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We propose a floating aerial LED signage technique by utilizing retro-reflection. The proposed display is composed of LEDs, a half mirror, and retro-reflective sheeting. Directivity of the aerial image formation and size of the aerial image have been investigated. Furthermore, a floating aerial LED sign has been successfully formed in free space.
© 2014 Optical Society of America
1. Introduction
Floating and aerial display is a prospective technique for digital signage to provide sensation to viewers. For example, aerial traffic signs can be placed in front of a driver's face. Since the sign is aerial, the sign cannot be crashed even if the car goes through it. Another application example is advertisement over a sidewalk and at eye level. There have been several techniques on floating display: floating display can be realized by use of gradient-index lens array [1], lens array based on integral imaging [2], and reflective optical elements [3, 4]. We have realized a floating display of LED signage by use of crossed-mirror array [5]. Although the crossed-mirror array is robust, it needs precise alignments. For the use of pervasive advertisements such as street advertisements and building wall signs, easy setups and alignment-free feature are required.
The objectives of this study are to propose a technique to form aerial LED sign with easy and quick installations. The proposed technique utilizes retro-reflection to form an aerial image. Main point is that there is no alignment problem in arrangements of retro-reflective material. We investigate the directivity of an aerial LED image when the angle of retro-reflective sheeting is rotated. Furthermore, we'll demonstrate floating aerial LED sign.
2. Principle of aerial imaging of an LED panel
2.1 Design issues for aerial LED signage
Imaging of a LED panel is not same as the conventional imaging, which aims to high resolution. In order to improve cost per display area, LED panels are composed of sparse LED lamps. Furthermore, each pixel in a large LED panel is consisted of a red, a green, and a blue LED lamp in different packages. Thus, color mixing and interpolation between LED pixels are important issues for imaging of a LED panel. One of the solutions is defocusing the LED panel, as shown in Fig. 1. The left side is a photograph of an LED panel with 20-mm pitch, which was taken in focus. The right side is a photograph of the same LED panel, taken intentionally defocused.
Fig. 1 Schematic diagram showing that defocusing changes the discrete LED dots into a smooth LED sign.
Thus, one of the solutions to represent a smooth LED sign is to perform blurred imaging. We have designed and fabricated a CMA for LED panel [5]. The CMA was composed of large apertures (4 mm × 4 mm) in order to defocus the LED lamps. The CMA was made of comb-shaped mirrors with 1-mm thickness to maintain stiffness. Mirror walls look like crossed pattern for viewers. The formed aerial image provides smooth motion parallax and binocular disparity to induce accommodation responses under binocular viewing. However there were three problems in forming aerial image by use of CMA for practical applications such as show window advertisements: (1) stray lights, (2) precise alignment in CMAs, and (3) crossed pattern. For use of window advertisements, stray lights annoys viewing from certain directions, precise alignments increase cost and time, and crossed pattern spoils image quality under a close (< 1 m) viewing distance. To make solutions for these three problems, we utilize retro-reflection for blurred imaging of an LED panel.
2.2 Aerial imaging by retro-reflection (AIRR)
We propose an aerial imaging method by utilizing retro-reflection. Retro-reflective material reflects light in reversely toward the incident direction. Retro-reflective sheeting is widely used for traffic signs in order to improve visibility under illumination with car headlight. Compositions and features of commercially available retro-reflective sheeting are shown in Fig. 2. Because the size of these structures is sub-millimeter, each structure is not noticeable for viewers. Retro-reflective sheeting of micro-beads type is more suitable for blurred imaging of an LED panel because of its wide reflected angle, high reflectance, and wide viewing angle.
Fig. 2 Compositions of retro-reflective sheeting. (a) Corner-cube type is consisted of micro-prism. (b) Micro-beads type is consisted of small glass beads in reflective material.
