# Holography

The above image was taken through a transmission hologram. The hologram was illuminated from behind by a helium-neon laser which has been passed through a diverging lens to spread the beam over the hologram.

Holography is "lensless photography" in which an image is captured not as an image focused on film, but as an interference pattern at the film. Typically, coherent light from a laser is reflected from an object and combined at the film with light from a reference beam. This recorded interference pattern actually contains much more information that a focused image, and enables the viewer to view a true three-dimensional image which exhibits parallax. That is, the image will change its appearance if you look at it from a different angle, just as if you were looking at a real 3D object. In the case of a transmission hologram, you look through the film and see the three dimensional image suspended in midair at a point which corresponds to the position of the real object which was photographed.

These three images of the same hologram were taken by positioning the camera at three positions, moving the camera from left to right. Note that the pawn appears on the right side of the king in the left photo, but transitions to the left of the king as you move the viewing position across the hologram so that you are viewing it from further to the right. This is real parallax, which tells you that the image is truly 3-dimensional. Each perspective corresponds to looking through the hologram at a particular point.

### What is the image on the film like?

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# The Holographic Image

Some of the descriptions of holograms are

• "image formation by wavefront reconstruction.."
• "lensless photography"
• "freezing an image on its way to your eye, and then reconstructing it with a laser"

A consistent characteristic of the images as viewed

1. The images are true three-dimensional images, showing depth and parallax and continually changing in aspect with the viewing angle.
2. Any part of the hologram contains the whole image!
3. The images are scalable. They can be made with one wavelength and viewed with another, with the possibility of magnification.
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# Three-Dimensional Image

Holograms are true three-dimensional images. This is evidenced by the fact that you can move your head while viewing the image and see it in a different perspective. This includes revealing part of the image which was hidden at another viewing angle.

Shown below are three images from the same hologram, obtained by looking through it at different angles. Note that the pawn appears in different perspective in front of the king behind it.

 Properties of holograms
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# When a piece is the whole

 Every part of a hologram contains the image of the whole object. You can cut off the corner of a hologram and see the entire image through it. For every viewing angle you see the image in a different perspective, as you would a real object. Each piece of a hologram contains a particular perspective of the image, but it includes the entire object. The image at upper right is the view through the larger part of the hologram, while that at lower right is through a small corner cut off the hologram. While the view through the small corner is from a particular point of view, it contains the whole object.
 Properties of holograms
 What is the image on the film like?
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# Scaling Holographic Images

The holographic image scales with the wavelength. This initially caused great excitement because one could imagine making holographic images with x-rays and viewing them with visible light, getting three-dimensional views of things on the scale of molecules. X-ray holograms have yet to be made, and there are practical difficulties with the scaling, but there is still the possibility that this feature of holograms will prove to be of great benefit.

 Example of scaling with mercury source
 Properties of holograms
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Holography concepts

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