Design procedure for ultra-thin free-form micro-optical elements allowing for large DHR values and uniform irradiance distributions of ultrathin direct-lit luminaires

Freeform (FF) optical elements offer new opportunities in many fields of applicationssince they allow the generation of tailored irradiance distributions, which would be difficult toachieve using conventional optical elements. However, FF optical elements are very challengingwith respect to mastering and replication, for which techniques like diamond turning, milling,grinding and polishing are common methods for the manufacturing of the tools required forinjection molding, which is a common method for fabrication. In contrast, the possibility ofmastering FF micro-optical elements (FF-MOEs) with a laser direct writing method and theirlarge-scale fabrication in roll-to-roll processes offers a cost-effective alternative. Still, for suchproduction techniques, the maximum height of the FF-MOEs must be limited, which requiresnew design strategies. On the other hand, besides producibility, such ultrathin FF-MOEs alsoallow for new strategies for the integration of optical elements in miniaturized systems andproducts. Here, we present an improved design concept for extremely thin FF-MOEs with aconfined maximal structure height of 50μm. They allow for uniform illumination in ultrathindirect-lit based luminaire systems with a DHR (distance between the LEDS : height of the system)ratio of 3 by seamlessly stitching the hexagonal-shaped irradiance distributions provided by theindividual FF-MOEs. The applicability of the as-designed FF-MOEs is demonstrated by opticalray-tracing simulations of a simple direct-lit luminaire consisting of an array of 14 LEDs anda target plane in a distance of 10 mm. The simulation results confirm a very high degree ofuniformity for the overall irradiance distribution on the target plane.