Illumination Optics VII

This work investigates different optical configurations based on either (1) total internal reflection, (2) aspheric lenses and freeform mirrors, (3) scattering volumes, to create a uniform light distribution when a system with sizes <25 mm is required. End-emitting and edge-emitting optical fibers are used as the light source. A comparison based on uniformity, efficiency, and cost, is provided for the different optical techniques.

Previous work has demonstrated the optimization of spatial distribution, angular pointing, angular widths, and design efficiency of light guide luminaires using prismatic light extraction elements. Increasingly, in the automotive industry—and for signal lighting in particular—light guide luminaires that use secondary arrays of micro-optics are becoming more common. Typically, these secondary micro-optics are used either to scatter light selectively (micro partially scattering arrays), or to steer light (for example, microprism arrays). In this work, we focus on the latter type of secondary optic arrays: microprism arrays. We build on previous optimization techniques to create signal lamps; used in conjunction with arrays of prismatic elements that are part of secondary lenses. The secondary lenses are used to refine the angular distributions further. These modifications help to meet intensity test point specifications while at the same time preserving quality visual appearance. Typical sizes of the arrays studied are 0.25mm to 1mm pitch, with tens to tens of thousands of elements. Geometry creation and simulation are performed in a a CATIA V5 based environment.

We present some examples of the design, tolerancing and fabrication of freeform plastic lightguides for applications like illumination and optical sensing. The design of optical lightguides relies on Nonimaging Optics principles and uses raytracing simulations for analysis and optimization. We examine the influence of fabrication parameters on the simulated performance, and show ways to minimize their impact depending on the fabrication technique used. The presented lightguides have been fabricated at the Photonics Innovation Center of VUB – B-PHOT.

Uniform illumination, including uniform irradiance and color uniformity, is of indispensable importance for numerous imaging and sensing applications. These illumination designs typically comprise one or multiple light-emitting diodes (LEDs), or feature halogen sources, which are preferred for spectroscopy applications. We present an overview of different design strategies enabling a uniform illumination, taking the extended source characteristics into account, while aiming to compare different optical design approaches with respect to cost, efficiency, complexity, scalability and robustness. Four case-studies will be compared: (1) considering geometrical optimization of the source positions, (2) combination of a multi-chip LED with a biconic lens array and Fresnel lens, (3) combination of a halogen source array with a biconic aspheric lens array, and (4) the combination of a multi-chip LED with a Shell-Mixer. A design-for-manufacturing approach is applied for all designs, considering integration and robustness, paving the way towards industrial uniform illumination optics.

The Gemini North Adaptive Optics (GNAO) facility is the upcoming AO facility for Gemini North providing a state-of-the-art AO system for surveys and time domain science in the era of JWST and Rubin operations. The adaptive optics bench receives both natural star and laser guidestar light from the telescope focus and relays it, via a path that includes a high-rate deformable mirror and a tip-tilt mirror, to science instruments, at 2x magnification. The relay splits off light in a bandwidth of 350 nm - 800 nm for use in three wavefront sensors: a tip/tilt sensor and a low order wavefront sensor using natural guidestar light, and a 4-channel laser guidestar wavefront sensor, supporting a narrow field and a wide field mode (12 arc seconds and 2 arc minutes respectively). The optical design requirements on the relay are demanding. In particular, exceptionally tight control of distortion was required for the science field, which precluded conventional relays using pairs of off-axis paraboloids. A novel optical system has been produced that meets all of the GNAO AOB requirements. This design is based on an asymmetric version of the Offner relay. A particularly novel aspect of this relay is that there is no collimated light path within the system, and in this paper it is shown that the system meets all performance requirements without needing such a space. Unlike the normal Offner relay, the “Modified Offner Concentric” (MOC) relay produces an accessible pupil, clear of the confusion of rays at the M2, allowing placement of a flat deformable mirror there. The MOC design is presented here and is shown to be competitive in this application to conventional AO relays, providing superior performance with reduced complexity.

There are several aspects of optimizing multi projection system illumination. A main driver in the illumination performance are the light sources, their spectral characteristics, luminance and etendue. The impact of these characteristics on the architecture of illumination design and their implications for key performance metrics such as luminous flux and projectable color space are addressed. The transformative role of solid-state light sources, which have advanced the development of illuminations with high luminous flux and wide color space is stressed. The emergence of new challenges posed by the latest generation of coherent light sources is highlighted and advanced illumination design strategies to overcome them are presented.

