The Truth About Holography — Part 1
Episode 1: These Aren’t The Holograms You’re Looking For
“Holography” is a word that is misused so much these days that we felt compelled to write a four-part blog about it. With a growing number of XR companies all claiming to have developed some form of “holographic display”, it begs the simple question: what is true 3D holography? Which of these new products can actually be defined as such? (spoiler: not many!).
In this series we will start by explaining the true meaning of holography, exploring how the misuse of the term came about and how true holograms are actually created. So stick around, grab some coffee and feel empowered by the truth.
In this first episode, we want to answer the simple yet complicated question: what is ‘holography’? What first comes to mind when you think about the term? Is it scenes from sci-fi films such as Star Wars, Iron Man or Minority Report? These films have all used CGI to create 3D portrayals of holography. But if you’re thinking of modern day tech companies that have advertised “3D holographic display” in their technology, then it is most likely this statement has been used very loosely. Very very loosely.
Looking at the etymology of the word ‘holography’, it originates from the Greek words holos (whole) and graphē (drawing/writing), which refers to the complete representation of a 3D object. In academic terms, a hologram is the calculated interference pattern designed to accurately recreate a desired 3D object/scene. Let me explain.
We can explain the concept of hologram generation by considering the fact that light rays are modelled as a wave. By using diffraction amongst a group of waves to trigger wave interaction (and thus interference), we can create an interference pattern in space (imagine light waves colliding with each other and merging to form certain characteristics at points in space). We can record a section of this pattern, either using optics or performing a direct calculation, to produce a hologram.
The image above represents the basic creation of an interference pattern. Light is emitted from a light source through slits (which cause diffraction). Using multiple slits at different locations causes diffraction waves that can collide and interact with one another. Certain light characteristics form during collision that depict aspects of a virtual object/scene, such as colour. Through controlling the diffracted wave pattern, it is possible to create a virtual 3D scene on a 2D “viewing screen” in space. This is the holographic image. In the past, producing commercial holographic displays has not been feasible due to computational inefficiencies and the difficulty of calculating these diffracted wave patterns at adequate speeds.
If done correctly, the resulting hologram should contain all the visual depth cues expected of a 3D object/scene. What are depth cues? Well, they are defined as visual indicators used by the human eyes that help perceive the world in three dimensions. There are two key types of depth cues: psychological and physiological. Whilst psychological depth cues trick your eyes in perceiving depth, physiological depth cues utilise the physical movements of the eye to perceive depth naturally.
In the image above, the painting is on a flat canvas yet you perceive it as 3D. Why? It is due to psychological depth cues. This type of depth cue is responsible for most of the “3D technologies” you see on the market today. It is simply a visual trick.
To make a true 3D hologram, you need the reconstructed image (or holographic projection) to have depth, which requires use of all physiological depth cues, including parallax and interposition. This is what helps satisfy the natural depth perception of humans and prevents problems like nausea, headaches or eye-strain. Controlling the interference pattern effectively enough will allow the construction of multiple depth layers thus creating a true 3D hologram. We will go into the key importance of depth information in the next episode.
This concludes the first episode on The Truth About Holography and we hope you’ve enjoyed this introduction. If so, stick around because there is plenty more to be uncovered. Controlling the interference pattern is not at all as easy as it sounds!
