True 3D visualisation has been theorised about and fantasised over for the past fifty years. Every science-fiction fan, not to mention the scientific community, knows that the best way to make 3D images is using holographic or similar techniques. The term ‘hologram’ is derived from the Greek meaning ‘whole image’. Holograms are able to capture and reproduce both the brightness and direction of light emerging from a 3D object or scene. This is unlike a photograph, which only stores brightness, without a sense of direction, thus yielding a two-dimensional representation.
So, what is holographic 3D? The scientific definition of holography is image creation using the physical principles of diffraction. Most of us are familiar with the notion of reflections off a mirror and the refraction or bending of light in water. Diffraction occurs when light scatters from structures that are the same size or smaller than the light. Diffraction can reproduce both refraction and reflection effects, making it a powerful optical phenomenon.
A brief history
The foundations of holography were laid by Dennis Gabor, a Hungarian scientist working in the UK, in a patent  and a series of papers written between 1948 and 1951 that were aimed at microscopy [2–4]. These introduced the notion of storing 3D information as a diffractive interference pattern that can subsequently be reconstructed through illumination. Holography remained somewhat obscure owing to its dependence on special coherent light.
However, things changed dramatically after the invention of the laser in 1960 , with coherent light, leading to a revival of the field. Lieth and Upatnieks reached a key milestone with the off-axis transmission hologram . At about the same time, Denisyuk pioneered the reflection hologram  viewable using ordinary light rather than lasers. Dennis Gabor was awarded the Physics Nobel Prize in 1971.
Unfortunately, holography has not lived up to the expectations driven by science fiction such as the iconic Princess Leia hologram in the original 1977 Star Wars film . While static analogue holograms were fashionable for a while during the 1980s, their popularity soon faded. It also became clear that dynamic holographic displays, with moving images, were still very far off and beyond the technology of the day. So, holography went “underground” in the 1990s with the research being performed by just a handful of small companies, the military and academic organisations.
Holograms for the Digital Age
Most holograms you see in everyday life are found on credit cards, banknotes, passports or product packaging. These simple holograms are mass-produced using industrial scale printing press-like machines. This is a billion-dollar industry, which is designed for the purposes of security or anti-counterfeiting, rather for imaging as such.
Holoprinters are holographic printing machines developed over the past 15 years by companies such as Zebra Imaging, Geola and Ceres Holographics. These devices can print digital holograms. It’s a dot-matrix printer that uses red green and blue lasers to print tiny sub-millimetre holographic dots into a light-sensitive polymer sheet. Each holopixel is encoded with a unique view of the scene. It takes a couple of hours to produce a page-sized digital hologram in this way. Each holopixel contains around 6M bytes of data and a page-sized hologram requires hundreds of Giga bytes of information to be computed, making it a graphics-intensive process.
Digital hologram technology is constantly improving where state of the art machines have holopixels a quarter of a millimetre wide, making it possible to create photo-realistic digital holograms . As we can control each ray of light emitted from the surface of the hologram, it’s possible to include some limited animation within the hologram so we can view different images from different angles (Figure above). The holograms produced in this manner are high-quality, full-colour reflection holograms, and it is even possible to produce digital holograms that can lie flat, allowing the viewer to walk 360 degrees around the image.
In principle, it is possible to create a digital hologram from any kind of 3D dataset. This could be a medical scan, a simulation, an engineering design or a computer graphics model. For replay (viewing), digital holograms behave just like any other reflection hologram, only requiring a bright lamp for illumination to bring out the 3D image.
Current applications of digital holograms include military terrain visualization , medical imaging (Figure above) , scientific data presentation and architecture. They are used for education and training; often presented at universities, conferences, trade shows, museums and science festivals. Although current holoprinters are somewhat bulky and expensive devices with large and powerful lasers, portable machines are on the horizon. There is every reason to expect desktop-sized holoprinters no larger than a typical laser printer within the next few years.
