Sensor Sense: Contrast sensors

May 11, 2006
Contrast sensors detect differences between two colors by measuring the amount of light reflected by the target.


Contrast sensors detect the difference in reflected light between two different colors. Here, a contrast sensor finds a defective safety seal on a medicine bottle. The white top produces a higher incidence of reflected light than the blue safety seal, indicating an improperly applied safety seal.

Contrast sensors detect differences between two colors by measuring the amount of light reflected by the target. In most situations the change in contrast corresponds to the difference between target and background colors. The threshold at which the sensor output changes state is typically halfway between the light values reflected back from the target and background. Anything lighter than the switching threshold is one state, while anything darker is the opposite state.

Contrast sensors often use RGB (red, green, and blue) technology to help improve differentiation between colors. Two colors, such as red and blue, might both reflect the same amount of white light. This makes it difficult to distinguish between the colors by contrast. However, red light reflects more strongly from the color red than from the color blue. Modern contrast sensors use color LEDs to emit red, green, or blue light while the receiver evaluates light the target reflects. Contrast sensors that operate on RGB principles automatically select the optimal color for detection based on the target and background colors.

A typical application for contrast sensors detects a registration mark on a web of media. The mark triggers a cutting, folding, or gluing process for individual wrappers or cartons as they speed trough a packaging machine.

Pepperl+Fuchs ( provided Information for this column.

Sponsored Recommendations

From concept to consumption: Optimizing success in food and beverage

April 9, 2024
Identifying opportunities and solutions for plant floor optimization has never been easier. Download our visual guide to quickly and efficiently pinpoint areas for operational...

A closer look at modern design considerations for food and beverage

April 9, 2024
With new and changing safety and hygiene regulations at top of mind, its easy to understand how other crucial aspects of machine design can get pushed aside. Our whitepaper explores...

Cybersecurity and the Medical Manufacturing Industry

April 9, 2024
Learn about medical manufacturing cybersecurity risks, costs, and threats as well as effective cybersecurity strategies and essential solutions.

Condition Monitoring for Energy and Utilities Assets

April 9, 2024
Condition monitoring is an essential element of asset management in the energy and utilities industry. The American oil and gas, water and wastewater, and electrical grid sectors...

Voice your opinion!

To join the conversation, and become an exclusive member of Machine Design, create an account today!