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Model behaviour

The use of optical modelling in the design of lighting systems has increased in recent years, largely due to the transition to energy-efficient LEDs. And, with the emergence of other lighting technologies, such as solid-state lighting and OLEDs, optical design software is a key tool for non-experts to create a range of innovative lighting products that are used in increasingly imaginative ways. 

One of the latest trends in the lighting market, according to Dave Jacobsen, senior application engineer at Lambda Research Corporation, is the move to more compact and higher performing luminaires. ‘Many of the new, smaller designs are taking advantage of the benefits LEDs bring to the table,’ he said, ‘and OLEDs are showing up in display applications, adding a new option for these types of applications.’

This need for smaller and more efficient light sources is important for applications such as architectural lighting, said Stuart David, group director of applications engineering at Synopsys’ optical solutions group. ‘Freeform lenses and freeform reflectors can reduce the physical footprint of light sources in architectural lighting,’ he said, ‘while also making it possible to achieve new photometric qualities.’

Size matters

Mark Jongewaard, president at LTI Optics, noted that many of the new trends in lighting systems are still driven by the prevalence of LEDs as the primary light source. He said: ‘The small size of LEDs compared to other sources like fluorescent or HID (high intensity discharge) has allowed for the design of smaller and more precise optical systems, as well as more flexible form factors for large, luminous surfaces. The range of spectrums available from LEDs has also led to the increased use of colour mixing in general lighting, whether for RGB mixing, variable white tuning, or tuning for specific reasons like circadian lighting for human health or plant growth cycles in horticultural lighting.’

As more off-the-shelf optical components become available for LEDs, Jongewaard continued, it is important to design custom optical systems that better meet the needs of a specific market to maintain product differentiation. When it comes to lighting design, one notable trend, according to Jacobsen, is the assignment of design tasks to engineers that do not necessarily have an optical design background. 

This could prove challenging, as there are a number of key considerations when designing a light source. David said: ‘If you are designing an LED source, you need to consider maximising the photon extraction from an LED die to improve luminance. Maximising output power or minimising the electrical energy is also important, as is optimising package size for a particular application and managing the output spectrum to improve colour rendering. Optical design software for lighting applications can help designers address all of these design considerations.’ 

Synopsis’ LightTools software, for example, has a set of capabilities for simulating colour rendering, or chromatic effects in an illumination system. Software such as this can be used to automatically optimise a design to achieve the best chromatic performance, based on the application’s particular requirements.

Design for life

When designing a new light source, the first step usually starts with a specification or a market need, according to Lambda’s Jacobsen. ‘This could be a certain illumination level, a lighting pattern, or a certain architectural look to the luminaire,’ he said. The next step is to select an appropriate source for the luminaire, which has the necessary light output and spectrum for the application. ‘Factors such as source life and serviceability may also need to be considered,’ added Jacobsen. ‘Then start the design process, which can be done using optical design and analysis software, or manually by hand. In either case, being able to draw on training or past experience gives the designer a leg up in getting started. After the design process has been completed it is time to build a prototype and test it to see if the finished product meets the initial requirements.’

LTI Optics’ Jongewaard believes that the specification and goal determining process should be a collaborative effort between marketing and sales, mechanical and electrical engineering and optical engineering. ‘The goals and their priority can include things like the intended application, required output, visual appearance, overall cost and any regulations the product must satisfy.

‘The optical and mechanical engineer must work closely to negotiate space and determine how to achieve those goals in an acceptable form factor. During these steps, the optical designer considers what light source and optical techniques to use, and how those can meet all the requirements. Once the concept is well defined, the optical and mechanical engineers will settle on space and optimise the design to achieve the goals. Only then can the form be designed.’

Designing a lighting system does not come without challenges, particularly with the move towards smaller systems. ‘As lighting systems get smaller,’ Jongewaard continued, ‘the design of the optical elements can get more challenging and the demand for higher precision manufacturing increases.’ This means that design tools such as software and manufacturing methods must be able to meet these needs. LTI’s Photopia parametric optical design tools are designed to create reflector and lens geometry based on the desired beam angles, as well as the amount of light to be directed toward each angle in the beam.

Life in colour

As well as size, colour is also a design consideration, and with colour mixing gaining popularity, creating optics that provide a uniform visual appearance and avoid markings in the beam pattern, while still maintaining as much beam control, becomes a challenge.  ‘The optical software,’ said Jongewaard, ‘must therefore be capable of accurately simulating LED spectral properties and material diffusing properties to provide qualitative visual and quantitative colorimetric results for the design.’

For Synopsys’ David, freeform optical elements offer freedom in design to achieve precise light control, innovative styling, compact system dimensions and energy efficiency. However, he said: ‘As with any type of lighting system, designers must continue to find ways to manage light extraction, defeat total internal reflection, and find the best optical materials to meet performance goals. This is the challenge.’

Jacobsen sees one hurdle of working with LEDs for lighting applications in their ability to sometimes create harsh effects in lighting, especially if the viewer is able to see into the fixture and see the source, either directly or indirectly. This is because there is a lot of light coming from a small area. ‘It is important,’ he said, ‘to have the capability to do photorealistic rendering and luminance plots in the optical design software, so that these visual effects can be analysed and minimalised if necessary. The spectrum of OLED sources can vary as a function of viewing angle, so it is important to be able to accurately model these effects.’

