Evaluation of the potential of high index chalcogenide lenses for automotive applications

Infrared cameras could serve automotive applications by delivering breakthrough perception systems for both in-cabin passengers monitoring and car surrounding. However, low-cost and high-throughput manufacturing methods are essential to sustain the growth in thermal imaging markets for automotive applications, and for other close-to-consumer applications, which have a fast growth potential. With the reduction of the pixel pitch of microbolometer detectors, their cost has decreased considerably and now the optical part represents a significant part of the system cost. Fast low cost infrared lenses suitable for microbolometers are already sold by companies like Umicore, Lightpath, FLIR… Chalcogenide glasses are widely used as materials for optics because they have many cost advantages, especially due to the possibility of mass molding the optics. However, with the reduction of the pixel pitch, it is more and more difficult to design high performance lenses with a limited number of optics. The possibility of molding the optics allows us to use many highly aspherical surfaces at affordable costs. However, Chalcogenide glasses have usually a lower refractive index than other more expensive infrared materials such as Germanium. Indeed, high refractive index materials are known to be effective in attenuating the amplitude of many geometric aberrations. In this presentation, we evaluate the interest of high index Chalcogenide lenses, especially TGG and TGS, to design optical systems meeting the needs of the automobile with a limited number of optics. TGG glass has an index of refraction of 3.396 at a wavelength of 10µm, i.e. its index of refraction is close to the Silicon one and was initially studied for space applications. TGS has a lower index of refraction (3.12@10µm) but can be used in a cost effective manufacturing process by using flash spark plasma sintering (SPS) on raw powder. Demonstrators with TGG glass have been made and their performance evaluated.