View source for Fairy Lights

==Introduction==

[[File:Fairy lights.png|thumb|Figure 1. Fairy Lights. (Image: Ochiai et al., 2016)]]
[[File:Fairy lights system setup.png|thumb|Figure 2. System setup. (Image: Ochiai et al.)]]
[[File:Fairy lights voxels.png|thumb|Figure 3. Rendering voxels (Image: Ochiai et al.)]]
[[File:Fairy lights applications.png|thumb|Figure 4. Example applications of laser-based graphics technology. (Image: Ochiai et al.)]]

Fairy Lights is term given to the images created by a method of rendering aerial and volumetric graphics using a femtosecond laser. The aerial plasma, induced by laser, emits light without interaction with physical matter. The practical result of the ionization of air molecules is crackling, photon-emitting pockets of plasma (voxels) that can be arranged to create moving and interactive images. The technology was developed by researchers at the University of Tsukuba, Utsunomiya University, the Nagoya Institute of Technology and the University of Tokyo and first published in a scientific paper called ''Fairy Lights in Femtoseconds: Aerial and Volumetric Graphics Rendered by Focused Femtosecond Laser Combined with Computational Holographic Fields''. According to Ochiai et al., the main contribution of the paper was “the production of an in-air SLM-based [Spatial Light Modulator] laser-plasma graphics that enables physical contact and interaction by ultra-short pulse duration laser.” <ref name=”1”>Ochiai, Y., Kumagai, K., Hoshi, T., Rekimoto, J. Hasegawa, S. and Hayasaki, Y. Fairy lights in femtoseconds: aerial and volumetric graphics rendered by focused femtosecond laser combined with computational holographic fields. Arxiv</ref>

There are two main methods to render graphics in air using a femtosecond laser. The first is holograms by spatial light modulation technology and the second is the scanning of a laser beam by a galvano mirror. Currently, the hologram size and workspace for these systems is 1cm<sup>3</sup> and 5 cm<sup>3</sup>, respectively. Although these sizes are still too small for the practical applications proposed in the research paper, the study will allow for further development and design of laser-based aerial volumetric displays that could be scalable according to the optical devices and setup used. <ref name=”1”></ref>

With volumetric displays, users can see the images produced from any angle. They can be divided into two categories according to the characteristics of the voxels. The first is voxels that emit light, and the second voxels that reflect light. Those in the first category can be LEDs, end points of optical fibers, or laser-induced plasma. The voxels in the second category that reflect projected light can take the form of a fog, water drops, or floating small particles. <ref name=”1”></ref>

In the study conducted by Ochiai and colleagues, they used laser-induced plasma which has some advantages: 1) “it does not require physical matter arranged and suspended in air to emit light,” 2) “it does not require wires and structures that possibly obstruct the line-of-sight because power is transmitted wirelessly”, and 3) “the laser can be precisely controlled owing to the progress in optical technologies.” By using a femtosecond-laser display system, the technology is also safer than using a nanosecond laser. <ref name=”1”></ref>

Previous studies pioneered the development of laser-plasma 3D displays using a nanosecond and femtosecond laser. Nevertheless, the studies were incomplete and Ochiai and colleagues intended to provide a complete discussion on laser-plasma graphics technology. <ref name=”1”></ref>

Three-dimensional (3D) displays have attracted the attention of researchers and the general public for decades. Holograms have been popularized by science fiction movies like Star Wars. Although there have been some public experiments with so-called holograms - CNN’s correspondent Jessica Yellin being projected into Wolf Blitzer’s Situation Room and projecting Tupac Shakur during 2012's Coachella - in fact, these were not true holograms. While CNN filmed the correspondent from different angles and superimposed the composite image onto the footage, Coachella used a technique know as Pepper’s Ghost. On the contrary, one of the exciting things about the technology developed by Ochiai and colleagues is that it has the potential to develop into something that can be compared to the popular image of a hologram in movies. Tangible, aerial 3D holograms have been created and further improvements will make them useful in countless different applications. <ref name=”1”></ref> <ref name=”2”>Floyd, D. (2015). “Fairy Lights”: floating, responsive 3D holograms. Retrieved from http://www.nasdaq.com/article/fairy-lights-floating-responsive-3d-holograms-cm492607</ref>

Fairy lights overcome the limitations of quasi-holograms and trick from the Victorian age like Pepper’s Ghost. There is no need of a medium for the image to be projected like glass, smoke, or water vapor since the image is made of ionized molecules. In spite of that, the image cannot be created in a complete vacuum since air needs to be ionized. It can also be reproduced in water or a fluorescent solution. Since the images created are 3D, they can be viewed from any angle. <ref name=”2”></ref>