The light tube concept is a means to characterize the illumination properties of an optical system. A light tube is an arrangement of two areas in space which defines an etendue value. Examples are the object / entrance pupil combination in an imaging system (such as for Köhler illumination), the coupling of an LED into a light guide, and even the sun / earth system. The etendue of a light tube is calculated via Hottel’s formula. We will derive approximations for important special cases. The search for a perfect illumination for a given optical system can be understood as a task to fill a light tube, more exactly to fill the phase space of the light tube with light. In our paper, we show various design approaches – imaging and non-imaging - to address this task. We apply them to obtain designs related to Köhler’s illumination principle.

Retinal imaging has always played a major role in the detection and management of ocular disease and a wide range of retinal imaging techniques exist. Most of them, however, focus only on high image quality (field, resolution, 3D) but neglect spectral information. We present a low-cost lighting module that can replace the bulb of most conventional fundus cameras and allow images to be taken in white light and at several wavelengths.

The arrival of ChatGPT, Google Bard, and other highly advanced artificial intelligence model show us just how brilliantly tasks can be reproduced by those engines. So, it's legitimate to wonder how our field (or any fields) might be affected in the future. We've already seen the beginnings of the possibilities, notably with LensNet [1], which provides optical designers with starting points for common cases; we can also study a solution space of certain type of lenses using deep learning [2]; and more recently, papers on the use of deep learning to simulate the entire chain of an optical system from object to final image processing, including tasks such as recognition. These latest end-to-end simulations have shown that in some cases, it is even necessary to redefine the optical optimization criteria to maximize certain computer tasks. In short, the computer doesn't necessarily need a good image in terms of MTF to perform its task. In this context, how the future will be affected or enhanced by these new AI approaches. In this presentation, I will first give a brief history of how AI has impacted optical system design since 40 years. Then I will use examples to discuss the extraordinary acceleration in works over the past 5 years, the choices that have or haven't been made, and the importance of having access to source code from publications. Finally, I will conclude with some thoughts on what may or may not lie ahead, and how we can introduce these new technologies into the training of future optical system designers. [1] Geoffroi Côté, Jean-François Lalonde, and Simon Thibault, "Deep learning-enabled framework for automatic lens design starting point generation," Opt. Express 29, 3841-3854 (2021). [2] Geoffroi Côté, Yueqian Zhang, Christoph Menke, Jean-François Lalonde, and Simon Thibault, "Inferring the solution space of microscope objective lenses using deep learning," Opt. Express 30, 6531-6545 (2022).

Freeform optics for illumination, pioneered over 20 years ago, are now widely used to light up streets, automobiles, architecture and more. But many questions remain: Do we have good, accessible design methods, especially for extended sources? Do we have proven processes to estimate and specify tolerances, to ensure full production yield without overengineering? Do we fully understand diffractive structures on freeform surfaces? The talk discusses the progress of design and manufacturing methods over the last 30 years, shows the knowledge gaps we’re suffering from, and concludes with an outlook to a non-obvious but exciting new approach for coherent light: What happens when we combine freeform surfaces with scattering and spatial light modulation?

Freeform surfaces are optical surfaces without linear or rotational symmetry. Their flexible surface geometry offers high degrees of freedom, which can be employed to avoid restrictions on surface geometry and create compact yet efficient designs with better performance. Therefore, freeform surfaces can endow beam shaping with more new functions and satisfy the ever-growing demand for advanced beam-shaping systems. The Monge-Ampère (MA) equation method converts the design of freeform beam-shaping optics into an elliptic MA equation with a nonlinear boundary condition. The MA method is considered as the most advanced point source algorithm, because it can satisfy the integrability condition automatically and can be implemented efficiently. In this talk, we will introduce the principles behind the MA method, and reveal the mathematical essence of illumination design based on ideal source assumption. Also, several interesting beam shaping systems will be given to show the effectiveness of the MA method in a wide variety of applications.

We introduce the development process of a new headlight module with perfect colour mixing and sharp upper edge for use as an additional module. Light is nearly collimated (<0.5° cone angle) for a variety of high-power automotive LEDs illuminating up to 1000 m street in front of the car. Key to success is an optimization process not only of the optical elements but also the materials and parameters of the injection molding process. A feedback loop of geometry measurement and simulation of real geometries does lead to acceptable tolerance values and geometry deviations. Finally, the optics in the module needs to be optimized with respect to mechanical tolerances and the inevitable fabrication errors. In this case it resulted in an additional element to be added at the entrance and exit surface of the initially designed optical structure. All steps were accompanied or carried out by simulations with LucidShape and FRED.

