  We present a novel approach for characterizing the properties and performance
of active matter in dilute suspension as well as in crowded environments. We
use Super-Heterodyne Laser-Doppler-Velocimetry (SH-LDV) to study large
ensembles of catalytically active Janus particles moving under UV-illumination.
SH-LDV facilitates a model-free determination of the swimming speed and
direction, with excellent ensemble averaging. In addition we obtain
in-formation on the distribution of the catalytic activity. Moreover, SH-LDV
operates away from walls and permits a facile correction for multiple
scattering contributions. It thus allows for stud-ies of concentrated
suspensions of swimmers or of systems where swimmers propel actively in an
environment crowded by passive particles. We demonstrate the versatility and
the scope of the method with a few selected examples. We anticipate that SH-LDV
complements estab-lished methods and paves the way for systematic measurements
at previously inaccessible boundary conditions.
