Today’s high-power illumination is mainly based on LED technology. Continuing trends are increased luminous power on decreasing emitting surfaces. This leads to growing technical requirements of optics that are useable for modern LED systems: Firstly, the increasing power densities require transparent materials that are durable when exposed to high temperatures or luminous fluxes. Here, polymeric materials as polycarbonate, PMMA or silicone quickly exceed their material limits when used in such conditions. Secondly, the smaller LED size allow for the design of smaller optical solutions. This means, the size of luminaires can be shrunk without any disadvantage for the optical performance. This comes with increasing requirements for system accuracy and geometrical deviations of the optics. Glass, shaped with modern manufacturing techniques, is a promising solution for these requirements. In this paper, we show two different approaches for automotive front lights. The first is non-isothermal glass molding of aspherical lenses. This method allows for the fast manufacturing of precision glass lenses including all structures needed for accurate mounting. These lenses can be used in imaging systems for modern front lights. The second method covers glass injection molding of complex 3D-optics. Here, tiny light guide optics are used for typical automotive front light distributions.
Modern illumination applications increasingly require adapted non-symmetric light distributions. Examples can be found in street lighting, architectural lighting, and also in more technical applications as automotive lighting. From an optical design aspect this leads to an increasing need for freeform lens design. Even though some design methods for freeform surfaces exist, the development of non-symmetric illumination solutions is still challenging. We investigated the applicability of the Cartesian oval method for the design and production of lenses with customized light distributions in Suprax glass. Of importance are both the manufacturability and the usability with extended LEDs. In the following paper we will show the basics and implementation of this method also using GPUs and discuss the pros and cons in the context of the usual requirements of illumination projects.
Today’s trends in illumination engineering clearly turn towards high power LED applications with a precisely controlled light output. The first requires glass optics which will withstand the increasing temperature load and lumen output of LEDs. The second requires tight control of production tolerances and defined surface structuring. Especially the surface structure – which can be realized for example as micro lens arrays – is of increasing importance. Using two different fabrication techniques we investigated the implementation of micro surface textures on glass optics. The first method uses directly molded glass from the liquid phase while the second is an imprint process. For both methods we determined the minimum replicable feature size and found current limits of only 50 μm for the imprint process.
Constant LED developments show increasing levels of luminous flux and power densities. In particular, automotive and entertainment industries are requesting mechanically and optically stable light guides for their new mid to highest-power lighting solutions. The switch from polymer to glass optics comes with improved temperature resistance, higher optical performance and better longevity of the systems [1, 2]. Even highest-power LEDs can be driven at maximum current obtaining best light output. The option of directly implementing micro structures on the output aperture of glass light guides gives the opportunity to customize final color mixing and light scattering over a wide range. This reduces the amount of required components and subsequently the total system costs.
In the past, the major part of transmissive LED optics was made from injection molded polymers like PMMA or PC. Recent LED developments now show constantly increasing levels of luminous flux and energy densities, which restrict the usability of such polymer optics due to their limitations in thermal stability. Thermal simulations have shown that light guiding/mixing structures (rods) made from polymer materials can easily reach temperatures above their melting point due to the absorption characteristics. However, there is a great demand for such light rods from the automotive and entertainment industry and thus glass is becoming increasingly important as an optical material. Glass has typical transformation temperatures of hundreds of degrees Celsius and therefore withstands the conditions seen with LED without any problems. Square-shaped glass light guides show temperature advantages over round light rods, which are known for being able to produce caustics inside the material causing absorption and temperature hot spots, respectively. This paper presents some comparative thermal simulations by means of the Finite Element Method for a light conductor as an example and gives corresponding assistance for an appropriate material and light guide shape selection for highpower LED optics.
Free-form reflectors are encountered in numerous illumination systems, especially in highly sophisticated applications. The construction of these kind of optics however remains a challenging task where only a few methods are available to derive the free-form shape. One such method is the multi-ellipse approach where a superposition of conic sections is utilized to create the desired illuminance or luminous intensity distribution. While it is useful in many areas one is not always interested in an illuminance or intensity distribution. Especially street lighting reflectors are often tailored towards a homogeneous luminance, taking into account the road's reflective properties, luminaire arrangement etc. While we used our implementation of the multi-ellipse method to design street lighting reflectors with a uniform illuminance before, we now extended this method to support the calculation of a roadway reflector with a homogeneous luminance. For a given roadway scenario we can quickly get an optimized reflector with a good performance compliant to roadway standards such as EN-13201 or IESNA-RP-8-00. Furthermore the optic can be quickly adapted to changing requirements.
The increasing market demand for LED illumination requires new design approaches for the replacement of
conventional recessed luminaries. In this paper we present a series of glass reflectors covering a broad range of beam
angles between 10° and 40°. The key feature of this development is the identical size of all reflectors making a modular
set-up possible complying to a Zhaga-standardized LED module. The reflector dimensions are comparable to halogen
MR16 lamps and allow an immediate use in existing downlight systems.
Here, we present light technical measurements of these reflectors and compare the performance to already existing
MR16 LED retrofit solutions.
For LED lighting applications, Fresnel lenses or TIR lenses are frequently made of optical plastics. Glass,
however, can offer a number of advantages, including higher resistance to heat, to UV light, and to chemicals
like solvents. In this work, several glass materials for transmission optics are compared. The transmittances
are evaluated, including Fresnel losses and absorption, as well as shifts of the chromaticity coordinates and of
the color rendering index. TIR lenses made of Suprax borosilicate glass and polycarbonate are compared
concerning their contour accuracies and their resulting photometric properties like luminous intensity
distributions, luminous fluxes, and chromaticity distributions.