Solar energy, when accidently focused, can cause wildfires and melt plastics. Solar irradiance concentration analysis can help to optimize wanted concentration or avoid unwanted concentration. This article provides a workflow on how to perform solar irradiance concentration analysis using light simulation software. As an example, an automotive headlight will be used. One analysis approach is to filter the object surface sensors for the maximum irradiance value of a given sun position. All maximum irradiance values can then be mapped in relation to the horizontal and vertical sun position, creating a solar irradiance map. If the maximum irradiance value of this map is below a given threshold, then the analyzed product is safe for sun exposure.

For a long time a key focus of automotive illumination optics design was on controlled free-form shapes. With the rise of matrix-headlights, new imaging tasks and specifications came up in automotive optics design. Most recent developments like high resolution micro-LEDs for digital headlighting and near field projection of signaling functions, as well as the use of Micro-Lens-Arrays boost the imaging optics methods in automotive lighting Digital functionality, design aesthetics, energy efficiency, complementary to mass manufacturability and cost effectiveness are defining the boundary conditions for automotive lighting system design. The presentation will elaborate optical design principles on the basis of a number of different application examples: Adaptive Driving Beam matrix headlights, Micro Lens Array carpet light patterns and projection signals; to show the specific interaction of imaging subsystems with illumination optics.

Rudolf Kingslake is widely regarded as one of the founders of modern optical design. When educating his students at The Institute of Optics, Professor Kingslake championed the importance of lens design fundamentals as a complement to computer-aided design. At that time, ray tracing speed was a major bottleneck in the lens design process. Now that lens designers can trace rays in fractions of a second and have access to powerful computational tools like global optimization and AI are these same fundamentals needed? Should we keep teaching them? One of Kingslake’s biggest fears was that we would forget “our laboriously acquired knowledge of geometrical optics and substitute for it the mathematical problem of optimizing a merit function”. There is no question that computers have done wonders for lens design and have enabled far more advanced designs than thought possible. The issue at hand is if mastery of both lens design fundamentals and computer software is required for success. Unfortunately, the current educational landscape places much more emphasis on the latter than the former, and many of the fundamentals impressed by Kingslake have been lost. However, three boxes of index cards belonging to Rudolf Kingslake were recently uncovered. Included in the collection are 171 lens design exam problems which present a fascinating perspective on lens design as it was taught in the pre-computer age. In this talk we’ll take a closer look at several of these forgotten problems and discuss how their solutions are still relevant for modern lens design today.

In 2017, the European Southern Observatory (ESO) awarded a contract for the Polishing, integration and final figuring of the Segment Assemblies of the primary mirror (M1) for the Extremely Large Telescope (ELT) to Safran Reosc. Since then, the design and commissioning of a production unit dedicated to ELT M1 has been accomplished and the plant has been producing many mirrors since spring 2022. We will introduce the smart factory, its processes and their automation that enabled reaching the current throughput of one mirror per day. We will then present the status of the project, some lessons learned and highlight the successes that have been achieved so far.

We previously proposed an iterative wavefront tailoring (IWT) method to solve the freeform lens design problem for irradiance tailoring, where the entrance surface can be predefined as a spherical, aspherical or freeform surface. Here, this method is adapted to address a more challenging design problem where the exit surface is predefined. We design a freeform lens with a fixed aspherical exit surface to demonstrate the effectiveness of the modified method.

Uniformity is a critical performance issue in illumination design. When Etendue is considered, the two main options to achieve uniformity are mixing rods and lens arrays. Mixing rods are quite effective, but they often require a large package size. We have explored a Turn-Mixer concept based on a turn prism with TIR surfaces and embedded partial mirrors. This approach provides uniformity in a small package size. And, by using two Turn-Mixers, both spatial and angular mixing can be achieved.

Combining the principles of geometrical optics with conservation of luminous flux, we can derive a fully nonlinear elliptic PDE, defining the shape and location of an optical surface. For some base systems, this equation is of Monge-Ampère type, and for others it is a so-called generated Jacobian equation (GJE). In addition, the transport boundary condition holds. We have developed an iterative solution method for the GJE. We first compute the optical mapping connecting source and target domains and next the location of the optical surface(s). Both optical map and optical surface(s) are computed in a least-squares sense. Our algorithm is very efficient and can handle quite complex target distributions.

A micro LED, which is small in size (< 5 µm) with high light extraction efficiency, is a big challenge to overcome. There is always a compromise. We present an ultra-small µLED, which is 1-by-1 microns in size, and have the light extraction efficiency of 70.4%. The distributed bragg reflector is used in the structure of µLED, which helps in enhancing the extraction efficiency of proposed structure. The systematic optimization approach used in which we have seen a dependency of LEE on the structural parameters. The proposed tiny and high efficient µLED is a viable candidate for future display applications.