One key requirement for highly efficient production processes are steady and robust process conditions because they are most likely to ensure a constant product quality at the desired level. In blown film extrusion, film properties are predominantly influenced within the bubble formation zone, where the molten polymer expands from a thick-walled tube to a solid thin-walled sensitive film bubble. The characteristic bubble shape or contour, respectively, represents the state of balance resulting from all underlying process conditions and therefore its inline detection offers potential for an advanced process monitoring. Against this backdrop, the use of different measuring techniques, such as thermography, image processing or distance measurement, aims at an extensive and accurate description of the expanding film bubble. As a first step, a concept for such an inline detection will be presented in this paper. Moreover, it will focus on the application of distance measurement by means of ultrasonic sensors, for example regarding the evaluation of the bubble stability. Initial findings from experimental tests on a small-scale blown film line will demonstrate their applicability. Future work in this current research project involve the development of a simulation model for the expanding film bubble that is based on the given sensor signals and allows process monitoring with regard to the film's quality.