By Andrew Wilson, Editor
Designers of optical systems can, to some extent, abandon the optical benches of the past in favor of PC-based optical-design software. With more than 20 optical-design and optical-system design and simulation packages available (see table on p. 36), developers can experiment with different optics and combinations of lenses and optomechanical systems and build prototype models that can graphically show the completed design of a lens, optical, or optomechanical system.
Of the numerous optical-design and analysis packages available, most operate on Windows-based PCs. However, in choosing a specific package for optical design, developers must carefully consider the performance and cost goals. Low-cost packages, which can be effective for several tasks, such as examining the structure of optics designs, may be limited in the number of off-the-shelf components available for designer selection. Designers using such packages also may find themselves limited if they need to export their designs to other computer-aided design (CAD) packages for further optomechanical analysis or three-dimensional (3-D) rendering.
Designers must also determine whether they require sequential or nonsequential ray-tracing or a combination of both. As camera lenses and microscopes can be developed in terms of a sequential list of surfaces, rays can be traced sequentially from the object surface to each surface in a specific sequential order using sequential ray-tracing.
Other optical systems such as prisms and faceted reflectors cannot be described by using sequential ray-tracing. Instead nonsequential ray-tracing, in which a ray may strike any object in any order, must be used. While some software packages offer only sequential ray-tracing, the more sophisticated (and more expensive) packages also include nonsequential ray-tracing, as well as extensive databases of manufacturers' and custom-based lens designs, solid modeling options, and support of multiple CAD formats for file output.
Many designers starting to build optical systems often approach lens manufacturers for help rather than purchase full-blown optical-design software packages. As a result, lens manufacturers, including Schneider Optics (Hauppauge, NY) and Linos Photonics (Milford. MA), have developed software programs that simplify the task of determining the component parameters needed for systems development and analysis. Such packages allow mechanical and electrical engineers to explore, analyze, modify, and specify the optics and structure of their products at any stage of design or manufacturing without direct assistance from on-site optical engineers.
Originally developed at Jos. Schneider Optische Werke Kreuznach GmbH, Schneider's GAUSSOPT_ik package can calculate depth-of-field and depth-of-focus tables at any object distance, magnification, and f-number, even when dealing with macro images. Using this package, a developer enters a few known parameters, such as height of subject, preferred distance from subject to lens, and size of film or sensor surface. The software then automatically calculates all other relevant parameters, suggests suitable existing lenses, and provides the charts, tables, and diagrams necessary to document the selection.
FIGURE 1. Developed as a tool for component selection and system development and analysis, optical-design software from Linos Photonics can create systems and custom components by specifying part numbers.
WinLens optical-design software from Linos Photonics is also available as a software tool for component selection, system development, and analysis (see Fig. 1). With this package, developers can create systems with both Linos and custom components using the company's component database. Software functions can handle systems with rotational or cylindrical symmetry; select such components as singlets, doublets, air-spaced doublets, triplets, blocks, and mirrors; and provide capabilities to import/export to/from files in other formats.
To estimate aberrations and quality of optical systems from more than 100 standard glasses made by Schott Glas (Mainz, Germany), the Optic program from Hohner Corp. (Ontario, Canada) can perform 260 ray-tracings at specific wavelengths set by the developer. In the image plane, it can calculate paraxial characteristics, geometrical aberrations, and wave aberrations, and plot optical schemes and positions of points for each ray traced through the optical system. Designers can also add custom glasses to the program by entering specific optical parameters.
To design and analyze optical systems, many design packages provide more sophisticated ray-tracing functions as well as access to hundreds of different standard lenses, mirrors, prisms, and gratings. These packages also allow developers to create new components or modify existing ones, add components to existing systems, and show the optical path length of each ray using two- and three-dimensional graphics.
In the Optica package from Wolfram Research (Champaign, IL), for example, a ray trace through an optical system is described by a list of objects that are individually created at each surface intersection with the ray. These objects contain parameters about the ray-trace intersection, including position, direction, intensity, and optical path length. The position is given as a 3-D vector, and the direction is given by direction cosines in three dimensions.
