3D Raster
The default method for producing tones of color on the cutting medium is called dithering. Instead of varying the cutting power and speed of the laser as it moves along the material, it pulses the laser at full power at varying densities. The denser the pulses, the darker the area looks. Since the pulses produce very fine dots of color, from far away it appears as if the laser was cutting at a fraction of full power and produces a different colored mark on the material. While this method is simple and can complete the entire image in a single pass, the pulses create a dotted texture that is visible if looked at closely.
The dithering and 3D rastering methods were compared with the same image to a separate method, coined “iso-tone”, to visualize the differences. The iso-tone method involved separating out different shades of grey in the original image and manually rastering each intensity one at a time. While, this process takes longer (6x longer for this example because 6 shades were isolated in the original image) it produces a better result. 3D rastering produced a smooth gradient of cutting depth, however, because each tone of color was cut at the same time, the material burned and blended the different shades together into a low contrast result. Meanwhile, dithering produced an ultra-high contrast result with large patches without any coloring.
Part of being a technician at the AACE lab involved writing instructions and making example projects to showcase the functionality of different equipment in the lab. These art panels were used to display how to prepare files and use the Universal CO2 lasers to raster complex images with different tones. This type of project was made because many art students wanted to raster images on wood and plastic, and needed a better solution than the default method for producing grey-tones between white and black.
To solve these issues, the Universal software has an advanced raster method called 3D rastering. Instead of pulsing the laser, it reduces the power output depending on the greyscale intensity of a certain area of an image. Because the power of the laser does not linearly correlate to engraving depth or material discoloration, this method requires calibration for desirable results. The calibration process involves rastering 16 different shades of gray that correspond to the intervals of the hex code greyscale values. Based on the results of these shades, the power percentages are changed to achieve a smooth gradient from one end of the scale (white) to the other (black). Most often, the power intensity needs to start high to achieve any noticeable depth or discoloration, and the higher end of the spectrum needs to be spaced out to have noticeable differences between different shades.
Dithering
3D Raster
Iso-tone