However as red-wavelengths are longer, they are deflected more than blue-wavelengths and so the image will have some color smear — the light spreads into a rainbow image. A rainbow hologram, recorded from a horizontal master strip and replayed with a light from above, will have horizontal parallax — enabling the spatial qualities when looking with two eyes or moving side-to-side.
But moving up-and-down produces no change in spatial perspective and viewer only sees a change of color. Recording a hologram requires the interference pattern to be stationary during the exposure. For this reason holograms are traditionally made of static sculptures on vibration isolated tables. A pulse laser produces an ultra short flash of light, thus freezing motion and allowing for holograms of live subjects. As the holographic recording process is dependent on the interference pattern between optical-waves the holographic image is the comparative shape between these waves.
Holographic interferometry utilizes this property to detected small variations in form, with applications in industrial non-destructive testing.
Holograms can also be used to direct light, which has commercial application in lighting design as well as being used for video projection screens and AR displays 6. This property has been incorporated into the process of making holograms by artists. Before the stenciling and digital printing techniques became widely practiced, Rudie Berkhout was creating spatially dynamic holograms using multiple holographic optical elements HOEs to shape light into dynamic abstract images.
Multiplexing is a technique of recording multiple holographic exposures across the surface of the master hologram. Lloyd Cross developed a one step process of recording multiplex holograms in , using a sequence of film frames to make a cylindrical hologram where the image appears in the center. The process combines cinematographic and holographic techniques to display a short animated image loop. These holographic images are like looking at a three-dimensional movie without having to use the special three-dimensional glasses.
There are many types of holograms, such as an embossed hologram , a reflection hologram and a rainbow hologram, to name a few. An embossed hologram is a transparent type of image that typically is used in security measures because the image is extremely complex and difficult to recreate.
Embossed images have one single color for each light source being used. After an image is created, a mirror effect is placed behind the original picture, which creates an effect that makes it seem as if the original image has been multiplied.
The image is embedded into an object by the heat and pressure of a metal stamp. Embossed holograms are more commonly seen in everyday life. A reflection hologram forms an image by reflecting light. The virtual image can be very sharp and deep. For example, through a small hologram, a full-size room with people in it can be seen as if the hologram were a window. If this hologram is broken into small pieces to be less wasteful, the hologram can be covered by a piece of paper with a hole in it , one can still see the entire scene through each piece.
Depending on the location of the piece hole , a different perspective is observed. Furthermore, if an undiverged laser beam is directed backward relative to the direction of the reference beam through the hologram, a real image can be projected onto a screen located at the original position of the object.
To mass produce holograms for authenticity applications such as security hologram stickers or the holograms you find on on credit cards, currency and passports, a two-dimensional interference pattern is pressed onto thin plastic foils. The original hologram is usually recorded on a photosensitive material called photoresist. When developed, the hologram consists of grooves on the surface. The shim is placed on a roller. Under high temperature and pressure, the shim presses embosses the hologram onto a roll of composite material similar to Mylar.
Embossed holograms are actually a combination many types of holograms. A transmission or reflection hologram can be made from a series of photographs usually transparencies of an object—which can be a live person, an outdoor scene, a computer graphic, or an X-ray picture. Each view is shown on an LCD screen illuminated with laser light and is used as the object beam to record a hologram on a narrow vertical strip of holographic plate holoplate.
The next view is similarly recorded on an adjacent strip, until all the views are recorded. When viewing the finished composite hologram, the left and right eyes see images from different narrow holograms; thus, a stereoscopic image is observed.
Recently, video cameras have been used for the original recording, which allows images to be manipulated through the use of computer software.
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