Pressure and Temperature Sensitive Paint (PSP/TSP) became recognized by the experimental fluid dynamics community in 1990. In the three decades since then, PSP and TSP have become indispensable technologies for aerodynamic design and CFD validation. Japan has played a major role in the development of this innovative technology and is now recognized as one of the key players in the world. As a researcher who has been involved in PSP/TSP research since its early days, this article reviews the activities of the early days, which are unfamiliar to the younger generation. By summarizing the current status and future challenges of PSP/TSP, it is attempted to examine the true innovations that have realized this technology. In addition, some of the ideas that have appeared over the past 30 years but remain unfinished are introduced to inspire readers who are interested in studying the next generation of PSP/TSP technology.

Japan Aerospace Exploration Agency (JAXA) Aviation Technology Directorate (formerly the National Aerospace Laboratory) began basic research of pressure-sensitive paint (PSP) in the 1990s, at the dawn of PSP technology, and has accumulated outcome comparable to those in Europe and the United States. These technologies have been applied to global pressure measurements in practical wind tunnel tests for development of aircraft and spacecraft, and have contributed greatly to the validation of aerodynamic design and numerical simulation (CFD) results. JAXA's PSP technology is also used in academic research fields to clarify fluid phenomena and develop fluid control devices. This paper briefly describes the development of PSP for practical wind tunnel tests of JAXA Aviation Technology Directorate and how the research and development targets have evolved. In addition, examples of recent applications in two areas (aeroacoustic measurement and in-flight measurement) that are expected to improve by applying state-of-the-art PSP technology are presented.

PSP has been applied to a wide range of applications, starting from steady-state flows in the transonic to supersonic range, to low-pressure regions such as high Knudsen number flows and Martian wind tunnels, and to cryogenic wind tunnel measurements. In order to create a PSP suitable for each experimental condition, it is necessary to consider the characteristics of the pressure-sensitive dye, binder, and particles. This paper discusses the characteristics of dyes, polymers, and particles required for PSPs for steady-state measurements, for cryogenic wind tunnel measurements in low-temperature and low-oxygen environments, and for unsteady measurements. The key points for developing PSPs with these characteristics are also discussed.

The development of pressure distribution measurement using Pressure-Sensitive Paint technique was supported by the "Research and Development of Thermo-Fluid Sensing Technology Using Functional Molecular Sensors" funded by the Special Coordination Funds for Promoting Science and Technology from 1999 to 2003. This project established the foundation of PSP research in Japan. Since then, PSP has been applied not only to the aerospace field but also to various other fields. On the other hand, new and significant challenges have arisen, such as the development of technology for non-intrusive and quantitative measurement of micro areas in MEMS devices and the measurement of high-precision small pressure changes in the low-speed range in the automotive industry. Furthermore, there has been a demand for measurement under special environments, such as for application to the development of an airplane that can fly on Mars, where the atmospheric density is about 1/100 that of the Earth. This paper describes some of the research and development activities conducted by the authors from 2004 to 2015 to solve these problems.

This paper describes the latest research trends in unsteady PSP measurement using fast-response PSP at Tohoku University, Asai and Nonomura laboratory. Measurement techniques for the low-pressure environment and the supersonic flow have been studied in an effort to expand the application limit of fast-response PSP of unsteady pressure measurement. It was clarified that the pressure sensitivity and frequency response of PSP generally decrease at low pressure. Furthermore, an example of unsteady pressure measurement on the surface of a cylinder at a low pressure of 1-22 kPa using AA-PSP is shown. In the application of unsteady PSP measurement under supersonic airflow conditions, PSP with low-temperature sensitivity, low photodegradation, and high-emission intensity is required in addition to fast response of a PSP. The pressure fluctuation of the supersonic unsteady phenomenon of approximately 18 kHz is successfully obtained by combining H₂TCPP-based AA-PSP with low-temperature sensitivity and fast response with a high-power laser.