The present invention relates to an imaging apparatus and comprises input
and output polarisers, a first polarising beam splitter and at least one
additional polarising beam splitter, a light sensitive detector and
focussing means arranged on an axis. The input polariser resolves
incident light into a single linear polarisation state. The first
polarising beam splitter receives light from the input polarises, and
resolves it into equal magnitude orthogonally polarised rays which are
mutually spaced and have a path difference therebetween. The or each
additional polarising beam splitter is arranged to receive light from the
first polarising beam splitter. The transmission axis of the output
polariser is parallel to or perpendicular to the transmission axis of the
input polarises to resolve the orthogonally polarised light rays having
past through the or each additional polarising beam splitter into the
same or perpendicular polarisation state as light resolved by the, first
polariser. The first polarising beam splitter, the or each additional
polarising beam splitter and the focussing means are mutually spaced such
that said mutually spaced rays are brought to coincidence whereby
interference fringes are produced, the detector being arranged to detect
the interference fringes. One beam splitter is mounted for movement
perpendicular to said axis, whereas the other beam splitter(s) is/are
rigidly mounted against movement.