the receiver.
Retroreflective and through-beam sensors are similar in the fact they depend on the target interrupting the light beam being reflected back to the receiver. When an object interrupts the light beam, preventing the light from reaching the receiver, the output will energize which is referred to as Dark-on (normally open) operate switching mode or normally open. Light-on (normally closed) operate switching mode or normally closed output in a reflex sensor is true when the object is not blocking the light beam.
Outputs from photoelectric sensors are typically either digital or analog. Digital outputs are on or off and are usually three wire PNP (sourcing output) or NPN (sinking outputs). The exception to this is a relay output that provides a dry or isolated contact requiring voltage being applied to one pole.
Analog outputs provide a dynamic or continuous output that varies either a voltage (0-10 volt) or current (4-20mA) throughout the sensing range. Voltage outputs are easier to integrate into control systems and typically have more interface options. The downside to a voltage output is it should not be ran more than 50 feet. Current outputs can be ran very long lengths without worry of electrical noise. As additional advantage of the analog output is that it has built in diagnostics, at its minimum there will always be some current at the input unless the device completely fails or the wire is damaged.
Some specialty photoelectric sensors will provide a serial or network communication output for communications to higher level devices. Depending on the network, IO Link, for instance, additional diagnostics can be provided or even parameterization of the sensors. 
Interested in learning more about photoelectrics basics? You can also request a copy of the new Photoelectric Handbook.
