Photoelectric Sensors, Opposed Mode:

First Choice

Opposed mode sensors should be your first choice. In opposed mode (sometimes referred to as thru beam) you have an emitter and a receiver. You are not reflecting light, but rather breaking a beam. The target breaks the effective beam, ceasing the transmitting of light into the receiver lens, thus triggering an output. Opposed mode offers far and away the greatest amount of excess gain. There are opposed pairs rated at 700 feet (sensing range). With this kind of power, it is easy to burn through a dirty or foggy environment.

Opposed Mode Sensing

The example at left is perhaps exaggerated for effect, but it makes a point. Opposed mode sensing is definite. There is no room for doubt. Either the effective beam is blocked, or it is not blocked. There isn't any in between. For that reason, opposed mode should always be your first choice. With opposed mode sensing, you are not subjected to the variables inherent with diffuse (proximity) sensing. Issues like color, texture and hysterisis are not much of a factor. You're either breaking the beam, or you're not breaking the beam. It's the best way.

Pros and Cons

There are a lot of pros to using opposed pairs. About the only negatives are cost and space restrictions. With opposed mode, you are not purchasing twice the electronics, just an extra housing. They cost a little more, but they aren't double the price of diffuse sensors! Space restrictions can present a problem, but there are some tiny opposed pairs available now, with NEMA 6 ratings. Without doubt, opposed pairs are the first place to look when you're using photoelectrics for your application.