SPEX, the Spectropolarimeter for Planetary EXploration, is designed to characterize aerosol and cloud particles in the atmospheres of Solar System planets from orbiting satellites, by measuring both the flux and the degree and direction of linear polarization across the visible spectrum.
SPEX measures the polarization using the spectral modulation technique by Snik, Karalidi, & Keller(2009), which allows both the flux and the linear polarization information to be measured simultaneously.
iSPEX uses the same technique as SPEX, only with cheaper materials and with a single modulated spectrum. The image below shows how the different components are combined in iSPEX. The slit and the grating create a spectrum. The quarter-wave plate, multi-order wave plate and polarizer cause the modulation of the spectrum. The lens and camera of the iPhone are used to record the modulated spectra, in order to perform a measurement. On board instruments (GPS, inclinometer etc.) of the iPhone yield the geographical location and direction in which you have pointed, which is recorded simultaneously with the measurement.