Influence of solar incidence angle on single-image photoclinometry for precision lunar topographic mapping

Photoclinometry is an important method of reconstructing the topography of planetary surfaces. The technique relies on the photometric response of the surface under known illumination and viewing conditions. Each image pixel embeds the geometric and radiometric information of the surface, thereby allowing the underlying surface to be inferred and reconstructed in full. Photoclinometry is renowned for its ability to reconstruct pixel-wise three-dimensional (3D) models from a single image. However, it is affected by various error sources, including the radiometric quality of the sensor and variations in the surface photometric properties (e.g., albedo). This paper investigated the optimal solar incidence angle for photoclinometric reconstruction using error propagations and simulation analysis, and validated the findings using up-to-date photoclinometric methods and Lunar Reconnaissance Orbiter Camera (LROC) Narrow Angle Camera (NAC) datasets. The results suggested that variations in albedo have a significant effect on the quality of the reconstructed topography. The findings agreed with previous studies, which implies a potential bottleneck in the development of photoclinometric methods. Moreover, the photoclinometric performance becomes stable when the solar incidence angle exceeds 30° and the optimal incidence angle is approximately 60°, depending on the overall topography. Too large an incidence angle results in severe shadows, which hides topographic details, whereas too small an incidence angle introduces errors. The findings enrich the theoretical understanding of photoclinometry and are useful for optimising data capture strategies for future planetary missions to maximise the ability of photoclinometry and provide a foundation for future development of the technique. The recommendations on solar incidence angle and assessment strategies can be directly applied to other airless planetary bodies, such as Mercury and asteroids, for topographic mapping based on photoclinometry.