Another dimension

Pretty well every time we buy anything today – at least using credit cards or bank notes – we are handling one of the most extraordinary, futuristic pieces of technology: a hologram. This would surprise Hungarian physicist Dennis Gabor, who invented it almost fortuitously when working on electron microscopes at the Thomson-Houston company in Rugby in 1947. Gabor developed the theory of how it was possible to ‘capture’ objects in 3d and used a mercury arc lamp to produce a tiny hologram showing the names of famous scientists. The importance of Gabor’s work was recognised with the 1971 Nobel Prize for Physics. To fully capture a real 3d scene you need to record not only the light’s amplitude and wavelength, as a photograph does, but also its phase (essentially, a stage in the period of a periodic motion, in this case the wave of light). Photography does not capture this, but holograms do. If the hologram is illuminated with the appropriate light, it diffracts part of it into exactly the same wave that emanated from the original scene, thus recreating it completely. To capture the phase of the light wave at each point in an image, holography uses a reference beam, which is combined with the light from the object, the object beam. If these two beams are coherent, optical interference between the reference and object beams generates intensity fringes that can be recorded on ordinary photographic film. The fringes form a kind of diffraction grating on the film, which is the hologram. The central miracle of holography is that when the recorded grating is later illuminated by a substitute reference beam, the original object beam is completely reconstructed, producing a 3d image.