Smart Freeform Optics Solution for an Extremely Thin Direct-Lit Application

摘要

Light-emitting diodes (LEDs) based lighting solutions offer many advantages like the huge potential for energy saving, long lifetime, high reliability and the compact size compared to incandescent light bulbs. Especially the last-mentioned aspect favors new concepts for the design and integration of light points and luminaires. In case of a direct-lit system for general lighting applications the LEDs are separated by a certain distance, arranged in a regular array and illuminate an out-coupling surface which is placed in a certain height above the LED array. One approach to fulfil the demand of a uniform luminance of the out-coupling surface is to replace the surface by a diffuser sheet. In order to allow for a flat luminaire design the distance of the out-coupling surface has to be modified synchronously with a variation of the distance between the LEDs. This means, in order to maintain the uniformity of the luminance the distance to height ratio (DHR) of the optical arrangement has to be kept constant. A (from the viewpoint of costs desirable) reduction of the number of the LEDs and as a consequence thereof an increase of the DHR value can be achieved by using additional optical elements like, e.g., freeform optics. However, this can cause additional design problems due to the size of such optical elements. In this contribution we discuss a smart design of an extremely flat direct-lit luminaire for general lighting applications. The main advantage of this concept is the increased DHR ratio compared to diffuser sheet only-approaches and a smaller thickness of the whole set-up compared to common freeform approaches. For this demand we have designed very thin freeform optical elements with a maximal height of 75 μm that allow to maintain a uniform illumination in direct-lit applications using an LED-array with a comparably large distance between the individual LEDs. The presented design concept in addition emphasizes the use of cost-effective manufacturing methods like grey scale laser lithography for mastering and roll-to-roll processing for large area manufacturing of these optical elements.