When is a hologram not a hologram? Well, pretty much always in the music world. We’ve heard quite a bit about holography lately. It all began last month when an apparition of rapper Tupac Shakur, who died in 1996, showed up at the Coachella music festival. And then, of course, there was the media frenzy that inevitably followed (“will the Tupac hologram go on tour?” “what other deceased rappers and rock stars will get the holographic treatment?”). The thing is, we never saw a hologram to begin with. A few days after the Coachella performance, the good people at Gizmodo reported that Tupac was in fact a 2-D CGI animation that rapped alongside Snoop Dogg by way of Pepper’s Ghost — a reflection technique named after mid-19th-century optics researcher John Pepper, which has found ample use over the years in magic shows and haunted houses. Unfortunately, this did nothing to stem the full-on “dead celebrity hologram” craze that’s rolled across the US and the UK. Early this month, for example, the BBC asked Queen guitarist Brian May whether frontman Freddie Mercury, who died in 1991, would be resurrected in holographic form. May — a trained physicist who, I’m convinced, lives to give me things to write about: see: We = Rock (You) and Rock ‘n’ Roll Optics — said that the band had in fact developed an effect that was set to appear onstage during an anniversary performance of We Will Rock You, the musical based on Queen’s music. He stressed, though, that this would be “an optical illusion of sorts,” most definitely not a hologram — a distinction that seems to have fallen largely on deaf ears. So, what will it take? Clearly, there’s a demand for life-size holograms of rappers and singers who have slipped the mortal coil — there has been for almost two months now. What do we have to do to make it happen? To try to get a sense, I reached out to a few of the folks who would know. Nasser Peyghambarian is chair of photonics and lasers at the University of Arizona College of Optical Sciences. He and colleagues are developing a holographic imaging technology, called three-dimensional telepresence, that can record a 3-D image in one location and show it in another, where it refreshes every two seconds (See: New Dimension Trends: Photonics Enters New Dimensions). The technology does not require special eyewear or any other auxiliary devices. I asked Peyghambarian whether a true life-size hologram that can writhe on stage while singing about moonlight drives and lizard kings, for example, were even possible. Not only is it possible, he said, it’s already in the pipeline. Using the telepresence technology, he and colleagues currently can produce an image with a maximum size of 1 ft. by 1 ft., but they have plans to go to a size of 6 ft. by 6 ft. — human size. “There is no fundamental ‘show stopper’ that would prevent such a development,” he said. “It requires funds and hard work.” Michael Bove, head of the Object-Based Media Group at the MIT Media Lab in Cambridge, Mass., picked up the story as it relates to bringing deceased rappers and rock stars to life to life. Two pieces are necessary for this to happen, he said: a 3-D model of the performer and a life-size holographic video display. The latter would be especially challenging, he said, as real diffractive displays require roughly 2 million pixels per scan line per meter of screen width. “That’s why all the dynamic holographic video displays you can see these days are either small or have a limited view angle” — the maximum angle at which a display can be viewed and still provide acceptable visual performance — “which relaxes the pixel pitch requirements a lot (but wouldn’t work on stage).” Finally, I touched base with Gregg Favalora, a principal at the engineering consultancy Optics for Hire in Arlington, Mass., and a recent contributor to Photonics Spectra (See: Hitting Every Angle with Autostereoscopic 3-D Displays), and asked what specifically we would need to build a life-size hologram of a Buddy Holly or a Biggie Smalls. He offered a list of requirements, including excellent 3-D cameras, advances in processing, and large arrays of wavelength-scale light modulators, which are used to project the images. The details here would vary depending on the size of the hologram, the required viewing angle of the audience and the image fidelity needed, among other factors. Favalora shares Peyghambarian’s belief that we are already seeing some of the tools needed to build such a hologram. Companies like imec in Leuven, Belgium are developing small modulators for holographic video display that could be used in arrays. At the same time, he said, the Object-Based Media Group headed up by Bove at the MIT Media Lab is developing a promising approach to electronically modulated diffraction that could facilitate holography on the necessary scale. How long, then, before we see a Rat Pack reunion tour, or Robert Johnson’s first-ever show at Madison Square Garden? Our panel of experts estimates that we could get there in anywhere from 5 to 20 years — not too far off, in the grand scheme of things. In the meantime, concert promoters can breathe a sigh of relief knowing there are other, non-classical-holography techniques available. Bove suggests, for example, a “proper autostereoscopic” — true 3-D that doesn’t depend on additional eyewear — Pepper’s Ghost, “unlike the 2-D Tupac.” Also, said Favalora, companies like Holografika and Third Dimension Technologies offer systems where images from a group of projectors are directed toward a selectively diffuse screen to provide a different perspective view across an audience. “At some point, it might be fair to say that displays with a sufficient number of ‘views’ sort of become ‘holography,’ “ he said. “My mind isn’t made up on that yet.”