“Day & night marking” is used in automobiles, aviation, instrumentation and computer keyboards to make buttons and controls (e.g. door locks, window controls, sound system adjustments, etc.) clearly visible under ambient illumination conditions varying from bright sunlight (day) to low light (night). Although it sounds simple, manufacturing these products cost-effectively in a small-batch production environment requires the use of a sophisticated, automated, laser-based tool.

What is Day & Night Marking?

Figure 1. Day & night marking of automotive interior controls. Under front illumination (day), the button markings appear dark against a light background. Under rear illumination (night), they appear light against a darker background.

Day & night marking solves the simple need to provide legends that are easily seen and clearly legible on plastic buttons and controls under widely varying illumination conditions. To achieve legibility for both day & night, a clear plastic button is first painted with a layer of semitransparent dark paint. Then, a second layer of opaque paint in a contrasting (light) color is overlaid on top of this. The top layer of paint is then selectively removed to create the desired button legend (mark pattern) by revealing the darker layer beneath.

When illuminated externally, the mark appears dark against a light background. A light source (typically LED) placed beneath the button illuminates it from within when the ambient illumination level is low. This light passes through the semitransparent dark paint, but is blocked by the opaque paint layer on top. This makes the mark appear light against a dark background. The exact opposite contrast (light mark on a dark button) can be achieved by making the first layer semitransparent light paint, and overlaying this with an opaque layer of darker paint.

Historically, day & night marking was performed using either two-part injection molding or rotary engraving. The injection molding method is still in use, especially for high volume production. But, this approach requires expensive tooling, and can’t readily produce high resolution marks on small buttons.

Laser engraving has emerged over the past 20 years as the preferred method for rapid production of custom day & night marks because it offers an unmatched combination of speed, operational flexibility and mark quality. In laser engraving, a high-power laser beam is scanned over the surface under computer control to ablate the necessary amount of material in the desired pattern.

One of the major advantages of laser processing is that it can remove material to the desired depth with very high precision. Specifically, it can ablate the top layer of paint from the part, without removing essentially any of the underlying paint layer. It also delivers high dimensional accuracy in the mark itself. The result is excellent quality and high part-to-part consistency in terms of mark appearance.

The Challenge

Figure 2. Day & night marking is accomplished by placing two layers of contrasting paint on a plastic button. Selective removal of just the outer layer of opaque paint creates the mark. This makes the mark visible under both front (day) and rear (night) illumination conditions.

Crestron Electronics produces custom automation systems for both consumer and commercial applications. Typical uses are controls for lighting, window shades, audio, security, teleconferencing, and displays. Frequently, these systems are operated using either wall mounted keypads or handheld remote controls. The legends on these devices often incorporate day & night marks.

In terms of manufacturing, key imperatives for Crestron Electronics are to produce highly legible and visually appealing marks that are congruent with their high-end brand image. Plus, increasingly, these marks involve relatively complex logos and graphics, rather than just traditional alphanumeric characters. The small physical size of these marks requires that the marking system deliver high spatial resolution and sharp, well defined edges.

The challenge for Crestron Electronics is that their products are usually produced in small quantity (often just one or two units). Thus, the marking process must be readily configured to produce a custom mark with minimal setup time, and to deliver an acceptable unit production cost in low volume. Their combination of small batch sizes and high mark quality means it is not cost effective to employ volume production methods such as two-part injection molding.

Furthermore, Crestron Electronics has empowered their dealers and end-users to create their own mark artwork which is then automatically transmitted to the manufacturing facility. Specifically, this means that customers must be able to create any graphic design they wish, without having to worry about how the laser system will fabricate it, and then have a simple means for transmitting their design to production personnel.

Day & Night Marking in Action

Figure 3. Crestron Electronics uses day & night marking on the remotes and control keypads of many of its automation products.

Crestron Electronics utilizes a Coherent LME-RM laser marking system because it offers the combination of mark quality and operational ease-of-use they require. This is a turnkey system that can be equipped with a wide variety of different laser sources for marking all sorts of materials (metals, ceramics, plastics, etc.).

For plastics marking, the source is most typically a solid-state (Nd:YVO4) laser, since these provide the short pulse duration that usually delivers the most pronounced color or contrast change in the plastic, without any thermal damage to the material. These lasers are available with infrared, green or ultraviolet (UV) output to match them to specific material absorption characteristics. This same advantage (minimal heat affected area) also makes the solid-state laser the best choice for day & night marking, even though it is actually an ablation process, rather than one in which a color or texture change is induced in the bulk material (like with foaming).

In practice, producing a day & night mark typically involves making several passes with the laser, where each pass hatches out the mark shape and removes a small amount of material (total paint layer thickness is typically 0.0025"). And, there may even be “cleanup” passes at the end of the process to polish up the underlying layer that has been revealed to give it the desired smooth texture and appearance.

The exact process recipe (number of passes, laser power, laser pulse repetition rate) varies depending on the materials being ablated and the layer thicknesses. In order to speed development of new process recipes, the Coherent LME-RM laser marking system can automatically run through several different sets of laser parameters to enable rapid identification of the optimum settings. Specifically, the system will automatically produce a grid of marks, where the laser power varies between each column, and the laser repetition rate varies between each row, so that a large number of unique combinations are explored. Then, the operator examines the grid of marks and picks the settings that produced the best results. Plus, the Coherent applications development engineers have established a large database of existing process recipes that they can supply to users in order to provide a good starting point for this kind of optimization.

Figure 4. The Coherent LME-RM is a compact, turnkey marking system that can be configured with a variety of different laser sources and options.

“Mark quality is our single biggest priority,” notes Keith James, Director, Strategic Supply Chain and Manufacturing Operations, at Crestron Electronics. “We wanted to offer customers the option of marking bitmapped images on our user interface products, but we had to first clear a technical hurdle to mark bitmaps with the same high level of clarity we achieve with text. One way the Coherent system solves this problem is by doing an outline around the bitmapped image after it has hatched out the mark fill. That eliminates any aliasing or ragged edges on the mark, and consistently delivers a sharp edge.

“The Coherent Visual Laser Marker (VLM) software also facilitates the whole process of producing custom marks,” he adds. Using it, our dealers can bring up a keypad layout on their computer screen, define whatever symbols or nomenclature their customer requests, and then simply e-mail all that to our production order processing plant. Our internal production personnel can then produce that mark on the part, and the VLM software handles all the background tasks necessary to make that happen with almost no operator intervention. It’s an extremely user friendly environment, which an operator can learn to use in minutes, so our productivity is very high.”

Conclusion

Day & night marking is widely employed with many consumer products, most notably automobiles and computer keyboards, because it delivers both attractive and functional marks. Advances in laser-based day & night marking systems now reduce job setup time and facilitate the process of translating custom artwork into an actual mark. This makes the technique readily accessible for even small volume manufacturers.

This article was written by Robert Boyes, Market Development Manager, Coherent Inc. (Santa Clara, CA). For more information, contact Mr. Boyes at This email address is being protected from spambots. You need JavaScript enabled to view it., or visit here .


Photonics & Imaging Technology Magazine

This article first appeared in the March, 2021 issue of Photonics & Imaging Technology Magazine.

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