07 Mar What Holography is NOT
While preparing for my TEDx talk in 2014, I decided to google several terms that I knew from academic publications and books: “Holography”, “Holographic Display”, “3D Holography”. I was surprised to see that on several pages of results, there were only two or three links actually related to the science of holography that I was researching.
Over the years, the term “Holography” lost its original meaning and has been used in different contexts.
This is the first of a series of posts that aims to clarify this and explain what the “academic” kind of holography actually is. Before we can even start explaining the science, we first need to take a step back and look at what holography is thought to be. The three posts in the series are:
- Part 1 – What holography is NOT – visiting various technologies that are famously called “Holographic”, and analysing how they work
- Part 2 – The Inception of Holography – to fully appreciate holography, it’s worth looking at its invention in 1947 and witnessing how that single idea gave rise to plenty of applications. Display is only one of them
- Part 3 – Computer-Generated Holography – the missing piece that shows us how we can use wave nature of light to create true 3D moving images!
Ready? Let’s get to it!
When you think about “Holograms”, what is the first thing that comes to your mind? Chances are, it’s Princess Leia begging Obi Wan Kenobi for help, Tupac coming back to life on stage, or witnessing holograms in front of your eyes using Microsoft HoloLens.
In reality, those are not “holograms” in the scientific sense. Instead, they are either clever optical illusions or old technologies under new names. Let’s tackle them one by one.
Princess Leia Hologram
By now, Princess Leia Hologram has become a symbol of what a next-gen technology should look like.
The vision is spectacular. Yet absolutely unphysical
A fundamental point that needs to be made here is that images don’t just pop up into existence in free space. In general, you need to be looking at something (like an array of light sources) or through something (like a piece of glass that delivers the image to your eye). The image can be in front or behind the surface of the actual display, but an optical element of some sort has to be there.
Now, even if there was a way to somehow focus the image into the shape of Princess Leia, look what happens to those rays as they go further out from the projector? They vanish into nothingness.
That may not feel so wrong to you (you say, “it’s sci-fi, anything is possible”). So think about rays of morning sun being cast on the fog. The rays cannot “stop”, there is no physical mechanism to make it happen.
The science fiction dream is as impossible today as it was when “Star Wars – A New Hope” was released. And back then, the frames were actually drawn by hand.
There are a number of holographic performances where a person is brought back to life and dances “amongst the living” The performance is a real-life Photoshop – it overlays the real signers/actors with images created digitally.
It gained popularity and fame quite recently (and by recently I mean 2010s, realising that I’m getting old …). The concept has been around for a while.
The trick was developed in the 19th Century and is accredited to John Henry Pepper. It uses a sheet of plastic that has a dual purpose of transmitting as well as reflecting light.
As a photographer, I have always been obsessed with reflections. To explain Pepper’s Ghost effect a little better, let me use a very old shot that I took whilst at a boarding school in Torun, below:
It may be confusing at first, but take a closer look. The camera on the tripod is looking outside of the window. But the window, made of some unsophisticated glass, reflects as well as transmits. As a result, you’re looking “out” as well as “in” (via the reflection) and see two images on the top of each other. One is the street, tree and brick-walled building covered in graffiti, the second is the camera and the bit of furniture with a precious storage of milk and food conserves.
Well, Musion Eyeliner (the stage used to show Tupac Hologram) is essentially a far more expensive version of this, using the same trick with a sheet of plastic.
Inherently, the principle is the same as a “Holographic Prism” that you can buy cheaply from eBay and stick on your phone.
Like most scientists, I enjoy categorising things – it brings me inner peace to be able to say “Element X belongs to set Y”. Well, in this case, HoloLens is utterly puzzling.
On one hand, it is a spectacular piece of technology. Microsoft managed to construct a fully-functional computer which “understands” and can orient itself in the space around it – and it fits on your head. It sets a high bar for what Augmented Reality (AR) should be and gives people a taste of what the technology will be capable of in the future.
Let me explain in a bit more detail what HoloLens is and what it does. It is essentially a personal computer, which replaced a mouse/keyboard by a combination of hand gestures plus head movements. Instead of using the monitor, a complicated optical arrangement feeds the image directly to the user’s eye. It is capable of mapping the geometry of the environment using an array of sensors and then places digital objects in that environment.
