Pepperl+Fuchs Blog

Photoelectric Sensors: A Contrasting View

Posted by Patricia Stafford on Thu, Jul 09, 2015

What is a contrast sensor?

Contrast sensors have the ability to reliably distinguish between two different colors. They are also called print mark or registration mark sensors. These sensors are good for applications such as verifying the presence of a weld seam, confirming that a label has been placed on a package, and detecting registration marks. Basically, these sensors indicate whether a contrast between two taught-in colors is seen or not seen.photoelectric-sensors-contrast-1

Sometimes the most subtle differences can make all the difference. Like when you want to verify that an envelope has an address printed on it, and the color of the print mark on the envelope varies from the color of the envelope itself so slightly that you may not even notice this difference with the human eye. A contrast sensor can detect this kind of print mark faster than you can blink.

Housing material for contrast sensors can be metal, stainless steel, or plastic. Maximum sensing range can vary from 1.5 mm … 800 mm. Light type can be either multi-color LED or red. Switching output options include PNP, NPN, PNP/NPN, and push-pull. Connection type is typically an M12 connector. Versions include a rectangular type and a type with miniature housing. Working voltage is DC. Special features include sensitivity adjustment, function input, static Teach-in, test input, and time function. Degree of protection is IP67.

Our DK12 contrast sensor offers four programming modes and has three color LEDs—red, green, and blue. During programming, the sensor automatically selects the best emitter LED color for the application.


What can you do with a contrast sensor? What kinds of applications are they used in?

Contrast sensors are well known in the following industries for their ability to recognize and record print marks by using precise object positioning, as shown in the following list:

  • Food & Beverage Industry – Contrast sensors locate print marks to determine where to cut wrappers, for example, the wrappers used to package separate sticks of butter.
  • Pharmaceutical Industry – Contrast sensors detect print marks to determine where to crimp tubes of toothpaste.
  • Printing, Paper, & Packaging Industries – Contrast sensors are used for proper labeling, to determine where one page ends and another begins, or to ensure that an address has been printed on envelopes.

photoelectric-sensors-contrast-3How it works

The above photo shows a roll of uncut package labels (sensor not shown). On the right side of the roll are a series of evenly spaced white marks. Every time one of these marks passes in front of a contrast sensor, the sensor quickly recognizes it and indicates the exact position of each mark. Each package label can be checked to ensure it has been correctly printed, at which point it is ready to be precisely cut based on the mark’s location.

Some advantages of contrast sensors include their low contrast resolution, high degree of switching reliability when contrast differences are minimal, and quick response times.

See a contrast sensor in action



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Topics: Photoelectric Sensors

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