Holographic Motion Video Displays
Holographic 3D video displays have been a staple of science fiction films from Star Wars to Prometheus and Avatar. However, the engineering realities of building such a display disagree with sci-fi fantasy, which typically violate the laws of physics. This is because engineers have to create a display device with structures smaller than the wavelength of light, around 500 nanometres for green in order to obtain diffraction; and these structures need to be changed in real-time. This is very challenging for display technology in terms of fabrication over a large area, computation of the interference pattern; and the bandwidth to send this pattern to the device. Although the research community has made significant advances over the past decades [11,12], the prospect of a true holographic display remains elusive . The challenge is to build a true dynamic 3D display based on holographic technology that is practical and commercially feasible.
Some of the best-known holographic display research conducted by the academic community is by MIT Media Labs. The Mark1 and 2 displays were made in the early 90s based on diffraction by acoustic waves in an optical material [14,15]. The latest display is based on fabricated acousto-optical devices . Other groups focus on rewriteable holographic materials based on a polymeric material that can be written to with lasers and erased in less than a second by applying an electric field . Similarly, a material based on fast liquid crystals is optically erasable in milliseconds .
The military has been an avid supporter of holographic technology since the 1990s, funding a series of projects including holoprinters as well as holographic displays. A prototype display was developed in the mid 2000s by researchers at Qinetiq, a UK military contractor. It was based on arrays of special micro-displays with tiny pixels together with a computing cluster for computation of the diffraction patterns [19,20]. This outcome was viewed favourably, but the work has since been discontinued. One of the most advanced 3D displays emerging around 2013 is the ZScape display by Zebra Imaging, based on a tiled array of complex electro-optical elements . The 3D display technology was developed for terrain visualization purposes and scenario simulations.
Given the complexities of engineering the ideal holographic display, there are still numerous challenges ahead. One way to overcome them is to apply some physical constraints to the problem, leading to considerable simplifications in the underlying holographic components, data and computation. The SeeReal approach  uses a LCD display as a light-steering mechanism that is combined with eye tracking to present a holographic image within the observer’s field of view. This optimization drastically reduces the required specifications of the holographic display in terms of the resolution, bandwidth and computation. Holoxica’s approach imposes constraints on the image instead, which again simplifies the technical requirements with the advantage of the images being viewable by multiple observers at the expense of image resolution [23,24]. Their volumetric approach is scalable across three generations of displays, where the images are brighter, bigger and better after each iteration, see Figure below. Leia Inc, an HP spin-off, applies constraints to the hardware using multi-directional grating structures with masking . No doubt there will be more technology start-ups to follow such as Holografika,Ostendo and RealView Imaging.
The Way Forward
The good news is that, after decades of research, holographic technology is now poised for a renaissance. The bad news is that this period of wilderness left a void that has been filled not only by sci-fi fantasy but also a great deal of ‘fake holographic 3D’, based on Pepper’s Ghost  and similar optical illusions; otherwise known as “fauxlography”. Any vague illusion of an image floating in space is labelled a hologram, whether it’s celebrities, politicians or a cardboard cut-out in an airport. So, beware of fake holographics – remember if there’s no diffraction then it’s not a hologram!
The time is now ripe for holography to claim its rightful place at the forefront of 3D imaging technology. Digital holograms are now commercially available for everyone to use and enjoy. Holographic video displays are edging ever closer to commercial reality. There is still some way to go and more investment is needed but its likely we’re going to get there sooner rather than later and certainly within the next five years.
Holography is one of those advanced technologies that is truly magical when done properly. The last millennium was all about mastering the electron. This century is all about harnessing the photon and there is no better way to experience this than through holography!
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Javid Khan is entrepreneur, founder and director of Holoxica Ltd, a high-tech company working on holographic 3D visualisation including digital holograms and holographic video displays. Javid has won several national and international awards in business and photonics. Areas of expertise include embedded systems, IC design, displays and photonics. He is an expert assessor for UK/European government R&D programmes. He was formerly a Scientific Officer at the European Commission defining research programmes and running international technology projects. Javid holds a doctorate in Photonics Engineering on Holographic 3D displays, with MEng and MSc degrees in Microelectronics and Telecoms Engineering.