Evaluation of lenslet fabrication technologies for micro-optical array projectors

Micro-optical array projectors are discussed as replacement for structured illumination in applications with critical space requirements. The concept bases on the fly’s eye condenser principle and a well-defined buried array of micro-dia. Their optical performance benefits from etendue conservation and the large depth of field of applied short focal lenses. As an established technique to generate microlens profiles, thermal reflow of binary patterned photoresist is known for more than three decades. This approach leads to lens arrays with filling factors up to ~90% when used in hexagonal arrangement. Further increment requires direct writing methods such as grayscale lithography. Recently a LED based projection stepper-like lithography system became competitive, because it allows structure depths beyond 50 microns. It utilizes an LCoS micro-imager as variable 8-bit reticle and a high dynamic dosage controlled illumination. This paper represents the evaluation of the technique for the generation of refractive lens profiles by means of metrology and optical performance of micro-optical array projectors. Micro-array projectors based on circular lenslets will be compared, followed by the analysis of closely packed square-shaped lenslets. The aim is to understand the impact of lens shape deviation, conical constant or statistical distribution of lens properties like sag height, radius of curvature on the projection. A correlation of imperfections and quality loss due to scattering, aberrations, and mismatch of images in the overlay of different projectorlets will be given. The work concludes with an outlook on further developments in mastering micro-optical profiles for illumination application.