Photon recollision probability and the spectral invariant theory: Principles, methods, and applications

The photon recollision probability (p) and spectral invariant theory (p-theory) have gained much attention in the vegetation remote sensing community over the past decades. A broad range of studies have been performed involving the derivation of theoretical principles, determination of the p value in field measurements and remote sensing studies, and application of the theory in various areas. This review synthesizes current developments in this theory and discusses future directions. The review begins with a brief introduction of the historical background of the p-theory, followed by a description of the general principles of the theory. Subsequently, various methods for estimating the p value in field measurements and remote sensing studies are reviewed. Next, applications of the p-theory in canopy reflectance modeling and vegetation parameter retrieval are illustrated. Finally, the general features of the theory and future research directions are discussed. This study aims to enhance our understanding of the p-theory, its development and applications in various disciplines.