Mary G. Turner
In the beginning there was light, which was followed shortly thereafter by humans who desired to control it. The humans invented optics, which is now spreading rapidly into numerous areas of science and engineering that require a controlled application of light. Two examples are biomedical and illumination engineering.
In biomedical engineering, the understanding of the interaction between light and biological tissue is a key factor in the development of new technologies, from the detection of biological hazards to new treatments for cancers. The field of illumination engineering has emerged as a key component in reducing global greenhouse-gas emissions by developing new devices to provide better lighting with less energy.
A model of a patient’s bladder, for instance, was created in the Advanced Synthesis Analysis Program (ASAP) from Breault Research Organization (BRO) and was used to model photodynamic therapy (PDT), a treatment for some forms of cancer (see figure). A significant advantage of PDT is that it does not have the after effects commonly found with traditional radiation therapies.
Doctors and scientists can use this model to understand the effects of various modalities on the bladder. Investigations can be carried out to analyze the effect of different forms of treatments, in terms of intensity, duration, and location. Available analysis can provide data about depth of response, total accumulated energy, as well as energy distribution. By using modeling software, significant strides can be made in treatments with commensurate reduction in risk to the patient. The model can also be used to help design new instruments that can provide more-directed treatments.
The field of illumination engineering has changed significantly in recent years. No longer is it acceptable to simply get the light to the desired location. In the areas of automotive and architectural illumination, visual appearance is also a significant element of design. In addition, designs such as “zero cut-off fixtures”-long desired by astronomers and others interested in enjoying the night sky-are of increasing importance in industrial lighting, with the realization that upward-directed light is wasted and results in significant unnecessary energy costs.
What these two very different examples have in common is that both can be fully designed and analyzed within standard commercial optical analysis programs.
Optical-software providers need to respond to these new applications and challenges along two paths. First, software must include the capabilities necessary for these expanding technologies and the programs need to be adapted to help new users-who are often from disciplines outside of optics-use the programs effectively and efficiently. Second, because more of the engineers who use these software tools come from disciplines other than optical engineering, it is important to provide sufficient training to help them understand the capabilities and limitations of the various software tools. The training programs need to provide not only an insight into the software tools, but also a strong understanding of the fundamentals of optical engineering.
There are three general types of optical-software programs that can be applied to a wide range of optical problems. There are also a wide range of programs that have been written to analyze a very limited or specific set of problems. The three types of programs in widespread use include optical-design programs, often referred to as sequential ray-tracing programs; optical-analysis programs, or unconstrained ray-tracing programs; and Maxwell-equation solvers, such as finite-difference time-domain (FDTD) or Eigenmode expansion programs.
Although each of these programs can be applied to a wide range of problems, they generally are not used to evaluate the same problems. For some types of design and analysis, it is possible that the entire range of interest can be fully modeled using one code. For other problems it is necessary to use a combination of two or all three types of tools. In using any of these programs, it is as important to understand its strengths and limitations as it is to understand the fundamental principles of optics. To provide this foundation to engineers and physicists from fields other than optics, there is a need for optical-software training courses designed to provide an understanding of optical engineering with a significant emphasis on how to apply these concepts within any of the optical-simulation tools. The course should include an understanding of the strengths and limitations of any optical code.
At BRO, we are developing such a training regimen. The new training is designed to provide a solid understanding of the nature of light and the interaction between light and matter. Although Maxwell’s theory of electricity and magnetism can be used to completely describe any nonquantum optical phenomena, many problems in optics can be fully investigated using techniques that require less mathematical rigor. An introduction to optical engineering starts from the principles of geometrical optics. Many optical systems can be fully evaluated using only a few basic postulates. Most optical-design and analysis programs use geometric optics for most calculations.
Beyond the limits of geometrical optics is physical optics. By considering the wavelike properties of light propagation, it is possible to analyze other details in the performance of the optical system. This analysis includes information about the phase relationships between superposed wavefronts and the differences in these effects seen in coherent and incoherent light. The training will also consider interference and diffraction effects, why they occur, when they occur and, of significant importance, when is it important or necessary to consider these effects.
The principles of geometric optics as well as wavefront superposition offer approximate solutions to Maxwell’s equations. Full solutions to Maxwell’s theory provide exact solutions to diffraction phenomena, at considerable computational expense. However, these solutions are necessary to provide a basis for understanding and analyzing polarization, which describes the oscillation of the light wave in the direction of the transverse electric field. An understanding of polarization and proper modeling of polarization is key in many areas of optical analysis.
Most scientists and engineers, while familiar with the basic ideas described above, are not as knowledgeable about the proper software tool to use for a particular analysis. A training course designed to expand on the basic understanding of optical concepts, as well as provide a thorough understanding of the capabilities and limitations of the available programs, is a significant first step toward providing the necessary information.