Data Matrix codes demand proper lighting
Steve Scheiber, Contributing Technical Editor -- Test & Measurement World, 5/1/2006
Since the government began requiring its suppliers to mark parts and assemblies with two-dimensional Data Matrix codes, the use of these codes has exploded even for products not destined for government applications. Data Matrix codes, which are imprinted on a part through direct-part marking (DPM), take up less real estate and can convey more information than conventional bar codes.
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| Fig. 1 The correct light facilitates the accurate reading of Data Matrix marks (red arrow) despite poor contrast against the reflective surface. Courtesy of Cognex. |
The data collected from such parts tracing also provides feedback that can be used for process improvement. In addition, using DPM to imprint a Data Matrix code directly on a part or board surface offers distinct advantages over attaching a label that might curl, come off, or otherwise end up less than useful.
Jamie Pearce, DataMan product marketing manager at Cognex, explained that the recent dramatic growth in DPM identification results from significant advancements in the tools available to read the codes. Today's DPM readers combine high-speed processors, advanced software algorithms, and specialized lighting to permit the reading of codes on a wide range of surfaces.
Pearce said that some readers are versatile enough to offer all necessary illumination types and to automatically select the one that will produce the best results in a given situation. Such tools use pattern-matching software algorithms to successfully read printed bar codes and poorly marked DPM codes. Pearce said users of Data Matrix marking techniques need a DPM reader that offers:
- ruggedness to withstand the factory environment,
- high-yield reading to ensure accuracy under even adverse conditions,
- fast trigger-to-read response, allowing the reader to be as convenient to use as a bar-code reader,
- the ability to handle both easy and challenging applications, and
- a choice of form factors—corded and cordless, as well as handheld and stationary.
In adopting DPM identification for parts traceability, Pearce recommended that a manufacturer consider the amount of available real estate before deciding how much information to include in the Data Matrix code. Consider the surface that will contain the mark and the marking method. Laser techniques, for example, can offer a small mark if space is limited, but the marks may not survive on a high-wear surface. In that case, a dot-peen mark may be more appropriate.
Manufacturers also need to ensure that a code will be readable. Codes marked directly on part surfaces often exhibit much lower contrast than conventional black-on-white labels do. In addition, the characteristics of the part surface and the mark's location can greatly affect its readability. The manufacturing process itself can also degrade the mark's appearance.
Lighting choices
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| Fig. 2 The three types of illumination permit the reliable reading of marks even under difficult surface conditions. |
In low-volume situations however, such as low-volume manufacturing, prototyping, preproduction, and failure analysis, the "large-system stationary light source" and fixed-mount camera system model breaks down. Adapting the lighting technology used by machine-vision systems into handheld devices enables manufacturers to more easily meet that challenge. Traditionally, handheld DPM readers have suffered from limited lighting options, but newer ones, such as the DataMan ID readers from Cognex, include an integrated illumination system that offers a range of lighting choices.
Verifying DPM Data Matrix codes can require one of several lighting configurations (Figure 2):
- Bright-field illumination projects light directly at the mark surface. Although this configuration is often the simplest and least expensive option for a handheld reader, it does not always produce the best results. It works best for high-contrast labels and DPM on less reflective surfaces.
- Dark-field illumination projects light at the mark surface at a low angle. For marking methods such as dot peen, the low angle provides superior contrast. The light can reflect off the mark surface and back to the reader.
- Cloudy-day illumination creates a diffuse, even light across a mark surface that is especially beneficial for marks placed on curved parts or highly reflective materials.
The usefulness of Data Matrix codes depends on the ability to both apply them reliably to a product and read them accurately during test, inspection, and data analysis. Accomplishing those goals, in turn, requires a variety of lighting and reader-configuration alternatives.
