Survey of Alternative Displays
(This is the online version of this article — if you prefer, a formatted PDF is available on my website here for easier reading offline
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An artist has a large range of ways they can display their work. Cave walls gave way to canvas and paper as ways to create portals into another human’s imagination. Stained glass windows were early versions of combining light and imagery. Electronic displays are our next continuation of this same concept. A photon is emitted; it travels until it reflects off of or passes through a medium. That photon then passes into your eyeball and excites some specialized cells — when enough of these cells are excited, your brain turns these into what you perceive as an image.
However, standard computer monitors, LED video walls and projection screens offer only a small glimpse of the range of possible visual illusions. Any traditional display can be augmented or used in an unusual way. New displays and technologies are still being actively developed and researched. Some content is suited precisely to being shown on a standard display, like a webpage. Other content is better suited to a space that exists beyond the screen’s surface and enables a sort of suspension of disbelief that this thing is really there. We continue to find new ways to construct the image of new destinations within the eye.
Knowing the range and limits of these different displays is similar to a painter really understanding their choice of paint and surfaces. Spray paint behaves very differently than oil, watercolor or ink. Drying times, color depth, texture, reflectivity, ability to blend colors — these are just some of the characteristics the painter must consider when choosing a medium for their new work. The textures of canvas, concrete, metal also impart a particular surface aesthetic. The same considerations can be a part of a digital artist’s practice when they work with displays.
Additionally, musicians use what is called extended technique to explore the absolute limits of what sounds are possible with their instrument. Mastering an instrument with classical training is one dimension. Extended techniques demonstrate a deep understanding of how these devices function and respond to human input. Things that may sound like mistakes at first can be honed into highly expressive new tools. Violins can be made to sound like cellos with the right bowing method. Video and film artists like Nam June Paik and the Vasulka’s have been exploring extended techniques for displaying video since their inception — but it is important to continue this tradition. There is still much to discover.
The purpose of this article is to collect and consolidate a list of these alternative methods of working with displays, light and optics. This will by no means be an exhaustive list of the possibilities available — depending on how you categorize, there could be dozens or hundreds of ways. There are historical mainstays, oddball one-offs, expensive failures and techniques that are only beginning to come into their own.
This document will hopefully serve as a reference for artists who are curious about pushing their content outside of a standard screen. Some implementations are incredibly practical and achievable on small budgets, and some require very specialized patented hardware that only exists in a lab somewhere. It is important not to get bogged down in the specifics of the technology, but to recognize that these all exist on a spectrum of information transference that employ light, medium, and brain. By keeping things in these simple terms, you are free to mix, match and re-appropriate to tell new stories.
Contents
- Notes on Standard Displays
- Brief Note on Holograms
- Pepper’s Ghost
- Projection on Static Transparent Materials/Scrims
- Projection on Water or Fog
- Volumetric Projection
- Diffusion and Distortion Techniques
- Transparent LCD/OLED
- LCDs with modified polarization layers
- Volumetric Displays (Mechanical/Persistence of Vision)
- Volumetric Displays (Layered screens)
- Electronic Paper
- Flexible Displays
- Laser Projectors
- Head Mounted Displays (VR/AR/Mixed Reality)
- Plasma Combustion
- Physical/Mechanical Displays
- Appendix and Other References
Notes on Standard Video Displays
It is worth discussing a few notes about the standard displays that most digital artists use. Many of the other things discussed in this article aren’t standalone technologies, but rather techniques that modify or adapt pre-existing technologies into new applications. Each of these technologies could fill several books, so we’ll just touch on some relevant bits.
Standard Monitors
These can be a range of different technologies. Cathode Ray Tubes or CRT displays were common up until about 2005 but are difficult to find these days — they do have a lot of unique properties (not necessarily good ones..) that aren’t available in many standard modern displays. Right now, the most common display is the Liquid Crystal Display or LCD and it is in most laptop screens, desktop monitors, commercial TV’s and so on. LCD’s have a backlight, a rear polarizer, a glass layer with electrodes and liquid crystals that react to electrical changes, and a front polarizer. Each pixel has a set of 3 sub pixels with red green and blue color filters that can be combined at different levels to recreate their millions of colors.
