Schematic of the optical path and measurement geometry of the three-color goniometer. M1, M2, and M3 are mirrors that direct the laser beams into the optical fiber. The angles i, e, α, and n denote the incident zenith angle, viewing zenith angle, phase angle, and surface normal vector, respectively. LP1 and LP2 are linear polarizers for the incident and viewing beams. λ/2 WP and λ/4 WP indicate the halfwave plate and quarter-wave plate, respectively.