An ordinary photo records only the brightness – or amplitude characteristics – of light reflected from the object being photographed. In holography, the phase of the light waves is recorded as well on an ordinary photographic plate or film, to give a three-dimensional representation of the object.
To make a hologram, a coherent source of light is needed – that is, the light is all the same frequency and in phase. A laser provides this. The laser beam is split into two. One beam, the object beam, illuminates the object and is reflected on to the photographic plate. The other beam, the reference beam, is aimed directly at the photographic plate. The two beams interfere, and rather than producing a photograph, an interference pattern is recorded on the plate – the hologram.
When this hologram is illuminated by coherent light of the same frequency as the original reference beam at the correct angle, it produces a real and a virtual image of the object, which together create a 3D image.