Please, stop calling those virtual images on stage holograms.

From the 2015 Spring Festival Gala to the recent G20 Gala, almost every large-scale performance gala in recent years has triggered a hype about the integration of holographic technology into stage art.

On the evening of September 4, the welcoming gala of the G20 summit, "The Most Memorable Place is Hangzhou", was held on the West Lake. This gala used virtual images in the stage performance, which attracted the media's enthusiastic praise for the holographic projection technology again. Some reports said that "holographic projection makes you see ghosts."

In the 2015 Spring Festival Gala, Li Yuchun's show "Shu Embroidery" left a deep impression on many people. During the performance, Chun Ge suddenly split into three clones, forming a strange scene of four Li Yuchuns on the same stage, which caused exclamations on social media.

Foreign netizens also had the same reaction. In 2012, at the concert of rap singer Dr. Dre and Snoop Dogg, the late American hip-hop star Tupac returned to the stage in the form of a virtual image. Tupac walked back and forth on the stage with his upper body naked and even greeted the audience. Everything looked very real, and the audience thought they had seen a ghost. This triggered a heated discussion among hip-hop fans on Twitter, and a funny account @HologramTupac was created.

Yes, the stage effects are great, however, they are not holograms.

It's actually a clever optical illusion called Pepper's Ghost, first described by a 16th-century Italian scientist and named after 19th-century Englishman John Pepper, who first used it for stage performance art.

This ancient technique combined with modern high-level computer rendering technology creates realistic images on stage. But it focuses on computer rendering technology rather than the progress of display technology.

"It's amazing, there's no doubt about it," said David Brady, director of Duke University's imaging and spectroscopy program, of the Tupac image. "What's most impressive is that it looks so real, so detailed and so natural, but that's only a result of advances in computer rendering, not display technology."

So, how does Pepper's Ghost work?

As shown in the picture above, the audience is looking at the virtual image on the stage. What they don't see is a transparent glass hidden on the stage and the real performance under the stage. The light shines on the real performer and is projected onto a specific area on the stage through the glass. Because the early lights were not very strong, the image the audience saw was low in brightness, like a ghost.

Modern Pepper's Ghost shows can be brought to life thanks to high-luminance lighting and excellent CG technology, but the principles of the display are still very similar to the original show.

For Tupac's performance, the production company behind it, AV Concepts, did not use glass, but a special Mylar film, also known as "holographic film". Many holographic performances in China are the same. Wang Zhigang EGO, a multimedia designer at Yuguo Animation, wrote in an answer on Zhihu,

The holographic technologies commonly used in business can be roughly divided into two categories: one is the rear projection method, where the projector directly projects the image onto the holographic film, and the other is the use of reflection, where the projected or LED image is reflected onto the holographic film placed at a 45-degree angle.

The picture above is a holographic projection stage made by Yuguo. They used the reflection principle, and the LED was on the ground, at a 45-degree angle to the holographic film.

CCTV has previously reported on how Li Yuchun's performance at the Spring Festival Gala was achieved. It was also a similar method. A holographic film at a 45-degree angle was placed on the stage, and the image source on the ground was refracted upwards.

The two holographic technologies mentioned by Wang Zhigang, the first one should actually be called projection, and the latter one is Pepper's ghost.

At present, most of the so-called holographic technology used in stage performances we see are Pepper's Ghost or projection, and should not be called holography. Holography can be said to be black technology, but Pepper's Ghost is not difficult to achieve. You can even DIY one yourself. For example, this Canadian used Pepper's Ghost to build a Halloween haunted house in 2007.

| If these are not holograms, then what are true holograms?

When writing about the virtual image performances at the Spring Festival Gala or the G20, many articles directly quote the explanation of holographic technology from Wikipedia or Baidu Encyclopedia in order to popularize science to readers. These explanations are actually the definition of true holographic technology, but they mistakenly quoted them in the article. In fact, they should have quoted the explanation of the term "Pepper's Ghost".

Holography was invented by British Hungarian physicist Dennis Gabor in 1947. Chinese Wikipedia defines holography as follows:

Holography, also known as holographic projection and holographic 3D, is a photographic technology that records all the information (amplitude, phase) in the light waves reflected (or transmitted) by the photographed object, and the light reflected or transmitted by the object can be completely reconstructed through the recording film, as if the object is there. By observing the photo from different directions and angles, different angles of the photographed object can be seen, so the recorded image can give people a three-dimensional vision.

Since the effect achieved by holography is similar to the visual effect achieved by Pepper's Ghost, many people confuse the two concepts. Not only do viewers call it holography, but the media also follow suit and report it without thinking. In fact, they are different technologies.

Many popular science articles refer to visual effects such as Pepper's Ghost as pseudo-holography. Regarding the difference between pseudo-holography and true holography, Zhihu user Za sar has a good explanation in plain language:

True holography means that only a flat surface on the ground can display images in the air above. Changing the viewing angle does not affect the clarity, and the human body can walk through the screen. True holography is only a concept now.

Pseudo-holography is a holographic visual effect created by refracting light sources with optical materials tilted at various angles. It can only be viewed at a designed angle, and if you try to see through it, you will hit your head. Pseudo-holography can now be achieved through homemade equipment using mobile phones and special videos.

So, next time you see a stage performance like this, please don't call it holographic easily. Even if you use this word, you should know that they are different. At least consider John Pepper's feelings in the afterlife.

This article about Tupac comes from Ars Technica

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