Cut a hologram in half and each piece still shows the whole scene
Tear a photograph in two and you get two halves of a picture. Split a hologram and each fragment still contains the entire scene, seen through a smaller window. That is because every point on a hologram records light arriving from every part of the subject, not just one spot.
A hologram is a frozen interference pattern. A laser beam is split in two: one part lights the subject, and the other, the reference beam, shines straight onto the recording film. Where the scattered light and the reference light overlap, they interfere, leaving a microscopic pattern that looks like meaningless noise under ordinary light. Shine the same kind of beam through it later, and the pattern diffracts the light into a copy of the original light field, complete with depth and perspective that shift as you move your head.
Recording is extraordinarily fussy. Unlike a camera, holography uses no lens, needs a special film with light-sensitive grains smaller than about 20 nanometres, and requires everything, laser, optics, film and subject, to stay still to within roughly a quarter of a wavelength of light. Living subjects therefore need an intense, extremely short laser pulse, which is hazardous, so holographic portraits usually go through an intermediate non-holographic step.
The Hungarian-British physicist Dennis Gabor invented the technique in 1948 while trying to sharpen electron microscope images at a company in Rugby, England, and won the 1971 Nobel Prize in Physics for it. The name comes from Greek words for whole and writing. Optical holograms of solid objects had to wait for the laser, invented in 1960; in 1962 Yuri Denisyuk, working in Soviet Russia, and a separate pair of researchers in Michigan both produced the first practical ones.
Early holograms absorbed much of their light until bleaching methods converted the pattern into variations of refractive index. Stephen Benton then devised rainbow holograms, viewable in ordinary light without a laser. Today cheap laser diodes like those in DVD recorders let hobbyists make holograms, computers can calculate patterns for scenes that never existed, and the method also serves in data storage, microscopy and measurement.
Source: Holography
