Tupac’s glowing image appeared on stage, rapping, dancing and interacting with Snoop Dogg and Dr Dre. Many media outlets, including the ABC, described Tupac’s image as a “hologram”, appearing in “vividly life-like” 3D. The show even generated a slew of Tupac-hologram memes, as evidenced below.
But was Tupac really a hologram? You might think so.
In a video post subtitled “Holographic Keynote”, CEO Nick Smith said their technology creates a “holographic-looking image”. He went on to say: “While it’s not 3D and it’s not holographic, it gives you … an illusion of that”.
Tupac, in fact, appeared courtesy of a very old stage-craft technique known as “Pepper’s Ghost”. A thin, transparent plastic sheet, ten metres across and four metres high, was lowered across the stage, slanting from the stage up towards the audience.
While Snoop Dogg and Dr Dre rapped behind the mylar film, Tupac was projected on it using high-definition video projectors reflecting off mirrors below the stage.
By carefully avoiding any stage lights glinting on the plastic, technicians kept the audience unaware they were looking at the (living) performers through the screen.
Most of the light from the projector passed through the screen, but a few percent reflected from the front and back surface of the film; the same partial reflection that lets you see yourself in a shop window. Because the projectors were very bright, and the spotlights on Snoop and Dr Dre well-controlled, everyone appeared with the same brightness, adding to the realism of the illusion.
But Tupac had as much depth as any other 2D projection – none – and the illusion only worked because the audience was too far back to see this.
The super-bright projector and mylar make this seem a minor technological marvel, but the same effect can be achieved with glass, lamplight and black paint.
Pepper’s ghost has been a popular theatre trick since the 1860s. In those days, an image reflected from a hidden, illuminated room appeared to be on stage, with the audience unaware they were essentially looking though a window.
This “ghost” was at least genuinely three-dimensional, with the audience viewing a reflection of a live actor, who appeared three-dimensional just as your reflection in a mirror appears three-dimensional.
UK company Musion Eyeliner holds a patent on the use of video projectors to make Pepper’s Ghosts, but the basic idea was outlined in the Renaissance-era science book, Magia Naturalis by Giambattista della Porta, in a chapter called “How we may see in a Chamber things that are not”.
Published in 1584, della Porta’s prior art predates the Musion patent by some 415 years.
A hologram, by contrast, is formed on a piece of photographic film or an electronic detector, but it does not record an image. A hologram instead records the full wavefield of light falling upon it.
You can’t see anything by looking at a “transmission hologram” – it appears like a piece of featureless grey film and even under a powerful microscope, is an incomprehensible pattern of tiny lines.
The magic happens when the hologram is illuminated by a laser beam. Whatever was hologrammed reappears in real 3D, floating in space. A hologram shows much, much more than even the latest 3D television.
A 3D TV gives the illusion of depth, but lacks “parallax” – an apparent difference in an object’s position when seen from alternate viewpoints.
If one of the Na'vi from James Cameron’s film Avatar is facing you on TV, looking at your 3D TV side-on won’t let you look in its pointy ears, and lying on the floor in front of the TV won’t give you a view up its blue nose.
Everyone sees the same thing, no matter which angle they look from. In a hologram, as in real life, what you see depends on where you look from.
Most people have never seen one of these “transmission” holograms because a bright laser is needed to view them. The 3D views produced are much more convincing than “reflection” holograms: the rainbow-coloured artworks in some museums, or the limited-depth hologram on your credit card.
Real transmission holograms aren’t hard to produce – my third-year physics students make them in an afternoon – but they are resolutely stuck in the technology of the mid-20th century.
Holograms are perhaps the last piece of advanced technology that works best on film; they can be made only on extremely fine-grained black-and-white film but, surprisingly, can store full colour information.
Digital camera pixels are ten times too large to record holograms: holo-cameras need a resolution of gigapixels. Even the iPad’s new “high-resolution” display is far too coarse to reconstruct a hologram. As a result, holography remains stuck in the pre-digital doldrums.
It needs another ten years to become a mainstream technology. MIT’s Media Lab has a holographic TV prototype with true parallax and depth (and, of course, no clunky glasses). But it is closer to TV circa 1930 than it is to R2D2’s Princess Leia, delivering coarse, jerky images in laser monochrome.
Nevertheless, I have little doubt we will get there. The computing power required for live holo-video is still formidable, but a decade ago it was simply inconceivable.
The real impact of ubiquitous digital holography will be on broadband networks. A single holographic videocall on a one-square-metre portal would require a raw data rate of about 200 terabits per second – two million times the maximum speed the NBN will provide.
And yet Malcolm Turnbull decries the NBN for providing “gigabit fixed line speeds for which [we] can’t yet envisage a use”.
Physicists envisaged holographic video almost immediately after the pioneers of modern holography, Emmett Leith and Juris Upatnieks, made their first 3D holograms, in 1964. There’s little doubt that as bandwidth expands, technologies such as holography will be there to make great use of it.