The field of illumination optics has an increasing demand for free-form optics that produce arbitrary light distributions. In various applications an asymmetric, e.g. rectangular illumination can be beneficial, such as street lights, shop lights or architectural lighting. Yet there are only very few construction methods for free-form surfaces, especially using extended sources. One such method utilizes a manifold of conic sections to derive a source-target mapping for a particular source and target distribution. Although it relies on the assumption of a point source it can be adapted to work with real, extended sources. We implemented the algorithm to construct glass reflectors for almost arbitrary light distributions, either prescribed in the near- or far-field. Starting with a point source, an initial surface is optimized in a second process with a feedback loop to produce the desired result with the actual extended source. Our method is quite robust and was used to design for example an asymmetrical street light reflector. It was manufactured at Auer Lighting GmbH out of borosilicate glass. Measured target distributions are in excellent agreement with the simulations. These promising results show that this particular design method can be applied to real world applications. It is a powerful tool whenever a highly optimized reflector for a non-trivial illumination is required.
We investigated guided mode extraction in organic light-emitting diodes and compare the experimental findings to transfer matrix (T-matrix) and finite difference time domain (FDTD) simulations. To this end, we patterned the indium tin oxide anode with Bragg gratings with lattice constants from 300 to 600 nm and varied the depth of the grating structures. The structuring was done by laser interference lithography and plasma etching. Both techniques allow for a rapid large area processing. We measured angle resolved electroluminescent spectra of the nanostructured devices and reference devices. To obtain the mode distribution in the devices we made use of T-matrix simulations. In addition we performed FDTD simulations of the emission characteristics of the patterned devices. The simulations are in agreement with our experimental findings and give insight into the outcoupling mechanisms.
We report on the fabrication of large-scale surface gratings by laser interference lithography and reactive ion etching
on which we evaporated a thin film of the organic semiconductor tris(8-hydroxyquinoline) aluminum doped
with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyril)-4H-pyrane. We created a thickness
gradient by using a rotating shadow mask evaporation technique. This allowed us to continuously tune the
emission wavelength from 606 nm to 661 nm on a single substrate. After encapsulation, we demonstrated the
usefulness of such low-cost and tunable organic semiconductor lasers by conducting simple fluorescence excitation
and transmission spectroscopy measurements using a minimal amount of additional optical components.
Metallic nanostructures have attracted large interest recently due to new optical properties caused by plasmonic effects.
The exceptionally high transmission of light through periodically structured metals is originated by interactions between
light and plasmonic resonances. These resonances are controllable by varying periodicity and geometrical dimensions of
the metal gratings.
Our aim is the utilization of these effects to improve the efficiency of conventional light-emitting diodes (LED). The
application of one-dimensional periodic metallic gratings as top electrodes of LEDs offers advantages such as efficient
and homogeneous current injection, enhanced light output, modified angular light emission characteristics and linear
polarization of the emission.
Based on finite-difference time-domain simulations, we optimized the parameters for gold and silver gratings on top of
InGaAs/GaAs/AlAs heterostructures. Fabrication of these structures was carried out using laser interference lithography
(LIL) and a lift-off process. We measured the optical transmission of these structures and were able to demonstrate a
polarization- and wavelength-dependence in good consistency with our calculations.
We demonstrate the feasibility of organic semiconductor lasers as light sources for lab-on-a-chip systems. These lasers
are based on a 1D- or 2D-photonic crystal resonator structure providing optical feedback in the active laser material that
is deposited on top, e.g. aluminum tris(8-hydroxyquinoline) (Alq3) doped with the laser dye 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM). We investigated different fabrication methods for the resonator
structures, like thermal nanoimprint, UV nanoimprint, and laser interference lithography. Different substrate materials
commonly used in lab-on-a-chip systems, e.g. PMMA, Topas, and Ormocer were deployed. By changing the distributed
feedback grating periodicity, we demonstrate a tuning range for a single material system of more than 120 nm.
The investigated organic semiconductor lasers are optically pumped. External optical pumping provides a feasible
way for one-time-use chips. Our recent success of pumping organic lasers with a low-cost laser diode also renders hand-held
systems possible.
As a further step towards the integration of organic lasers in sensor systems, we demonstrate the coupling of an
organic laser into polymeric waveguides which can be combined with microfluidic channels. The integrated organic
lasers and the waveguides are both fabricated on the same polished PMMA substrate using thermal nanoimprint
lithography and deep-UV modification, respectively. We could demonstrate the guiding of the laser light in single-mode
waveguides.
The interaction of surface acoustic waves (SAWs) and light is spatially restricted to a region close to the surface approximately given by the acoustical wavelength. Therefore optical waveguides very close to the surface are required for high-frequency i.e. short-wavelength acoustic waves. In contrast to existing collinear integrated acoustooptical devices we are aiming at the regime where the optical and acoustical wavelengths are comparable. The periodically modulated refractive index caused by the SAWs may serve as a tunable and switchable optical add/drop comparable to fiber Bragg gratings, though not static. Another aspect of this regime is the phonon energy, which is non-negligible compared to the energy of the photons. So a significant energy shift i.e. wavelength conversion caused by scattering processes can be exploited. Existing integrated optical waveguides based on silica, SOI, lithiumniobate or III-V semiconductors are not suitable for a realization of such components, due to small piezoelectric coefficients or weak optical confinement. In contrast, heterostructures made of II-VI compounds are promising candidates for the proposed applications. Using Beam Propagation simulations we developed an optimized ridge waveguide structure based on a CdSe/CdS heterostructure, grown by molecular beam epitaxy. The waveguide is defined by wet-chemical etching using a standard photoresist mask. The mode field dimensions are about 1 μm x 2 μm, which requires fiber coupling using lensed fibers. We present measured coupling and propagation losses and discuss the integration with acoustical waveguides.
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