Summary: An advanced optical technique based on digital version of $μ$-PIV and speckle technologies has been applied for flow microstructure analysis in a semi-transparent model of a PEM fuel cell. The line-of-sight speckle photography has been used for quantitative measurements of the instantaneous temperature fields in 3D unsteady flows. Using $μ$-PIV, the sequence of flow images has been visualised by micro-particles. The images obtained have been evaluated then by cross-correlation and autocorrelation techniques and instantaneous temperature, velocity and vorticity maps have been obtained with sub-millimeter spatial resolution. Up to 250,000 quantitative velocity vectors are reconstructed simultaneously using one digitally recorded specklogram.