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sannyg

2006-09-06 10:19

Optical Design software: which package do you need?

If you're designing an optical system - be it a single light-bulb or a petawatt laser - there is a software package that can help. But how do you choose the right one for your needs? Michael Hatcher looks at six of the most popular packages used by the optics industry.

As the processing power of PCs has rocketed over the past decade, so optical design software has evolved. Once a highly specialized tool that had to be operated on mainframes and UNIX workstations, now such software is accessible on the desktop of any average optical engineer.

Whether you are designing cameras or radiotelescopes, a wide choice of packages is available to help you. We have taken a look at six of the most widely-used design programs to help you decide which is the most appropriate for your application.

Optical design software falls into two broad categories: "classical" lens-design software, and illumination packages. Lens-design software is used to optimize image-forming optical systems. First, you must provide a starting design that includes data such as the shape, size and position of lenses and other optical elements in the system, the system's aperture and field of view, and the wavelengths being used. The program then traces the paths of light-rays entering the system, optimizing the optics using a mathematical algorithm until your target criteria are met.

Illumination packages are used for more detailed analysis of an optical system - to predict the effect of sunlight on a telescope, for example. Output from lens-design software will often provide the input for an illumination package, enabling you to produce a more detailed analysis of system performance. Illumination packages are also used when designers want to analyse complex light distribution - for instance, that produced by car headlights.

Lens-design packages

Three of the major packages are ZEMAX, OSLO and CODE V. OSLO and CODE V are older programs, dating back to the 1960s. ZEMAX is a much newer program - the first commercial version appeared in 1990.

For an independent viewpoint, Opto & Laser Europe contacted Scott Lerner, an optical engineer at the Lawrence Livermore National Laboratory, US. Lerner uses all three lens-design packages to design a wide range of systems.

Broadly speaking, says Lerner, ZEMAX is an entry-level package suited to less experienced designers, whereas CODE V is a high-end solution suited to more demanding tasks. OSLO fills the middle ground.

"Given a specific problem, you could use any of the three codes to generate a solution. But I guess my rule of thumb is that if it's not too complex I'll do it in ZEMAX," Lerner told Opto & Laser Europe. "ZEMAX is accepted to have the lowest learning curve, and it is also the least expensive of the packages. So typically, if you have a relatively simple system to design that doesn't require extensive analysis then ZEMAX is the program of choice - it is cheaper and your customers are more likely to have access to it."

ZEMAX is not best suited to every situation, though. Lerner continued: "However, if you're doing extensive analysis of a system, or if you are developing 10 systems that are fairly similar and analysing all of them, then the macro language used by CODE V becomes extremely valuable. OSLO is kind of the middle ground."

Your preferred style of working may come into the equation too. "There's a knack to using each package and in the end, it can simply boil down to personal preference. CODE V tends to be faster, but that's not always what you want," said Lerner.

Dave Hasenauer is product manager for CODE V at ORA, US. He said: "We are the high-end package and we strive for those applications where the optics are critical to the success of the product."He adds that the applications that best suit CODE V are those that require high-precision optimization and tolerancing. "For example, people use CODE V for microlithography stepper systems and for reconnaissance systems," he said.

A crucial point to consider, says Hasenauer, is the level of technical support that you're going to need. How much you get depends largely on the way that the different packages are sold. CODE V, for example, is sold on a leasing basis. This means that the customer pays a monthly fee for a package that includes full technical support.

On the other hand, ZEMAX customers pay for their package outright. The deal includes one year of technical support, after which there is a smaller fee for subsequent support and upgrades.

Mark Nicholson, technical director of ZEMAX vendor Optima Research, UK, says that ZEMAX has the advantage of having been written specifically for a Windows user-interface. "It's a very easy-to-use package, although you still need to be a good designer to use ZEMAX," he said.

Nicholson says that in the right hands the program has no limitations. He believes that because ZEMAX offers users the freedom to write their own features, the package can match anything that its more expensive competitors can produce.

OSLO, sold by Lambda Research Corporation (and also by LightTec in Europe), is available on either a leasing or a purchase basis. Lambda claims that OSLO was the first program used for "serious" optical design on desktop computers, and G Groot Gregory, Lambda's vice-president, says that non-experts are making up an increasingly large proportion of customers. Bearing this in mind, one of OSLO's strong points is said to be its interactive nature, which makes the user interface more intuitive.

OSLO is described as an object-oriented Windows program that has been developed exclusively for desktop computers. It features a programming language that allows users to customize the user interface and functionality. Three versions are available, the most advanced being OSLO Premium, which includes features such as polarization raytracing and analysis of thin-film coatings. Gregory says the package is easy to customize and adapt, and claims that because it has greater overall flexibility than its rivals, difficult problems can be solved more easily.

Illumination packages

Once your optical system has been designed and optimized, you may need to take a closer look at it. This is where illumination packages come in. Common programs are ASAP, LightTools, TracePro and ZEMAX (Engineering Edition).

According to Lerner, such packages are complex, demanding a greater degree of understanding and a higher learning curve than lens-design packages.

Illumination software is typically used in the automotive field for headlight design, or in the cellphone industry to ensure uniform backlighting of LCDs. But its uses are not restricted to commercial applications. Lerner uses ASAP, the oldest of the illumination packages (sold by Breault Research Organization, US) for stray-light analysis for the National Ignition Facility project.

Going strong for 21 years, ASAP is seen as the industry standard in illumination software, and is the most powerful package available. Until relatively recently its user interface was seen as a potential weakness. However, recent versions of ASAP now feature a graphical user interface (GUI), making it more appealing to younger engineers familiar with Windows-based packages.

Breault admits that in the past it has concentrated on the power and speed of ASAP at the expense of its GUI, but both command-driven and GUI user modes are now available. "Almost everything possible in command form can be done through the GUI, and users can switch between these two user modes," Breault told Opto & Laser Europe.

Mike Hayford is product manager for LightTools, the illumination package from ORA. He says that this sector of the market has not yet developed to the same extent as the lens-design segment, and that it is less easy to define packages as high- or low-end. So while ASAP is seen as the standard, the distinctions between TracePro and LightTools are less clear.

Hayford says that although the feature lists of LightTools and TracePro look similar, the extra cost of LightTools guarantees its users a robust, accurate, reliable piece of software that suits a wide variety of designs. "LightTools uses solid modelling [rather than surface modelling], which makes the program more suitable for engineers without a background in optics," Hayford said. He adds that LightTools can also be used as a design tool. "You can modify the system geometry and re-analyse."

For automated operations, LightTools features a user interface that allows the program to be driven by Microsoft applications such as Excel. According to Hayford, this means that engineers unfamiliar with optical design tools can still use the software.

TracePro, from Lambda Research, is the cheaper of the two, and according to Lambda president Edward Freniere its emphasis is on ease of use, flexibility and interoperability. He disagrees with Lerner's comments on the complexity of illumination packages: "With a well-designed user-interface, illumination software can be easier to use than lens-design packages. This is what we strive for in the design of TracePro." TracePro is routinely used in biomedicine and to design LCD panels, lightpipes and projectors.

According to Nicholson, ZEMAX can also be used for stray-light analysis, although the physical optics engine required for this feature is only included in ZEMAX EE, the package's Engineering Edition.

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