Optical 3d geometry measurement is a vital part of the process control when manufacturing hybrid components. These so-called Tailored Forming components are meant to be inspected fast, in hot state and with high accuracy. In this paper, we examine the effect of a pressure reduction on the optical measurement data of hot objects, obtained with a self-developed fringe projection system. For this purpose, the fringe projection system has been attached to the window of vacuum chamber, housing a heatable cylinder as measurement object. Additionally, basic information on light propagation in inhomogeneous refractive index fields is provided next to a short review of previous work – including a theoretical analysis of a pressure reduction on the refractive index of air.