Optical lenses have been invaluable to mankind for centuries and have found application in numerous fields, be it astronomy, photography, medicine or the everyday use of eyeglasses. In recent years, additive manufacturing, in particular stereolithography (SLA) 3D printing, has made astonishing progress, making it possible to produce complex geometric structures with high precision. In this experiment, the question was raised as to whether optical lenses can be produced using SLA 3D printing technology. Transparent resin is used in an SLA printer to produce the lenses and then they are machined using a specially designed lens grinding machine. This grinding machine follows the proven principles that have been used for many years in the manufacture of optical lenses. In this article, we will take an in-depth look at this fascinating experiment and explore the possibilities and challenges of producing optical lenses using SLA 3D printing.
Basic considerations for optical lenses
A crucial factor to consider when manufacturing optical lenses with 3D printing is the light transmission of these lenses. The relationship between the material structure and light transmission plays a decisive role here. An ideal lens consists of a completely homogeneous material, which ideally means that an amorphous liquid is required. The term „amorphous“ refers to the structure or state of a material in which there is no regular, ordered crystal structure. Unlike crystalline materials, which have a defined lattice structure, amorphous materials are characterized by a disordered, random arrangement pattern of atoms, ions or molecules.
Unfortunately, the production of a completely amorphous lens with 3D printing is in no way achievable. The 3D printer creates the lens in individual layers, and it can be assumed that the molecules align during the curing process and form a lattice structure. This inevitably leads to a problem as the individual layers will have different density differences. When light passes from a medium with a higher density to a medium with a lower density, it is refracted at a certain angle. Since there are many layers in 3D printed lenses, it is extremely difficult to predict how the light will be refracted.
These challenges are the biggest problems in producing optical quality lenses with 3D printing and are crucial when it comes to ensuring light transmission.
The 3D printing
An SLA 3D printer is required to create the basic shape of the lens. In my case, I used the Anycubic Photon Mono X. An essential requirement for printing transparent objects is the use of resin that does not yellow after curing in the UV station. In this experiment, I used Formfutura Platinum LCD Series Clear Resin. I set the exposure time for this resin to 3 seconds. The printing was done in a cool basement with a temperature of about 18°C.
I find that this resin is excellent for 3D printing lenses. Before printing the lens, it is very important to let the freshly filled resin rest for a while so that all air bubbles can escape from the resin. Air bubbles are absolutely unacceptable in an optical lens.
To minimize the problem of layers as much as possible, I concentrated on printing the lenses with as few layers as possible. The number of layers required depends on the print orientation of the lens. If the lens is placed completely vertically on the build platform, this requires the maximum number of layers. However, if the lens is printed at an angle to the build platform, fewer layers are required. I tried and extensively tested different printing angles, which will be explained in more detail later.
Specifically, I printed lenses at angles of 0°, 10°, 30°, 70° and 90° to the vertical. In theory, the lens at 90° should be the most translucent, and my experiments actually confirmed this. The more oblique the lens is printed, the clearer and more translucent it becomes. Unfortunately, it was difficult to show the subtle differences between the lenses in a photograph. To the naked eye, however, these differences are very clearly visible.
However, it would be too easy to solve the problem simply by printing the lenses at an angle. Another challenge arises when the lenses are printed at an angle. Let’s take a closer look at a print cycle for one layer:
1. the printer lowers the lens into the resin until it reaches the LCD display and exposes the layer to UV light to cure it.
2. after exposure, the printer lifts the lens slightly to release it from the display so that new resin can flow in. At this moment, liquid resin sticks to the lens and runs down it.
3. the printer moves the lens towards the LCD display again and exposes the next layer. As the resin is transparent, the UV light penetrates the overlying layers and hardens the resin that has previously flowed down the top of the lens.
As a result, the top of the lens no longer has the desired convex shape. This effect occurs with every layer that is printed and results in the top surface essentially being printed undefined.
