The quality and the deposition efficiency of coatings produced in thermal spraying processes are influenced by many parameters. Therefore normally the process optimization is laborious and time consuming. To reduce these efforts the low cost In-situ diagnostic method PFI was developed. A stationary CCD camera records the radiation of the hot propellant carrier-medium and of the luminous particle flux. A connected PC immediately converts the brightness distributions to a false colour image corresponding to the temperature and density profiles of the carrier-medium and of the particle flux. Utilizing a reference image the parameters can be adjusted for a prompt and successful optimization of the spray process. The performance of the PFI- method is demonstrated in some examples. Paper text in German.