Things like Quantum Dots are on the horizon to further improve LCD’s color reproduction and accuracy by allowing more precise tuning of light wavelengths. Plasma displays were contender for LCD for a while, but they have become less popular. Pixels in plasma displays are individually lit which result in a deeper contrast compared to LCD’s. Organic Light Emitting Diode displays or OLED displays operate in a similar principal to Plasma and have started to become more and more common. OLED has a lot of interesting properties since it can be made smaller and thinner than LCD or Plasma, this means that flexible displays and transparent displays are a much more viable option with OLED. OLED is still quite expensive in comparison to LCD’s at the moment, but this will change as the market shifts. MicroLED is another technology that works in a similar fashion but is still very new.
Standard monitors are affordable for most applications, are high resolution which makes them ideal for applications when the view is standing up close, have a decent color and dynamic contrast range, accept a variety of inputs, and are long lasting. Their brightness is suitable for primarily indoor applications. Brightness of these is generally measured in nits or candela/sq meter — most laptop screens are around 300nits at maximum. For outdoor applications, you have to source specially made outdoor monitors that are weatherproof, can withstand a variety of temperature fluctuation, and have a considerably higher brightness rating — some available ones can do 1500 nits or more which would be almost painful to look at up close in an indoor setting.
Of course, these displays have their limitations. They are only viable up to a certain size for a single unit. Most of the largest max out at 120in or 305cm of diagonal image. Past this, they must be tiled together to form a larger video wall, and there are inevitable lines or bezels between adjacent units. Even those larger video walls start to reach a limitation at a certain point where Projectors or LED video walls are a more economical choice. The color and dynamic range of these monitors appears to be decent, but it is actually not as good as you might expect — we are missing out on a whole range of visible colors. Most standard displays are also locked at 60hz refresh rate (the speed that the screen is redrawn every second) which is perfectly fine for most applications like movie watching, but things like gaming monitors have started jumping to 144hz or more. Even though our brain’s visual refresh rate is about 60hz (a huge oversimplification), there are some intriguing things that can be done with a higher refresh rate. Imagine scrolling this page up and down and having it look as natural as a piece of paper moving up and down instead of the commonly jittery experience. There are also researchers looking into using high frame rate or high temporal resolution displays to do things like turning normal displays into higher resolution displays — here is an incredible survey of a range of options with computationally augmented displays. Consumer displays are also typically two dimensional and flat, even if displaying 3D content with glasses or another method.
Projectors
I have covered projectors in depth in another article so I won’t go into detail with them here. It is important to remember that they are not much more than a fancy implementation of a light source, an imaging element and a lens. They are best for darker environments, but they tend to be the most economical choice for large scale imagery. It is also easier to blend multiple projectors together more seamlessly.
LED Video Walls
LED Video Walls are another common option for displaying digital art on a large scale (also called LED Displays — not to be confused with LED Monitors where the light source is simply the LED backlight). These are usually comprised of individual tiles that are linked together and driven by a special display driver box that addresses the tiles from a standard monitor input. The tiles are generally either single all-in-one RGB LED’s or larger individual R,G and B LED’s that are placed close together. The primary spec of an LED wall is its pixel pitch, measured in millimeters. If you are viewing a wall close up, you want a low pixel pitch — some of the lowest available are around 1.6mm. Larger pixel pitch like 16mm to 20mm is perfectly acceptable if your viewer is really far away from the screen because their eye won’t be able to discern individual pixels as easily. LED walls are also one of the only display types that can be viewable in direct sunlight. Some of them are 3000nits or more of brightness, which explains why they are the display of choice in places like Times Square.
They have a wide variety of models and applications. Some are used as jumbotrons in stadiums, as high end storefront signage, or are used as sculptural stage elements. Some move towards the spectrum of lighting elements and are extremely high pixel pitch. These large pixel pitch tiles can be used almost as “transparent” elements because when the audience is far away, they are able to see through the frame — as in this video wall. Stage lighting examples are the LightSlice, Vanish, and the Saber. Some manufacturers also provide custom LED tile work and can do more unusual shapes like spheres or triangles.
The primary drawback of LED walls is cost, although the prices have been dropping rapidly in the past few years as these become more commonplace. Finding price points for certain elements isn’t usually publicly available but it can cost around $2000 for an individual tile and the driver box can be $5000–10,000. Most LED walls are typically rentals due to the large cost of purchasing them. They do last a long time in the case of purchasing, but even a modest sized wall at a high resolution can run into the hundreds of thousands of dollars very quickly. The cost of installation (for rental or permanent) can also be an additional hurdle since you typically need an experienced technician to set them up and get the pixel mapping established. They also have a particular aesthetic that is suited to viewing from far away. Up close they can be uncomfortably bright, and their pixels can be a distraction. Some stage designers will overlay a black or dark grey rear projection material or even acrylic overtop of the LED’s to soften them and provide a more diffuse look.
