# High-Luminance Mid-Air Image Display for Outdoor Viewing by Focusing Sunlight
## 2 Related Work  
Several methods are available for forming mid-air images; these include those that use a Fresnel lens [1,2], a concave mirror [3], a dihedral corner reflector array (DCRA) [4,5], an ASKA3D plate [6], a roof mirror array (RMA) [7,8], and aerial imaging by retro-reflection (AIRR) [9]. FLOATS V [1] and FuwaVision [2] are available as techniques using Fresnel lenses. These methods can display three-dimensional (3D) images by presenting different images to the left and right eyes. However, the following problem is encountered: the position of the viewpoint is limited in the mid-air image of the Fresnel lens. Therefore, using it in a situation in which multiple people view the image is difficult. Concave mirrors can be combined with lenticular lenses to form 3D mid-air images. However, large concave mirrors are not widely available, and the issue of distortion is also encountered. The DCRA [4,5] reflects the light twice in the transparent material on the grid, whereas the ASKA3D [6] plate reflects the light twice in two orthogonal mirrors and displays the real image at a position that is plane-symmetrical to the light source. The RMAs [7,8] can be used for realizing a mid-air image with a large number of V-shaped roof mirrors arranged in parallel. Although RMAs appear to be easy to manufacture, the required components are not commercially available at present. The AIRR [9] is a method of reflecting and transmitting light from a light source by means of retro-reflective elements and half-mirrors to form a mid-air image at the plane-symmetrical position of the light source with respect to the half-mirror. Although the optical element of this method is available in the market, it exhibits large brightness attenuation. In this study, we use Aska3D, which is already commercially available and is easy to purchase in large sizes. This is because this device is brighter and more luminous than others. Owing to its structure, of this deviceis referred to as a micro mirror array plates (MMAPs) herein. Several interaction systems with a mid-air image using MMAPs exist; these include MARIO [10] and Scoopirit [11]. MARIO changes the image formation position in 3D by moving the light source up and down in relation to the mid-air image optical element. The user interacts with the CG character in real space. Scoopirit is a system that allows users to scoop up mid-air images that are displayed upright in or above water with their bare hands. These methods are good examples of how mid-air images displayed by MMAPs can interact with CG in the real world. However, the mid-air images displayed by these methods have low brightness and can only be used in dimly lit rooms. Although the use of MARIO has been demonstrated in public spaces, these demonstrations were limited to the interior of the National Museum of Emerging Science and Innovation. Scoopirit has also been demonstrated, but the actual mid-air image has a maximum brightness of 6.46 cd/m\({}^{2}\). Therefore, it is necessary to increase the brightness of the mid-air images to use such optical designs in bright rooms and outdoors. Although luminance attenuation is a major challenge for mid-air images, few technologies have been developed to solve this problem. Research has been conducted on increasing the brightness of general displays but not mid-air images [12]. For example, to obtain a high-luminance active-matrix organic LED display that is compatible with high dynamic range technology, the effects of basic components were investigated and a high-luminance display with a peak luminance exceeding 3000 cd/m\({}^{2}\) was fabricated. A low power, high-brightness display is realized by creating a highly efficient backlight. Furthermore, many problems, such as heat generation, must be solved for outdoor use. In this study, we propose the use of LCDs, which are available extensively in the market and have reached technological maturity.