Photoelectric Sensors, Diffuse Mode

Diffuse Mode

Sometimes referred to as "proximity mode", diffuse mode sensors are probably the most commonly applied photo eyes of all. In diffuse mode, the sensor depends on the target to direct light back into the receiver lens. There is no opposed pair, no reflector, just the sensor and the target.

Pros and Cons

Diffuse mode sensors are popular for obvious reasons. They consist of only the one piece. They cost less than an opposed pair or even a retroreflective sensor and a reflector. They are easy to align, because you only need to align them squarely to a target. You do not have to align them to a receiver several feet away, or even a reflector. But diffuse mode sensors are also are subject to the most problems. Variables such as color and material come into play. A diffuse mode sensor will pick up a white smooth plastic target a lot easier than a dark blue cloth, for example. What about clear objects?

Versions of Diffuse Mode Sensors

To combat these variables, many versions of diffuse mode sensors are available. Standard diffuse mode sensors generally have ranges from a few inches to a few feet, and they're used commonly to pick up anything from cardboard boxes to plastic bottles. But clear targets and targets with bad contrast call for specialized types of diffuse sensors. The image shown here is a divergent sensor, good for picking up clear or difficult targets.

Hysterisis

Hysterisis can be a problem with diffuse mode sensors. The sensor needs a certain amount of its emiteed light back in order to trigger an output. Hysterisis is a state where the sensor is in a borderline situation. It's having trouble knowing when to trigger, and "chatter" occurs. The amount of light getting back to the receiver lens is right on the threshold. Hysterisis can result in unreliable performance.