Real time control, by multiwavelenglh phase modulated ellipsometry (PME), of the growth of plasma deposited optical structures is presented here. The transparent multilayers consist of SiO2 and SiNx alloys. Various on-line methods are compared. The best results are obtained with a feedback method based on comparison between the real time PME measurements and pre-computed target trajectories. It can provide the high precision required to deposit high performance optical coatings. In particular, an overall accuracy better than 1% is obtained on a 15-layer quaterwave filter, designed at 670 nm. This real time procedure, which is not limited to transparent materials or plasma processes, appears a useful tool for process control.