Holography

Holograms are photographic images that are three-dimensional and appear to have depth. Holograms work by creating an image composed of two superimposed 2-dimensional pictures of the same object seen from different reference points. Holography requires the use of light of a single exact wavelength, so lasers must be used. In reflection holograms, the kind of holography that can be viewed in normal light, two laser beams and a photographic plate are used to take an image of the object. Both laser beams used in a holograph go through beam spreaders, which spread the laser light out like a flashlight. The coherence of the beam is lost, but it remains an exact wavelength.



One beam illuminates the object from the side. The other beam, known as a reference beam, travels through a photographic plate and hits the object head-on, similar to the way in which a conventional camera takes a 2-D image. The reflecting light from the reference beam leaves an image, or hologram, on the photographic plate, but so does the light reflected by the object from the side beam. The result is a photographic plate that registers two images simultaneously, creating a hologram. When viewing any object, the human eyes each receive a distinct image, from slightly offset reference points. The brain combines them into a three-dimensional image. The hologram produces the same effect artificially.



When developed, the hologram is printed onto a set of ultra-thin curved silver plates, which are made to diffract light. Diffraction is what a prism does. It splits multi-wavelength white light into each specific wavelength. When white light hits the silver plates, the reflections diffract into each specific color, creating two offset images made up of colors that roughly reflect what the original object looked like. The result is a three-dimensional image, a hologram. The hologram is composed of the interference pattern between the two original images.



Another kind of common hologram is the reflection or Denisyuk hologram. This is capable of multicolour image reproduction using a white light illumination source on the same side of the hologram as the viewer.One of the most promising recent advances in the short history of holography has been the mass production of low-cost solid-state lasers. This is typically used by the millions in DVD recorders and other applications, but which are sometimes also useful for holography. These cheap, compact, solid-state lasers can under some circumstances compete well with the large, expensive gas lasers. They were previously required to make holograms, and are already helping to make holography much more accessible to low-budget researchers, artists, and dedicated hobbyists.


Holography dates from 1947, when British scientist Dennis Gabor developed the theory of holography. He did this while working to improve the resolution of an electron microscope. Gabor coined the term hologram from the Greek words holos, meaning "whole." And then gramma, meaning "message". Further development in the field was stymied during the next decade. This was because light sources available at the time were not truly "coherent."


In 1972 Lloyd Cross developed the integral hologram. He did this by combining white-light transmission holography with conventional cinematography. It produced 3-dimensional images. Sequential frames of 2-D motion-picture footage of a rotating subject are recorded on holographic film. When viewed, the composite images are synthesized by the human brain. Just like a 3-D image.

Another major advance in display holography occurred in 1968. Dr. Stephen A. Benton invented white-light transmission holography while researching holographic television at Polaroid Research Laboratories. This type of hologram can be viewed in ordinary white light creating a rainbow image from the seven colors which make up white light. The depth and brilliance of the image and its rainbow spectrum soon attracted artists who adapted this technique to their work and brought holography further into public awareness.Benton's invention is particularly significant because it made possible mass production of holograms using an embossing technique. These holograms are "printed" by stamping the interference pattern onto plastic.

Hologams, I believe, are quite interesting. There are many different kinds. And also there have been many upgrades and discoveries about it. They have shown us a differnt way to get an image. Plus it is 3-D. Holography puts photography.