Stereoscopic white-light imaging of a large portion of the inner heliosphere has beenused to track interplanetary coronal mass ejections.At large elongations from the Sun,thewhite-light brightness depends on both the local electron density and the efficiency of theThomson-scattering process.To quantify the effects of the Thomson-scattering geometry,we studyan interplanetary shock using forward magnetohydrodynamic simulation and synthetic white-lightimaging.Identifiable as an inclined streak of enhanced brightness in a time–elongation map,thetravelling shock can be readily imaged by an observer located within a wide range of longitudes inthe ecliptic.Different parts of the shock front contribute to the imaged brightness pattern viewedby observers at different longitudes.Moreover,even for an observer located at a fixed longitude,adifferent part of the shock front will contribute to the imaged brightness at any given time.Theobserved brightness within each imaging pixel results from a weighted integral along itscorresponding ray-path.It is possible to infer the longitudinal location of the shock from thebrightness pattern in an optical sky map,based on the east–west asymmetry in its brightness anddegree of polarisation.Therefore,measurement of the interplanetary polarised brightness couldsignificantly reduce the ambiguity in performing three-dimensional reconstruction of localelectron density from white-light imaging.