Advanced optical modeling is essential for optical engineers who must create designs within ever-shorter product-development cycles. For many projects, illumination modeling is becoming as important as the imaging optics.
FIGURE 1. Biaspheric lens to be modeled with illumination software is first optimized using a lens-design program. Intermediate image plane must be modeled as a specular surface.
Illumination systems include many optical systems such as compact desktop projectors, fiberoptic systems, automotive headlights, backlights, and homogenizers. In some of these systems, both illumination and imaging optics must be optimized to work together for maximum performance.
Traditional lens-design software has long been used to optimize, analyze, and tolerance imaging lenses for cameras, projectors, and electronic imaging devices. Although lens-design software varies in price, ease of use, and sophistication, these programs all share a similar set of features. Users can expect damped-least-squares optimization, ray-tracing, access to off-the-shelf components, and a host of image-analysis options such as modulation transfer function, spot diagrams, aberrations, and others. Many lens designers can switch design programs with minimal effort. These design codes contain features that allow a designer to create optimized imaging designs and to feel confident that the designs will perform properly when fabricated.
Lens-design codes cannot, however, analyze systems that have scattering components such as diffusers. When scattering or complex source modeling is required, many designers turn to illumination software.
Intended primarily for tracing a very large number of rays and examining the spatial or angular ray distribution at some surface, illumination software is useful in designing condenser systems, light pipes, homogenizers, and systems with complex geometries. It is also useful as an interface with optomechanical techniques that determine system layout and appropriate placement of light-blocking baffles. One very powerful feature is the ability to import complex three-dimensional geometry from mechanical computer-aided design (CAD) packages in the form of IGES, SAT, STEP, or STL files.
Illumination systems are not designed like conventional lenses, where a designer optimizes a merit function over a range of individual field points (point sources). Instead, illumination systems create a distribution of light with a required illuminance and uniformity. There are no automatic merit functions available to the illumination designer, and the sources used are not always well defined. Although several traditional lens-design programs offer nonsequential ray-tracing options, setting up a system with these programs can be tedious.
By modeling the entire system, including lens mounts, coatings, and accurate source models, designers using illumination software can predict collection efficiency and uniformity and can control stray light for complex systems. However, illumination software does not allow for automatic optimization or automatic tolerancing and offers limited image-analysis op tions. Many users require a standard lens-design code in addition to illumination modeling software.
Choosing a design package or combination of packages requires the user to understand the main requirements of the system. Although there are significant differences between illumination code and lens-design codes, many lens-design programs are building in illumination design capabilities for systems that do not have scattering components.
When selecting an illumination package, there are several issues to consider: How easy is the program to use? How fast does it trace rays? Does the program offer CAD import/export and/or lens-design import? What are the analysis features of the program? and How is the user support and program documentation?
CAD and lens-design import - Integration with lens-design software and mechanical CAD software is an important aspect of any nonsequential ray-tracing code. Designers turn to illumination software when scattering surfaces, advanced source simulations, or nonsequential ray tracing are important items. Ray splitting-which cannot be achieved with standard lens-design codes-enables designers to model the diffuse surfaces important in many illumination applications.
For example, a confocal lens is designed for a scanner application. The lens is a biaspheric element that is decentered with respect to the optical axis. A light-emitting diode (LED) is imaged to a scan plane and then received by a detector. The design parameters are optimized on a traditional lens-design code to achieve the smallest spot size (see Fig. 1).
If a reflector is placed at the scanned media, the return signal is 100% because the design is confocal. However, in this example, the scanned media could be a variety of diffuse material including several with different degrees of diffuse and specular return. Analysis of a partially diffuse material requires the use of illumination software.
Rather than rebuilding the lens model from scratch in the illumination code, a translator supplied with the illumination program can be used. This saves time in rebuilding models after initial optimization. Almost all illumination programs now offer lens-design and CAD translators. Although they are fairly robust, there are still occasional import problems for nonstandard surfaces.
Also, users should be aware that some illumination programs only provide translators for specific CAD files (IGES, SAT, STEP, STL) and lens-design codes. Designers should make sure that their illumination software has the required translators for their specific needs. Using illumination software, diffusion properties can be added to the scanned media and the collection efficiency can be calculated for a variety of materials (see Fig. 2).
In addition to scattering capabilities, illumination software can model complex extended sources. Users can enter both spatial and angular apodization to create accurate source models that can predict the performance of an optical system. In the above example, specific angular LED distributions were modeled, and the power reaching the detector was calculated.
The ability to simulate both source distribution and scan media allowed for more-advanced simulation of this system than a traditional lens-design program would allow. However, lens-design software was used in the optimization of the lens surfaces.
The use of convenient translators makes it easy for a designer to iterate the design and then try a variety of scan media and sources to optimize the collection efficiency. Without these translators, this task would be more tedious for the designer and would require a new model to be built after every iteration.
Prototyping even this simple plastic lens would be expensive and would take many weeks. With the integration of lens-design and illumination software, the designer can optimize the lens, analyze real-word performance, and readjust the design as needed. These simulations can be done in days instead of weeks and save time on cost on iterating prototypes.
While nothing is a substitute for the actual hardware, this virtual prototyping process allows the designer to save time on hardware iterations and get a more detailed understanding of potential design limitations. The time saved in the design of complex optical systems can be even more significant than for the simple example given here.
FIGURE 2. Illumination software models the intermediate image plane as a diffusely reflecting surface (left). Some of the returning rays are refocused, while others miss the lens entirely. Rays shown in blue are secondary reflections. Illumination software renders the lens in three dimensions (right).
MICHAEL THOMAS is president of Optical Product Development Inc., Cambridge, MA; e-mail: [email protected].