Understanding the flow around helicopter rotors is one of the greatest challenges in modern aerodynamics. The flow field plays a key role in the rotorcraft performance and operational safety, and it is characterized by highly unsteady and three-dimensional phenomena. State-of-the-art computational fluid dynamics (CFD) is applied during the design of future rotorcraft and offers remarkable capabilities, including the simulation of entire helicopter configurations in maneuvering flight. Nevertheless, experiments are still essential for the understanding of complex flow regimes, and for the validation of numerical results. An ever-increasing level of detail in CFD studies motivates the development and refinement of experimental methods, and combined experimental-numerical efforts have been particularly rewarding in recent studies.Starting with early rotorcraft-specific research topics, for example the systematic characterization of pitch-oscillating airfoils in the 1960s, experimental techniques have undergone continuous improvement. This particularly holds true for optical methods, which have developed from providing qualitative and “simple” snapshots of the flow into quantitative and time-resolving diagnostic tools. Optical methods require few modifications of the rotor or rotorcraft under investigation. They are particularly suitable for an application on multiple scales, ranging from small-scale laboratory studies to full-scale free-flying helicopters. This thesis concentrates on the development, validation, and application of the differential infrared thermography (DIT). The DIT method is able to determine the moving position of the laminar-turbulent boundary layer transition, which is a relevant aerodynamic feature on rotor blades, accounting for the unsteadiness introduced by the different inflow conditions on the advancing and retreating sides of the trimmed rotor plane in forward flight. Additional helicopter-relevant applications include the study of pitch-oscillating airfoils or small-scaled rotors in laboratory or wind-tunnel environments. Furthermore, it will be shown that the DIT principle can be adapted to other rotor-relevant topics beyond transition research, such as dynamic stall investigations. DIT is a valuable addition to the larger family of optical measurement techniques for aerodynamic applications.