# Time-Division Color Holographic Projection in Large Size Using a Digital Micromirror Device +
###### Abstract  
Holographic projection is a simple projection as it enlarges or reduces reconstructed images without using a zoom lens. However, one major problem associated with this projection is the deterioration of image quality as the reconstructed image enlarges. In this paper, we propose a time-division holographic projection, in which the original image is divided into blocks and the holograms of each block are calculated. Using a digital micromirror device (DMD), the holograms were projected at high speed to obtain the entire reconstructed image. However, the holograms on the DMD need to be binarized, thereby causing uneven brightness between the divided blocks. We correct this by controlling the displaying time of each hologram. Additionally, combining both the proposed and noise reduction methods, the image quality of the reconstructed image was improved. Results from the simulation and optical reconstructions show we obtained a full-color reconstruction image with reduced noise and uneven brightness.  
holography; hologram; computer-generated hologram; holographic projection; time-division; digital micromirror device +
Footnote †: Copyright: © 2021 by the authors. License MDP, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license ([[https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/)]([https://creativecommons.org/licenses/by/4.0/](https://creativecommons.org/licenses/by/4.0/))).  
Footnote †: Copyright: Copyright: © 2021 by the authors. License MDP, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CCbut needs improvements for phase-only and binary holograms [14; 15]. Optimization of CGH using the Gerchberg-Saxton algorithm [16; 17] and the gradient descent method [18] obtains a reconstructed image with good image quality, however, it is computationally expensive due to iterative calculations.  
This paper proposes a time-division holographic projection projected on a large screen. The proposed method divides an original image into blocks, and the CGHs of each block generated is calculated. Then, a digital micromirror device (DMD) is used to project the CGHs at high speed to obtain the entire reconstructed image. The CGH to be displayed on the DMD needs to be binarized. However, the binarized CGHs lose the brightness information of the divided original image, causing uneven brightness in the reconstructed image. To solve this problem, the proposed method changed the display time of the CGHs proportional to the average brightness value of the divided original image. Additionally, combining our method with the multi-random phase [10] and pixel separation methods [11], we improved the image quality of the reconstructed image. From the simulation and optical reconstructions, we obtain a large-color reconstructed image with reduced noise and color unevenness. The rest of the paper is organized as follows: Section 2 describes each aforementioned corrective method. Section 3 presents the simulation and optical results. Section 4 holds the conclusion.