Characterization of high aspect ratio miniature reflectors using negative shape polymer filling and image processing

Abstract

Optical reflectors are essential components used in spectroscopy applications that require high-intensity and uniform sample illumination. Typically, reflector parameters such as curvature and dimensions are optimized to ensure efficient light direction from filament lamps to the sample under test. Elliptical reflectors are often used to collect all the light emitted from one focus and direct it to the other. However, the curvature of miniature reflectors can be challenging to evaluate using standard measurement tools and methods due to its high aspect ratio, which can present mechanical and optical limitations when trying to access and scan it. In this work, we report a characterization procedure to evaluate the different optical and dimensional parameters of a fabricated miniature elliptical reflector with a high aspect ratio of width to depth. We compare two fabrication methods, Plastic Molding, and Diamond Turning, to assess their effectiveness. The reflector's curvature is characterized using the negative shape filling method, where melted polymer is used to fill the reflector and allowed to cool until solidified, resulting in a negative convex shape of the reflector. The reflector's curvature is then calculated by fitting the shape to the appropriate elliptical function using image processing. This method shows good accuracy in evaluating the reflector's curvature. Furthermore, the reflector's optical performance and illumination spot are characterized by imaging the spot onto a target screen and detector, validating the good performance achieved with low-cost plastic molded reflectors compared to DT reflectors. The image quality and optical power identify surface roughness and coating quality, where the molded reflectors show better results compared to the DT reflector.