Optimising optics

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Gemma Church looks at the range of optical design software on the market and how it can help engineers create cutting-edge photonics-related products without the need for numerous costly prototypes

The benefits of modelling anything, from nanobots to jumbo jets, are easy to fathom. By optimising designs before reaching the prototype stage, designers can save a small fortune both in monetary terms and time spent optimising their ideas.

Optical design software is no stranger to these conventional and more specific advantages it brings to optical engineers. The software helps such engineers to design a range of optical systems, such as imaging and illumination systems.

But there are many different ways in which one could segment the optical design software industry, as Tom Settimi, owner of optical design software company Sky Scientific, explains: ‘Optical design software programs fall into one of two general types: general purpose imaging software is used to design products, such as camera lenses, astronomical optics or even beam shaping optics for laser diodes; special purpose software programs, on the other hand, perform functions that treat a single specific aspect related to the behaviour of light.

‘Examples include software for designing illumination systems or optical coatings. In the latter case, the software is intended to improve the performance of optical systems by helping to design coatings to reduce stray reflections and increase efficiency of the optics by transmitting more of the incident light through the system.’

Robert Hilbert, president and CEO of Optical Research Associates, breaks down the optical design software offerings further: ‘Optical design software falls into three main categories: software for the design of imageforming optics, for illumination systems, and for waveguide optics. Optical Research Associates develops software packages for the design of image-forming and illumination optics, which have very different goals. Imaging (or imageforming) design requires mapping each point of light to a precise place, such as when taking a photo, where the image is a faithful depiction of the object. Illumination design revolves around efficiently transferring radiant energy from one space to another, often so the “image” has a more uniform distribution of light than the light source.

‘Many optical systems, such as digital projectors, require detailed consideration and integrated analysis of both imaging and illumination design goals.’

Neil Barrett, general manager of Optima Research, also sees the optical design software market as segmented into different categories: ‘Broadly speaking there are two types,’ he says, ‘geometric optics (ray tracing) software, and physical optics software. However, both types can be further subdivided into two different categories. For geometric optics there are sequential and non-sequential ray tracing tools, and physical optics software can be based on scalar or vectorial (FDTD) calculations. The different categories are intended to solve different types of problems.’

With all these different ways in which optical design software can be used, it is surprising that the optical design software vendors are relatively few and far between, with four main companies involved in the market space: Breault Research Organisation (BRO), with its illumination software called ASAP; Lambda Research, with illumination offering Tracepro and imaging software OSLO; Optical Research Associates, with imaging software CodeV and illumination software LightTools; and the Zemax Development Corporation with its dual illumination and imaging software Zemax, distributed in EMEA by Optima Research.

There are also some smaller vendors, including Optenso with imaging software Optalix, Photon Engineering with illumination software FRED, and Sky Scientific with its dbOptic Optical Design Database Program.

The application areas such software can be used within are expanding though, as Optical Research Associates’ Hilbert explains: ‘The field of imaging optics includes applications in virtually every industry where a precise depiction of the object is required. Examples include projection systems, digital and film cameras, camcorders, photocopiers, and microlithographic lenses for imaging ultra-fine lines on computer chips. The field of illumination optics includes applications in industries where light must be controlled, including automotive instrument displays, LCD televisions, LCD computer monitors, cell phone displays, vehicle exterior and interior lighting, general lighting, and LEDs and other light sources.’

Optima Research’s Barrett says: ‘Traditionally sequential ray tracing packages are used for imaging optics design and non-sequential ray tracing packages are used for non-imaging optical design and stray light analysis. The true range of applications is massive; Zemax has been used to model micro-optical systems, car head lamps, architectural lighting, huge laser systems (including those being built for ITER), space-based telescopes, such as the JWST, and everything in between.’

