Wood Engraving Shade Dependence on Laser Beam Focus

Engraving Wood with CNC Lasers


The technology of laser engraving consists of the controlled generation of a focused beam of light whose energy heats the point of impact on an opaque material at a specific wavelength. Depending on the beam profile, the amount of energy applied, the exposure time and the base material, the surface of the material changes with different gradients and intensities (1). The effects of laser beam on the surface of the material are only apparent above a certain power level and are generally irreversible (2). The most important changes in the material during engraving are the loss of material during burning (the depth) and the change in the chemical composition of the surface layer (the tint - carbon).

The ability to determine the intensity of these changes in advance is one of the key areas of quality control in laser engraving. The shade produced by the laser beam on wood depends not only on the intensity (the power) and the profile of the laser beam (the cross-section). The material itself is used in combination with its environment (the air) to produce the desired pigmentation (the tint) of the material. So, it also depends on the type of wood, its temperature, its humidity, the hardness, and the current chemical composition of the engraved layers (the age of the wood and its surface treatment).

In addition to this, wood is a natural composite (3) and a living material, and even after its processing it is constantly changing (degrading – decomposing – absorbing or loosing water) (4). Wood dimensions can change as well (5).

Use of semiconductor emitters in laser engraving

For a system capable of fine-tuning the intensity of the laser beam depending on the current properties of the base material and its environment, it is appropriate to use a sensitive and fast (pulse frequency) transmitter. Nowadays, engraving photos in wood is mainly done with a CO2 laser. CO2 lasers have high power, allowing high production speeds and material cuts (6). However, they produce longer wavelengths (10600 nm), so they are not as accurate as visible spectrum diode lasers (455 nm) (7). The use of semiconductor lasers has seen a huge increase with the use of CD (8; 9).

They find their application here mainly due to their physical dimensions, purchase price and higher safety given by the wavelength and output power. Thanks to the dramatic increase in the power of laser diodes in recent years, this emitter is becoming an important part of the technology (10; 11). Diode lasers are increasingly appearing in industrial applications (12; 13). Moreover, this potential is further enhanced by the rapid production due to the high purchase price. Diode lasers can be controlled in power, and their beams can be focused relatively precisely (14).

This allows much finer detail to be imparted to a wood surface and potentially calibrates minute changes in the material, environment, and ubiquitous wear of the emitter. However, diode lasers still have up to two orders of magnitude less optical power, so production is significantly slowed. A big advantage is their size, which is up to three orders of magnitude smaller. This makes it easier and cheaper to assemble semiconductor lasers into multibeam systems that can be controlled simultaneously (increasing power) or separately (engraving multiple lines at once) (11).

Features of PLH3 Compact Design

Laser module with combined emitters

Semiconductor lasers thus have the potential to outperform CO2 lasers not only in terms of accuracy, but also in terms of speed and economy of production in the application of photoengraving on wood. The factor that ultimately makes them a more suitable emitter for producing engraved photos on wood is their higher efficiency and effectiveness (15) in calibrating the beam based on the current properties of a material and its environment.

The use of semiconductor lasers is increasing year by year, and nowadays they are the most widely used type of laser emitter. This is due to the wide range of applications and the significant increase in their maximum power over the last few decades. The total global revenue from the sale of laser emitters was $10.4 billion in 2016 (16). Of this, 45% was from semiconductor laser sales alone (17). The current trend, shown in Figure 1, suggests that this technology currently has a more promising future in terms of attracting investment in research and development.

Profit from worldwid sales of lasers

Semiconductor LASER sales

Increasing quality of laser engraving

Two basic chemical processes that take place in wood material during laser engraving are burning and carbonization. These processes take place simultaneously and with proper control considerable number of different tints can be achieved. Multiple approaches can be used to achieve proper control. One option is the different focus of the laser beam.

With focused beam

Results of focused and defocused LASER

In engraving with a focused beam, the material is cut off, and the palette of shades changes from light brown to dark brown. For engraving with a de-focused beam, we get up to rich black shades. In this way, we can get different palettes of shades and get a larger number of colors when engraving with a suitable combination. This leads to greater graphic depth and improved transfers of photos and graphics to wood. With the right tuning, it is possible to achieve similar qualities as with standard printing. This makes laser engraving into wood very interesting from the point of view of the quality and ecology of the whole process. There is no ink, no chemistry. Such products can be described as eco-friendly and have a much lower negative impact on the environment. The only negative components are the laser light, which is miniature and lasts tens of thousands of hours and the energy needed.

Shades Wood

Different results of source photo(middle) with focused(left) and defocused(right) laser beam

In the image above, we can see different palettes of shades that were achieved only by different focus of the beam. The production code, control unit and laser power settings were identical. It should be mentioned that the blur of the beam also leads to slightly higher dimensions of the engraved point, thanks to which lower details can theoretically be achieved. It always depends on the performance and optics of the engraving head. It is necessary to focus the beam considering the desired resolution. However, when making photos and graphics into wood, the engraved point will have a limited minimum size (depending on the power and optics), as the wood burns slightly around the engraved point as well. This provides some clearance to adjust the sharpness relative to the engraved point.

The blurred beam provides an increase in the darkness of the resulting graphics, as shown in the graph below. At the same time, the shade curve is sharper, so the resulting darkening occurs faster. Therefore, the maximum emittor power should be adjusted appropriately so that the entire depth of the shades is evenly distributed throughout the histogram of the input graphic. It should be mentioned that a blurred beam needs more power to change the material than a focused beam. This is due to the lower energy density of the blurred beam and can be seen in the following graph for laser power values 0-20%.

Laser output 0-100%

Shade depth of laser engraving with burned and carbonized surface

The resulting surfaces have different compositions. The difference between those to chemical processes can be easily seen on the electron microscope scan below. The amount of black carbon left on wood surfaces rapidly changes with increasing carbonization of surface. This is expected since the black carbon is responsible for black surface shade. The depth of a surface change only slightly since the main laser energy is used for wood carbonization rather than wood sublimation.

Black Carbon Amounts

Differences in black carbon amounts under electron microscope for focused laser beam(left) and defocused laser beam(right)

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The use of semiconductor lasers is growing every year. Thanks to raising power output of semiconductor lasers, they can be used in more and more cases. The physical dimensions of emmiters allows their combination. They can be combined to higher power outputs or faster raster engraving (engraving of multiple rows at the same time). With defocused beam darker shades can be achieved for engraving of wood. This leads to greater depth of resulting graphics. With proper combination better quality of image transfer onto wood surfaces can be achieved. It is important to optimize laser engraving process to different species and states of wood.

With the same laser and power output settings different quality is achieved. Focus of the beam is only one variable. Humidity and the age of wood itself are other variables witch takes a role in enhancement of engraved results.

Article’s Author:

Martin Jurek


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