iSPEX consists of an add-on and corresponding app for the iPhone. By attaching the iSPEX add-on in front of the smartphone camera this transforms it into an optical sensor that is suited for the measurement of the macro- and microphysical properties of atmospheric aerosols. In order to achieve this, iSPEX measures the (intensity) spectrum and the degree of polarization for visible light. Its measurement principles are based on that of the Spectropolarimeter for Planetary EXploration (SPEX) that is designed to measure aerosol and cloud particles in atmospheres of planets within our solar system.
The add-on is essentially a slit spectrograph that uses a transmission grating foil and a plastic lens in addition to the lens inside the smartphone camera. In addition, it contains a combination of stretched (and hence birefringent) plastic sheets and Polaroid film to modulate every spectrum by a sine curve. The relative amplitude of this sine curve directly scales with the degree of linear polarization (DoLP), and its phase is determined by the polarization angle. As such, all the information on both the spectrum and the linear polarization of the light entering the slit is obtained in a single shot. The iSPEX app disentangles the spectral information from the polarization information.
This way, the degree of linear polarization (DoLP) of the cloud-free sky can be measured as a function of wavelength and, by pointing the phone at different directions in the sky, as a function of scattering angle. This DoLP as a function of both wavelength and scattering angle yields unique information on fundamental aerosol properties, including the amount of aerosol, and also the particle size distribution and the chemical composition (through the refractive index).
Snik et al., Mapping atmospheric aerosols with a citizen science network of smartphone spectropolarimeters, scientific paper, Geophysical Research Letters, 2014.