An optical coherence tomography (OCT) imager comprising: a low coherence light source that provides light; a photosensor for imaging light; optics that directs a first portion of the light along a first optical path so that it is imaged on the photosensor and a second portion of the light along a second optical path so that it is reflected by material in the target to the photosensor where it is imaged to interfere with light from the first portion to form an interference pattern; and a mask through which light from at least one of the first and second portions of light passes that generates a relative phase perturbation between portions of light that passes through it.