Hyperpixels: pixel filter arrays of multivariate optical elements for optimized spectral imaging

We introduce the concept of `hyperpixels' in which each element of a pixel filter array (suitable for CMOS image sensor integration) has a spectral transmission tailored to a target spectral component expected in application-specific scenes. These are analogous to arrays of multivariate optical elements that could be used for sensing specific analytes. Spectral tailoring is achieved by engineering the heights of multiple sub-pixel Fabry-Perot resonators that cover each pixel area. We first present a design approach for hyperpixels, based on a matched filter concept and, as an exemplar, design a set of 4 hyperpixels tailored to optimally discriminate between 4 spectral reflectance targets. Next, we fabricate repeating 2x2 pixel filter arrays of these designs, alongside repeating 2x2 arrays of an optimal bandpass filters, perform both spectral and imaging characterization. Experimentally measured hyperpixel transmission spectra show a 2.4x reduction in unmixing matrix condition number (p=0.031) compared to the optimal band-pass set. Imaging experiments using the filter arrays with a monochrome sensor achieve a 3.47x reduction in unmixing matrix condition number (p=0.020) compared to the optimal band-pass set. This demonstrates the utility of the hyperpixel approach and shows its superiority even over the optimal bandpass case. We expect that with further improvements in design and fabrication processes increased performance may be obtained. Because the hyperpixels are straightforward to customize, fabricate and can be placed atop monochrome sensors, this approach is highly versatile and could be adapted to a wide range of real-time imaging applications which are limited by low SNR including micro-endoscopy, capsule endoscopy, industrial inspection and machine vision.