Of course, not all onstage holograms are posthumous ethical conundrums. The technology has been used to simulcast performances by Janelle Monae and MIA, to throw the Gorillaz avatars on stage with Madonna, and to bring fictional stars, like Hatsune Miku, to life.

Sorry, They Aren’t Holograms

Let’s clear the air real quick. There’s a lot of debate on what is or isn’t a hologram. So, for argument’s sake, we’re going to stick with a very simple definition for the word hologram.

Holograms are freestanding 3D light structures. They aren’t projected onto a surface (that would make them 2D), but they can be diffused by glass, sci-fi moon crystals, or whatever object gets the job done.

So, Princess Leia’s secret message in Star Wars? That’s a hologram. The ghost of Michael Jackson? That isn’t a hologram—it’s projected on a flat surface and exists in 2D (but we’re still going to refer to these as holograms to keep things simple).

Either way, these holographic concerts are a step in the right direction. But they aren’t exactly a new idea. The holographic performances by Tupac, Janelle Monae, MIA, and others are based on an 1860s parlor trick called Pepper’s Ghost. It’s a simple trick that was used extensively at Victorian fairs, plays, and parties. You’ve seen it in action at Disney’s Haunted Mansion if you’ve ever been to Disneyland.

The Pepper’s Ghost trick is literal smoke and mirrors (well, minus the smoke). A reflective pane of glass is set on a stage and angled down toward a hidden booth. When the hidden booth is illuminated, it reflects an image onto the pane of glass, which then reflects the image toward the audience. At eye level, this image would look squished (remember, the glass is angled). But because the audience looks up at the stage, the image looks “correct,” with a ghostly, translucent quality.

Of course, your garden-variety Pepper’s Ghost trick requires an actor. Last time we checked, Michael Jackson was dead, so we can assume the technology has changed a bit, right?

Musion Eyeliner Projections

Musion Eyeliner sounds like a crappy local band, but it’s actually a patented, modernized version of the Pepper’s Ghost trick. And, in a way, it’s even simpler than Pepper’s Ghost.

Rather than relying on secret rooms, actors, and glass to project humans onto a stage, the Musion Eyeliner trick simply requires a projector and a thin mylar sheet.

First, the mylar sheet is placed at the front of a stage at a 45-degree angle. Then, a projector in front of the stage shoots an image at the mylar sheet.

And that’s all there is to it—kind of. There also needs to be a source video for these projections. Ideally, the source video is completely still, creating the illusion that a performer is on the stage. This can be done by recording a performance with a still camera, or by creating an expensive 3D model and then rigging it to sing and dance (the Tupac, Jackson, and Roy Orbison holograms are 3D models).

Problems with Tech

Aside from obvious ethical dilemmas, Musion Eyeliner has a lot of technological shortcomings and vulnerabilities:

Phase Issues: The most elaborate Musion Eyeliner holograms use multiple projectors to make an image as wide and detailed as possible. But these projectors need to work perfectly with one another. If one falls out of phase, it ruins the image. Wavy Screens: Musion Eyeliner holograms rely on a thin mylar screen, which “waves” like a flag when hit by a good gust of wind. This is very easy to observe in the Michael Jackson hologram video, where the entire stage looks like it’s underwater. Viewing Angle: Again, the audience’s viewing angle determines whether a Musion Eyeliner hologram looks “correct” or “squished. ” When viewed from the side, these projections can look flat, like paper. Illumination: Musion Eyeliner projections work best in dark or dim environments. The problem is, they always create bright images, which isn’t a big deal on its own. However, holograms in dark environments can look ridiculously bright and flat—especially when real people wander on stage (as shown in the Tupac performance). Cost: It doesn’t cost much to set up a Musion Eyeliner hologram. But re-creating famous people in 3D costs a ton of money (the Tupac 3D model cost about $400 k). Even with a sold-out auditorium, it’s hard to recoup that kind of expense.

You probably shouldn’t pass judgment on Musion Eyeliner holograms for their technical shortcomings. But the fact that wind can ruin these projections is a sign of just how young this technology is.

The Future of Holograms

Right now, most of your favorite electronics corporations are spending oodles of money on augmented reality. From Instagram filters and Pokemon Go to creepy undead musicians, we’re inching closer and closer to the inevitable: genuine 3D holograms.

It’s hard to know when genuine holograms will become common, but they may be used for entertainment over the next few decades. We already know there’s a market for hologram concerts. The BBC is also currently researching hologram TVs (which are, essentially, small-scale, 3D versions of the Pepper’s Ghost trick).

At the moment, we’re just waiting for the technology to mature a bit. When that happens is anyone’s guess. In the meantime, we’ll just have to live with (and get used to) creepy posthumous concerts and Hatsune Miku.

Sources: Christie Digital