Optics Software for Layout and Optimization (OSLO) of optical systems from Sinclair Optics (Fairport, NY) is also targeted at scientists and engineers designing lenses, reflectors, optical instruments, and illumination systems (see Fig. 2). In addition, it can be used for simulation and analysis of optical systems using both geometrical and physical optics. For educational purposes, the company provides a free version of OSLO, which can be downloaded from its Web site (www.sinopt.com). Features of the package include evaluation and optimization of basic zoom systems, nonsequential ray traces of surfaces or element groups, and the ability to develop user-defined surfaces, gradients, and ray tracings.
FIGURE 2. Using the Sinclair Optics OSLO package Autodraw window, designs can be visualized and analyzed. Data can then be exported to other CAD packages in DXF and IGES formats.
While many inexpensive optical-design packages allow developers to analyze optics designs, more-sophisticated packages also allow 3-D visualizations of completed designs. Using both electro-optical and mechanical CAD software, such packages are useful in developing complete electromechanical optical systems. Perhaps the best known of these packages, Advanced Systems Analysis Program (ASAP) from Breault Research Organization (BRO; Tucson, AZ), CODE V from Optical Research Associates (ORA; Pasadena, CA), and ZEMAX from Zemax (San Diego, CA), are multifunction packages that are offered in conjunction with additional modules to perform specific optical design, analysis, and rendering functions.
BRO's ASAP, for example, is an unconstrained, nonsequential ray-tracing program that includes capabilities for modeling physical optics, imaging, and illumination systems. Graphical tools allow cross-sectional or 3-D visualization of model geometry, ray-tracing, and analysis of results. The ASAP package can determine scattering, diffraction, reflection, refraction, absorption, polarization, nonsequential ray-tracing, and Gaussian-beam propagation. Moreover, the package's interactive display features reveal potential problem areas while optical analysis is in progress.
The combination of 3-D surface and structure modeling techniques with ray-tracing and coherent/incoherent beam propagation algorithms is useful for modeling mechanical structures and their interactions with the optics, three-dimensional modeling of optical and mechanical components, and real-time rendering of system geometry, ray traces, and movable light sources. Sold in modules that start with ASAP/Basic, other modules include ASAP/Optical for coherent imaging and lens translators, ASAP/CAD for computer aided design, and ASAP/ELTM for standards compliance testing.
ORA's CODE V is also an integrated system of modules, allowing a variety of optical computations to be performed on a common input lens database. The various functions and major capabilities of CODE V are grouped into options, including some used to analyze the geometrical optical performance of the optical system. These include third-order aberration analysis, real ray-tracing, modulation transfer functions, square-wave responses, radial energy distribution, line spread distributions, and detector energy analysis. Options are also included for the computation of lens transmission (including the effects of single or multilayer coatings and polarization), weight and center of mass, and ghost image analysis.
Computations that aid in evaluating the performance of the total optical system are also included in CODE V. For example, the spectral-analysis program cascades specified detector, blackbody, or filter responses to provide a system spectral response curve, and computes appropriate sampling wavelengths and weights for polychromatic computations. A multilayer option also can be used to analyze or optimize the structure of a multilayer coating stack made from dispersive or nondispersive layers, with or without absorption. Antireflection coatings and various types of filters can be designed with this feature.
To conceptualize, design, optimize, analyze, tolerance, and document optical system, the ZEMAX package can model optical surfaces that are reflective, refractive, or diffractive (see Fig. 3). In addition, surface properties, such as variable transmission due to thin-film coatings, can be modeled in detail. ZEMAX also supports sequential and nonsequential ray-tracing in the same system, allowing sequential surfaces to be mixed with nonsequential objects of arbitrary shape, orientation, and position.
FIGURE 3. ZEMAX graphical user interface can highlight spreadsheets, lens designs, layouts, and point spread and modulation transfer functions of an objective lens.
For sequential ray-tracing, sources are defined as either field points or extended bitmaps on the object surface. Nonsequential sources can be more complex than sequential sources, are generally three-dimensional, and are defined to have an output flux in either watts or lumens. A user-defined number of rays are generated by each source to control the source sampling. Separate controls are available for the number of rays shown on layouts and analysis windows.
In addition to using optical-glass catalogs, ZEMAX can be supplied with other catalogs that include infrared materials, plastics, and natural materials. Stock-lens catalogs from companies such as Edmund Industrial Optics (Barrington, NJ), Linos Photonics, and Melles Griot (Longmont, CO), are also included.
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