Mask-aligner lithography is a technology used to transfer patterns with critical dimensions in the micrometer range from below 1 micron for contact printing to a dozen of microns in proximity printing. This technology is widely used in the fabrication of MEMS, micro-optical components, and similar ﬁelds. Traditionally, the light sources used for mask-aligners are high-pressure mercury arc lamps, which emit in the UV rang of the spectrum with peaks at 365 nm, 405 nm and 435 nm, respectively the g-, h- and i- lines. These lamps suﬀer from several disadvantages (ineﬃcient, bulky, dangerous), which makes alternatives interesting. In recent years, high power UV LEDs at the same wavelengths appeared on the market, opening the door to new illumination systems for mask-aligners. We have developed a modular 250 W LED-based illumination system, which can advantageously replace a 1 kW mercury arc lamp illumination. LEDs, arranged in a 7x7 grid array, are placed in the entrance apertures of individual reﬂectors, which collimate the individual irradiation to an output angle of 10◦. A subsequent ﬂy’s eye integrator homogenizes the illumination in the mask plane. It is followed by a Fourier lens, superimposing the individual channels in the mask plane, and a ﬁeld lens to ensure telecentric illumination. This multisource approach allows the shaping of the source by switching individual illumination channels, determining the illumination angles and the spatial coherence in the mask plane. This concept can be used, for example, to do source-mask optimization. Compared to mercury arc lamp illumination, our system is simultaneously more eﬃcient, compact, versatile, economic and sustainable. In our contribution, we present the design of the system as well as lithographic test prints done with diﬀerent illumination patterns.