Image-Based Barcode Readers

Image-based barcode readers use an area array sensor similar to those found in digital cameras to acquire a picture of 1D and 2D barcodes. Then a microprocessor, running special image-processing software, locates and decodes the code before distributing the resulting data across a network.

Image resolution

figure explaining field of view FOV for reading barcodes with sensors

One of the biggest differentiators for choosing an image sensor, or camera, is resolution. Image resolution refers to how many individual pixels make up each image.

When it comes to matching an image-based reader’s resolution to an application, one of the most common criteria is pixels per module (PPM). PPM refers to how many pixels it will take to cover one cell or module of the code, and will confirm whether the camera has enough resolution to read the code. PPM is calculated by dividing the camera’s resolution in one direction (for example, 752 pixels for a standard resolution reader) by the Y-field of view in millimeters (78 mm), and then dividing the code size in millimeters by the modules (12 mm/22 modules). Finally, multiply these numbers together (5.26 PPM). It may sound complicated, but a configurator app or image-processing software running on these readers can quickly calculate PPM.


figure explaining machine vision liquid lens technology

The optics of an image-based barcode reader are key for acquiring a good image of the code. Quality readers offer both S- and C-mount lens options, depending on the amount of resolution required at a given working distance to acquire an image of the code. The latest readers offer autofocus or liquid lens technology, which allows the reader to adapt to changes in working distances. It works in the same way as the human eye, reshaping and bending in order to focus. This refocus is also accomplished through software, thus avoiding an operator manually adjusting the lens in the field or on the line.


figure for machine vision lighting options bright field, dark field, dome lighting

Lighting also has an important part to play in acquiring a good code image. A lighting technique involves a light source and its placement with respect to the code and the reader. Industrial barcode readers offer various combinations of integrated and external lighting options based on the environment and application. Consult the Cognex Lighting Advisor to explore the effects of different lighting techniques and positions.

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