The optical configuration of the aerial imaging method is shown in Fig. 3. The setups are composed of light sources, a beam splitter, and a retro-reflective material. The basic setups are used for inverting pseudoscopic images [6]. Maekawa pointed that use of retro-reflector of corner-cube type needs four reflections, which are triple reflections at retro-reflector and one reflection on a half mirror and reduces brightness compared to roof-mirror grid array [7]. Current LED panels for digital signage have extremely high brightness, for example, we have developed a full-color LED panel with 5000 cd/m2 [9]. Such high brightness enables us to utilize retro-reflection for aerial imaging. Nowadays, retro-reflectors become pervasive to traffic signs. A large scale, such as tens of meters, retro-reflector is commercially available at a reasonable cost. This availability of large-scale optics is good for an aerial imaging of LED panels. Recently we have realized aerial imaging with wide point spread function by retro-reflective sheeting [6]. Light rays that are reflected on the half mirror impinge the retro-reflective material. After the retro-reflection, the lights travel reversely toward the light source. About a half of the retro-reflected lights are transmitted through the half mirror and form the aerial image of the light sources. Because the retro-reflection is maintained even if the retro-reflective material has curves, as shown in Fig. 3(b), there is no alignment problem in installing the retro-reflective material. The position of the aerial image is the mirrored position of the light sources about the half mirror. This is a good advantage in installation of a large LED signage compared to aerial imaging with crossed-mirror array [5] because it is difficult and too expensive to fabricate a flat CMA in a large scale. Furthermore, AIRR can show 3D images when the light sources are located three-dimensionally.
Fig. 3 Principle of aerial image forming based on retro-reflection. (a) A half mirror separates the aerial images from the light sources. (b) The proposed display is free from alignment problem in retro-reflective sheeting.
4. Experimental results
4.1 Directivity
Experimental setups to investigate directivity are shown in Fig. 4. An LED lamp and a retro-reflective sheeting that is pasted on a plate are located in a side of a plate beam splitter. The retro-reflective sheeting is rotated and luminance of the aerial LED image is measured with a luminance meter.
The experimental results are shown in Fig. 5. Horizontal axis indicates the rotation angle, which is the rotated angle of the retro-reflective sheeting from the condition parallel to the beam splitter (0 degrees) toward clockwise direction (positive angle). There is no significant change in luminance between 0 degrees and 80 degrees. This result suggests that there is 80-degrees tolerance in the incident angle. The full-width half maximum (FWHM) is more than 90 degrees, which corresponds to the viewing angle of the aerial image formed with a planar retro-reflector. This wide directivity enables imaging with a curved retro-reflective material, which is illustrated in Fig. 3(b). Note that the luminance of the aerial LED image is about 80 cd/m2, which is the equivalent of the luminance of laptop computer in normal condition. Thus, AIRR forms aerial image that is observable under ordinary room lighting.
Fig. 5 Relationship between luminance intensity of the aerial LED image and the rotation angle of a retro-reflective sheet of micro-beads type.
4.2 Blurred imaging
Divergence of the retro-reflective sheeting is investigated by use of He-Ne laser beam. Experimental setups are shown in Fig. 6. Width of the reflected beam is shown in Fig. 7. The width increased linearly with the distance of the screen from the retro-reflective sheeting.
Fig. 6 Experimental setups to measure width of the reflected beam on the retro-reflective sheeting of micro-beads type.
In order to confirm the blurred imaging function, the width of the aerial image of an LED lamp has been investigated by changing the pop-up distance, which is defined by the distance of the aerial image from the beam splitter as shown in Fig. 8. The direction of the retro-reflective sheeting was adjusted toward the aerial image position. We placed a screen on the aerial image position and measured the image width. Experimental results are shown in Fig. 9. The image size linearly increases with the pop-up distance.
4.3 Demonstration of floating aerial LED sign
A full-color LED panel that is consisted of 6-mm pitch LED dots, shown in Fig. 10, was used for the light sources. Experimental results on aerial image formation of an LED panel are shown in Fig. 11. Aerial image of a Japanese character is floating over the palm. Note that the black regions between LED lamps (about 3mm) are filled with blurred LED images.
5. Conclusion
We have proposed aerial imaging technique based on retro-reflection. We have investigated directivity by use of micro-beads type retro-reflective sheeting. Aerial image of an LED lamp was observed over 90 degrees. A floating aerial LED sign was successfully realized by use of an LED panel. The floating aerial LED screen was visible under room lights in a wide range of viewing angle in horizontally and vertically. The main advantages of out technique include alignment-free feature, wide viewing angle, and wide point spread function to smooth LED signs without additional diffusers. The wide directivity enables us to cover an LED panel and form a large aerial image.
Acknowledgments
This work has in part been conducted within CREST, JST, Japan.
References and links
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