Spotlighting is one illumination field where the application of light emitting diodes (LED) creates many advantages. Commonly, the system for spot lights consists of a LED light engine and collimating secondary optics. Through angular or spatial separated emitted light from the source and imaging optical elements, a non uniform far field appears with colored rings, dots or patterns. Many feasible combinations result in very different spatial color distributions. Several combinations of three multi-chip light sources and secondary optical elements like reflectors and TIR lenses with additional facets or scattering elements were analyzed mainly regarding the color uniformity. They are assessed by the merit function U<sub>sl</sub> which was derived from human factor experiments and describes the color uniformity based on the visual perception of humans. Furthermore, the optical systems are compared concerning efficiency, peak candela and aspect ratio. Both types of optics differ in the relation between the color uniformity level and other properties. A plain reflector with a slightly color mixing light source performs adequate. The results for the TIR lenses indicate that they need additional elements for good color mixing or blended light source. The most convenient system depends on the requirements of the application.
The scope of the present paper is the derivation of a merit function which predicts the visual perception of LED spot lights. The color uniformity level <i>Usl</i> is described by a linear regression function of the spatial color distribution in the far field. Hereby, the function is derived from four basic functions. They describe the color uniformity of spot lights through different features. The result is a reliable prediction for the perceived color uniformity in spot lights. A human factor experiment was performed to evaluate the visual preferences for colors and patterns. A perceived rank order was derived from the subjects’ answers and compared with the four basic functions. The correlation between the perceived rank order and the basic functions was calculated resulting in the definition of the merit function U<sub>sl</sub>. The application of this function is shown by a comparison of visual evaluations and measurements of LED retrofit spot lamps. The results enable a prediction of color uniformity levels of simulations and measurements concerning the visual perception. The function provides a possibility to evaluate the far field of spot lights without individual subjective judgment.
Many LED-based applications would benefit from more efficient and/or high lumen output devices that enable
usage in both white and single color illumination schemes. In the present article we briefly review the materials
research history leading to optical ceramic converters and discuss their typical characteristics. Recently
demonstrated high performance values in terms of efficacy and external quantum efficiency in orange (amber)
spectral region are described.
Photonic crystals (PhCs) are known to diffract guided modes in a light-emitting diode into the light extraction cone
according to Bragg´s law. The extraction angle of a single mode is determined by the phase match between the guided
mode and the reciprocal lattice vector of the PhC. Hence, light extraction by PhCs enables strong beam-shaping if the
number of guided modes can be kept to a minimum. InGaN thin-film micro-cavity light-emitting diodes (MCLEDs)
with photonic crystals (PhCs) emitting at 455 nm have been fabricated. The GaN layer thickness of the processed
MCLEDs with a reflective metallic p-contact was 850 nm. One and two-dimensional PhCs were etched 400 nm into the
n-GaN to diffract the guided light into air. The farfield radiation pattern was strongly modified depending on the lattice
type and lattice constant of the PhC. Two- six- and twelve-fold symmetry was observed in the azimuthal plane from 1D
lines, hexagonal lattices and Archimedean A7 lattices, respectively. The emission normal to the LED surface was
enhanced by up to 330% compared with the unstructured MCLEDs. The external quantum efficiency was enhanced by
80% for extraction to air. The flux from PhC-MCLEDs in a radial lens was 15.7 mW at 20 mA and 36% external
quantum efficiency was measured at 3 mA. High order diffraction was found to contribute significantly to the
enhancements in efficiency and directionality. The experimental results are compared with FDTD simulations.
Keywords: light-emitting diodes, photonic crystal, cavity, InGaN