HoloLens Display mechanism
In short, its light engine produces an image on a very small screen, which then gets magnified and fed into your visual field. It presents two images to both eyes and uses a very clever optical arrangement to deliver it to your eyes. While the image delivery is amazingly advanced, the underlying concept, known as stereoscopy is an old technique. Some of you may have even seen it in action, if you ever used 3D TV, saw a 3D movie in the cinema, or played Nintendo 3Ds.
While it works to a degree, what is missing is the depth cue called accommodation (the ability of the eye to focus at different distances).
HoloLens within the bigger picture of Augmented Reality
Given how many companies tried their luck in AR and went under (DAQRI, ODG, Meta…), HoloLens is doing quite well commercially. As Daniel Wagner (ex-CTO of DAQRI) and his colleagues tell us, Augmented Reality is Hard. In the light of this, HoloLens continuously manages to keep the dream alive.
On the other hand, Microsoft’s marketing has positioned HoloLens very much towards the mainstream benchmark of Princess Leia & Tupac. As a scientist working in both holography and augmented reality, I am disappointed to see it happening. The reason why I’m hesitant about definitely categorising HoloLens as “non-holographic” (in line with an academic understanding of holography) is because of its waveguide combiner.
A combiner’s main purpose, as the name suggests, is to overlay the image generated digitally with the real world. It sounds trivial, but the technology is not at all simple. As it happens, HoloLens’ combiner is very clever: it uses diffractive optics that extends the “viewing zone” of the image. Details can be found in the SID publication by Bernard Kress and William Cummings or Bernard’s book.
Final verdict on HoloLens?
HoloLens is clever, and great. Some would argue that it’s holographic based on its combiner technology. But it achieves the effect of 3D by presenting two flat images to your eyes in a stereoscopic (non-holographic) fashion.
1. Term “holographic display”, although originated from academic labs backed up by a great deal of science, has lost its original meaning
The term “Holographic Display” proliferated into pop-culture thanks to Star Wars. It was a blessing and a curse. The general public automatically became very excited by this new vision for display. And of course people began fantasising about how such a display will work and what it can offer (once it finally becomes available).
“Holography” became a buzzword that companies use to add a “cool-factor” to their display technologies. The misuse of the word has often caused a great deal of suffering to those who devoted a portion of their lives to developing the science of holography. Examples can be found all over the internet, for instance here, or here.
While the term “Hologram” has been used to mean multiple things, “Holographic Display” using Computer-Generated Holography was developing slowly within academic labs.
2. Holography is not synonymous to 3D
It’s often the case that displays which look somewhat 3D are called “Holographic”. Yet, those words, in their original meaning, are not synonyms. It is true that a holographic display method can produce a 3D image, but it is not the only route available. There are plenty of non-holographic technologies out there that can show some form of 3D, I don’t even need to look that far to give you a taste of them:
- Vari- and Multi-focal Near-eye displays
Scientists realized how much of a problem Vergence-Accommodation Conflict poses and current approaches to address it include Varifocal and Multifocal displays. Varifical means that the image changes its depth either by following the viewer’s gaze or it sweeps through focus quickly enough for our eyes not to notice the motion. Multi-focal displays present multiple focal planes simultaneously. We’re approaching a fairly lengthy topic which can become a book of its own. If you’re interested, I recommend this talk from Electronic Imaging by Douglas Lanman.
- Volumetric displays
Volumetric displays generate a solid image strictly enclosed within a volume. A way to do this is to utilise a relatively slow reaction time of the eye and sweep elements of the image fast enough so that the display element becomes practically invisible, but the image appears solid. Like this display from Voxon Photonics.
- Light Field Displays
Light Field Displays can be thought of as flat displays that can encode directional information of light by using additional optical components (like microlenses). Depending on the angle of viewing, a user is capable of seeing a different view of the same object. Examples include Looking Glass Factory, or Fovi3D
- Laser-induced plasma display
This is probably my favourite type of display from the category of “borderline insane, don’t do it at home”. Imagine focusing your light down to a spot so dense that it heats up the air and ignites it to form plasma. Well, it has been done. I have some doubts how practical this can be, but the effect is truly impressive (*1).
3. Holographic Display that utilises Computer-Generated Holography is coming soon
To finish off, Computer-Generated Holography is currently undergoing a great deal of development and we’re finally approaching an era where actual Holographic Displays of The Future will soon be commonplace. Stay tuned to my next posts to learn more!
(*1) It should be mentioned that Laser-induced plasma display actually uses computer-generated holography to shape the laser beam. But the actual image formed is more volumetric-like rather than holographic-like.