Light emitting Diodes (LEDs) which use phosphor conversion might be a neat alternative to the more costly 3 chip RGB
(red, green, blue) LEDs. Almost any color of phosphor conversion LEDs (pc-LEDs) can be adjusted by combining a
light-emitting semiconductor chip and one or more phosphors. Depending on the ratio of unconverted and converted
light, it is possible to verify both unsaturated and saturated colors. A very common type of phosphor conversion LED is
composed of a reflector cavity, which contains a blue light emitting chip and is filled by a phosphor containing resin. At
a fixed concentration, parameters like the thickness of the phosphor filled resin layer (conversion layer) above the chip
and the wavelength influence the final color of the pc-LED. It is necessary to reduce the variation of the influencing
parameters to be able to control the color and prevent yield losses in the production. A new phosphor coating technique
developed at OSRAM OS makes it possible to precisely control the thickness of the conversion layer above the chip. A
layer of a hard resin is applied on top of a wafer and afterwards its thickness is milled accurately to the desired value.
With this new technique the color distribution can be reduced significantly compared to the common techniques.
LEDs using light conversion have gained importance for a variety of applications. As an example, white conversion LEDs were introduced by Osram OS in 1998 as dashboard illumination devices. Since then they have evolved into a powerful light source for projection and lighting applications. Today the quality of white LED light can be adjusted from a cool bluish white to a warm white with very good color rendering. Combining several wavelengths emitted by both chips and phosphors makes a variety of LED colors accessible. The concept of realizing any customized color by light conversion is called "Color on Demand" (COD). Another advantage for certain colors is, that the conversion LED can be more efficient than the purely chip based solution. In particular conversion LEDs with a dominant wavelength of 563 nm can be offered due to the strong converter R&D platform of Osram. The efficiencies of these LEDs, which consist of a blue emitting chip and a green emitting nitride phosphor, are 10 times higher than the ones of the corresponding InGaAlP-based solution. All COD LEDs have proven an excellent stability even in accelerated life time tests and thus are suitable for automotive applications.
Recently near UV conversion LEDs with an excitation wavelength of around 400nm have attracted increasing interest due to their high efficiency and output power. Favorably is also the great variety of efficient phosphors available for near UV excitation. For the generation of saturated colors this method is very efficient especially for dominant wavelengths from 480 nm to 570 nm. Direct light emission from InGaN chips show a strong temperature and current dependency increasing with the emission wavelength and may cause disturbing color shifts in applications. For the wavelength range between 530 nm to 570 nm the physical limitations of the InGaN system are reached and the efficiency of the InGaAlP system is not yet satisfying. Conversion LED's based on near UV-light emitting chips provide solutions for both problems. But up to now a disadvantage of these LEDs is the residual near UV-light emission which could lead to severe eye damages over time. OSRAM OS developed a eye safe solution by avoiding the near UV-peak while maintaining the high luminous efficiency. A luminous efficiency of 28 lm/W for λdom of approx. 560 nm was demonstrated, a value more than ten times higher than the efficiency of green emitting InGaAlP diodes. For these LEDs no more restrictions because of eye safety regulations are expected.
Recently new phosphors from various material classes have been developed for LED applications by Osram OS and partners. Excitation wavelengths of these phosphors range from below 400 nm to 470 nm, enabling the creation of purple and unsaturated LED colors and even the efficient conversion of near UV-radiation into white light. By addition of red and green phosphors to white LEDs, a warm white color impression can be achieved. These LEDs are suitable for all purposes of general lighting, where a high color rendering is required. An outlook to new applications with unsaturated and warm white LEDs will be given.
The synthesis of transparent nanomers by the incorporation of nanoscaled tantalum oxide into an organic-inorganic composite matrix and their subsequent characterization are presented. The matrix materials used consist of a mixture of organically functionalized silanes and polymerizable monomers. The mixture does not exhibit phase separation, even down to the lowest nanometer scale, as revealed by SAXS measurements. The addition of nanoscaled Ta2O5 particles (mean particle diameter: 4 nm as determined by photon correlation spectroscopy) aims to increase the refractive index of the nanomers. The preparation of the oxide sol and the optimization of the synthesis with respect to compatibility with the matrix material, thereby avoiding agglomeration effects, is described. After incorporation of the particles in the monomer mixture, a photopolymerization step, followed by curing with a temperature program up to 90 degrees Celsius, led to colorless and transparent monoliths. The volume shrinkage, caused by polymerization, decreases from 8.2% for the unfilled matrix material to 5.8% for a nanomer containing 30 wt.% tantalum oxide. The shrinkage decreases linearly with increasing filler content of tantalum oxide. The increase in refractive index is about 7.4 X 10-4 per wt.% oxide (measured at a wavelength of 546.1 nm). The coloration of the monoliths is expressed as yellowness index G according to DIN 6167. Color values attained for nanometers with up to 15 wt.% tantalum oxide are comparable to values for commercial optical polymer materials. Nanomers containing 15 wt.% tantalum oxide show transparency losses at a wavelength of 850 nm below 0.1 dB/cm.
Sol-gel derived organic-inorganic hybrid materials and their potential for the production of refractive optical elements are presented. The main components of the investigated compositions are precondensed silanes with polymerizable double bonds [e.g. methacryloxypropyltrimethoxysilane (MPTS)] and co-condensates thereof. Dimethacrylates like tetraethyleneglycoldimethacrylate (TEGDMA) were employed as organic monomers. Molar ratios of silanes to organic monomers between 10:90 and 90:10 were investigated. Nanoscaled titania was incorporated in the homogeneous mixture of silanes and organic monomers. The combination of different molecular hybrid matrices and inorganic nanoparticles allows the adjustment of material properties, for example: impact strength between 2 and 15 kJ/m2, Youngs moduli between 0.8 and 3.7 GPa and universal hardness in the range from 40 to 170 N/mm2. Phase separation could be kept in the nanometer range to minimize optical losses due to scattering effects. Depending on composition, ne could be varied between 1.50 and 1.54, whereby the corresponding Abbe numbers ranged from 57 to 45. Ophthalmic lenses were prepared in less than 10 hours by simple mould techniques and by applying a combination of photochemical and thermal curing processes.
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