  Structured light illumination is an active 3-D scanning technique based on
projecting/capturing a set of striped patterns and measuring the warping of the
patterns as they reflect off a target object's surface. As designed, each pixel
in the camera sees exactly one pixel from the projector; however, there are
exceptions to this when the scanned surface has a complicated geometry with
step edges and other discontinuities in depth or where the target surface has
specularities that reflect light away from the camera. These situations are
generally referred to multipath where a given camera pixel receives light from
multiple positions from the projector. In the case of bimodal multipath, the
camera pixel receives light from exactly two positions from the projector which
occurs when light bounce back from a reflective surface or along a step edge
where the edge slices through a pixel so that the pixel sees both a foreground
and background surface. In this paper, we present a general mathematical model
and address the bimodal multipath issue in a phase measuring profilometry
scanner to measure the constructive and destructive interference between the
two light paths, and by taking advantage of this interesting cue, separate the
paths and make two separated depth measurements. We also validate our algorithm
with both simulation and a number of challenging real cases.