Optimising the benefits

Optical design software is bringing a range of benefits to the photonics industry. One obvious advantage is that the software speeds up the development process and, therefore, companies spend less cash on expensive prototypes, as Hilbert explains: ‘Optical design software helps engineers create virtual prototypes of optical systems much more quickly and costeffectively than building physical prototypes. Once an initial virtual prototype is created, engineers can also use the software to analyse system performance and make adjustments to the design as needed to achieve performance requirements. Many complex aspects of this iterative process, such as the optimisation and tolerancing of optical systems, can be automatically handled by optical design software with great speed and precision. These capabilities help companies get the best quality product to market faster.’

Barrett also adds that optical design software allows engineers to be more creative with their designs: ‘Optical modelling is cheaper and faster than building prototypes and this allows engineers to explore many different design choices,’ he says. ‘The result of this is that companies can produce better designs and get them to market more quickly. Optical design software should automate the optimisation of the designs, systematically searching for the best possible design.’

But not all packages provide such automation, as Barrett adds: ‘Some packages only provide analysis; they can tell you the performance of an optical design, but not help you improve it. The design aspect is crucial, and Zemax is excellent at optimisation. Further, once designed the system can be toleranced, i.e. the effects of finite manufacturing precision can be included. This ensures the designs work as expected in real life. This can eliminate costly rework.’

Settimi agrees optimisation is a key factor for a piece of optical design software and says: ‘Automatic optimisation is a useful feature that permits the designer to start with an approximate design, then allows the program to make small systematic changes to the design automatically until the calculated image quality is the best possible. Another useful feature of some programs is the imaging of extended objects, which can provide a realistic predicted rendition of the image that a lens system is likely to deliver.’

Accuracy and challenges

The accuracy of optical design software is improving to the point where it is only hampered by the initial assumptions of the calculations used. ‘Provided the model is set-up to correctly reflect the real-life situation then the accuracy can be exact,’ explains Barrett. ‘The limitations always stem from the physics on which the calculation is based.’

Hilbert agrees that accuracies are on the up, adding: ‘Optical design software is capable of producing accurate results that can solve even the toughest engineering challenges. For example, engineers from Optical Research Associates used the Code V optical design software package to design the primary null lenses used for the highly successful Hubble Space Telescope first servicing mission, which dramatically improved the telescope’s image quality. In this project, Code V established the prescription against which the Hubble Space Telescope’s second Wide Field and Planetary Camera was built. In orbit, the measured performance results exactly matched those predicted by the software.

‘For illumination systems, simulations using millions of rays can produce predicted performance that can match the photometrically measured output of a prototyped system very closely.’

But there are some challenges within the optical design software industry, including catering for all levels of knowledge and keeping up with increasing demands of users. ‘Maintaining ease of use while providing enormous power is a perceived challenge,’ says Barrett. ‘Some users are novices, some are expert, some use the software occasionally, some use it all day every day, and all points in between.’

Optima Research’s Zemax Optical Design Program. Here three filament lamps in parabolic reflectors illuminate detectors are shown.

Hilbert also identifies user-friendliness as an area of improvement within the industry. ‘Making continued improvements to ease of use remains as one of the biggest challenges to developers of optical design software,’ he says. ‘This is in spite of the dramatic strides that have been made over the last few years. Ease of use is still a major challenge, because of the increasing diversity of types of optical and illumination systems and the increasing optical engineering responsibilities of engineers without formal optics training, and without years of experience in optical design.

‘Providing optical engineering software that can be productive for engineers less familiar with optical design theory is one of the key determinants where software vendors are going to be most successful over the next several years.’

Changing times

The optical design software arena has changed over the last few years with vendors trying to overcome the challenge of making the software easier to use and subsequently altering how it looks and feels to users. ‘Optical design software has become more user friendly,’ says Hilbert, ‘with a combination of smart default values to help guide the design process, standardised interface designs for improved ease of use, and automatic design calculations that streamline workflows by enabling rapid evaluations of optical behaviour.’

