Practical spectral photography II: snapshot spectral imaging using linear retarders and microgrid polarization cameras.

Despite recent advances, customized multispectral cameras can be challenging or costly to deploy in some use cases. Complexities span electronic synchronization, multi-camera calibration, parallax and spatial co-registration, and data acquisition from multiple cameras, all of which can hamper their ease of use. This paper discusses a generalized procedure for multispectral sensing using a pixelated polarization camera and anisotropic polymer film retarders to create multivariate optical filters. We then describe the calibration procedure, which leverages neural networks to convert measured data into calibrated spectra (intensity versus wavelength). Experimental results are presented for a multivariate and channeled optical filter. Finally, imaging results taken using a red, green, and blue microgrid polarization camera and the channeled optical filter are presented. Imaging experiments indicated that the calculated spectra's root mean square error is highest in the region where the camera's red, green, and blue filter responses overlap. The average error of the spectral reflectance, measured of our spectralon tiles, was 6.5% for wavelengths spanning 425-675 nm. This technique demonstrates that 12 spectral channels can be obtained with a relatively simple and robust optical setup, and at minimal cost beyond the purchase of the camera.

Mantis shrimp-inspired organic photodetector for simultaneous hyperspectral and polarimetric imaging.

Combining hyperspectral and polarimetric imaging provides a powerful sensing modality with broad applications from astronomy to biology. Existing methods rely on temporal data acquisition or snapshot imaging of spatially separated detectors. These approaches incur fundamental artifacts that degrade imaging performance. To overcome these limitations, we present a stomatopod-inspired sensor capable of snapshot hyperspectral and polarization sensing in a single pixel. The design consists of stacking polarization-sensitive organic photovoltaics (P-OPVs) and polymer retarders. Multiple spectral and polarization channels are obtained by exploiting the P-OPVs' anisotropic response and the retarders' dispersion. We show that the design can sense 15 spectral channels over a 350-nanometer bandwidth. A detector is also experimentally demonstrated, which simultaneously registers four spectral channels and three polarization channels. The sensor showcases the myriad degrees of freedom offered by organic semiconductors that are not available in inorganics and heralds a fundamentally unexplored route for simultaneous spectral and polarimetric imaging.

Organic-based photodetectors for multiband spectral imaging.

Using organic photodetectors for multispectral sensing is attractive due to their unique capabilities to tune spectral response, transmittance, and polarization sensitivity. Existing methods lack tandem multicolor detection and exhibit high spectral cross talk. We exploit the polarization sensitivity of organic photodetectors, together with birefringent optical filters to design single-pixel multispectral detectors that achieve high spectral selectivity and good radiometric performance. Two different architectures are explored and optimized, including the Solc-based and multitwist-retarder-based organic photodetectors. Although the former demonstrated a higher spectral resolution, the latter enables a more compact sensor as well as greater flexibility in device fabrication.

Internal defect scanning of sweetpotatoes using interactance spectroscopy.

While standard visible-light imaging offers a fast and inexpensive means of quality analysis of horticultural products, it is generally limited to measuring superficial (surface) defects. Using light at longer (near-infrared) or shorter (X-ray) wavelengths enables the detection of superficial tissue bruising and density defects, respectively; however, it does not enable the optical absorption and scattering properties of sub-dermal tissue to be quantified. This paper applies visible and near-infrared interactance spectroscopy to detect internal necrosis in sweetpotatoes and develops a Zemax scattering simulation that models the measured optical signatures for both healthy and necrotic tissue. This study demonstrates that interactance spectroscopy can detect the unique near-infrared optical signatures of necrotic tissues in sweetpotatoes down to a depth of approximately 5±0.5 mm. We anticipate that light scattering measurement methods will represent a significant improvement over the current destructive analysis methods used to assay for internal defects in sweetpotatoes.

Phase-shifting interferometry-based Fourier transform channeled spectropolarimeter.

Channeled spectropolarimetry is a snapshot technique for measuring the spectral dependence of the state of polarization of light. However, it suffers from two major limitations, namely, its high sensitivity to environmental perturbations and its susceptibility to channel crosstalk. These limitations reduce the polarimetric reconstruction accuracy of the spectropolarimeter. A new calibration technique for channeled spectropolarimetry is presented that utilizes the concept of phase-shifting interferometry to accurately acquire and demodulate the retardation phase factors, thereby improving the accuracy of the Stokes data reconstruction as well as enabling more robust performance. The new technique also enables the acquisition of high-resolution intensity spectrum by adopting a dual-scan measurement technique for reducing crosstalk. Experimental results show that calibrations using phase-shifting interferometry yield higher data reconstruction accuracy as compared to the self-calibration technique.

Microwave photonic mixer with high spurious-free dynamic range.

A new linearized photonic mixer structure, which can fully eliminate the third-order intermodulation distortion, is presented. It is based on an integrated dual-parallel Mach-Zehnder modulator to which an optimized RF split and an optimized optical phase shift are applied, in series with a Mach-Zehnder modulator driven by the LO. The mixer achieves a very high spurious-free dynamic range performance, it enables essentially infinite isolation between the RF and LO ports, and it has the ability to function over a multioctave frequency range. Experimental results demonstrate a record measured spurious free dynamic range performance of 127 dB·Hz(4/5), which is over 22 dB higher than that of the conventional dual-series Mach-Zehnder modulator-based microwave photonic mixer.