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Retro-reflective photoelectric sensors: polarised or not?

We are going to talk about the differences and the applicability of photoelectric retro-reflective sensors with a polarized and a normal reflector. Finally, we give an idea of what is the best choice for your application
The ILR 31-46 R is a reflector for Telco Sensors reflector photocells.

Retro-reflective photoelectric sensors can perform a detection after an interruption of the light signal from a reflector. Is everything said now? No, because a distinction can be made between normal reflector sensors and polarized reflector sensors. In this article we expose these two kinds of reflective photoelectric sensors, when these are used and which one is the best choice in what application.

Why a retro-reflective sensor?

With a photoelectric sensor without reflector, such as a diffuse proximity sensor, an object is used as a source of reflection. It happens that the emitted light is reflected in every direction and barely or no reflection will arrive in the sensor.

In applications where a bigger distance has to be covered, they tend to choose for a photoelectric sensor with a reflector. With this sensor, more assurance can be given for a good reflection of the emitted light. The reflector is a stable and consistent surface compared to for example a wall or the products that pass with different shapes and colors.

What is a polarized retro-reflective sensor?

In the previous paragraph is discussed why you could choose for a retro-reflective sensor. What can a polarized reflector offer on top of that? The answer is a more reliable solution.

In the essence, a polarized reflector works exactly the same as a usual reflector. The difference is in the way the emitted light is rebounded. A retro-reflective sensor with a normal reflector can suffer interference in an application with multiple light sources. The reflector than also sends the light of these sources as a reflection to the sensor, which could lead to a wrong detection.

A retro-reflective sensor with polarized reflector is provided with a so called polarisation filter. This filter is making sure that light with a given wavelength is reflected and the rest of the wavelengths not. By using this attribute, only the light with the wavelength of the emitted light is reflected. Than it is less likely that false reflections of the undesired lightsources are rebounded to the sensor. In this way, you can be more sure that the sensor only receives the light that it emitted itself.

A couple of applications for retro-reflective sensors

The theory of these sensors can be reflected to some reality examples. Below are given a couple of applications in which interpratation is given to the product attributes of retro-reflective sensors with a normal or with a polarized reflector.

Detection on conveyors

A lot of products are transported industrial via a conveyor from the one process to the other. Here a retro-reflective sensor could be used. In the application a detection will be done at the moment that a product interrupts the emitted light. A retro-reflective sensor with a normal reflector is sufficient in these kind of applications. Think of the SPPR 2910 5 from Telco Sensors with a potentiometer which makes it possible to set up the sensitivity.

When these are products with a higher reflectivity or when different sorts of products are passing and the chance of a false detection is possible, the better choice is a polarized retro-reflective sensor. An example is the QERN/BN-0F from Micro Detectors with which a reliable detection can be guaranteed.

Vehicle detection

Not only products but also vehicles can be detected with a retro-reflective sensor. Think of acces control with the installation in a passage such as barriers. When a vehicle has passed the gate the sensor knows this and now can make sure that the gate will close. This same sensor application can be used for counting the number of cars that enter a parking garage or another public space. A thinkable sensor is the SPPR 2910/1 5 from Telco Sensors.

When to use which photoelectric sensor?

Below a number of frequent applications are given with the most obvious retro-reflective sensor to use. Is your application not given below or you rather talk to an expert and receive free advice? Please contact the experts to the right of this article!

 Normal reflectorPolarized reflector
Detection of shiny objects X
Detection of matte objectsX 
Detection of transparant objects X
Measuring range12 m10 m
Level detection in transparant containers X
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