Brief Note on Holograms
To get this out of the way early, It should be mentioned that none of the displays mentioned below are in line with the definition of a hologram. A hologram is closer to a photographic medium as it captures an imprint of the light waves that bounce off an object. Most of the media headlines these days with the word “hologram” are typically talking about simple optical tricks or AR. Holograms have taken on a cultural meaning that differs from the scientific definition, similar to the cultural rebranding of “synesthesia” or “literally.” This article by Oliver Reylos has a concise summary of what is considered holographic and what isn’t. In his words:
When viewing close-by objects, there are six major depth cues that help us perceive three dimensions:
- Perspective foreshortening: farther away objects appear smaller
- Occlusion: nearer objects hide farther objects
- Binocular parallax / stereopsis: left and right eyes see different views of the same objects
- Monocular (motion) parallax: objects shift depending on how far away they are when head is moved
- Convergence: eyes cross when focusing on close objects
- Accommodation: eyes’ lenses change focus depending on objects’ distances
Almost all of the displays or techniques in this article have some holographic properties like parallax or multiple viewing angles, but are primarily in a class of their own. Would you call an oil painting a sculpture?
Pepper’s Ghost
Pepper’s Ghost is a classic illusion — it has been around for over a century and is still making headlines. 99% of the time, when you see a headline with the word “hologram” it is talking about Pepper’s ghost.
Historically, the effect comes out of Phantasmagoria, a fascinating tradition of theater illusions that were developed in the 18th and 19th centuries that frightened audiences with never before seen images of spirits and floating otherworldly beings. The Magic Lantern is another one of these early theater effects and it is one of the earliest forms of the projector. The name Pepper’s Ghost comes from John Henry Pepper who popularized the effect in the mid-1800’s with his friend Henry Dircks (who arguably developed it before Pepper). However, the illusion was first described in the 1600’s by an Italian scholar named Giambattista della Porta in his book Natural Magic:
Wherefore to describe the matter, let there be a chamber wherein no other light comes, unless by the door or window where the spectator looks in. Let the whole window or part of it be of glass, as we use to do to keep out the cold. But let one part be polished, that there may be a looking glass on both sides, whence the spectator must look in. For the rest do nothing. Let pictures be set over against this window, marble statues, and suchlike. For what is without will seem to be within, and what is behind the spectators back, he will think to be in the middle of the house, as far from the glass inward, as they stand from it outwardly, and so clearly and certainly, that he will think he sees nothing but truth. But lest the skill should be known, let the part be made so where the ornament is, that the spectator may not see it, as above his head, that a pavement may come between above his head. And if an ingenious man do this, it is impossible that he should suppose that he is deceived.
Pepper’s ghost is very easy to implement. The simplest version involves a transparent reflecting surface (a sheet of glass, plastic, or a half silvered mirror), and an image source (a monitor, projection screen, or a lit source). There are two versions of this effect that are commonly used — the classic one from the 19th century typically involves two separate physical spaces and specialized lighting. The modern version of Pepper’s ghost involves a digital screen (monitor, or projected image) and a half silvered mirror or specialized film designed to be invisible to the viewer. This version is also used for teleprompters where the camera lens is positioned behind the mirror facing the speaker. Both are essentially the same in principal.
Glass mirrors are the most accessible way to achieve this effect (it can even be done with reflective plastic and a smart phone), but at a certain point it becomes difficult to scale the glass to be large enough. For stage productions, there is specialized plastic film that can be employed to reflect much larger surfaces. Musion is the primary company that comes up while searching, and another is Arena 3D. It is worth noting that Musion claims a patent on a version of this 100+ year old technology and has hit “imitations” with lawsuits in the past. It is also easy to source your own film from 3M or other sources in Asia — another version of the film is manufactured by DuPont.
Carefully controlled lighting is essential for this effect to look its best. The source of the image must be bright in comparison to the surroundings behind the transparent surface. The observer should also be in a very dark space so their own reflection doesn’t show up in the mirror. It is also helpful to have something slightly visible behind the transparent surface so that your floating image has something to float overtop of and give the viewer the parallax depth cue. The effect can be striking if combined with props behind the mirror — like a person sitting on a chair or animations that swirl around an object. However, there are limitations to this depth effect.
