What are Freeform Optics?

Optical components come in many shapes and sizes. Traditional lenses have a spherical shape and were originally the only type of optical surface that could be fabricated for many years. Over time, nonspherical optical surfaces have emerged with complex curvatures and advanced optical properties.

Freeform optics is the ideal approach for optical systems that traditionally have a high number of lenses or off-axis components. Modern-day lens design, optical engineering, and optics manufacturing make it possible to build innovative and more complex elements, enabling better optical performance while improving compactness.

Many new optical systems need to be small, but compacting multiple lenses in an optical system can affect its optical and image quality. The freeform optics approach provides better optical performance over spherical lenses and can reduce the number of lenses, reflectors, or mirrors in an optical element. This enables manufacturers to create smaller optical components and provides a way of removing optical aberrations. Because of these benefits, freeform optics are in use in the aerospace, automotive, and consumer electronics industries.

Some key applications for freeform optics include:

• Heads-up displays (HUDs)
• Telescopes
• Imaging systems
• Beam shaping and illumination applications
• Vehicle headlights
• Infrared lenses
• AR/VR headsets
• Diffractive optics
• Aberated wavefronts

While freeform optical design is sometimes implemented to optimize and improve the status quo of optical systems, many applications wouldn’t be commercially feasible without freeform surfaces.

For example, telescopes used on satellites require multiple mirrors to guide light. If freeform surfaces are not used when reducing the size of the telescope, the image quality will suffer. Because of this, it’s almost impossible to develop advanced telescopes without freeform optics. 

HUDs also require freeform optics to perform well, as their reflective optical components would not fit under a vehicle's dashboard. These systems would require many more mirrors than are used today and would unlikely fit in the designated space.

Freeform optics have also become useful for external lighting in vehicles by providing an optimal light beam path that doesn’t illuminate drivers on the other side of the road while using fewer components. They are also used in automotive mirrors to provide a large field of view to the driver and have become ubiquitous in AR/VR headsets due to the need for high-performance and compact components. So, without freeform optics, many emerging optical technologies would not be possible.

The number of nonspherical surfaces in development today is increasing, with more complex geometries available than ever before. Aspherical surfaces (aspheres) were the first type of nonspherical lens to be developed. Today, nonsymmetrical freeform surfaces can be designed and manufactured to provide more versatility for advanced optics and photonics applications.

Freeform optics are high-performance optical surfaces with a lack of symmetry and nonconstant curvature that are more geometrically complex than aspheres. This is a broad classification that encompasses many different types of freeform surfaces that are named based on the mathematical description of the surface geometry of the lens.

Within each of these freeform surface classes, a number of surface geometries can exist so long as they follow the mathematical function. 

Some of the common freeform surfaces used today include:

• XY polynomials
• Zernike
• Chebychev
• Q-type freeform

The type of freeform used for an optical system is governed by the needs of the system and the end application. Many XY polynomials can be created due to the potential x and y variations in the surface profile of the freeform lens. 

Zernike freeforms are a popular choice among designers because they are created by combining different modular “bricks” together to form a freeform pattern with differing degrees of freedom based on the intended application. The range of possible combinations makes Zernike a very versatile option for many freeform optics applications.

Zernike building blocks are circular in nature, so lenses that require strong left and right surfaces or top and bottom asymmetrical rectangular surfaces rely more on XY polynomials and Chebychev surface profiles. Q-type freeforms are a newer freeform lens design that can be designed with Ansys software solutions. These were developed due to direct requests from end users.

Freeform optics are designed through simulation, but they require different manufacturing processes depending on the materials used to fabricate the lenses into complex surface shapes. For many freeform optics — such as substrates that include metals — the tools used to fabricate these lenses include CNC machining and diamond turning (also known as diamond machining).

In diamond turning, the tip of a diamond turns very quickly in every direction to remove any unwanted material on the lens and define the surface profile of the lens. For plastic optics, a range of injection molding options (including diamond-turned mold inserts) can be used to make freeform surfaces with specific geometries. The surface roughness and other surface and optical properties are then determined by metrology and interferometer techniques to ensure that the lens performs well and contains the desired optical characteristics.

The main manufacturing challenge lies in the tolerance of the process against the properties outlined in the design method. A simulated design has a high level of optimization and is an ideal version of an optical system. When these components start to be manufactured, the tolerances of the manufacturing process can affect the final product’s properties and surface shape. 

Different manufacturing constraints exist depending on the technique used. The design process needs to be robust and factor in these potential tolerances during the design stage to ensure that there is not a performance disconnect between the optical component’s design and the manufactured components.

Advanced simulation tools can be used to design any kind of optical element: spherical, aspherical, or freeform. Ansys solutions such as Ansys Zemax OpticStudio software and Ansys Speos software can easily simulate different types of freeform surfaces for different applications. While the freeform choice is currently completed manually by engineers, the freeform selection process could automatically be handled by AI algorithms in the future.

OpticStudio software is an efficient tool in designing freeform optical systems because it contains a built-in sequential mode that enables imaging system design without the need to manually choose each surface within the design. Speos software can be used to determine complex illumination patterns through a design.

OpticStudio software includes built-in constraints for freeform surfaces that enable the design to compensate for the capabilities and tolerances of the manufacturer and the specific manufacturing process. OpticStudio software also contains a true freeform option that doesn’t rely on a specific mathematical function for optimization and tolerance, enabling engineers to create a truly freeform surface by manipulating the grid control points in the design. 

Ansys simulations also consider the wider environmental parameters that freeform optics are exposed to, such as the local pressure and temperature, to see the bigger picture.

Advanced simulation tools enable engineers to change the parameters of a freeform design to see how it impacts the real-world performance of an optical component — including accounting for any irregularities that might arise due to the manufacturing process. Simulation enables engineers to see if their systems will pass quality control, hit desired performance targets, and determine whether their products can be manufactured at scale.

Ansys has a wide range of simulation tools for modeling all freeform optic eventualities. Ansys solutions offer a robust way of designing optical components that factor in the manufacturing aspects of the physical product and any tolerances and sensitivities that might be present during fabrication. 

Discover how Ansys software solutions can help you develop your freeform optics by contacting our technical team today.

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