Night vision cameras can save lives and are therefore already included in many higher-priced cars. However they typically consist of multiple IR-lens elements which bring about large adjustment efforts.
Fraunhofer IOF has therefore developed a compact lens design consisting of one single element. The work was done within the research project fo+. The material of this monolithic element is a novel IG-glass. This chalcogenide glass is suitable for injection molding. With pressed pre-forms, the material volume and time of ultra-precision milling is reduced, resulting in the production process becoming cheaper for mass production. This offers an advantage in comparison to the typical IR material germanium. However its refractive index is in the range of 2.6 and thus comparably low for this wavelength range, making the requirements on the optical design larger. The element is described by four optical surfaces, of which the entrance and exit surface are refractive, while the two remaining surfaces are metalized and used as mirrors. Compactness is achieved by a folded ray path which breaks the rotational symmetry. Thus, the imaging application requires the use of a freeform surface. Typically the optical designer starts with an optical system that is already close to the aimed specifications. The final design is then reached with an iterative optimization process. However, freeform surfaces come with a huge variety of possible system geometries, while the number of existing starting configurations is very small. Therefore the starting configuration for the monolithic design is built from aspherical surfaces, which allows a perfect image for one monochromatic on-axis point. In particular an off-axis parabolic mirror focuses the incoming light bundle of the pilot ray into an intermediate image plane, which is then imaged by an ellipsoid onto the sensor. The optical design is optimized until it reaches the desired field of view of 37° x 25° with help of a freeform surface. The freeform is described by a Zernike polynomial in Fringe convention. The bandwidth is expanded to 8 to 14 μm. As a result, good image quality over the whole field of view with 17 μm pixel pitch is achieved together with an f-number of 1. The real intermediate image allows a compact lens volume of 2 cm x 2 cm x 1 cm. In addition a design with 60° x 40° field of view has been tested successfully.
The design is sustainable for manufacturing with help of ultra-precision milling. Therefore the position of the surfaces is optimized in a way that two neighbored surfaces can be manufactured simultaneously. The design thus supports the calculation of a continuous tool path over the two neighbored surfaces on each side of the monolith. Furthermore the rotational axes of these two surface compounds are aligned and the monolith can therefore be produced as one turning work piece. This novel method significantly reduces the manufacturing expense and efforts of the tooling adjustment.