In the 1950s, differential-interference contrast (DIC) systems were designed as an alternative technique to phase contrast (see Milestone 5). In DIC, polarized light is separated into two beams, which take slightly different paths through a sample depending on its optical density. When the beams are recombined, their interference reveals interfaces between regions of different thickness and/or refractive index and gives the illusion of a three-dimensional image. DIC allows for high-resolution imaging of unstained and living cells and organisms, and for 'optical sectioning' of thick samples. Another advantage is that there is no 'halo' artefact in DIC images, unlike that in phase-contrast images.