Spectropolarimetry is a powerful technique for remote sensing of the environment. It enables the retrieval of particle shape and size distributions in air and water to an extent that traditional spectroscopy cannot. SPEX is an instrument concept for spectropolarimetry through spectral modulation, providing snapshot, and hence accurate, hyperspectral intensity and degree and angle of linear polarization. Successful SPEX instruments have included groundSPEX and SPEX airborne, which both measure aerosol optical thickness with high precision, and soon SPEXone, which will fly on PACE. Here, we present a low-cost variant for consumer cameras, iSPEX 2, with universal smartphone support. Smartphones enable citizen science measurements which are significantly more scaleable, in space and time, than professional instruments. Universal smartphone support is achieved through a modular hardware design and SPECTACLE data processing. iSPEX 2 will be manufactured through injection molding and 3D printing. A smartphone app for data acquisition and processing is in active development. Production, calibration, and validation will commence in the summer of 2020. Scientific applications will include citizen science measurements of aerosol optical thickness and surface water reflectance, as well as low-cost laboratory and portable spectroscopy.
Spectropolarimetry, the simultaneous measurement of spectrum and polarization, provides a wealth of information on environmental processes, particularly aerosols and water-borne particles. The SPEX method of measuring polarization through spectral modulation is used to accurately measure aerosol optical depth on ground-based, air-based, and space-based systems. This incudes SPEXone, a spectropolarimeter on NASA's new climate satellite PACE, as well as iSPEX, a smartphone-based single-beam implementation originally designed in 2012.
iSPEX was previously used in citizen science projects with thousands of volunteers to measure aerosol optical depth across the Netherlands and Europe. However, it was limited in accuracy by the smartphone camera and the single-beam SPEX implementation. Furthermore, it only supported a few smartphone models.
We present iSPEX 2, a completely new design with universal smartphone support as well as a dual-beam design for full spectroscopy and linear polarimetry. Using the SPECTACLE method for spectral and radiometric calibration of consumer cameras, any smartphone can provide quantitative spectral and polarimetric data. Due to its low cost, iSPEX 2 is well-suited to use in underprivileged areas, large-scale deployment, and citizen science projects.
Beetles of the families Scarabaeidae and Hybosoridae display left-handed circular polarization due to the Bouligand arrangement of chitin fibers in their cuticle. We intend to verify and understand, or disprove, the apparent and hitherto unexplained universality of left-handed polarization in beetles that exhibit circular polarization. We here report the first results of our ongoing massive survey of one of the largest beetle collections in the world, covering ±10.000 beetle genera. To facilitate the scanning of museum drawers full of beetles, drawers were placed in a purpose-built setup and automatically photographed in RGB colors through polarization filters mounted on a motorized wheel. A total of 6 photographs were taken, through left- and right-handed circular polarization filters, and four linear polarization filters mounted at 0, 90, 45, and -45 degrees. From these, the full Stokes parameters were calculated across the drawer. Images were segmented to obtain polarization parameters representing individual insects. Taxonomic information was obtained for each drawer, by automatically reading the database accession number. Specimens showing circular polarization in taxa for which this has hitherto not been studied are selected to be studied in more detail later, using a custom designed spectropolarimetric integral-field unit. Polarization can then be analyzed as a function of wavelength, resulting in both detailed hyperspectral imaging data and fully resolved Stokes parameters per spectral bin. Our two-stage high-throughput optical screening approach allows for bioprospecting for biophysical properties of valuable and potentially irreplaceable museum specimens, such as type specimens, or specimens of rare or even extinct species.
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