In this step, you'll be making the lens for the GelSlim 4.0 sensor, which is highlighted in the diagram below. First you'll cut several test lenses to see which size fits your sensor body. The final lens size should just barely not fit into the sensor body because you'll remove some material by polishing the lens edges. Polishing the lens edges is important because the edges will have ridges after being laser cut. These ridges can distort the light shining through them in an undesirable way. The final lens edges will have a cloudy finish, which helps diffuse the light shining through.
Test Cuts
Every laser cutter and every 3D printer is different, so it’s important to test that your parts fit together before you make too many of them in the wrong size! To facilitate this, we have provided a cut file with several different sizes of laser lenses. Before starting this step, make sure you have a sensor body 3D printed with the same infill settings you want to use in your final sensor.
The actual diameter of the lens cavity is 2.8cm. You’ll need to adjust for the kerf on your laser cutter (learn more here) when cutting out the lens that fits into this lens cavity. The diameter of the lens you choose from the test cuts will likely be larger than 2.8cm. We have provided the cut files to cut 2.8cm, 2.81cm, 2.82cm, 2.83cm, 2.84cm, and 2.85cm lenses. If none of these sizes fits just right, we have also provided the CAD file for you to generate new test cut sizes by changing the boxed dimension shown below.
Load 1/4" clear acrylic onto laser cutter bed
Note: Leave the paper backing on both sides of the acrylic when you cut it for a cleaner cut.
Load compatible lens_test_cuts file into laser cutter software
Note: In the files provided, cut lines are red (R: 255, G: 0, B: 0) with weight 0.01pt. If your laser cutter software uses a different stroke color/stroke weight for cut lines, there are several ways to change the files provided to fit your setup. For example, you can load lens_test_cuts.ai into Adobe Illustrator and change the stroke color/stroke weight of the cut lines to match the settings of your setup.
In laser cutter software, set material to cast acrylic with 1/4" thickness
Note: Setting the material and material thickness in most laser cutter software should load the default laser power and speed for this material. It's likely that you will have to change these settings to achieve a clean cut. You can perform several test cuts with smaller parts until you're satisfied with the cut settings.
In laser cutter software, align cut geometry with available space on the acrylic sheet
Execute the cut
Label the test cut lenses with their dimensions (2.8cm, 2.81cm, 2.82cm, 2.83cm, 2.84cm, and 2.85cm)
Note: Leave the paper backing on both sides of the lenses.
Starting with the 2.8cm lens, insert the lens into the lens cavity of the sensor body, as shown below
Remove the lens by pressing against the lens via the camera hole with a <1cm cylindrical object, as shown below
Move up in lens size until you arrive at a lens size that just barely doesn't fit into the lens cavity
This lens will be the size you use for your final cuts
Final Cuts
Once you've picked the lens diameter that just barely doesn't fit into the sensor body, you can cut more lenses at that size. Make sure to keep the paper backing on all of your unpolished lenses.
This section is accompanied by video instructions, which are embedded below.
The edges of the lens will be rough after being laser cut, as shown below. In the final sensor, rough inner curvatures can distort the light that shines through them in an undesirable way. Rough outer curvatures can prevent the lens from fitting into the sensor body. Therefore, you’ll need to sand down the inner curvatures to a smooth translucent finish (shown below) and the outer curvatures to a snug fit into the sensor body.
Leave the paper backing on both sides of the acrylic lens
Don PPE
Put on gloves to protect the skin on your hands from being abraded by the sandpaper
Put on safety glasses to protect your eyes from dust and splatters
Tear small squares of sandpaper (about 1.5in per side) from the larger sheet for each grit of sandpaper: 120, 240, 500, 1000, 2000, and 2500
Note: Use whatever grits of sandpaper you have available to create a progression from coarse to ultra-fine grit sandpaper.
Arrange the sandpaper on the table from coarsest to finest grit
Wet the coarsest grit sandpaper using the spray bottle of water
Sand the edges of the acrylic lens according to the diagram below, focusing on the inner curvatures
Note: If the sandpaper becomes clogged with acrylic dust, use the spray bottle of water to remove the dust and re-wet the sandpaper
If you have difficulty sanding the outer curvatures (especially on the coarser grits), try placing the sandpaper flat on the table and pulling the lens across it while rotating the lens to avoid creating flat spots
If you have difficulty sanding inside the inner curvatures, try bending the sandpaper around the fingertip of your dominant hand, pressing it into the curve, and rotating your finger back and forth along the curve as shown below
Dry off the lens with a soft towel to inspect your progress, referencing the diagram below for examples of when to move to the next grit
Note: There may be small chips and low spots at the very edges between the curvatures and the paper backing. It is okay to leave the chips at the very edge. They do not impact the final sensor as much as the ridges on the inner curvatures do.
Move to the next grit and repeat with increasingly fine grit sandpaper until the inner curvatures of the lens are smooth and the outer curvatures have major ridges removed
After you've sanded with the finest grit of sandpaper, check that the lens fits into the lens cavity in the sensor body
Press the lens into the lens cavity, as shown below
If you cannot press the lens into the lens cavity with firm pressure, continue sanding the outer curvatures until you are able to firmly press the lens into the lens cavity
If the lens fits, use a <1cm cylindrical object to press through the camera hole and remove the lens from the sensor body, as shown below
Peel paper backing off each side of the polished lens
If the top and bottom of the lens sustained any scratches during edge polishing, buff them out using acrylic polish and a clean microfiber cloth
Note: If your lens does not have any scratches on the top and bottom, you can skip this step. However, inspect closely. Because of the small sensing area, any chips will show up as large defect in the final sensor image. Do not buff the inner and outer curvatures that you just sanded. Those surfaces should remain cloudy.
Lay the soft cloth flat on a smooth surface
Wet a small area of the cloth with the spray bottle of water, about 2x the diameter of the acrylic lens
Squeeze a pea-sized amount of acrylic polish onto the wet spot of the cloth
Press the scratched surface of the lens firmly into the polish
Pushing the lens firmly into the cloth, move the lens in small circles to buff out the scratch
Note: Use your other hand to stretch the cloth and keep it flat so that you're able to move the lens smoothly across the cloth
Repeat with the other side of the lens if necessary
Clean any residual polish off the lens