Non-intrusive measurement techniques are based on optical techniques and have tremendous advantages, but as all measurement techniques they have their limitations and drawbacks. Aside the arduous technical issues associated with data processing and analysis, their main physical limitation is simply based on visual access of the experimental domain of interest and also the ability to guide the light/laser source for sufficient illumination. This depends a lot on the experimental facility and sometimes the working section has to be redesigned and equipped with sufficiently large windows with high optical quality glass or Perspex which is a strict requirement for classical interferometry techniques.