Randomized Micro Lens Arrays for Color Mixing

摘要

Color homogeneity is a key issue for LED lighting. To achieve sufficient flux, efficiency and color rendering, LED luminaires have to use multiple LEDs, whose brightness and color properties differ. To mix greenish white LEDs using phosphor with red monochromatic LEDs is an especially promising approach to achieve both high effiency and good color rendering at warm white colors. However, even for luminaires using only white LEDs, color and brightness of the LEDs varies due to random selection within LED bins, or by using LEDs from different bins. For diffuse illumination, color mixing is not too difficult, but for collimated light, good color mixing is a key challenge to the optical system. Micro lens arrays are known to provide extremely good color mixing while increasing beam divergence only slightly. However, in their standard forms with (i) hexagonal lenslets or (ii) circular lenslets with blackened triangles, they create beam patterns with (i) sharp edged hexagonal or (ii) less efficient sharp edged circular distributions, instead of the round, soft edged beam shapes needed for general lighting. This is due to the fact that the lenslet shapes are imaged to the far field. It is not possible to choose the lenslet shapes freely: the lenslet edges depend on the lenslet positions, forming a Voronoi diagram. We present various approaches to perturb the regular lenslet positions, forming a smooth round beam, while keeping the superior properties of standard micro lens arrays.