Since 1994, PROGRA2 instruments (Optical Properties of Astronomical and Atmospheric GRains) have been taking measurements of light scattered by clouds of irregular solid particles in suspension or deposited on surfaces. Levitating measurements 
are carried out in zero gravity during parabolic flights aboard the A300 ZeroG then the A310 ZeroG within the framework of campaigns funded by CNES and ESA. Additional measurements were made in the laboratory for the smallest particles using a lifting technique by air injection. 
The measurements are carried out for different wavelengths between 450 and 1600 nm.
Hundreds of light scattering curves were obtained under these different conditions for characteristic samples of materials found on Earth, in the atmosphere and in space (volcanic ash, basalts, sands, carbons, meteorites, silicon carbide, …). 
Some samples are made in the laboratory, such as cometary and interplanetary analogues, and tholins. All these data help to interpret measurements of sunlight scattered by particles as observed from telescopes, space probes and instruments under atmospheric balloons. From those, it is possible to better characterize certain physical properties of these solid particles, such as their size, their nature and their porosity. This project was primarily funded by CNES.