Goal: To provide an introductory overview of each topic we will be addressing in this blog series. As a result, we will be able to dive deeper into each item to better understand aspheres and how they are manufactured.
Aspheres have allowed optical designers to create systems and products that push the limitations of performance across several fields. These systems find application in products such as night vision goggles, laser rangefinders, high power laser fabrication tools, medical instruments, and many more. Aspheres are not limited to specific applications; instead, they can increase performance by correcting aberrations with fewer surfaces than would be required using only spherical lenses. Along with new possibilities, aspheric lenses have also brought much disruption to the design and fabrication processes. It's one process to design an asphere that works in an optical system, and it's another process to develop an asphere that is manufacturable. The purpose of this blog series is to address the following topics:
- What is an Aspherical Lens?
- Asphere Design, A Manufacturers perspective
- Manufacturing Processes for Aspheres
- Geometries of Aspheres
- Tolerancing of Aspheres
- Qualifying your Asphere Utilizing Inspection Data
What is an Aspherical Lens?
Let us start by discussing what an aspherical lens is. An aspherical lens is a spherical lens with a constantly changing local radius. In our illustration below an asphere has the best fit sphere (BFS), which is a radius that touches the edge and center of the lens.
Departure is the distance from the BFS to the aspheric form. Form error can be explained by how much the manufactured asphere contrasts with the theoretical aspheric form. Slope error is the derivative of form error and quantifies how quickly the form error changes. The aspheric surface is typically defined using an even aspheric equation. Other forms of defining aspheric surfaces exist although most optical fabrication machines utilize the even aspheric equation.
Look, Mom, I Designed My First Asphere!
Over the years we have seen some interesting asphere designs. We preface the following comments by stating; clearly, we are not optical designers; so please use your best judgment when designing. With that said, here are 10 considerations to keep in mind when you are designing your next asphere from a manufacturing perspective. Some of these tips are specific to ground and polished aspheres.
- Include a sag table with your asphere so the manufacturer can confirm the prescription. Also, please increment the sag table by a reasonable step such as 0.5 or 1mm. We are looking to match our sag table with yours, and this helps make sure we are looking at the same part.
- Define which aspheric equation you are using. Most manufacturers prefer the even aspheric equation. Other forms would include Q-Forbes and Q-Con among others.
- Avoid using odd term coefficients.
- Attempt to avoid using an A2 term as this changes the vertex radius and causes the vertex radius tolerance to become slightly insignificant.
- Include a tolerance for the following:
- Vertex Radius
- Form Error
- Slope Error
- Geometric Tolerancing
- Non-Aspheric Tolerancing
- Consider equivalent materials. Considering equivalent materials can potentially save money as well as speed up material availability leading to quicker lead times.
- Get your manufacturer involved early on in the design phase so you can include manufacturability in your design considerations.
- Do not tolerance the deformation terms or conic constant.
- Watch the minimum local radius on concave aspheres, minimum 15mm.
- The edge thickness at processing diameter, which is typically 2-4mm outside the final diameter should be 1mm or more. This allows us to hold onto the outer diameter as well as providing more options to manufacture the asphere.
How It's Made
There are several different methods of manufacturing aspheres. Each method has strengths and weaknesses, which depends on material, specifications, size, and quantity to name a few. Below is a table highlighting each technique.
Types of Geometries
Aspheres geometry can come in several different flavors. Each class presents specific challenges and considerations for manufacturers. These challenges will also be different for each type of manufacturing process. For example, a gull-wing asphere with mounting flanges may not be very difficult for precision molding, but the same lens would be impossible to manufacture using a grinding/polishing method. Below is a table that highlights different potential geometries along with benefits and considerations for each.
Materials are typically chosen based on the end user application. Several considerations need to be accounted for such as the wavelength of the application, thermal and mechanical stability, cost, manufacturing method, and any other possible concerns. Each material presents manufacturing challenges that need to be in consideration. Below is a chart that provides which materials can be made with each process.
Tolerancing an aspheric lens is perhaps the single most significant point of confusion, and one could easily write an entire article on each tolerance. Over the course of the next few blogs that are we what we are planning to do. In the meantime, here is a chart that describes several asphere specific tolerances along with a button below to download our tolerancing chart on precision optics for spherical and aspherical tolerances.
How do you know that your asphere meets design specifications? What if your design needs to be tweaked explicitly to each lens to squeeze that last bit of performance from your system? Furthermore, inspection data can be extremely beneficial. We have a complete guide on inspection data that explains all aspects of qualifying optics. The industry standard has been a profilometer trace for aspheric-specific inspection data. A profilometer trace involves using a precision stylus that has been atomically balanced, which traces the surface of the lens. The resulting trace is compared with the theoretical design of the aspheric surface and the form error, vertex radius error and slope error can also be identified. Feel free to check out interferometry blog if interested in learning more about the various forms of interferometry and optical measurements.
Two potential problems with profilometry can occur. First is you can potentially miss out on asymmetric error induced on your part; and second, it is possible you can scratch the surface of the part with the stylus. 3D non-contact metrology is an excellent alternative for this potential risk. LaCroix specifically uses an Optipro Ultrasurf to provide 3D measurements of aspheric surfaces. This instrument is capable of seeing the entire surface of the asphere rather than a single trace. With the complete data set, we can correct the asymmetric error and provide a higher quality 3D aspheric surface. The data below is of a 6-inch asphere that our profilometer showed a peak to valley form error measurement of roughly 0.4 um of form error; however, the profilometer did not capture the asymmetric error of the part. Our 3D analysis revealed the asymmetry and form error was 1.3407 um. After a quick 3D correction, the 3D form error was reduced to .294um and .133 um in 2D with less than 0.1 um/mm of slope error using a 1 mm integration window.
3D Measurement 1.3407 um after 3D Correction .294 um Form Error
2D Form Error vs 3D Form Error
Our hope with this series is to shed light on aspheric manufacturing; specifically, to help enable optical designers and engineers to gain a better understanding of how aspheric lenses are manufactured as well as how to properly tolerance aspheres to meet their performance goals and obtain manufacturable lenses. Optics is an exciting field, and we are passionate about helping you with your next project. Please contact one of our engineers today if you have a project you would like some feedback or a ROM quote.