The problem becomes more serious the more the lens is printed at an angle. In extreme cases, if the lens is printed at an angle of 90° to the build platform (i.e. parallel to the build platform), the resin hardly has a chance to flow off and it hardens on the top of the lens. The ideal printing angle for me was therefore 30° to the vertical. This is a good compromise between the number of layers printed and the resin flowing off.
I hope that this explanation makes the problem understandable.
The Lens Grinding and Polishing Machine
Another decisive factor influencing the light transmission of the lenses is the surface quality of the lenses. After printing, the surface is relatively rough and the individual layers are visible on closer inspection. When a beam of light hits this rough surface, it is deflected in various, undefined directions, which means that the lens does not appear transparent. The aim is to grind and polish the surface of the lenses as smoothly as possible without changing the basic convex shape of the lens. This cannot be achieved with manual grinding, as every movement would change the convex shape of the lens, even with the highest precision. In addition, manual grinding would be a time-consuming task that would take hours, which is not very pleasant.
For this reason, I designed and built a 3D-printed lens grinding machine. With this machine, it is possible to grind and polish the lens precisely and in a comparatively short time, without affecting the original convex shape of the lens. This method makes it possible to significantly improve the surface quality of the lenses and thus increase light transmission.
The lens grinding machine I have developed is not a completely new invention, but is based on proven principles that have long been used in the optics industry to grind and polish lenses. The basic design of the machine is quite simple: a swivel arm that swivels around the vertical axis using an eccentric can also be moved around a horizontal axis. This allows the swivel arm to adapt perfectly to the shape of the lens.
The lens itself is glued to a turntable which rotates on its own axis. A sanding block is placed on the lens, which has the exact shape of the lens. This grinding block can be moved in all directions using a vertical rod that has a round shape at its end so that the grinding block can rotate freely. Weights are placed on the rod to press the sanding block onto the lens. A sanding paper is attached to the sanding block using a clamping ring.
The rotation of the lens also causes the grinding block to rotate. It is crucial that the grinding block is completely free to move without a mechanical device steering it into a predefined path, as this could change the shape of the lens.
As a result, the swivel arm constantly moves back and forth, the lens rotates around its own axis, and the grinding block rotates in the same direction as the lens due to its free bearing. In addition, the sanding block is not exactly centered on the lens. The combination of these three movements results in an extremely complex grinding movement on the surface of the lens. The actual movement of an abrasive grain on the lens in relation to these movements is indeed difficult to visualize and understand.
Attachment of the lens
Due to the considerable forces that act on the lens during the grinding process, it is very important that the lens is mounted concentrically and stably. In order to center the lens, I have developed a special centering disc with which the lens can be mounted completely centrically on the rotary or turntable. You can see this in the video.
The lens is securely fixed to the rotation or turntable using a hot glue gun. Hot glue makes it possible to remove the lens from the holder easily and without leaving any residue. The lens can then be attached to the rotary drive using a plug-in system. This design allows the lens to be removed after each sanding process and cleaned before changing to finer sandpaper. The centering disc and the flexible fastening system thus help to ensure that the lens remains stable and precisely aligned during the sanding process.
Eccentric drive of the swivel arm
The swivel drive of the swivel arm is powered by a 24V geared motor that works in conjunction with an eccentric. A major advantage of this eccentric design is that it enables infinitely variable adjustment of the eccentricity. This means that both very small and very large lenses can be ground and polished with this swivel arm. This flexible design makes it possible to adapt the machine to different lens sizes and thus to process a wide range of optical lenses efficiently.
The sanding block
The grinding block is certainly one of the decisive components in lens grinding, as it must have the exact convex shape of the lens. I have devised a method to achieve this precision: I use Fimo, a modeling clay, to imprint the shape of the lens. After the polymer clay has been baked in the oven for about half an hour at 110°C, it hardens and becomes relatively firm, which makes it ideal for sanding the lenses.
The exact details of this process can best be illustrated in my video, and I don’t want to go into too much detail here. In practice, this results in a solid sanding block that reproduces the exact shape of the lens and into which a sandpaper can be clamped using a clamping ring. This enables precise and efficient processing of the lenses to achieve the desired surface finish.