Lightening the load

The right optical design software can play a big part. Jacobsen said: ‘With the diffusion of lighting design tasks to individuals that may have little or no experience, it is important that the software be intuitive and easy to use with a short learn curve. There should also be numerous catalogues of properties, such as material, surface, and source catalogues pre-loaded with the software. It is also important to have the ability to add new properties as required. The ability to automatically optimise a design to a given goal is very important, and the software should also have the capabilities that more advanced users may require.’

With lighting being used in new ways, such as adaptive lighting, flat panel and holograms, and new technologies like solid-state lighting and OLEDs being developed, design software also helps to allow design freedom. Jacobsen elaborated: ‘As new light sources come to market, optical design software needs to evolve to cope. Typically, this will be the ability to allow the designer to accurately model these new sources in the software. Without this capability, any resulting design may be suspect. A common way of doing this for LED sources is to use rayfiles, generally available from the LED manufacturers, including the new IES LM-25 rayfile format.’ 

Many of these optical design software programs are based on 3D solid modelling, which means the user can choose to make a 3D model of the source and then apply the appropriate optical properties and, with new sources, new design forms may also become more popular, according to Jacobsen, such as TIR hybrid lenses for LED sources. ‘Any optical design program aimed at the lighting market,’ he said, ‘should be able to handle these types of systems, as well as have the flexibility to work with any new design forms that may come along. As new designs and tighter requirements come into play more often, the ability to automatically optimise a design to a user defined goal, such as flux level, illumination pattern and uniformity, is also a key factor. This allows the user to test multiple designs in a short period of time, without user involvement.’

Design freedom

For Jongewaard, the most design freedom – in terms of optical component-types, materials and lamp types – stems from the software providing a flexible interface on top of an extensible raytracing foundation. He said: ‘Providing optical design features directly in the CAD software allows designers to create any type of geometry they prefer, as well as leverage design tools we provide. Their CAD models can even include all mechanical features that interact with the light and affect performance. The simulations must start with accurate input to obtain meaningful results. Because of this, we have invested heavily in specialised measurement equipment to characterise both light sources and material scattering properties. As new technologies and application types come into the industry, having extensible raytracing and output features allows us to support the new technologies more quickly.’

The use of freeform surfaces to tailor output distribution is something that allows designers to match the source distribution characteristics to the desired lighting output distribution, David noted. ‘The LightTools advanced design module includes a set of specialised tools to enable fast, robust modelling of reflective and refractive freeform optics in both single-surface and segmented configurations. The LucidShape software for automotive lighting design includes capabilities that automatically calculate and construct optical geometries based on user-defined illuminance and intensity patterns,’ David said. ‘This is a functional approach that enables designers to focus on overall design objectives, rather than the implementation details of complex optical components.’

With the evolution of lighting technologies and applications, the software also had to evolve, and Jacobsen believes this will continue, at least in the short term. ‘The software will evolve to include more and more properties as they become available, such as new light sources and new diffusers,’ he said. ‘Increased ease-of-use will allow software to be used by a wide base of users, either with or without optical design experience. The ease of file sharing and/or integration with CAD software – such as SolidWorks, PTC Creo, Inventor, Catia – is also very desirable and important, as it allows most of the design work to be done in the CAD environment; then the optimisation and analysis to be done in the optical design software.’

Ever evolving

LTI Optics’ Jongewaard agrees that software will evolve as lighting becomes ever more innovative and is used in new ways. ‘Software will always need to evolve to accurately model any new technologies in the industry, including light sources and materials,’ he said. ‘In the coming years this could mean the adoption of OLED or laser diode-based lighting, and the expansion of micro-structured or holographic optical systems. Furthermore, design tools will continue to expand in scope to cover a broader range optic styles, performance criteria and constraints, while also working to provide the most tolerant designs to aid manufacturing. 

‘Optical design software will also continue to leverage all computational hardware available via multi-core CPUs, GPUs and cloud computing, to make simulations as fast as possible. This provides designers with more refined optimisations and more realistic product visualisations.’ 

Tom Davies, chief photonics engineer at Photon Engineering, believes that simulation tools will play an ever-increasing role in the development of illumination systems, providing predictive analyses during product design and diagnostic forensics during the manufacture and test process. ‘Correspondingly,’ he said, ‘it becomes increasingly important that the software used should provide the highest fidelity results possible, in the shortest amount of time possible. That’s why we recently invested a lot of time creating a new core architecture for our FRED software to utilise the massively parallel computing capabilities of GPUs.’

For David, advances will be made by combining simulation tools. ‘Illumination design software will increasingly combine macro- and nano-optics simulation, previously considered different modelling disciplines. Synopsys is already making progress in this area with an optical design workflow for nano-textured diffractive optical elements that enable smaller, lightweight AR/VR devices with improved displays and immersive experiences,’ said David. ‘This combination of simulation approaches will help foster smaller overall device packages and the ability to simulate more diverse operating environments.’  

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