An interesting characteristic of the fairy lights is that they are interactive (Figure 1). A person can touch them, feel them, and control them. The haptic feedback is a result of the shockwave created when plasma interacts with material. The current displays are very small, the spatiotemporal resolution is relatively high (up to 200,000 voxels), and the image framerate varies according to the voxel density needed. <ref name=”3”>Doss, H.M. (2015). Plasma Fairies: femtosecond laser holograms. Retrieved from http://www.physicscentral.com/explore/action/femtosecond-hologram.cfm</ref> <ref name=”4”>Ackerman, E. (2015). Femtosecond lasers create 3-D midair plasma displays you Can touch. Retrieved from http://spectrum.ieee.org/tech-talk/consumer-electronics/audiovideo/femtosecond-lasers-create-3d-midair-plasma-displays-you-can-touch</ref>

==Creating Fairy Lights==

The basic setup to generate a simultaneous-multi-point volumetric display consists of a femtosecond laser source, an XYZ scanner (galvano scanner + varifocal lens), and a liquid crystal on silicon SLM (LCOS-SLM) displaying a CGH for simultaneously addressed voxels (Figure 2). <ref name=”1”></ref>

To generate fairy lights, there is a need for a medium that can reflect or emit light at precise locations throughout a given volume. The researchers used air, turning it into plasma. Focusing a laser very precisely, it is possible to produce a small, isolated volume of plasma, thereby creating a voxel. These can be seen as the 3D equivalent of pixels and stand for “volume element” (Figure 3). <ref name=”3”></ref>

Previous successful attempts at creating aerial plasma used nanosecond laser pulses. These type of lasers had the effect of burning tissue since the energy associated the lit voxels was high. Ochiai and colleagues used femtosecond (a quadrillionth of a second) laser pulses to generate short lived, light-emitting plasma voxels that would not burn tissue when touched, although they can still burn softer tissue like eyeballs. Since plasma emits ultraviolet and infrared radiation, the researchers still advise the use of glasses with infrared filters as a precaution. <ref name=”1”></ref> <ref name=”3”></ref>

Each voxel is generated by a laser with pulse durations of 30-100 fs and 269 fs, resulting in a safer technology that doesn’t produce any relevant skin damage “unless the laser is firing at that same spot at one shot per millisecond for a duration of 2,000 milliseconds.” <ref name=”4”></ref>

According to Doss (2015), “the location of the voxels is determined by using a computer-generated hologram, which is a method of digitally creating interference patterns that are printed on a mask or film. The mask or film is also called a spatial light modulator and its role is to alter the phase and location of the coherent light source illuminating it.“ He concludes that using the femtosecond laser, it was possible “to create multiple plasma voxels with one pulse and create voxels at a faster rate, leading to a well defined, bright 3D holographic image safe enough to touch, and feel.” <ref name=”4”></ref>

==Limitations==

The first main limitation of this technology that needs to be addressed in future research in the system’s scale up, which is dependent on the optical devices used and the setup. The fairy lights produced are only 1cm<sup>3</sup> in volume, with 5 cm<sup>3</sup> to move around in. A Japanese firm - Aerial Burton - has a technology that projects large images through ionization but they are too dangerous to touch and loud. <ref name=”1”></ref> <ref name=”2”></ref> <ref name=”3”></ref>

According to Ochiai et al., three factors need to be developed in order to scale up their system: 1) increasing power of laser source; 2) shortening pulse width to increase peak power; 3) and increasing scanning speed. The development of these makes possible to “have an amount of simultaneously addressed and scanned voxels within a single frame, keeping visible and touchable features.” <ref name=”1”></ref>

Another limitation of the system is the safety issue. Plasma has high energy and can be harmful to humans. While the use of a femtosecond laser was proven to reduce the damage to tissue, the plasma can still damage the retina. <ref name=”1”></ref> <ref name=”2”></ref>

Currently, it seems that there is a trade-off between large and dangerous displays and small, interactive ones. Furthermore, the color of the fairy lights cannot be changed, and plasma can be a little noisy. <ref name=”2”></ref> <ref name=”3”></ref>

==Applications==

The viable implementation of laser-induced plasma aerial images (i.e. Fairy lights) has many useful applications such as spatial aerial augmented reality (AR), aerial user interfaces, and aerial volumetric images (Figure 4). These would be effective means to display 3D information, and could even be applied to wearable materials and spatial user interactions. In the AR field, this technology offers an advantage since the content is generated in a 3D space instead of a 2D computer display. <ref name=”1”></ref>

==References==
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