Stationary imagers can be permanently installed at the appropriate focal distance and angle for optimal lighting to read low-contrast direct part marks.
The handheld imager can be rapidly adjusted to deliver three types of illumination: bright field, dark field and diffused.
Because of the many variables in direct part marking, adhering to a list of absolute rules will be nearly impossible. However, a few specific guidelines can be applied to the large majority of direct part marks:
- Percussive marking methods: Percussive marking methods rely on changes in depth to create the light and dark elements of the Data Matrix symbol. If the surface of the part was removed to create the mark, then the dark field lighting solution will heighten the contrast for optimal readability. Percussive marking methods that typically create low-contrast symbols include dot peen and some types of laser-etch.
- Textured surfaces: Marked parts with surface texture benefit from the use of diffused lighting. Examples of surface texture include cast and brushed metal parts. According to industry specifications, the element size of the Data Matrix code must be five times larger than the surface texture in order for the symbol to be considered "readable." Diffused light is recommended for reading marks on textured and scratched surfaces. The diffused light helps to minimize indentations and raised areas on the surface, whereas dark field would emphasize surface abrasions.
- Curved surfaces: Marks applied to curved parts that resemble the shape of a cylinder typically require diffused light in order to neutralize the glint that usually appears when light is shown directly onto the curved surface. To further improve readability, mark the symbol as small and format it into a rectangular shape instead of a square shape. This will minimize the amount of distortion to the symbol caused by wrapping. Industry specifications require that the symbol be no more than 16 percent of the diameter of the part.
- Reflective surfaces: Diffused illumination is an effective way to neutralize the specular reflection when reading marks on polished chrome or even steel and aluminum. One solution actually uses the light reflecting back at the reader by the substrate as a white background, to increase the contrast between the light and dark elements of the symbol.
If the symbol applied to the shiny or polished substrate is marked with a percussive method, dark-field illumination is often the most effective for improving readability. By directing light at the symbol at a shallow angle, the light reflects off the edges of the indentations, illuminating the elements of the Data Matrix code.