Settimi agrees that improved usability is an up-and-coming trend. ‘Expect to see major improvements in the usability of future optical design software packages,’ he adds. ‘This is becoming more important, as so many mechanical and electronic systems contain optical components. The mechanical or electronic system designer is often responsible for the optical subsystems as well and currently available optics software could be friendlier for such users.’

The software is also much speedier than it once was. ‘It has become faster and more powerful at doing all the things it used to do,’ explains Barrett, ‘and it has added new capabilities into regions unimaginable only 10 to 20 years ago. For example, 20 years ago, the optimisation of a zoom lens would have been a task that took months, and is now routine. Instead today, it is full opto-mechanical stray-light analysis that consumes huge computational power. Twenty years ago no one could have imagined performing opto-mechanical stray light calculations on a desktop computer.’

And Hilbert echoes this viewpoint. ‘Software performance has become much faster with support for parallel and distributed processing,’ he says, ‘which takes advantage of today’s powerful computers to process complex optical calculations, such as design optimisation, more quickly. In addition, algorithmic research and implementation has evolved over the years, making optical software instrumental in the design of high-performance applications, such as micro-optics for medical devices, lithographic imaging systems, and lenses containing diffractive optical elements (DOEs).’

Interface standards have also helped the software become more streamlined. ‘Optical design software is also doing a better job facilitating a streamlined engineering environment by providing tighter integration with other CAD and analysis packages,’ says Hilbert. ‘This has been accomplished through the continual improvement of interface standards, such as STEP and IGES, as well as more integrated approaches, which range from including optical functionality directly in mechanical CAD products to linking optical design software to mechanical CAD software and enabling bidirectional, real-time communications between the software packages. The latter approach has the advantage of combining the geometric modelling capabilities of a CAD package with the full optical capabilities of optical design software.’

But all these changes have taken place gradually, with the development of optical design packages being viewed as ‘evolutionary rather than revolutionary’, according to Barrett.

There have been some leaps forward within optical design software though, according to Hilbert. ‘The introduction of fully integrated automatic design optimisation for illumination applications is a major technical breakthrough in the industry,’ says Hilbert, ‘which often involves “noisy simulation” not seen in imaging optics.

‘Optical design software has for many years provided automatic design optimisation for imaging systems, and has now expanded that technology to specifically target illumination performance requirements. This is an important new development in illumination design software that allows designers to achieve solutions that may never before have been practical. As the use of illumination optimisation becomes more prevalent, the sophistication and performance of illumination systems will advance more rapidly, particularly for small light sources, like LEDs.’

Optimistic future

Optimisation seems to be one trend that will keep on going over the next few years. ‘Optimisation and tolerancing features will continue to be key capabilities for both imaging and non-imaging engineers,’ says Hilbert. ‘These are capabilities that require the speed and analytic capabilities of a computer, and can’t be duplicated or even augmented without the benefit of excellent optical software. Those engineers that use the best optimisation tool are likely to produce the end product with the best optical performance.

‘Those that leverage good tolerancing algorithms will be able to manufacture their optics most reliably and at the lowest cost. There are still substantive advances and benefits to be made in these areas across both imaging and illumination applications – but clearly illumination software lags imaging in terms of development and use in elapsed years in these regards, and therefore has the most potential to benefit as these features are implemented and improved in optical software for illumination.’

Barrett predicts fewer approximations will be used within future optical design software. ‘Exact calculations will continue to replace approximations,’ he adds. ‘For example, noone will tolerance a lens with first-order extrapolations of performance when the exact calculation can be done quickly.’

And Settimi agrees with this prediction about increasing accuracy. ‘Available software and modern computer workstations have turned optical system design into a very exact science,’ he concludes. ‘There is virtually no gap between the performance of an optical system as predicted by software versus the performance of the actual system produced. If there is any gap, it is due to the inability of the manufacturer to generate, figure and position the elements of the system within the tolerances as prescribed by the software program design.’