Mechanical CAD (computer-aided design) programs have become very sophisticated during the past few years. Unfortunately, there is still a portion of the engineering spectrum that cannot be handled well in a traditional CAD program: optical modeling. If you are creating a complicated optical system (think of a camera zoom lens), then it is best to perform almost all of the design in a specialized optical design software program and then transfer the optical design to a CAD program for the later stages of the design process where items like housings, threads, cams, and motors are designed and integrated into the model.

Figure 1. Cree XLamp 7090 LED Module modeled in SolidWorks.
On the other hand, if you are designing a simple indicator light for the outside of a computer housing, then the optical analysis would be a small portion of the overall design, which would be performed almost entirely within a CAD program. The projects that lie in between these two extremes are the ones that typically require using some type of optical software in addition to CAD software. Ideally, the optical and CAD software also have to exchange model data back and forth between them in order to facilitate the iterative cycles that the engineering process typically entails.

Interoperability and Data Integrity

Figure 2. LED modules aligned in a Ring Formation on a PC board
A typical application that requires iterative exchange between optical and CAD software would be the design of specialized or technical lighting. A good example would be an LED ‘ring light’ that mounts on the outside of a camera lens, providing broad, even, and consistent illumination for macro photography, robotic vision, or even medical procedures. In these cases, both the optical and mechanical aspects of the ring light design need to be considered together.

Depending on the capabilities of the CAD and optical software, the design of a ring light model could follow four different workflows.

First, the model could be exchanged back and forth between the optical and CAD software using external data files or translators. Passing the model directly between programs would require either a common data file format or import/ export translators to translate from one file format to another. Using a common data file format would be ideal as long as the optical software program respected the integrity of the CAD data in the file, and vice-versa. Unfortunately, most software programs use proprietary formats. On the other hand, using translators to import/export solid models has a large drawback in an iterative workflow environment. Optical properties (i.e. reflective coatings) would need to be re-applied every time the model is imported back into the optical software, and mechanical design properties (i.e. tolerances and parameter dependencies) would need to be re-applied every time the model is imported back into the CAD software.

Second, the CAD and optical software could transfer model changes directly using data exchange protocols. DDE or COM protocols often are used to exchange data among Windows programs. This approach typically requires specialized macro programming to make sure that the desired data is transferred and applied on the receiving end.

Third, the optical software could work as a sub-program entirely within the CAD software (or vice versa). This approach typically limits the capabilities of the subprogram due to user interface, data manipulation, and memory management limitations of the “parent” program.

The final option is to have an approach where you primarily perform design work in program “A” and utilize a sub-program, or bridge program, to apply specialized properties for program “B”. Model transfer only goes one way (from “A” to “B”), but program “A” (in combination with its bridge or sub-program) maintains all properties for both programs “A” and “B”. Lambda Research has adopted this final approach through the development of TracePro Bridge for SolidWorks.

Practical Design of an LED Ring Light

Figure 3. LED Ring Assembly integrated into housing in SolidWorks.
The design of a custom, high-intensity LED ring light provides a good example of the iterative workflow enabled using an add-in bridge software methodology.

Figure 4. Light Ring Assembly in TracePro showing analysis plate.
It is a good idea to start the design by determining if off-the-shelf components are available. Vendor-supplied LEDs are available as complete modules, including an integrated mount housing and cover lens. Some vendors even offer multiple LEDs mounted in a ring configuration on a PC board, and it is up to the designer to place them in an optically, mechanically, and electronically viable system. Depending on the choice of LED, CAD, or optical software models might be available from the manufacturer or third-party vendors; otherwise, the model can be built in CAD using supplied dimensions. Our selected LEDs are Cree XLamp 7090, which we modeled as individual modules in SolidWorks (see Fig. 1). Details on the LED lens, die, and reflector can be obtained from the appropriate Cree datasheet.

While building the LED module model, it’s important that the optical design keeps track of every feature (e.g. thickness, radius, fillet, etc.). This allows the designer to change any individual feature at a later time, but requires the designer to treat each individual LED module as an assembly of components, with each component having separate properties.

Once the LED module design is complete, the designer can access the add-in bridge software to apply optical properties such as source definitions, glass or plastic materials, paints, surface roughness, and reflective coatings to each component. A property database shared by the optical design and bridge programs ensures instantaneous update and consistency among property definitions. Once the properties are applied to the LED modules (including the number and distribution of rays that they emit), the model is saved as a TracePro file within SolidWorks. The same file is opened in optical design software for comparison against module specifications through ray tracing and optical analysis.

After the modeling is complete in the optical design software, the designer can integrate the LED models onto a PC board design (see Fig. 2), and continue designing the plastic housing in the CAD program, which will accommodate the requisite electronic components, including a power source (see Fig. 3). To complete the assembly, a ring of acrylic is provided as a protective cover for the LED modules. Since this cover can be formed into any number of shapes, it is common to use this cover to re-direct the light from the LEDs and is therefore called a “lens” (even though it might not look like your classic concept of a lens). The design of the electronics will not be considered here, but the outside of the housing and the acrylic cover lens can have properties applied to them through the bridge software.

Optical Analysis

Figure 5. Polar Candela Plot in TracePro analyzing the illumination field.
When the final model is opened in the optical design software, the only undefined parameters for the ring light relate to ray tracing — wavelength, ray splitting flags, thresholds, etc. A pair of parameters is critical to verify the optical performance of the model: angular distribution towards the intended object, and stray light back into the sensor/camera. Instead of modeling the actual camera, optical design software typically replaces the camera’s primary lens with an observation plate to determine the amount of stray light (see Fig. 4).

Figure 6. Irradiance Map in TracePro analyzing the straylight reaching the region of the camera lens.
The designer then traces a low number of rays in the model (~35,000) to get a rough idea of performance. Figure 5 shows a Polar Candela plot of rays in front of the ring light. Note that the intensity is fairly uniform over the 80° viewing angle. Figure 6 shows the irradiance on the analysis plate and indicates that less than 0.5 percent of the initial light energy (stray light) is directed towards the region of the camera lens.

At this point, the design can be adjusted in the main optical design software package, including the number or type of LED modules, the height of the ring assembly, the cover lens shape (diffusing elements can be added), and the overall housing design. These changes can be driven by either optical or mechanical requirements such as beam pattern or ruggedness of the application. The simulation can then be redone and new results generated on the path to improving the camera’s ring-light design.

As this example shows, using an efficient optical design software that integrates with SolidWorks via add-in bridge software greatly reduces the amount of time designers spend moving back and forth between mechanical and optical design environments in an iterative design process, with the final result of shortening product design schedules and quickening time to market.

This article was written by Dr. Leo R. Gardner, Director of Marketing, and Patrick Le Houillier, Applications Engineer, at Lambda Research Corporation. For more information, contact Mr. Le Houillier at 978-486-0766.

Photonics Tech Briefs Magazine

This article first appeared in the May, 2006 issue of Photonics Tech Briefs Magazine.

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