360-degree mixed reality volumetric display using an asymmetric diffusive holographic optical element.

Volumetric display technique has a great advantage of displaying realistic three-dimensional contents with a 360-degree viewing angle. However, most volumetric displays cannot provide mixed reality because their screens inside the displays obstruct the external scene. We design a 360-degree mixed-reality volumetric display using an asymmetric diffusive holographic optical element (ADHOE). The ADHOE has wavelength selectivity, and it diffuses the light with the only specific wavelength for the virtual object, so it is possible to optically combine the virtual object and the real scene. Also, the ADHOE has different vertical and horizontal diffusing angles, and it is suitable for a horizontal-parallax-only application. In our system, the parallax images are generated by the DMD, and they are projected sequentially on the ADHOE. The ADHOE is shaped as a slanted curved surface with respect to the optical axis, and some annoying color dispersion is observed due to the mismatch between the diffraction peak points of two different wavelengths. In order to solve this problem, the carrier frequency is applied to green elemental images and the proper Fourier filter cuts off the unwanted diffraction peak points. The Fourier transform with 2f optics is built to record the ADHOE where the angular spectral bandwidth is determined by adjusting the width of the incident object light. A 360-degree see-through display with ADHOE is implemented and the feasibility of mixed reality is verified successfully.

Holographic printing for generating large-angle freeform holographic optical elements.

We propose a holographic printing technique for generating highly efficient large-deflection-angle freeform holographic optical elements (HOEs). For industrial device applications, the optical efficiency and deflection angle of HOEs are critical. To fabricate a high-frequency volume grating in a hogel, we design an optomechanical hogel recording system with a high angle deflection capability, which contrasts with the conventional printing scheme, the wavefront holographic printing technique featuring a paraxial deflection angle. With the proposed system, a large-deflection-angle HOE is experimentally demonstrated, and short-throw holographic caustic projection patterns are realized.

Inverse conversion algorithm for an all-optical depth coloring camera.

Three-dimensional (3D) metrology has received a lot of attention from academic and industrial communities due to its broad applications, such as 3D contents, 3D printing, and autonomous driving. The all-optical depth coloring (AODC) camera has some benefits in computation load since it extracts depth information of an object fully optically. The AODC camera represents the depth of the object as a variation of wavelength, and spectroscopy is generally required to measure the wavelength. However, in the AODC camera, the color vector in RGB color space is convertible inversely into the wavelength after projection on the normalized rgb plane because the detected spectrum through the gating part has a narrow bandwidth as a result of the width of the slit in the projection part. In this paper, we propose an inverse conversion algorithm from RGB color to depth without spectroscopy. Experimental results are presented to confirm its feasibility. Also, some practical limitations are discussed, resulting from the nonlinearity of the response of the image sensor and the widths of the slits in the projection part and the gating part.