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Most Effective Way to Improve Read Rates: Lighting

barcode reader lighting DPM code

Barcode readability is primarily dependent on how well the barcode itself is printed, the capability of the barcode reader, and whether the barcode is illuminated in a way that maximizes its contrast. The quality and appropriateness of the lighting is an often underappreciated way of improving read rates. In fact, appropriate lighting can result in significant read rate increases at a moderate cost.

For standard printed high-contrast barcode labels, bright-field illumination aimed directly at the surface typically shows best results, though even here there are small changes that can improve readability.

Many codes are not printed on such labels, but are instead direct part marked (DPM) on the object itself by a physical method such as dot peen or laser etching. Such codes can be low contrast, small, or on curved or reflective surfaces, and are becoming much more common in some industries. Increasing readability for DPM codes requires specific lighting solutions. Light can be shown on the code at various angles, be collimated or diffuse, and be of various colors and intensities.

The goal is to choose a lighting type that creates a homogenous, high intensity, and glare-free image that maximizes the contrast both within the code and between the code and its background. It is worth time and effort to make the right choice of lighting.

Bright-field illumination

For a barcode printed on a label, a bright on-axis or slightly off-axis directional light is usually the best solution for enhancing barcode contrast. Even with well-printed barcode labels, it is worth paying attention to lighting configuration. Slight modifications in intensity, angle, filtering, and color can have noticeable effects. If there is any surface reflection that could cause hot spots in the image, diffuse illumination would be a better choice.

Dark-field illumination

Dark-field lighting is particularly well suited to reading DPM codes. Dot peening involves indenting a surface with a series of disconnected dots, and thus has a third dimension, unlike printed barcodes. In laser etching, a focused laser produces changes in color, texture, or depth on the object’s surface.

Dark-field lighting involves illuminating the DPM code at a low angle, typically 30 degrees. Like a rising or a setting sun, this angle emphasizes texture differences and dimensionality. The camera, directly above the code, receives only light reflected from the edges of the DPM code. Light from the surface is reflected away from the camera, minimizing any problems with a shiny surface.
Depending on the specific qualities of the surface holding the DPM code, choosing a specific color, such as blue, can result in increased contrast. This type of lighting has to be close to the code being read, and can be produced by a variety of light sources, including bar, ring, or spot lights.

Diffuse illumination

A curved or reflective surface will create uneven light reflections from an on-axis light such as a co-axial ring light, resulting in hot spots and darker areas. This overwhelms the contrast in the barcode itself. Diffuse lighting, also called full bright-field or cloudy-day lighting, creates an even background, making the code more readable. This is particularly effective when the light can be placed close to the barcode. Some printed labels, or labels that have to be read through a transparent wrapping, also suffer from reflections and hot spots, and so also benefit from diffuse lighting.

Quick setup and distance detection

Of course, barcode lighting solutions are not limited to simple fixtures. In many barcode reading applications there may be frequent changes in working distance, different kinds and orientations of barcodes, and smaller lots of varied products that require variations in lighting. It can be cumbersome to establish optimum read setups that take all factors into account, and even after the setups are established, switching from one to another can take time, reducing throughput.

Bright-field illumination with an integrated distance sensor and quick lighting reconfiguration, such as that provided by the Cognex High-Powered Integrated Torch (HPIT) light for fixed-mount barcode readers, can be a great solution in such cases.

As distances to the barcode change, a light with distance detection can instantly increase or decrease intensity to keep barcode illumination constant and optimize read time simultaneously with camera focus and other changes. HPIT dynamically enables multiple read setups based on distance sensor measurements, saving a lot of time on variable applications.

HPIT lighting automatic read setups
 
In general, production engineers often lack the time and resources to investigate the full possibilities of lighting improvement in their applications. The HPIT light’s flexibility makes it easy to investigate possible lighting changes to further improve performance.

The importance of testing

The HPIT light makes testing lighting setups easy. But every lighting setup needs to be checked and modified periodically. Initial setup may well have been optimal, but conditions can change over time. The texture or contrast on printed labels can be different depending on printer condition. Changes in the line can bring intrusive lighting from other equipment. A decline in read rates can often be traced back to poor lighting conditions, and quickly modified.

For something so visible, lighting is surprisingly neglected when it comes to optimizing barcode reading. Giving illumination the attention it deserves will pay dividends in increasing read rates and throughput. 

Download the DataMan 370 with HPIT datasheet to learn more about optimal integrated lighting for your application.

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