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

Abstract

Freeform (FF) optical elements offer new opportunities in many fields of applications since they allow the generation of tailored irradiance distributions, which would be difficult to achieve using conventional optical elements. However, FF optical elements are very challenging with 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 for injection molding, which is a common method for fabrication. In contrast, the possibility of mastering FF micro-optical elements (FF-MOEs) with a laser direct writing method and their large-scale fabrication in roll-to-roll processes offers a cost-effective alternative. Still, for such production techniques, the maximum height of the FF-MOEs must be limited, which requires new design strategies. On the other hand, besides producibility, such ultrathin FF-MOEs also allow for new strategies for the integration of optical elements in miniaturized systems and products. Here, we present an improved design concept for extremely thin FF-MOEs with a confined maximal structure height of 50 µm. They allow for uniform illumination in ultrathin direct-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 the individual FF-MOEs. The applicability of the as-designed FF-MOEs is demonstrated by optical ray-tracing simulations of a simple direct-lit luminaire consisting of an array of 14 LEDs and a target plane in a distance of 10 mm. The simulation results confirm a very high degree of uniformity for the overall irradiance distribution on the target plane.