Omnidirectional mid-air imaging with a single biconical-ended cylinder array plate

Mid-air imaging is an augmented reality (AR) display technique that forms real images in free space, enabling multiple users to view virtual content without head-mounted or handheld devices. In this study, we propose a single-plate retro-transmissive optical element, termed the biconical-ended cylinder array plate (BCAP), that provides omnidirectional (360°) azimuthal observability while inherently suppressing ghost images. Conventional optical elements for mid-air imaging, such as micro-mirror array plates (MMAP) and dihedral corner reflector arrays (DCRA), often suffer from limited viewing angles and unwanted ghost images. In contrast, the proposed BCAP employs refraction at conical surfaces combined with total internal reflection within cylindrical elements, enabling omnidirectional visibility while eliminating ghost images. To analyze the imaging characteristics, we performed physically based rendering simulations with a parameter sweep. Image quality was evaluated using peak signal-to-noise ratio (PSNR), structural similarity index measure (SSIM), learned perceptual image patch similarity (LPIPS), and modulation transfer function at 50% (MTF50), allowing us to identify favorable ranges of the cone apex angle and cylinder height. A prototype fabricated by 3D printing was used to experimentally validate the omnidirectional visibility of the proposed structure, and its performance was compared with that of a conventional MMAP. The results demonstrate that the BCAP produces ghost-free mid-air images observable from all azimuthal directions, although with a moderate reduction in sharpness compared with mirror-based designs.