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Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

New Light Shines on Direct Part Mark Identification

Advancements in optical engineering enable handheld imagers to overcome reading challenges for aerospace and military aviation applications
Stationary imager
Stationary imagers can be permanently installed at the appropriate focal distance and angle for optimal lighting to read low-contrast direct part marks.
handheld imager
The handheld imager can be rapidly adjusted to deliver three types of illumination: bright field, dark field and diffused.
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By Susan Snyder

Direct part mark identification is one of the fastest growing applications for smart cameras. While stationary smart cameras have evolved to meet the optical requirements of low-contrast direct part mark applications, handheld imagers have struggled. A new solution in optical engineering now enables aerospace and military suppliers to read direct part marks that in the past were nearly impossible to read reliably with a handheld imager.

Data Matrix is the most commonly used symbol for direct part marking applications. Resembling a checkerboard, it is a binary code constructed of light and dark data modules (1s and 0s), also referred to as elements. Among the features that make Data Matrix attractive for direct part mark applications is its scalability, robust Reed Solomon error correction, which corrects for matrix code damage, and its ability to be applied to a broad range of substrates by various marking methods. The versatile nature of Data Matrix has allowed aerospace manufacturers to apply it to a wide range of parts, from turbine blades to transmission gears to bearings. The flexibility of Data Matrix has also produced direct part marks with a broad range of characteristics, and as a result, different optics and illumination patterns are required to read them.

For example, dot peen codes present an algorithm challenge because they rely on changes in depth rather than color to create the light and dark elements of the Data Matrix symbol. Lighting and the angle of lighting is critical in order to decode the symbol. Because the cell fill is usually relatively small, grid mapping is also difficult. Marks applied to substrates with a lot of random texture, such as cast parts, often require a different type of lighting in order to minimize the image noise. Marks applied to polished metals have yet additional lighting requirements to neutralize the highly specular nature of the surface.

Stationary imagers have the advantage over handheld imagers in addressing these application challenges. Because the focal distance and angle of the reader in relation to the symbol are critical for reading direct part marks, stationary imagers can be permanently setup at the optimal angle for lighting and reading the symbol. Handheld imagers must rely on the operator?s experience and skill in finding the optimal focal distance and angle. It is also not feasible to apply lighting to a handheld imager if needed, as can be done with a stationary imager.

For the aerospace industry, this is unacceptable. The manufacturing environment for a commercial airplane or military aircraft is different than the highly automated automobile manufacturing facilities. As a result, they rely heavily on handheld imagers to read their direct marked parts instead of stationary imagers. It is considerably more expensive to transport an 800-pound helicopter engine to a fixed position imager than the other way around. In order for programs such as the aerospace SPEC 2000 and Department of Defense UID policy to succeed, handheld imagers must be able to reliably read direct part marks applied to many surface types such as cast and machined metal parts.

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