Micro-optics offers great potential for light shaping for LED light sources. Well-known examples are fly's eye condensers realized as tandem microlens arrays or holographic diffusers. The developed low divergence, large area LED light source consists of an array of collimated LEDs and a micro-optical telescope array. Efficient collimators with homogeneously illuminated pupil, low aberration and minimum stray light were designed and realized. Nonetheless, this LED light source has to be accomplished by a telescope producing a sharply defined circular top-hat spot in the farfield. With large area sources, usual single-aperture spatial filter layouts fail to achieve a small form factor design. Thus, a multi-aperture micro-optical array approach was chosen to realize a telescope array with minimum thickness. The basic design consists of Kepler telescopes with a pinhole to properly tailor the light beam. Aperture sizes, element thicknesses, and radii of lens curvature scale linearly with the permissible decentration errors, thus, a trade-off between required precision and system length has to be established. In our case, comparatively large 1 mm diameter lenslets require lateral alignment precision in the 10 micrometer range. A challenging task is suppression of adjacent channel cross talk inside the filter. AR coating and peculiar design of apertures help to minimize this spurious effect.
The system was realized with two 8“ borofloat glass wafers equipped with photolithographically structured black aperture layers buried under the lens arrays. The lens arrays are polymer-on-glass elements replicated from reflow master structures by UV reaction molding.
To achieve correct placement of the wafer with respect to each other, alignment marks for lateral adjustment and a replicated spacer structure for correct focusing are added.
This research was commissioned by Coelux S.r.l. from Como, Italy
Authors: Peter Schreiber, Chen Li, Peter Dannberg, Bernd Höfer, Stephanie Fischer