Variable Ring Light Imaging: Capturing Transient Subsurface Scattering with An Ordinary Camera

Subsurface scattering plays a significant role in determining the appearance of real-world surfaces. A light ray penetrating into the subsurface is repeatedly scattered and absorbed by particles along its path before reemerging from the outer interface, which determines its spectral radiance. We introduce a novel imaging method that enables the decomposition of the appearance of a fronto-parallel real-world surface into images of light with bounded path lengths, i.e., transient subsurface light transport. Our key idea is to observe each surface point under a variable ring light: a circular illumination pattern of increasingly larger radius centered on it. We show that the path length of light captured in each of these observations is naturally lower-bounded by the ring light radius. By taking the difference of ring light images of incrementally larger radii, we compute transient images that encode light with bounded path lengths. Experimental results on synthetic and complex real-world surfaces demonstrate that the recovered transient images reveal the subsurface structure of general translucent inhomogeneous surfaces. We further show that their differences reveal the surface colors at different surface depths. The proposed method is the first to enable the unveiling of dense and continuous subsurface structures from steady-state external appearance using ordinary camera and illumination.