Key Takeaways
The right illumination is critical to barcode readability—and small lighting changes can deliver big gains in read rates and throughput. Matching lighting type, angle, and configuration to the barcode and surface is essential, especially as DPM codes become more common.
For standard printed high-contrast barcode labels, bright-field illumination aimed directly at the surface typically shows the 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, as in dot peen or laser etching. Such codes can be low contrast, small, or on curved or reflective surfaces, and are becoming much more common. Increasing readability for DPM codes requires specific lighting solutions. Light can be shown on the code at various angles, collimated or diffuse, or 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 can result in increased contrast. This type of lighting must be placed 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 aren't limited to simple fixtures. Many barcode reading tasks have frequent changes in working distance, different kinds and orientations of barcodes, and smaller lots of varied products, all of which require variations in lighting. It can be hard to find one right configuration to take all factors into account, and switching from one to another can take time, reducing throughput.
Bright-field illumination with an integrated distance sensor and quick lighting reconfiguration 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. This enables multiple read setups based on distance sensor measurements, saving a lot of time on variable applications.
The importance of testing
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.