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Figure 13.( a) Principle of a retro-reflective surface, and( b) principle of a cube-corner retro-reflector [ 54 ].
3.4.4 Floating 3D display of retro-reflective film
The principle of a retro-reflective surface is illustrated in Figure 13a [ 54 ]. An optical ray incident on the surface with a direction( a, b) isreflected in the same direction within a narrow diffusion cone( w, u). Two primary solutions exist to achieve this specific geometrical effect. The first involves the use of microbeads, where the incident optical ray is focused and reflected from the rear surface of a spherical lens. This solution is commonly employed in safety jackets but suffers from relatively low efficiency and cannot be easily encapsulated. The second, more widely adopted solution – particularly in road signage – is the cube corner geometry. Figure 13b illustrates the operating principle of this optical element, which consists of three mirrors arranged mutually perpendicular to one another [ 54 ]. Due to the geometric configuration, an incident light ray that enters the element and undergoes successive reflections from each mirror is returned along a path parallel to its original direction. This optical component has been well known and widely manufactured at the macroscopic scale for many years; however, its fabrication at the microscopic scale – particularly for the development of thin retro-reflective films – remains an active area of research. Finally, the retro-reflective ray regenerates a floating 3D image. As a cutting-edge 3D display technology, this floating display has a wide range of applications, such as advertising, entertainment, educational training, and navigation systems. By imaging various content forms in mid-air, this technology provides a unique visual experience that captures increased attention and interest [ 55, 56 ].
In 2020, Christophe Martinez proposed a 360 ° volumetric display system based on a transparent film with a sparse cube corner array. By modifying the Pepper’ s ghost optical configuration, images are projected onto a retro-reflective transparent surface with high angular selectivity, creating floating virtual imagery as shown in Figure 14 [ 54 ]. The system enables multi-user, multi-view stereoscopic viewing without moving parts, offering high brightness, high transparency, and low-cost advantages, with broad application potential in artistic exhibitions, remote collaboration, and augmented reality.
In the fast-growing realm of the metaverse, this floating 3D display serves as a pivotal technology, enabling the seamless integration of virtual and real environments. It offers unique advantages in several aspects. First, it delivers seamless integration. Unlike VR, which creates a fully virtual setting, medium-free 3D technology merges virtual images with physical objects and actual surroundings, creating a unique spatiotemporal experience. Second, it significantly enhances user experience by addressing typical constraints of VR and AR – such as spatial limitations, discomfort from wearable devices, motion sickness, and reliance on dynamic compensation. Through the synergy of sensing technology, content creation, and intelligent algorithms, it delivers more adaptable and immersive interactions. Third, it enables wearable-free interaction. While VR and AR often require users to wear devices for both visualization and engagement, floating 3D technology integrates virtual images directly into the environment without the need for user-worn hardware.
3.4.5
Floating 3D display of plasma
In 2015, researchers at the University of Tsukuba in Japan introduced a breakthrough technology that creates 3D plasma pixels in mid-air using femtosecond laser pulses, enabling interactive 3D images that respond to human touch [ 57 ]. This floating 3D display technology, known as“ Fairy Lights” offers a promising new mode of interaction for the future as shown in Figure 15.
The plasma-based floating display system uses precisely calibrated lasers to selectively ionize air molecules, producing white light. The team also integrated a touch function, making the 3D system both visible and interactive to touch. The system works by directing a laser beam at specific points in the air, ionizing the gas and creating floating plasma. Earlier prototypes used nanosecond laser pulses, which overheated the plasma and caused skin burns [ 57 ].
The updated technology uses femtosecond laser pulses, which generate shorter plasma bursts at higher frequencies, preventing prolonged focus on one area and avoiding skin burns. The brightness increases upon touch, and this effect has been incorporated into the system’ s touch functionality. When the hand touches the projected image, the system recognizes it instantly, images different pictures, and even simulates a sense of physical presence.