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When I think of a hologram I imagine this

enter image description here

where you shine a beam of laser light onto a holographic film to reproduce an object beam and see a virtual image of an object that can be viewed from many angles.

I don't understand how flat holograms, like that seen on money (shown below), can even be in the same category as the hologram in the image above. I am very very confused any clarification would be much appreciated.

enter image description here

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  • $\begingroup$ Hmm, i dont really see the physics question here, its more the ethymological question why two different things have the same name. And thats probably just because: "It looks different if you look at it from a different angle". $\endgroup$
    – Anedar
    Mar 18, 2018 at 21:43
  • $\begingroup$ Not an expert on holograms but the hologram shown in your second picture is a "reflection hologram" which has the feature that it is visible under (incoherent) white light. It also has the advantage that it can be inexpensively made by just a stamping process, whereas many other types of holograms require high resolution films. I suggest getting a good book on holograms and reading up on the subject. $\endgroup$
    – user93237
    Mar 18, 2018 at 21:51

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The term "hologram" is used in a lot of ways. There are "3D" holograms, "2D" holograms, "E-BEAM" holograms, all of which use diffraction to produce an image and/or color. The term has been stretched in recent years to include anything that gives the impression of a 3D image.

So, don't worry about it. The diffractive images on money are usually made by electron beam lithography, but could as well be made by interfering object and reference laser beams. It is okay to call them "holograms".

Originally, though, the term applied only to the kind of image Denis Gabor envisioned, which you yourself would happily call a hologram.

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  • $\begingroup$ They use e-beam lithography to make holograms on currency? Thought that e-beam lithography was a relatively slow process mostly used for things like making master masks for optical lithography (where the slowness of the e-beam lithography process is not too much of a concern). Surprised to hear that e-beam lithography is used for mass production of holograms on currency. Would have thought that it would be too slow and expensive for that application. $\endgroup$
    – user93237
    Mar 18, 2018 at 22:02
  • $\begingroup$ Yes. E-beam lithography is used to make a master in resist, from which an embossing "shim" is made. The shim is used to mass-produce the holograms which are then transferred to the currency. $\endgroup$
    – S. McGrew
    Mar 19, 2018 at 2:58

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