Reflection Holograms

With a reflection hologram, the image is stored in a thick emulsion and can be viewed in white light. The simplest such hologram to make is the direct beam reflection hologram. In this case the direct beam through the film serves as the reference beam.

Rainbow Holograms

Rainbow holograms are holograms which can be viewed in white light. They are made by a double holographic process where an ordinary hologram such as a transmission hologram is used as the object and a second hologram is made through a slit. A horizontal slit limits the vertical perspective of the first image so that there is no vertical parallax in the resultant rainbow hologram. This slit process removes the coherence requirement on the viewing light so that full advantage can be taken of the image brightness obtained from ordinary room light, while maintaining the three-dimensional character of the image as the viewers eye is moved horizontally. If the viewers eye is moved vertically, no parallax is seen and the image color sweeps through the rainbow spectrum from blue to red, hence "rainbow hologram".

Denisyuk Holograms

Denisyuk contributed to the development of the hologram by using Lippmann's photographic process to make reflection holograms which could be viewed in color if more than one coherent source was available. In his method the beam was passed through the photographic emulsion and back reflected from the object. The emulsion could then be mounted on a reflective backing for viewing by reflected light.

Above are two views of a Denisyuk demonstration hologram. It is composed of a very thick emulsion mounted on a reflective backing. At first glance it will look like a black, shiny piece of film. But with an overhead light, you can find a position where the image will appear, as shown above right. You can see the image with ordinary room light because of the action of the thick emulsion.

Optimum viewing of the holographic image was found with a small flashlight held back as far as practicable from the image. The photo at left was taken under those conditions. The depth of field of the image depends upon the vergence of the light from the source.

This view of the same image was photographed with the flashlight close to the hologram. Although it is brighter, there is severe loss of depth of focus. The marching band figurines are also viewed from a different angle, showing the true 3-dimensional nature of the holographic image.

These two images were taken with the illumination of a small penlight to more nearly approximate a point source of light. The one on the left shows the loss of depth of field when the penlight was held close to the hologram (about 40 cm). Greater depth of field was obtained at right by holding the light about 100 cm from the hologram.

When you moved the light source across the hologram, you got dramatic changes in the perspective of the image, showing the true three dimensional nature of the holographic image.

Hecht describes this kind of hologram as a "volume hologram" because the emulsion thickness is so great that the object beam and reference beam overlap to produce a three-dimensional pattern of standing waves in the emulsion. The holographic information is stored throughout the volume of the emulsion.

References:
Holography Virtual Gallery

Hologram History

Historical notes:

Gabor was the discoverer of the hologram and received the Nobel Prize in Physics for it in 1971. This was pre-laser holography of course, and he made holograms only of transparencies where his reference beam was the unobstructed light which went through the clear parts of the transparency.

Leith and Upatnieks developed the off-axis reference beam method which is most often used today. It permitted the making of holograms of solid objects by reflected light.

Denisyuk contributed to the development by using Lippmann's photographic process to make reflection holograms which could be viewed in color if more than one coherent source was available. In his method the beam was passed through the photographic emulsion and back reflected from the object. The emulsion could then be mounted on a reflective backing for viewing by reflected light.

Benton is credited with the development of the rainbow hologram. Since it can be mass-copied and viewed with incoherent white light, it has become the most common type of hologram. Rainbow holograms have appeared on National Geographic's cover and on millions of credit cards as a deterrent to counterfeiting.

Composite holograms are made as multiple strips so that you get a different image from different angles. They can be viewed in white light.