Direct Part Marking (DPM) verification is greatly dependent upon special lighting options. The right light makes all the difference when it comes to scanning and verifying 2D barcodes. Verifiers use several different lighting angles. 45-degree angle is the most common lighting angle because it is the standard for non-DPM (paper, or regular label) verification. All camera-based verifiers must include 45-degree lighting in order to grade these regular printed codes. DPM (direct part mark) verifiers have the additional lighting options of 30 and 90-degree lighting that makes illuminating symbols on the most challenging of surfaces possible. These lighting options were introduced to the verification process in the “AIM DPM” grading standard in 2006, which is now also an ISO document (ISO/IEC 29158).
Here is an example of how the same barcode looks completely different when viewed 
using different lighting. The image on the far left shows a symbol laser etched into metal illuminated by 45-degree lighting. The background is dark because the shiny surface reflects the incoming light away, and not up, towards the camera. The marked area is not shiny and is scattering some light, some of which does go up into the camera. On the right, the 90-degree light, which is coming straight down, is reflected back up into the camera; therefore, the shiny background appears light. However, the marked area of the code is not very reflective and therefore looks dark. Note that the image on the left is how this code would look as a non-DPM symbol, and has low contrast. The image on the right, on the other hand, appears much brighter and has higher contrast. This was one of the main drivers behind the development of the AIM DPM grading method.
When working with codes that are directly printed onto the surface of a part or onto a curved surface the addition of 30-degree lighting makes a big difference. The DPM verifier series offer 30Q, 30T, 30S and 90 in addition to 45-degree lighting options. 30Q refers to all four sides illuminated by lights at a 30-degree angle. 30T is lighting from two sides and has two options; it can be light from the top and bottom or light from the left and the right. 30S is light from a single side and as you can imagine, there are four different lighting options.
Two sided light or 30T, is ideal for curved and/or
textured surfaces. The images to the right are an example of an object with a brushed surface texture. The image on the left shows light shown from the left and the right. On the left, we see the effect of the brushed surface, which reflects some of the light from the left and right lights from “tangential” point on the brush strokes. The image on the right shows light from the top and the bottom. To get the best image the light should be oriented parallel to the brush stroke. When it is parallel, the light reflects away, as it does on a shiny flat surface. The ability to use lighting from two, and not four, sides was another driving factor behind the development of the AIM DPM grading method.
Webscan is the leader in DPM verification and offers 3 different verifier models to fit into even the most challenging applications. Click here to learn more about the different models we offer. Or for more information on the challenges of DPM verification click here.
