Readability is a measure of how easy or difficult it is for a direct part mark identification (DPMI) reader to successfully read a code. If a code is not readable, the part is not processed and the production line stops. Until recently, manufacturers implementing DPMI have lived with varying levels of read rates, in some cases approaching the upper 90 percent level. However, this level of performance is no longer acceptable.

Today, Six Sigma read rates--a level of quality that equates to only 3.4 missed reads per million opportunities--are essential. To ensure these results, it is important to plan, understand and implement a marking system that builds in quality. A user looking to implement DPMI must understand all the factors that affect the readability of the code. A good baseline for this is to understand the design of the Data Matrix symbology and associated issues that might affect readability.

The features that comprise the Data Matrix symbol are the quiet zone, the finder pattern, the clocking pattern and the data region. Each individual element is referred to as a module or cell. The actual appearance of the code depends on the type of mark placed. For example, a Data Matrix code formed with a laser marking machine or printer would appear with a "continuous L pattern" and square modules. However, dot peen and ink-jet markers produce codes that have a "noncontinuous L pattern," with a data pattern made up of round modules.

A code pattern with individual modules that are consistent in shape and size, and modules that are distinctively different in shape and size from other features on the surface of the part, provide the basis for robust and reliable code location and reading. However, this can be challenging in DPMI applications, due to variations in the surface texture, variations in part presentation during the marking process, and inherent variability of the marking machine. For example, the bumps on the surface of a cast part will show up in an image. If these bumps are similar in size and shape to the dot peen marks of the code, readability suffers because the code blends in with the bumps in the surrounding image.

An objective verification of the code at the marking machine will help ensure that the Data Matrix mark that is produced meets the specifications of the standards, and greatly contributes to readability success downstream.