Catadioptric sensor concept with interlaced beam paths for imaging and pinpoint spectroscopy

This paper presents the concept, optical design, and implementation of a catadioptric sensor for simultaneous imaging of a scene and pinpoint spectroscopy of a selected position, with object distances ranging from tens of centimeters to infinity and from narrow to wide adjustable viewing angles. The use of reflective imaging elements allows the implementation of folded and interlaced beam paths for spectroscopy and image acquisition, which enables a compact setup with a footprint of approximately 90mm×80mm. Although the wavelength range addressed extends far beyond the visible spectrum and reaches into the near infrared (∼400nm to 1000 nm), only three spherical surfaces are needed to project the intermediate image onto the image detector. The anamorphic imaging introduced by the folded beam path with different magnification factors in the horizontal and vertical directions as well as distortion can be compensated by software-based image processing. The area of the scene to be spectrally analyzed is imaged onto the input of an integrated miniature spectrometer. The imaging properties and spectroscopic characteristics are demonstrated in scenarios close to potential applications such as product sorting and fruit quality control.