1. Tupac is not a hologramIf a 2D picture is worth a thousand words, then a 3D image is worth a million. With holography, it is possible to reconstruct 3D images using holograms, and the process is unlike anything found in traditional display technology. Even though it was invented over 70 years ago, holography remains the best candidate for achieving true 3D displays. Here we present six things you may not know about the strange and wonderful world of holography.
When you see Tupac, Michael Jackson, or anyone for that matter projected in the way you see at concerts or similar, they are not holograms. It’s a trick, and the fundamental hologram has no relation to this whatsoever. The trick that is being used was invented in the 1800s by John Pepper to wow unsuspecting audience members that a ghost had appeared next to the actors on stage (pictured above). In reality, a clever illusion was employed, whereby a piece of glass at an angle placed between the audience. The stage was used to reflect the light from an actor below the stage toward the audience, but still allowing them to see through to the stage ahead. Because the glass is effectively transparent, we think there is a ghost hovering on stage. As such, most “holograms” seen on TV are some variant of this Pepper’s ghost trick.
2. Only a hologram is hologram: it is remarkably different to anything else
Consider you’ve just taken a photo of a scene. You’ve taken your camera, pointed, clicked, and captured some information. From an optics point of view, you’ve stored some time-averaged amplitude of the light-field emanating from that scene using some form of sensor (in separate RGB channels). As a result, a vast amount of information within that light field has just been thrown away. Collecting just this information is effectively capturing a tiny percentage of what’s there. A hologram (invented in 1947 by Dennis Gabor, holography (from the Greek meaning “whole-drawing”) in its most basic sense, is the recording then reconstruction of all the light-field information such that when viewed, the observer is unable to tell the difference from the original scene because the hologram is ‘giving’ the observer all of the original information.
For a really amazing trip into this wonderful world, we recommend the following video:
Now you would naturally ask: how can we do this? Well, if you take the object you want to display, illuminate it with a laser, and interfere this scattered light with another laser (see Figure), a recording of this pattern created is the hologram [2]. It is capturing the amplitude, phase, and wavelength information of the object. Now if we looked at this pattern under the microscope, we would just see these interference fringes, which is uninteresting. However, if we illuminate with the same source, the light is scattered from all fringes simultaneously and interferes with itself to reconstruct the original object’s light-field.
The beauty of this technique is that it is still the only way to truly reconstruct 3D information and achieve real 3D displays. Yet, this technique was originally done nearly 70 years ago to form static holograms. But why can’t we just dynamically change the holograms and effectively create a holographic display? This is discussed in the next section.
3. 3D Holographic displays in your home are still decades off 
The problem with creating 3D holographic displays is that the amount of information a typical hologram contains is vast; light contains a lot of information! As an example, it is thought that the order of a million-trillion pixels are required in order to achieve a pure 3D holographic display [1,3], and with a typical refresh rate of, say, 30 fps, this is a staggering amount of data. Not only this, we also need technology that can record (in real-time) all of the complex information of the light field, communications technology capable of transmitting this huge amount of data, and then a computer in order to process this data. Considering we are just about entering the 4K TV era (which is a screen made of approximately 10 million pixels), we are some way off.
4. A hologram can be generated, and displayed, with computers
As discussed, we are dealing with a lot of information. Current state-of-the-art methods of displaying dynamic holograms are called spatial light modulators (SLMs). They are essentially small, television-like display devices in which holograms are shown on them, laser light is shone though or reflected off, and the pattern is formed on the other side.
Now, how do we calculate a hologram? Ideally, we could record all the information of the light field of a scene, yet we have no commercial technology to do this. We could do full electromagnetic wave simulations of a simulated scene to discover what the light-field scattered from an object looks like at all points in space, and then record this information to form a hologram. However this is computationally a nightmare with current technology. A seemingly better way (until full wave simulations can done quickly) is we can be clever about things and look deeper into the fundamental math behind the phenomena.
Essentially, we make an approximation. It turns out, when light diffracts, if you are far enough away from the point of diffraction, the pattern you see is related to the Fourier transform of the mathematical representation of the diffraction object. What this means is that, because our computers can currently perform FFTs quickly, we can rapidly generate computer-generated holograms on the fly. Therefore, by displaying this on an SLM, we can diffract light to form arbitrary images at will. This area is called computer-generated holography. And now that computers are getting faster and more efficient, is becoming a hot area of research.
5. The best attempt at a holographic TV was done a decade ago and cost a fortune
Qinetiq developed a holographic display prototype based on spatial light modulation technology 12 years ago. It used an active tiling system with two different spatial light modulators to provide all the depth cues needed to produce a 3D image. It was expensive to produce and was discontinued shortly after development, yet still is the closest true holographic display to be demonstrated.
6. Holography is not just for your TV
Even though we have discussed the fact that 3D holographic displays are still some way off, holography as a discipline is invaluable and has applications in many areas. Here are just a few examples:
- Electron imaging: By observing the phase shift of electron interference (due to electric and material field) as they pass through thin film materials, it is possible to determine the composition of materials.
- Data storage: Conventional optical discs store information on the surface. However, with holography it is possible to record information throughout the volume of a material and at different angles — hence it’s possible to store orders of magnitude more information that conventional optical data storage techniques.
- Holographic Optical tweezers: Optical tweezers use forces of light in order to move about small particles (mainly for biological applications) and create optical traps. By using computer generated holograms, researchers can manipulate large arrays of particles over small distances.
- Security: holograms have been used on bank notes and credit cards for decades. They are typically used because the technology required in order to produce such structures is fairly advanced.
References
[1] J. Geng, Adv. Opt. Photonics 5, 456 (2013).
[2] B. C. Kress and P. Meyrueis, Applied Digital Optics (Wiley, 2000).
[3] M. Lucente, in SMPTE 2nd Annu. Int. Conf. Stereosc. 3D Media Entertain. – Soc. Motion Pict. Telev. Eng. (2011).
No comments:
Post a Comment