Optical engineers can construct and analyze systems of optical components as well as mechanical structures with LightTools, a three-dimensional (3-D) modeling program from Optical Research Associates (ORA, Pasadena, CA). Unlike traditional optical-design software in which systems are entered and evaluated as a series of surfaces, each system component is specified as a 3-D object that can then be manipulated in space through simple drag and drop operations. Nonsequential ray-tracing capabilities, including refraction, reflection, and amplitude splitting at each surface, allow the designer to introduce user-defined ray patterns into the system at any point and observe how the light is propagated. Currently the software runs on SUN Sparcstations; a version for Microsoft Windows is planned for mid-1995 release.
Components are either specified through the use of an icon toolbox containing common optical shapes or drawn using mouse-based, mechanical CAD-style operations. Mechanical and optical surfaces can be given refractive or reflective characteristics. They can also be specified as binary optics or diffraction gratings, requiring the software to simulate scattered light from surfaces.
Because each object in the program is a complete 3-D shape, the program is able to analyze the interaction of light with the entire component. For example, by making a lens edge reflective, the user can quickly determine if light reflected from that edge propagates through the system. This technique can be applied to the mechanical elements of the system such as lens barrels, retaining rings, or screw heads, allowing designers to assess their contributions to system obscuration or stray light.
Fast track to real world solutions
John Tamkin of Polyscan Inc. (Tucson, AZ) worked with LightTools in the optomechanical design of his company’s laser-based direct-imaging equipment, which is used to manufacture printed-circuit boards, multichip modules, and flat-panel displays. As in many industrial applications, space is critical in Polyscan’s products, and designers frequently resort to complex folded optical paths in order to minimize system size. Using LightTools to model these systems, Tamkin has been able to quickly identify mechanical interferences in the ray path (see figure). Such obstructions can be difficult to visualize with traditional optical-design programs.
Once a rough optomechanical design is achieved by the optical designers, it is imported to AutoCad for refinement by mechanical engineers. Tamkin likes the capability of LightTools to transfer data to AutoCad through the DXF file format as a useful asset, although he would like to see this function further developed to include more sophisticated model parameters.
The ability to fully model both mechanical and optical structures of system prototypes allows designers to converge quickly on a real-world solution. "The software allows an optical-design team to explore mechanical constructs before turning the design over to the mechanical engineers," says Tamkin. Optical designers are thus able to come up with system configurations that the mechanical designers can use more readily.
Object-oriented approach
Researchers at the MIT Artificial Intelligence Laboratory (Cambridge, MA) have used LightTools in the design of eyeglass-mounted virtual-reality displays. Project member Phillip Alvelda was able to model the complete environment in which the system would operate, including the wearer’s face and mechanical elements of the eyeglasses such as hinge screws. The program allowed him to analyze optical obscurations caused by facial features and pinpoint potential sources of glare or stray light. In Alvelda’s opinion, no other currently available software would provide this capability while remaining easy to use.
Alvelda believes that the utility of the package stems from its object-oriented approach. "LightTools reflects how you work with real elements," he says. "Using it is just like constructing an actual prototype. It really allows you to spot potential problems in manufacturing, assembly, and operation before they occur." The MIT group may also use the program as a teaching tool, helping students visualize optical systems and understand how reshaping and adjusting elements affects system performance.
Probing for stray light
LightTools helped engineers at Ball Aerospace (Boulder, CO) design the Near-IR Camera Multi-Object Spectrometer (NICMOS) destined for use in the Hubble Space Telescope. Project engineer Michael Kaplan created the original optical design in ORA’s CODE V design program. The file was then imported to LightTools, where mechanical structures were added to the model.
Stray light is a major concern in this tightly packaged, multipath system. Using the software, Kaplan introduced various ray sets to probe for potential stray light problems and test the effectiveness of different baffle designs. He found the ability to arbitrarily specify the reflectivity of any surface highly useful in identifying potential problems with specular reflection.
To improve the software in future releases, Kaplan suggests that ORA add the capability for importing objects that already exist in other CAD packages. By adding this feature and further refining the interface between LightTools and CODE V, he says, ORA could create a seamless development environment for optomechanical design.