Thin direct-lit application for general lighting realized by freeform micro-optical elements

Common direct-lit systems for general lighting applications are using LEDs as light sources, which are placed in a certain distance in a regularly arranged array. In order to achieve a homogenous light distribution a diffuser sheet has to be placed on the out-coupling side in a certain height above the LED array. The position of the diffuser sheet is strongly correlated to the distance between the LEDs and is responsible for the positional homogenization of the LED spots, while the rough side of the diffuser averages the angular light distribution. In order to maintain the uniformity of the luminance the distance of the LEDs compared to the height of the diffuser sheet placement (DHR ratio) is of relevance. DHR values of 1 are hardly achievable. To overcome this limitation additional optical elements like freeform lenses are necessary. In this contribution we discuss a smart design concept for an extremely flat direct-lit lighting system. It is characterized by an improved distance (LEDs) to height (diffuser sheet) ratio compared to diffuser sheet only-approaches and a smaller thickness compared to common freeform approaches. For this demand we designed very thin freeform lenses with a maximal height of 75 μm that allow to maintain a uniform illumination in a flat direct-lit backlight using an LEDarray with a comparably large distance between the individual LEDs. The concept emphasizes the use of maskless laser direct write lithography for the cost-effective fabrication of the thin freeform micro-lens array.