Advantages: They're easier to install and adjust as only one end of the device requires wiring in, typically the reflector doesn't need power. They're not significantly affected by the object's colour or movement path. They can detect translucent or transparent objects, as the light beam travels through the object twice. Disadvantages: They have a potential dead zone at a very short range ? They can't detect highly reflective objects, as the amount of light bouncing back from anything mirror-like would make it appear that no object was present.