The most popular photoelectric sensor used is the retro-reflective or reflex mode sensor. One of the advantages of the retroreflective style photoelectric sensors is the light emitter, or light source, and the receiver are in the same enclosure, thus you only have a single device to mount and wire making installation easier and faster. The output of the sensor changes state when the object blocks the light beam between the sensor and the reflector.
Reflectors can be either a special reflector, you really don’t want to use the one off of someone’s bicycle, or reflective tape. The reflector or tape can be out off alignment by as much as 15º and the maximum distance a retroreflective sensor can sense is often based off a certain size reflector. Generally, reflective tape will be 25 – 30% less reflective than the reflector, which is about 3000 times more reflective than white paper. The key thing to remember is the object you are trying to detect must block all of the light coming back from the reflector.
I have seen many applications where a retroreflective sensor was used to detect a shiny object such as a finished wheel, a skid that has been shrink wrapped or shiny finished product and there were many false triggers because of the reflected light from the object instead of the reflector. There have been some applications in dusty environments where the reflector has had a layer of dust causing many false triggers; there is another better solution for these applications.
If you are trying to detect the wrapped skid then the answer is the polarized retroreflective sensor. In this case, there are polarizing filters in front of the light source and the receiver that are ninety degrees out of phase with each other. The reflector will depolarize the light thus allowing the light to reach the receiver. The polarized retroreflective sensor will also typically have a shorter sensing distance than a non-polarized sensor.
Retroreflective sensors have solved many applications over the years with great success and will continue to solve various applications. Just remember to make the target larger than the reflector and watch those shiny surfaces.