Overview of "holographic" displays
TL:TR Most are Illusion based and have little to do with how Sci-Fi envisioned them except Lases induced plasma emission
Unlike static holograms whose production has been perfected for over 50 years. Ever since the Hungarian physicist Dennis Gabor made the first hologram in 1947 and was awarded the Nobel Prize in Physics in 1971 the world was amazed by the technology and ideas of how to make a movable version became abound. But it’s not that easy.
It has long been the dream of any SciFi fan to look upon a true holographic screen. From Princess Leia in Star Wars to William Morgan Sheppard as The Professor in SeaQuest, and culminating in Star Trek the Next Generation with full-blown holodecks with interactive matter made using a transporter-based replication.
Regardless of where you saw it, it has become engraved into modern culture and exuberant amounts of money are being spent on achieving it. Mostly without success. There are ways of seeing 3D images in reality with Augmented Reality (AR) glasses but even they are severely limited in their Field of View and produce an image that mostly looks like a large TV screen some distance from you. But that is not what Hologram is, its definition according to Wikipedia is:
A holographic display is a type of display that utilizes light diffraction to create a virtual three-dimensional image. Holographic displays are distinguished from other forms of 3D displays in that they do not require the aid of any special glasses or external equipment for a viewer to see the image.
So, no glasses, no aids just eyeballs.
First off, why is it so difficult to create a holographic display? Well, it comes down to how our vision works. Our eyes are passive sensors, they receive light, not emit it. For an object to be visible light needs to hit that object and bounce and enter our eyes, only then is that object visible to us.
Air is mostly transparent to light, and due to the nature of light the interaction of photons and atoms is probabilistic and highly dependent on the density of the medium it travels through or hits. With air being of low density, the chances of photons hitting the atom and bouncing are minuscule at best. Also, light has a duality about it, depending on how we approach it can behave as a light or a wave.
One of the earliest and easiest ways to stop a light is to put something in front of it, duh.
Mist
First, let’s call it the first practical, way to create a holographic display was the mist table which as the name suggests projects an image onto a mist.
This hologram is more of an illusion as it’s still a 2D image with a 3D illusion achieved by the motion of the object projected. This approach can be significantly scaled up with water walls becoming an attraction.
There are two different approaches to this effect. One is bottom-up with a water curtain like the one firefighters use. The other is top-down by just creating a slightly downward diffused water jet that arcs above the projection area. The top-down produces better results as it’s more diffuse and has no high contrast streaks that the water wall makes.
Lenticular
Lenticular displays use lenticular lenses to split interlaced images or videos for the left and right eyes. I love this technology as it is simple but really effective if used correctly. It is part of light field technology that has the ability to recreate the path of light or encode different images depending on the use. Unlike full light fields, lenticular lenses are horizontal or vertically aligned.
I have a selection of lenticular lenses and tried to recreate the effect using Photoshop to create two different images on LCD when viewed from different locations. It worked.
Mirrors
Mirrors are original instruments of illusions and as such have found many applications in creating illusions of holograms, and can be easily DIYed. All you need is a plane of glass, transparent foil, or anything that is transparent and reflective in addition to an LCD screen like a TV, Computer LCD, Tablet, or a simple mobile phone screen.
Smoke and Mirrors is not just an expression, it has historical roots with the first recorded use in 1770 and came back to the public mind with TuPack hologram in 2012, at the Coachella Music Festival.
This effect can be easily made by anyone. I used thin plexiglass or overhead transparencies cut into triangles and connected with sticky tape to form a pyramid.
Or even simpler, just use the top of a clear plastic bottle.
I made one example using Blender
But there are many that can be found on Youtube.
Transparent LCD
Volumetric
Now, this one is very interesting. It uses an LCD display that oscillates really fast up and down creating an illusion of a floating image just don’t put your finger on it as it could really hurt you, that’s why it’s behind the glass.
The idea here is very similar to the rotating blade holograms that have started to appear all over the place, having also the same limitation regarding your fingers. Do not touch.
Lightfield
Lightfield technology is one that is the closest to reproducing a true holographic display. I did a whole post about them
The leader in this field is The Looking Glass Factory which already has retail versions of 4k displays and based on some reviews they are incredible. It uses a “similar” idea to lenticular lenses. For more information about them take a look at their patent filing.
The laser induces plasma emission
And I saved the best for last. Here is true hologram technology. An “image” in mid-air without any optical gimmicks.
This works by focusing a laser to a point in mid-air causing atoms in that point to become more energized, and ionized, as that is not a normal state of those atoms they need to get back to a lower energy state, and to do that they need to release energy and if everything is set up correctly does so by emitting a photon in the visible spectrum. The threshold for ionization to produce sufficiently visible light is when the laser intensity is greater than 1014 W/cm2.
The first lasers used to create such ionization were nanosecond laser in 2006 but with the advent of a femtosecond laser, the field of laser-induced plasma displays have found a new life. The first displays had the disadvantage of being slow with only 100 “pixels” or voxels per second and the second disadvantage was they were loud with the creation of plasma accompanied by a sound like a small explosion. The femtosecond laser can create 1000 voxels per second accompanied by more high-pitched sound but is hampered by the maximal power of such laser to reach larger distances as current power ranges need an objective lens for the fine focus of the laser beam to produce sufficient PW/cm2 to induce plasma making them suitable for only small displays like 1cm3. The larger aperture of the objective lens would permit a larger angle range resulting in a large display area.
The issue with these kinds of displays is that the energy needed to create plasma out in mid-air is kinda dangerous to touch. With the femtosecond laser, the pulses don’t last long but it merited a whole section on skin damage in the research paper. It’s a promising technology but it still has a long way to go.
There is a modification of this approach where laser induces microbubbles in a liquid medium as it needs less energy but is not that impressive and suffers from visible light paths.