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.
Transparent, rare-earth doped fluorozirconate-based glasses and glass ceramics are attractive systems as up- and downconverters
to increase solar cell efficiency. For down-conversion applications, the efficiency of a silicon solar cell could
be significantly increased in the ultraviolet spectral range by placing a europium-doped glass ceramic on top. High
transparency is a key issue here to avoid scattering losses and to obtain high light output. Transmission spectra of fluorozirconate
glasses, which were additionally doped with chlorine ions to initiate the growth of BaCl2 nanoparticles
therein upon thermal annealing, show that the absorbance in the visible spectral depends significantly on the annealing
conditions. For up-conversion applications, erbium-doped fluorozironate glasses have been investigated. 2-dimensional
intensity mapping of the up-converted fluorescence yielded information on the homogeneity of the glass sample and the
erbium distribution therein. Depth scan experiments showed that the position of the focus of the excitation laser beam
plays a crucial role since saturation of the 2-photon up-conversion occurs for high excitation power.
Transparent glasses as up- or down-converters are attractive systems to increase the efficiency of solar cells. Er-doped
fluorozirconate (FZ) glasses show an intense up-conversion upon excitation at 1540 nm. Transmission spectra show that
the absorbance at 1540 nm grows linearly with the Er-doping level. In Eu-doped FZ glasses, which were additionally
doped with chlorine ions, the growth of BaCl2 nanocrystals can be observed upon thermal annealing. For high annealing
temperatures a phase change from hexagonal to orthorhombic phase BaCl2 can be seen. Upon excitation in the ultraviolet
(UV) spectral range these glass ceramics emit an intense blue emission. A combination of a silicon solar cell and an
Eu-doped FZ glass ceramic as a down-converting top layer shows an increase in the short circuit current in the UV
spectral range compared to a solar cell without a down-converting top layer.
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