Imaging photoplethysmography (IPPG) has the advantages of high cost performance, simple equipment operation, and continuous automatic measurement and observation of subjects. IPPG has become an important means to deeply understand the optical properties of biological tissue and explore the pathological mechanisms related to complex cardiovascular diseases. IPPG is developed on the basis of traditional single-point photoplethysmography (PPG). IPPG uses imaging equipment to record the tiny changes in skin colors caused by the diffuse reflected light carrying the pulsation information of the heart after the interaction between light and skin tissue in the form of continuous images. Then, through video and image processing technologies, human vital sign information such as pulse, heart rate, and heart rate variability are extracted from the video stream. IPPG can reveal the dynamic and small changes in biological tissue during physiological and pathological processes. Therefore, IPPG can be used to better understand basic life activities and realize highly sensitive diagnoses and high-precision quantitative characterization of diseases. IPPG is applicable to large-scale clinical detection and physical health monitoring in daily life and other scenarios. It is a research hotspot of human daily physiological status monitoring in the new era of medical health.

In recent years, due to the continuous improvement of imaging sensor resolution and various signal processing technologies, IPPG has made great progress in the detection and application of human physiological parameters, such as heart rate, respiratory rate, and heart rate variability, as well as the application of disease diagnosis, such as arterial disease, stiffness and aging, and chronic microcirculation disease. However, for the further monitoring and classification diagnosis of complex cardiovascular diseases, IPPG still faces the challenges of lack of pathological feature analysis, complex optical feature parameters, and the manifestation of different types of pathological mechanisms of cardiovascular diseases on pulse waves. Therefore, we summarize the basic research and application of the existing IPPG and continue to explore the optical mechanism and pathological mechanism of IPPG. These efforts are very important for guiding the future development of IPPG.

Based on a comprehensive investigation of a large number of relevant literatures on the monitoring of human physiological parameters by IPPG in China and abroad in the past 20 years and our long-term research work, we first introduce the optical principle of IPPG. Then, we analyze the new method of IPPG signals in video image processing and that of improving the signal-to-noise ratio (Table 1), including a series of mainstream methods for selecting imaging sites in different areas of skin tissue and those for motion artifacts and blur. Finally, the clinical application of IPPG is introduced in detail, mainly including the extraction of IPPG heart rate under adaptive focal length, detection of living skin, measurement of blood oxygen saturation under visible light (Fig. 7), monitoring of fatigue status, evaluation of psychological stress, and analysis of the pathological mechanism of IPPG signals under cardiovascular diseases (Fig. 9).

IPPG is gradually developing towards miniaturization and intelligence. The monitoring indicators and accuracy of IPPG are continuously improving, which has important research significance and application value in biomedical research, clinical medicine, and daily life health monitoring. As an effective tool for early diagnosis of diseases and individual precision medical treatment, IPPG still needs in-depth and detailed exploration to promote its further development in academic and engineering fields.