Eidophors used an optical system somewhat similar to a conventional movie projector, but substituted a slowly rotating mirrored disk or dish for the film. The disk was covered with a thin film of transparent high-viscosity oil, and through the use of a scanned electron beam, electrostatic charges could be deposited onto the oil, causing its surface to deform. Light was shone on the disc by a striped mirror consisting of strips of reflective material alternating with transparent non-reflective areas. Areas of the oil unaffected by the electron beam would allow the light to be reflected directly back to the mirror and towards the light source, whereas light passing through deformed areas would be displaced and would pass through the adjacent transparent areas and onwards through the projection system. As the disk rotated, a doctor blade discharged and smoothed the ripples in the oil, readying it for re-use on another television frame.The Eidophor was a large and cumbersome device and not commonly used until there was a need for good-quality large-screen projection. This opportunity arose as part of the NASA space program, where the technology was deployed in mission control.