Cornelius Neumann

LED color mixing with diffractive structures

Lighting solutions with colored LEDs provide many opportunities for illumination. One of these opportunities is to create a color tunable light source. In this way different kinds of white light (color temperature) as well as discrete colors may be realized. This opens the field for applications as mood lighting. But there is always a spatial separation of the distinct LEDs that might get converted into an angular separation by any collimating optics. This angular separation causes such problems like color fringes and colored shadows that cannot be accepted in most applications. Conventional methods to solve these problems include e.g. mixing rods or dichroic filters. A new approach is the use of the dispersive effect of a diffractive structure to compensate the angular separation of the different colors. In this contribution the potential and limitations of diffractive structures in LED color mixing applications are discussed. Ray tracing simulations were performed to analyze such important parameters like efficiency, color performance and the cross section of the color mixing optics. New means for the estimation of color mixing performance were developed. A software tool makes it possible to detect the color distribution within ray trace data and it provides a quality factor to estimate the color mixing performance. It can be shown that the spectral band width has a large influence on the mixing process. Ray tracing simulations are compared with results of an experimental setup such that both measured as well as simulated data is presented.

Optimizing nonimaging free-form optics using free-form deformation

The increasing efficiency of high power LEDs has resulted in many new applications in general lighting. To take full advantage of the properties of LEDs, free-form surfaces can be utilized to create compact non imaging optical systems with high efficiencies and high degrees of freedom for optical designers. One of the commonly used methods to do optical design for this kind of systems is optimization. Appling this powerful tool allows the enhancement of given optical elements to achieve a desired performance. In this way, free form surfaces which are usually represented by NURBS, can be optimized and applied even close to an extended LED light source. However, using optimization for free-form surfaces is far from being straight-forward and requires a lot of experience mostly due to the high amount of possible optimization variables for NURBS. This comes along with high, computational effort and difficulties concerning the choice of boundary conditions and merit functions. This contribution presents a novel non-imaging optical design approach using the concept of free-form deformation (FFD) in conjunction with customized optimization algorithms to create efficient optical free-form surfaces for extended LED light sources. Within this framework, specific coordinate system transformations are used to modify the global shape of free-form surfaces. In this way, optimization techniques relying on relatively few and easily accessible variables can be applied successfully. All presented concepts are implemented in a flexible and fully automated FFD optimization software tool incorporating a commercial raytracer and numerical optimization techniques. Several examples are presented in detail and the scope of FFD based optimization is demonstrated.

Determination of tailored filter sets to create rayfiles including spatial and angular resolved spectral information

To simulate and optimize optical designs regarding perceived color and homogeneity in commercial ray tracing software, realistic light source models are needed. Spectral rayfiles provide angular and spatial varying spectral information. We propose a spectral reconstruction method with a minimum of time consuming goniophotometric near field measurements with optical filters for the purpose of creating spectral rayfiles. Our discussion focuses on the selection of the ideal optical filter combination for any arbitrary spectrum out of a given filter set by considering measurement uncertainties with Monte Carlo simulations. We minimize the simulation time by a preselection of all filter combinations, which bases on factorial design.

Optimization of freeform surfaces using intelligent deformation techniques for LED applications

Abstract For many years, optical designers have great interests in designing efficient optimization algorithms to bring significant improvement to their initial design. However, the optimization is limited due to a large number of parameters present in the Non-uniform Rationaly b-Spline Surfaces. This limitation was overcome by an indirect technique known as optimization using freeform deformation (FFD). In this approach, the optical surface is placed inside a cubical grid. The vertices of this grid are modified, which deforms the underlying optical surface during the optimization. One of the challenges in this technique is the selection of appropriate vertices of the cubical grid. This is because these vertices share no relationship with the optical performance. When irrelevant vertices are selected, the computational complexity increases. Moreover, the surfaces created by them are not always feasible to manufacture, which is the same problem faced in any optimization technique while creating freeform surfaces. Therefore, this research addresses these two important issues and provides feasible design techniques to solve them. Finally, the proposed techniques are validated using two different illumination examples: street lighting lens and stop lamp for automobiles.

Enhancing the luminance of converted green LEDs in LED projectors

Abstract High-power light-emitting diodes have successfully found their way into projection applications. Owing to their long lifetime, small size, and easy electrical drive, they are getting more and more attractive for use in various kinds of projectors. The challenge to achieve higher brightness levels lies in the étendue limitation, which is determined mainly by the size of the microdisplay inside the projection system. In this paper, a new approach is presented to increase the output flux of LED-based projectors by enhancing the luminance of converted green LEDs.

