# Causes and Corrections for Bimodal Multipath Scanning with Structured Light

## TL;DR

This paper models bimodal multipath in structured light 3D scanning, proposing a method to separate and correct for dual light paths to improve depth measurement accuracy in complex geometries.

## Contribution

It introduces a mathematical model for bimodal multipath and a novel algorithm to separate and correct dual light paths in phase measuring profilometry.

## Key findings

- The model accurately predicts bimodal multipath effects.
- The algorithm successfully separates dual light paths in simulations.
- Experimental validation shows improved depth accuracy in challenging cases.

## Abstract

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.