We present an ultra-compact system approach for snapshot, multispectral imaging. It is based on a slanted linear variable spectral filter mounted in close proximity to the entrance pupil of a micro-optical, multi-aperture imaging system. A compact demonstration setup with a size of only 60 × 60 × 28 mm3 is developed, which enables the acquisition of 66 spectral channels in a single shot and offers a linear spectral sampling of approximately six nanometers over an extended wavelength range of 450-850 nm. The spatial sampling of each channel covers up to 400 × 400 pixels. First, the concept, the optical design and the fabrication are detailed. After the optical performance characterization, a comprehensive calibration strategy is developed and applied. An experimental demonstration is performed by acquiring the spatial and the spectral information of an imaged test scene.

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Snapshot multispectral imaging is a rising non-invasive and contact-free analysis method and technology to discriminate or identify objects based on their spectral characteristics. We demonstrate a versatile system approach for compact and real-time capable snapshot cameras for the visible (VIS) and the near-infrared (NIR) or the short-wave infrared (SWIR) wavelength range based on a micro-optical multi-aperture system and various spectral filter approaches. In addition, the manufacturing, the calibration, and the limitations of the demonstration systems are described.

Hyperspectral and multispectral imaging have been used for a couple of decades in applications such as satellite imaging, air reconnaissance, and other not overly price sensitive markets. The advent of alternative approaches makes spectral imaging attractive for volume and consumer markets, for example cancer detection, precision farming with unmanned aerial vehicles (UAV) or directly at the plant, or food testing in supermarkets. Alternative approaches comprise wafer-level coated sensors with fixed wavelength bandpass filters. Thin film coatings on glass substrates that can be patterned during deposition (in situ), or by using a photolithographic process over the coating to block the addition or subtraction of materials deposited on the substrate surface are also common. These micro-patterning techniques allow filters in a 2D mosaic struc-tors that offer several advantages and benefits over conventional approaches:  huge aperture compared with grating and prism,  higher transmission than grating and prism,  short measurement time,  high suppression of stray light,  excellent signal to background ratio,  3D and snapshot capability. Fig. 1 shows the transmission characteristics of a CVBPF that covers a center wavelength range in VIS/NIR with a bandwidth of approximately 2 % of its center wavelength. In a wide wavelength range, the transmission is higher than 90 %. But even more important than the peak transmission all undesired radiation from 200 to 1150 nm is suppressed better than OD4. ture (suited for the snapshot technique, i.e. acquisition of the hyperspectral data cube with only one camera exposure). A smart combinationA novel snapshot multispectral camera with high spectral performance and high spatial resolution was achieved by combining a micro lens array imaging system by German Fraunhofer IOF and a continuously variable bandpass filter by Danish Delta Optical Thin Film. The continuously variable bandpass filters (CVBPF) developed and manufactured by Delta Optical Thin Film offer high transmission and are fully blocked in the light-sensitive wavelength range of silicon-based detectors. The combination of CVBPFs with silicon detectors (Fig. 2) allows the design of compact, robust, and affordable spectral imaging detec-A multispectral imaging concept based on a multi-aperture system approach using a customized microlens array combined with a slanted continuously variable bandpass filter and a silicon-based image sensor helps to overcome the restrictions of scanning techniques or wafer-level coated detectors.Snapshot multispectral camera demonstrator (Source all images: the authors)

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