Peppers ghost is still very much a 2D effect and does not present an image in three dimensions. It is just a mirror reflecting another flat plane. Parallax between the reflected image and the background is what gives our eyes the illusion of the content floating in mid air. A false sense of 3D can be achieved depending on your source and how the reflecting surfaces are arranged. There are some implementations of the effect that put 4 mirrors in a pyramid shape under a monitor (some have marketed themselves as holograms — sparking controversy). By having the monitor display a different image for each mirror, the observer gets more of a 3D view as they walk around — even if it is just 4 discrete viewing angles. Head or eye tracking would have to be employed to make the effect a little more convincing, but then it would only work for one observer at a time. As it usually functions, the effect may look best from one vantage point, especially if you are trying to align it with an object behind the surface. This misalignment can be minimized by having your observer be further back so when they move their head, the parallax isn’t as great as if they are right in front of the screen.
Projection on Transparent Materials and Scrims
Projecting on semi-transparent materials is essentially a variation on the Pepper’s ghost illusion. It is also an effect that has been used in theater for a long time. In contrast to Pepper’s ghost, this technique uses a transparent material to catch (not just reflect) the light from a projector. The viewer can still see through the material, but the projected light is scattered and appears to be transmitted from the material. Viewers can still see through the material allowing for a depth effect from parallax, but the illusion is still flat and two dimensional.
The implementation of this technique is one of the cheapest and most accessible on this list. You will need a semi transparent material and some means of projecting an image. The material you use depends on the scale or size of the end result and the type of effect you are going for. You also must consider whether you want to use front or rear projection. Rear projection (with the viewer facing the projector lens) will produce a noticeably brighter hotspot depending on the material used and where the projector is, and front projection means the image will spill behind the surface a little bit which may result in some doubling.
As far as materials to use, on a small installation you may be able to get away with just a piece of fabric like tulle or netting — things like bridal veil material. White fabric will catch and transmit light the best, but sometimes black can still work and give you a similar effect with the fabric appearing more “invisible.”
If you are trying to have an image appear on a storefront window or piece of glass, you will need a specially engineered film that is nice and transparent but still collects a lot of light from your projector. The proper film for glass can be very expensive for large pieces, so keep that in mind. One source has it at almost $1200 for a piece that is 2.2m by 1.2m. Here are some possible vendors for this kind of film: [One] [Two]. You can get away with cheaper materials, of course, but the effect may be very different. Cheaper or DIY material may be either more opaque (yielding a brighter image but less transparency) or too transparent (yielding a faint image). Projecting on glass will certainly show something if there is significant dust on it, but the effect will be very dim.
To achieve much larger images for theater or stage, fabric is the most economical choice for a scrim. You can get very large seamless swaths of fabric for the purposes of stage projection. Some fabric will have larger holes in its netting which will make it more transparent but will also cause your projected image to be less bright as well, in addition to dropping the sharpness and fidelity of the image. Here is a great resource for more info on stage scrim projection materials, including silvered fabric. You can also layer these materials to get several planes since the light is passing through. The cone of light from the projector will cause the image to be larger or smaller on each depth layer depending on whether you are projecting from the front or the back. You can also only go so far with layering before your light runs out or just gets out of focus.
Similar to the requirements for Pepper’s Ghost, this technique requires very controlled lighting. You will need to balance the ambient light that is hitting your fabric so you can preserve the illusion of a floating image — otherwise it can just look like a standard projection screen that you can see through. Contrast is key here. It also helps to have the space behind the image not be completely dark to give the image more dimension. If the viewer can see behind the image then they get the layered effect and the sense of parallax that helps it appear more 3D even if it is still just 2D.
Content that works best on any of the semi-transparent materials tends to be imagery that does not fill the entire projected rectangle. The optimal approach is to have your content sit on a field of black, so that that it appears to have no bounds. A vignette or feathering on the edges can also help if you have elements that enter and exit from the sides, otherwise the viewer will see harsh edges. Semi transparent material also causes the projections to have a slight glow to them — the light beams get slightly diffused when passing through the material which tends to soften the sharpness of the image a little bit.
Projection on Fog or Water
For this technique, instead of a static material like cloth, you can use water, haze or another atomized fluid to catch light and provide a semi-transparent screen.
Water Screens
There are two types of water based projection surfaces — either the water is moving upwards or falling downwards. For an upward blast, these rely on a high powered water jet and a special attachment that spreads the water into a large flat half circle screen of water and mist. The size of the screen is limited by physics and the power of the water pump — most companies can generate screens that are in the range of 20–30m wide and about 6–10m high. This mist is then usually hit with rear projection by a high powered projector. This results in a semi transparent screen that can be hidden or revealed at the flip of a switch in the middle of a body of water.
