How to Choose the Best Type of Sensor for Clear Object Detection

Summary: Clear objects can be challenging to detect because they allow light to pass through them.  Some sensing technologies may work better in certain conditions or may be more effective with certain materials. 

This article compares three types of sensor technologies frequently used for clear object detection: LED-based sensors, laser-based sensors, and ultrasonic sensors.   Learn how each technology works to detect transparent objects and how to choose the right technology for your application.  

Retro-reflective sensors are the most commonly used LED-based photoelectric sensor in clear object detection applications. Each sensor houses an emitter and a receiver. The emitter directs a beam of light to a reflector which redirects the light beam back to the receiver. An object passing between the sensor and reflector will either block or attenuate a perceivable percentage of the emitted light.  

Retro-reflective photoelectric sensors with a coaxial optical design emit and receive light along a single narrow axis. This optimizes the detection of transparent objects by dramatically reducing the risk of false detection from reflected light. Sensors using this technology will have a small bright spot that simplifies sensor alignment and enables very precise leading edge detection. 

A laser-based sensor is another option for detecting clear objects.  A laser-based sensor operates with a laser instead of an LED, allowing for easier alignment, greater precision, detection of smaller objects, and detection of smaller gaps compared to an LED-based sensor. Some laser-based sensors, like the Q4X from Banner Engineering, also do not require a retro-reflector or receiving unit. 

In addition to measuring distance, laser sensors with dual mode capability can also detect changes in light intensity from a stable background condition.   In other words, a laser distance sensor with dual mode can detect not only when the target is present within a certain distance but also when it returns a certain amount of light to the receiver. 

For this to work, a stable reference surface must be taught, and the distance and intensity of the reference surface are recorded and used as a baseline. The presence of a transparent object entering the sensing range of the beam alters the perceived distance and light intensity from the background condition.  

Ultrasonic sensors use sound waves to detect objects. Most ultrasonic sensors detect objects and measure distance by listening for the return echo of an emitted sound wave reflecting off of a target or background condition.

Opposed mode ultrasonic sensors, like the M25U from Banner Engineering, detect an object whenever the signal between the emitter and receiver is blocked. These sensors are faster and provide greater levels of precision than most ultrasonic sensors.  

Because ultrasonic sensors utilize sound rather than light for detection, they are not susceptible to many of the conditions which typically challenge optical sensors.