Lightburn Calibration for Greyscale Engraving
I spent a lot of time going through a calibration process to enable me to laser engrave using Lightburn’s greyscale image function. The greyscale algorithm is advanced and depends on a well-calibrated machine to produce a good quality image. I have documented the process so that others can enjoy the full capabilities of Lightburn and the Snapmaker laser. The process is as follows:
1. Find your optimal focal length
Lightburn has a focus test under Tools>Focus Test which allows you to find your optimal focal length. It allows you to vary Z axis and power over a user defined number of increments. It also allows you to test at a user specified speed. I tested my laser on cardstock paper in 1 mm increments from 5 to 10 mm at first and kept power constant at 75%. I found that my optimal focal length occurred between 7 and 8 mm, and I then tested between 7.0 and 8.0 mm at 0.1 mm increments (by setting the number of increments to 10). This allowed me to establish 7.9 mm as my optimal focal length. (You could go further to dial it in, but since the Snapmaker’s default minimum step is 0.1 mm, and the stated dimensional accuracy is 0.2 mm, it doesn’t appear to be useful.
2. Find your optimal line interval.
The line interval is the spacing between horizontal lines in a bitmap engraving. In Lightburn you can enter this parameter as either the line interval in mm (the distance separating horizontal lines from center to center), or as the resolution in dots per inch as you might see in a graphical image (also referred to as the DPI). Do not think of this as the DPI of your greyscale engraving as it will confuse you later. To find the optimal line interval I drew a series of eleven squares of roughly 50 mm per side. I then set the cut settings for each square individually to “fill” and varied the line interval from 0.10 to 0.20 mm in 0.01 mm intervals. Power was set consistently to 40% and speed to 300 mm/min.
I burned the squares onto cardstock paper, then used a x10 magnifying lens on a dissecting microscope (a good magnifying glass should work as well) to look for the line interval that shows each line and a burned area adjacent to it without over-burning the paper between the lines.
If you notice entire chunks of paper burned through, the line interval is too tight. If you can see distinct lines of undarkened paper between the engraved lines without a magnifying glass, you’re not tight enough. I found my line interval is .150 mm.
During this test I discovered that using the bidirectional path option in the cut settings under “fill” will result in irregular spacing between horizontal lines. It appears to be a bug in the software. I disabled “bidirectional” paths for all my engravings from this point forward and got much better results.
3. Enable and calibrate Scanning Offset Adjustment
The scanning offset adjustment allows the software to compensate for the time it takes the laser to fire. This will typically exhibit as ghosted edges on cuts with the bidirectional cuts enabled where the even and odd lines will not begin at the same x coordinate. To calibrate the Scanning Offset Adjustment I printed a series of squares and reenabled bidirectional cuts. I maintained the power at 40% and varied the speed from 1000 mm/min to 50 mm/min in 100 mm/min intervals.
Then I cut the previous test squares I used to establish the line interval in half to create a ruler. Using the 10x lens of my microscope I established the correct offset by estimating the adjustment required using my “ruler”. The adjustment is calculated as the difference between odd and even lines, divided by two.
I retested four times, changing adjustments until I felt all speed ranges were printing accurately.
- Find your optimal combination of speed and power.
There are many templates available that vary power and speed for standard engraving. These will not work for you. I was able to develop a simple rectangle in Inkscape (I set mine to 50 mm high, roughly 150 mm long) with a greyscale gradient from zero (white) to 256 (black).
I then copied this rectangle multiple times in Lightburn and added text denoting speed and power to keep track of settings. My initial tests were on cardstock. Because the paper’s color and thickness do not match the material I was going to engrave (dark cherry wood), I decided to test on cardboard to get into the range, and then fine tune on the cherrywood block itself.
I ran the test at 60% power with 12 intervals from 100 mm/min to 1200 mm/min and set the cut setting to “fill” and the image option to “greyscale”. I found that the settings created a gradient that was either way too dark or had vertical gaps indicating the head outpacing the laser along the X axis. The cardboard was burned through on the dark side and looked black or dark grey on the white end in the speed range where resolution seemed to be good.
I then retested using eleven rectangles varying power from 50 to 20% and speed from 320 to 390 mm/min. My initial test revealed that 30% power at 350 mm/min was fairly optimal. It had a good gradient with decent contrast. The 50% power lines were more than half blackened, which indicates the image would come out too dark.
I then ran a similar test on the side of the wood block with similar settings and found the same settings seemed to work well. These settings may be a good starting point, but most likely they won’t work for you. They didn’t work for me.
5. Pre-process the image
I at first decided to bring in the image straight into Lightburn and adjust the settings in the software by changing the image properties. It was helpful to preview the image and confirm the image’s white spaces (skipped) matched expectations. After about half a dozen tests it became clear that the results were not panning out. One problem was that the original image was still printing too dark. When I decreased power sections of the image would not be burned. I believe this is due to the laser not activating below some threshold level that I have not identified.
I then attempted to increase speed to around 700 mm/min and power to 50%, but found the resulting quality was not good and the image was still over-burned in areas.
Ultimately, the solution was to pre-process the image in GIMP. I set the image to greyscale, and adjusted the brightness, set the DPI to match what Lightburn was going to use as “DPI”, and then adjusted the contrast, highlights, shadows, and levels to lighten the image without whitewashing the darkest parts. Increasing contrast was not enough, as some faces came in fairly dark and others not, so use of the highlights and black levels was key to getting a good image for engraving.
6. Import into Lightburn
After the image is exported (I exported to PNG with transparency levels enabled) import into Lightburn and preview the image. If sections of the image that you want engraved are shown as travel moves adjust the image settings very slightly until the white levels are dark enough to trigger firing of the laser.
If you have a scrap piece of wood I would attempt the first engraving there. If not you may have to try and sand the blank multiple times until you dial in the image pre-processing setting, power, and speed. Ultimately, I sent my settings to 40% power, 350 mm/min and was able to provide a good quality engraving.
I hope this post can assist you in dialing in your own machine, and enjoying the capabilities that it brings. It is a process to go from a blank slate to engraving in greyscale, but once it is calibrated the process would require steps five and six with perhaps some testing of greyscale power/speed settings if the material is new.