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I was watching this video on how a Micromagnetic Piston Display works. After some research I can still not get my head around on how this display actually shows a holographic image. In my opinion, Francesco Pessolano in the video explains it like there are 'pixels' on the silicon piece, which can be made in two different positions: higher or lower. In this way, the silicon piece can somehow show an image from different angles. If I think about it in this way, it is impossible to achieve something like that while using pixels in 'only' two different positions.

Anyone knows how these pixels allow the hologram to be seen?

Qmechanic
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P.Yntema
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3 Answers3

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The use of "binary diffractive", or so called kinoform lenses is quite an established technology.

Recall Bragg's law: a locally sinusoidal variation in the transmissivity / phase of a wave passing through a medium sets up a diffracted wave ("ray") with a direction that is well defined by Bragg's law. This situation changes little if we replace sinusoidal variation with square wave variation: the grating looks like a superposition of sinusoids, which each diffract the incoming field in different directions. Often these directions are different enough that the different order images are kept separate, and so the main effect of the square wave is loss (to higher order diffractions). So we simply use modulation of the grating frequency in this technology to steer the output waves, where we would modulate the lens thickness in refractive technology to steer our output waves.

Furthermore, a system of Fresnel zone plates forming a lens is another example of binary diffractive optics.

Selene Routley
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  • But I don't understand how to look at these 'pixels' they use in the video. Can I see them as a kind of stairs, where one pixel is located in a higher position than the other? – P.Yntema Sep 20 '16 at 12:39
  • @P.Yntema The staircase is a good visual image for a reflexion mode kinoform: the pistons are little mirrors whose relative axial positions can be changed. For a transmission mode kinoform, the pixels are clear / opaque, or else impose different phase delays, again through a "staircase" kind of picture. – Selene Routley Sep 20 '16 at 22:34
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A surface relief hologram, like the ones you find on credit cards, has a rippled surface with ridges and troughs spaced 0.5 microns to a few microns apart. In order to produce a 3D holographic image, a piston display "only" needs to duplicate those ridges and troughs. Actually, it's not trivial to design a binary "high/low" surface that does a good job of producing the same image as a laser-recorded hologram whose surface height varies continuously. If you would like to understand this in depth, you can start by developing an understanding of Huygens' Principle.

Edit 12/11/18: The type of hologram you're asking about is a "binary hologram". There are many, many explanations of binary holograms online, but this is a good one. The basic idea is this: diffraction angles are controlled by the spacing and orientation of grating lines. When a grating is "clipped" to make a binary grating, the spacing and orientation are not changed. Effectively, higher spatial frequency components are added to the grating by doing the clipping. Those higher-frequency components diffract light through larger angles than the original grating, so diffracted light due to the high frequency components is not visible at the angle of the image produced by the unclipped grating.

S. McGrew
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I am not sure, but taking in account how holography works https://en.wikipedia.org/wiki/Holography what you are doing when two consecutive pixels go up and down, you procuce either constructive or destructive interfierence(he says that distance they move is quater of the wavlength, this produces this effect). A lot of this pair of pixels togehtet and then you can produce a reflective surface that can produce a hologram. Usually holography works with high resolution films wear each pixel is reflective or not, same as in the video. Holography is very cool

Victor
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