The grinding and polishing process
Now we finally come to the decisive step in lens production: grinding and polishing. This process starts with coarse sandpaper and progresses to finer and finer grits, always using water. I used a selection of 9 different grits, up to a 10000 grit sandpaper, which is really very fine – many toilet paper manufacturers can take a leaf out of my book 😉
As a final polishing step, I used a polishing agent that is supplied in block form. I scraped small pieces of this block with a knife
with a knife and rubbed it finely. I then applied the polish to a microfiber cloth and polished with it. You can see how I did this in the video.
It is extremely important to use sufficient water during sanding. Sanding should never be done dry. On the one hand, the water has the function of cooling the lens to prevent it from overheating, and on the other hand, it serves to remove any sanding particles that have been removed.
After each sandpaper, it is extremely important to rinse the lens thoroughly and switch to the next finer sandpaper with fresh water. It is really impressive to see how the lens becomes clearer and more transparent after each sanding process.
Another important factor is the force with which the sanding block presses on the lens, which is realized by different weights. It is advisable not to apply too many weights, otherwise the sanding block may develop unwanted vibrations. It is often more effective to use a little less weight and grind for a little longer.
Other parameters that influence the grinding and polishing process are the swivel speed of the swivel arm and the rotation speed of the lens. These can be infinitely adjusted using PWM controllers to achieve the desired results.
Determining the focal point of the lenses
To determine the focal point of the manufactured lenses, I designed and 3D printed an optics bench. In the first test, all the lenses were examined for their focal point. To do this, I clamped a lamp, in this case a Ledlenser, into the optics bench and focused it through the lenses onto a black paper. I then measured and documented the focal points. These measurements make it possible to understand and analyze the optical properties and focusing of the lenses produced.
As a second experiment, I clamped a photoresistor (LDR) into the optics bench to determine the exact focal point of the lenses. In this context, I carried out two experiments. I wanted to test whether it makes sense to varnish the lenses with clear varnish, as clear varnish often gives an improved impression at first glance. After varnishing, the lenses often appear much clearer and I wanted to find out whether this would also allow more light to pass through.
It’s important to know that the clear coat fills in the fine pits that remain after sanding and polishing, creating a smooth and clean surface.
First, I measured and documented the resistance value of the photoresistor without varnish using an Arduino. In the second pass, the lenses were painted with clear lacquer and tested again with the photoresistor. The measurements showed that the resistance value increased only minimally, and this increase was so small that it could also be a measurement error. It has therefore been shown that the coating does not contribute significantly to light transmission, although it can bring about visible improvements to the eye. The focal point also shifted only minimally, in the millimeter range. It became a few millimeters shorter for all lenses, but measurement errors could also play a role here, although the results were roughly the same for all lenses.
The question of whether it is feasible to produce optical lenses using 3D printing led me on a fascinating journey of discovery. I conducted some experiments to produce transparent lenses of relatively good quality. As you can see, there are many aspects to consider, but with this guide you will be able to not only conduct experiments but also realize impressive projects such as telescopes, magnifying glasses and much more.
A lens grinder is essential for perfecting the lens shape after grinding and polishing. If you already own an FDM 3D printer, you can easily download all the necessary .STL files and build your own grinding machine. I provide you with all the construction plans and data for mechanical engineering to download.
Dive into the world of optical lens manufacturing and discover the possibilities for your optical projects. My instructions will guide you through every step and help you turn your own visions into reality. Start now and turn your optical ideas into reality!
Have fun experimenting!
You can download the building plan here:
You will get the following digital files:
- All .stl files you need to print the machine.
- Assembly drawing in .pdf, .dxf -format.
- The complete 3D model of the machine in .step – format.
- A detailed parts list of all installed parts including internet links to the supplier.
- An instruction video with all building steps.
- I will do my best to help you with any problems or questions.
- .stl File for convex Lense diameter = 40mm, radii= 200mm