Sensor Sense: Contrast sensors

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 (am.pepperlfuchs.com) provided Information for this column.