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 than 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.

The Holographic Image

Some of the descriptions of holograms are

• "image formation by wavefront reconstruction.."
  通过波前重建实现成像
• "lensless photography"
  无镜头摄影
• "freezing an image on its way to your eye, and then reconstructing it with a laser"
  在图像传达到你眼睛之前将其冻结，然后用激光重建它

A consistent characteristic of the images as

1. The images are true three-dimensional images, showing depth and parallax and continually changing in aspect with the viewing angle.
   这些图像都是真实的三维图像，表现出深度和视差，并且随着视角的变化而不断改变其外观。
2. Any part of the hologram contains the whole image!
   任何 hologram 的一部分都包含整个图像！
3. The images are scalable. They can be made with one wavelength and viewed with another, with the possibility of magnification.
   这些图像可以缩放。它们可以用一种波长制作，用另一种波长观看，有可能被放大。

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.

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. 每一张全息图都包含整个物体的图像。你可以剪裁全息图的角落，通过它仍能看到整个图像。对于每一个视角，你看到的图像都是从不同角度呈现的，就像观察一个真实的物体一样。全息图的每一部分都包含图像的一个特定视角，但它包含了整个物体。 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. 右上角的图像显示的是通过 hologram 较大部分的视角，而右下角的图像则是通过截去 hologram 一角的视角。虽然通过小角的视角是特定的观察点，但它包含了整个物体。

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.