Development of a compact gradient bandpass filter for hyperspectral imaging from the VIS to SWIR range

In this work, a gradient bandpass filter is developed with spatially varying central wavelength ranging from 440 nm to 1700 nm within a lateral range of 10.24 mm. This filter is a central component of the Rainbow instrument which is a satellite based hyperspectral imaging instrument. The project is supported by ESA + program to provide commercially viable hyperspectral data for agricultural smart farming services. The filter was deposited using the PVD coating system EOSS ®. This is a rotary table batch coating system that can already implement gradient coatings on lenses and planar substrates in research and industry [1l However, due to the compact design of the filter, a steep gradient of up to 27.8% layer thickness increase per mm was prepared for this work. In order for the filter to meet the requirements for each central wavelength, an identical gradient slope had to be implemented for all optical coating materials in the filter, and was adjusted through special masks in the coating process. Using a digital twin of the coating system, the correct shape of the mask could be determined The required shapes and adjustments of masks and substrates were verified with trial coatings. To reduce the effect of the widening of the filter bandwidth according to the shift of the central wavelength, this filter was divided into three ranges, which are deposited in consecutive coating runs on a single substrate. In order to control the position of the deposited steep gradient bandpass on the substrate, the sample positions were adjusted with a micrometer stage, reaching accuracies up to 20 μm in the coating chamber. UV lithography was used for structuring of the filter ranges, where a lift-off process was successfully applied for filters with a thickness of up to 17.6 μm, achieving edges below 25 μm width. This result is not only interesting for the gradient bandpass filter but also for microstructured high-performance pixel filters. In order to measure the transmittance in the gradient band pass, a microphotospectrophotometer was developed. This measuring system uses mirror optics to achieve a spot size of approximately 50 μm. The filter position in the measuring system can be precisely adjusted in the μm range using XYZ translation stages. This enables a quick and precise characterization of the steep gradient coating.