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