Falling water screens are much more manageable to install indoors. These have a mechanism that just pushes water through spaced out nozzles on the top piece and collects and recycles the water in a basin on the bottom. Some systems are even able to selectively open and close the top nozzles to allow water to fall in different ways.
The effect of water screens is very unique due to the haze of smaller water mist particles causing a halo and giving the 2D image more volume. There is also a textural quality to the water and mist that you should plan on, as it can add some glow and reduce sharpness a bit. Rear projection works best on these screens, so there will be a persistent hotspot behind the content, but this may not impact too much depending on your setup. Front projection is possible, but you run the risk of doubling the image onto other surfaces behind the semi-transparent screen.
Fog Screens/Laminar Flow
These screens rely on a steady controlled flow of haze or water mist to create a thin layer of semitransparent fog that can be rear protected. A series of valves directs the mist into a narrow sheet, and the projected light is refracted off the particles. The haze can be water or oil based.
This technique works best indoors because of minimal air currents and the light contrast needed for the best illusion. Due to physics, this technique is limited by the screen size that can be created. A lot of commercially available screens can only get to something like a 2m by 1.5m size. The width can be extended with multiple mist units, but the height is the primary hindrance since the mist gets less dense after a certain distance from the valves and fans. Also, since this screen is so transparent, the viewer will get a strong hotspot from the projector and the content will shoot right through onto adjacent surfaces. Commercially available units are available, but aren’t cheap — some are almost $20,000 or more. DIY options also exist, but require a lot of materials. Getting the haze production right in a DIY setup is probably going to be the biggest challenge since most fog machines tend to accumulate in an enclosed space rather than dissipate.
Volumetric Projection
Volumetric projection is a technique that is a much more technical application of projecting into a thin sheet of fog. Instead of having light come from a single point, it uses multiple light sources or specialized optics. By combining these sources with the additive quality of projected light, this technique is able to create dimensional images with multiple viewing angles. There are a few scientific papers out there on similar processes, and we’ll discuss laser plasma displays later on that share some characteristics.
Light Barrier by Kimchi and Chips is likely the first piece to use this technique. With Light Barrier, the artists project images onto an array of parabolic mirrors. Using custom software that analyzes where the pixel’s light ends up after hitting the curved mirrors, they can approximate the path of light from each projected pixel. When this is done for the entire array of mirrors, they can calculate where in 3D space each pixel path intersects another after hitting the mirrors. The projection area above the mirrors is filled with haze from a fog machine — the medium for these intersecting light beams. If more beams illuminate a particular location in 3D space, then that spot will appear brighter. By hitting several of these overlapping spots together, the combined focal point becomes brighter, and images can be formed in the haze. There are other ways to achieve similar variations on the effect that involve multiple projector sources, but this gets logistically complex and expensive very quickly.
This technique currently has its limitations. Making recognizable images requires a calculation engine and custom software, meaning you can’t just drop in any content and have it show up in 3D. The workflows for generating content based on depth maps are improving, but there is also going to be an upper limit to how fine the details can be. It may take several dozen converging pixels to make a recognizable voxel — so once we have higher resolution projectors we might be able to put together even more complex visuals. Full color projection with this technique is also a challenge because the overlapping colors add together and change the colors for different viewing angles. White is also going to show up the best for an effect that is already going to be slightly faint compared to other projection methods. Nonetheless, it is an exciting area of discovery and still has a lot of potential to explore.
There are also variations on this idea that don’t involve fog or specialized optics. A simple way is to use layers of fabric — the image will be the same on each layer but will get larger or smaller on each pass. There is also the method employed by the Lumarca where a grid of thin strings are stretched to make a large volume. Each string in the volume can then be precisely mapped by a few columns of pixels from the projector. When the location of each column is then mapped to a known 3D space, it becomes easy to render simple graphics on the array of strings that appear to have volume. The strings transmit the light a bit, so it is easy to see from all sides. This method also has some density and fidelity limitations but is also easy to scale.
Diffusion and Distortion Techniques
This is a challenging category to compartmentalize, but there is a lot of great work to consider here. Sometimes you don’t need a cutting edge display or experimental hardware to do something new. By placing different optical materials in front of a monitor, projection or LED video wall you can create something that doesn’t feel like a standard display at all. Of course it can be a little more challenging to show sharp coherent images with a techique like this, but sometimes a piece is much more about playing with texture and motion more than legibility.