Design of compact freeform LED flashlight capable of two different light distributions

Free-form optical surfaces are designed for desired intensity requirements for applications ranging from general to automotive lighting. But a single compact free-form optics which satisfies two different intensity distributions is not presented so far. In this work, a compact LED flashlight fulfilling two different intensity requirements that could be used in potentially explosive atmospheres is designed and validated. The first target is selected after a study on visibility analysis in fog, dust, and smoke environments. Studies showed that a ring-like distribution (5°- 10°) have better visual recognition for short distances in smoky environments. The second target is selected to have a maximum intensity at the peak to provide visibility for longer distances. We realized these two different intensity requirements by moving the LED with respect to the optics along the optical axis. To fulfill the above- required intensity distributions, hybrid TIR optics was designed as free-form curves calculated by combining several geometric optic methods. We validated the free-form TIR hybrid optics using Monte Carlo ray trace simulation. The overall diameter of the optics is 29 mm and 10 mm in thickness. The simulated results showed an optical efficiency of about 84% to realize both target light distributions in a single optics. Then we designed a whole flashlight consisting of LED, PMMA hybrid optics, PC glass casing and a housing including the critical thermal management for explosive environments. To validate the results, a prototype for the designed optics was made. The measured results showed an overall agreement with the simulated results.

Hyperspectral LED models for optical simulations

Precise spectral and colorimetric simulations in commercial ray tracing software require realistic light source models, which provide spectral information as a function of angle and spatial dimension. We describe and validate a general workflow to create hyperspectral LED models as a linear combination of spectral models. The workflow only requires user defined precisions and rayfiles obtained with different optical filters. The rayfiles are transformed into histogram based models, whose precision is evaluated by normalized cross-correlation values of their intensity distributions in the near-, mid- and far field. Additionally, the concept is evaluated with a spatial and spectral well defined test device.

Automotive Lighting Continues to Evolve

The move to LED sources is only one step in a long series of changes that have defined auto exterior lighting’s functionality—and its sense of style.

Development in illumination optics

Even for somebody, who is working in the field of LED technology and illumination optics for more than 15 years, the current innovative speed in research and application is amazing. Recent progress has been driven by the rapid development of the LED as a powerful light source, new sensing and computing capabilities and better injection molding manufacturing technologies. The optics and applications one can find on the market today are far beyond what has been possible at the (not so far behind) beginning of this century. In this issue of Advanced Optical Technologies you will find a small selection of what is going on in the field of illumination optics. Three of the articles deal with automotive lighting. They can be described briefly as seeing and be seen. Seeing is the major task while driving at night time. The headlamp should improve night vision for the driver with emerging new functions and technologies. So the vision of the perfect headlight is pushing the development further on. Today, sensing, recognizing and illuminating are typical functions of an advanced headlamp system. In the far field the light has to give the best possible illumination, without glaring other drivers. In the near field new functionalities will support the driver with comfort and information. The two contributions, High resolution vehicle headlamps: technologies and laser scanning prototype by Kloppenburg et al., as well as High resolution headlamp by Gut et al., present an overview of different concepts, technologies and applications. Signal functions are responsible for the recognition of vehicles and their behavior at night. Tail-, stopor indicator lamps show other participants where I am and what I intend to do. Beside this important task, signal lamps can transport additional attraction. Today new optical systems are designed to show new styling and corporate identity during dayand night-time. You will find many beautiful examples of styling opportunities with new optical systems in the article Signal lights–designed light for rear lamps and new upcoming technologies by Mügge and Hohmann. Freeform technology, another strong driver of illuminating optics, is represented by two articles. The strong asymmetry of a flashlight illumination, or the low beam function in automotive application demand special solutions for the calculation and manufacturing of free form lenses presented in Free-form illumination optics by Mohedano et al. Additionally, different freeform design and optimization methods are tested and evaluated for the design of light guide (TIR) reflectors by Talpur and Herkommer in Review of freeform TIR collimator design methods. A specialty of LED lighting is color. With the additive RGB(W) LED mixing, a large color gamut can be used for illumination tasks. The last article of our topical issue, Angular and spatial LED color mixing with dispersive mixing rods by Bonenberger et al., presents a spatial and angular mixing light guide rod adopting the concept of chaotic billiard systems in classical mechanics. I would like to thank all the contributors to this issue and hope that you will be attracted by the presented topic Illumination optics.

High-resolution headlamp

Abstract The following article shall describe how human vision by night can be influenced. At first, front lighting systems that are already available on the market will be described, followed by their analysis with respect to the positive effects on traffic safety. Furthermore, how traffic safety by night can be increased since the introduction of high resolution headlamps shall be discussed.