The Rasnik system is a 3-point optical displacement monitor with sub-nanometer precision. The CCD-Rasnik alignment system was developed in 1993 for the monitoring of the alignment of the muon chambers of the ATLAS Muon Spectrometer at CERN. Since then, the development has continued as new CMOS imaging pixel chips became available. The system's processes and parameters that limit the precision have been studied in detail. We conclude that only the quantum fluctuations to which the light level content of sensor pixels are subject to, is limiting the spatial resolution. The results of two Rasnik systems are compared to results from simulations, which are in good agreement: the best reached precision of $\SI{7}{pm/\sqrt{Hz}}$ is reported. Finally, some applications of high-precision Rasnik systems are set out.
