iSPEX 2: A universal smartphone add-on for spectropolarimetry (Conference Presentation)

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.