Optical See-Through 2D/3D Compatible Display Using Variable-Focus Lens and Multiplexed Holographic Optical Elements
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hv photonics Article Optical See-through 2D/3D Compatible Display Using Variable-Focus Lens and Multiplexed Holographic Optical Elements Qinglin Ji 1, Huan Deng 1,*, Hanle Zhang 2, Wenhao Jiang 1, Feiyan Zhong 1 and Fengbin Rao 1 1 College of Electronics and Information Engineering, Sichuan University, Chengdu 610065, China; [email protected] (Q.J.); [email protected] (W.J.); [email protected] (F.Z.); [email protected] (F.R.) 2 School of Instrumentation Science and Optoelectronics Engineering, Beihang University, Beijing 100191, China; [email protected] * Correspondence: [email protected] Abstract: An optical see-through two-dimensional (2D)/three-dimensional (3D) compatible display using variable-focus lens and multiplexed holographic optical elements (MHOE) is presented. It mainly consists of a MHOE, a variable-focus lens and a projection display device. The customized MHOE, by using the angular multiplexing technology of volumetric holographic grating, records the scattering wavefront and spherical wavefront array required for 2D/3D compatible display. In particular, we proposed a feasible method to switch the 2D and 3D display modes by using a variable- focus lens in the reconstruction process. The proposed system solves the problem of bulky volume, and makes the MHOE more efficient to use. Based on the requirements of 2D and 3D displays, we calculated the liquid pumping volume of the variable-focus lens under two kinds of diopters. Keywords: optical see-through 2D/3D; variable-focus lens; holographic optical elements Citation: Ji, Q.; Deng, H.; Zhang, H.; Jiang, W.; Zhong, F.; Rao, F. Optical See-Through 2D/3D Compatible 1. Introduction Display Using Variable-Focus Lens Augmented reality (AR) is a technology which allows computer generated virtual and Multiplexed Holographic Optical imagery to exactly overlay physical objects [1,2]. AR display mainly includes optical Elements. Photonics 2021, 8, 297. see-through type [3], video see-through type [4] and reflection type [5,6] display devices. https://doi.org/10.3390/ At present, the optical see-through display device has become the mainstream technical photonics8080297 solution, because it allows the user to directly watch the real environment light through an image combiner, and improves the user’s perception of the real world. As a kind of Received: 28 June 2021 true three-dimensional (3D) display technology, integral imaging has a compact structure Accepted: 23 July 2021 and can provide various physiological depth cues [7,8]. It is conductive for the natural Published: 27 July 2021 integration of virtual 3D images and real scenes, and is suitable to be integrated into AR display devices [9,10]. However, due to optical modulation of the lens array, integral Publisher’s Note: MDPI stays neutral imaging is not compatible with two-dimensional (2D) display. Although 3D display with regard to jurisdictional claims in can provide depth information that is absent in 2D display, the current 3D resolution published maps and institutional affil- is severely degraded. The 2D display has the advantage of high resolution and large iations. viewing angle, but it can’t replace 3D display. Therefore, some researchers have proposed 2D/3D compatible displays, such as switching backlight units by using polarized pinhole array [11,12] and polymer dispersed liquid crystal (PDLC) [13], or using dynamic parallax grating technology [14], but these technologies cannot be applied to optical see-through Copyright: © 2021 by the authors. AR display devices. Licensee MDPI, Basel, Switzerland. The technology that is key to realizing optical see-through AR display is the image This article is an open access article combiner that overlaps the virtual images and the real scenes together. The image combiner distributed under the terms and can be further divided into two categories: reflection-type devices [15,16] and diffraction- conditions of the Creative Commons type devices [17,18]. Compared with reflection-type devices, the diffraction-type devices Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ have many advantages, such as unique angle selectivity and wavelength selectivity and 4.0/). flexibility of design, as well as high diffraction efficiency and transparency. It can replace Photonics 2021, 8, 297. https://doi.org/10.3390/photonics8080297 https://www.mdpi.com/journal/photonics Photonics 2021, 8, x FOR PEER REVIEW 2 of 9 biner can be further divided into two categories: reflection-type devices [15,16] and dif- fraction-type devices [17,18]. Compared with reflection-type devices, the diffraction-type Photonics 2021, 8, 297 2 of 9 devices have many advantages, such as unique angle selectivity and wavelength selec- tivity and flexibility of design, as well as high diffraction efficiency and transparency. It can replace one or several optical elements in the system and is an ideal image combiner [19].one orYeom several et al. optical proposed elements a 2D/3D in the converti systemble and projection is an ideal system image using combiner see-through [19]. Yeom in- tegralet al. proposedimaging based a 2D/3D on convertibleHOE. The use projection of prism system reduced using the see-through number of integral projectors imaging and enabledbased on a HOE.compact The form use of factor, prism but reduced the utilization the number of the of projectors HOE was and only enabled 50%, that a compact is, one halfform of factor, the HOE but thewas utilization used for 2D of thedisplay HOE and was the only other 50%, half that for is, 3D one display half of [20]. the HOE Chou. was et al.used proposed for 2D display a 2D/3D and hybrid the other integral half for imaging 3D display display [20]. based Chou. eton al.a proposedliquid crystal a 2D/3D mi- cro-lenshybrid integralarray (LCMLA), imaging displayin which based the LCMLA on a liquid has the crystal focusing micro-lens effect arrayor no (LCMLA),optical effect in determinedwhich the LCMLA by the haspolarization the focusing state effect of orthe no incident optical effectlight. determinedTherefore, bythe the system polarization added devicesstate of thesuch incident as twisted light. nematic Therefore, cell the and system polarizer added to devicescontrol suchthe polarization as twisted nematic state, and cell increasedand polarizer the toredundancy control the [21]. polarization In our previous state, and work, increased we proposed the redundancy attaching [21 a]. PDLC In our filmprevious to the work, lens wearray proposed HOE toattaching constitute a a PDLC 2D/3D film projection to the lens screen. array The HOE lens to constitutearray HOE a 2D/3D projection screen. The lens array HOE and PDLC film were used to realize integral and PDLC film were used to realize integral imaging 3D display and 2D display, respec- imaging 3D display and 2D display, respectively. However, the PDLC film degraded the tively. However, the PDLC film degraded the transparency of the system, and the opti- transparency of the system, and the optical see-through properties could not be achieved cal see-through properties could not be achieved in 2D mode [22]. in 2D mode [22]. In this paper, we propose an optical see-through 2D/3D compatible display by using In this paper, we propose an optical see-through 2D/3D compatible display by using variable-focus lens and multiplexed holographic optical elements (MHOE). Based on variable-focus lens and multiplexed holographic optical elements (MHOE). Based on angular multiplexing technology, the MHOE records the scattering wavefront of a dif- angular multiplexing technology, the MHOE records the scattering wavefront of a diffuser fuser for 2D display and the spherical wavefront array of micro-lens array (MLA) for 3D for 2D display and the spherical wavefront array of micro-lens array (MLA) for 3D display. display. The incident angles of the projected angles vary with the changes in the varia- The incident angles of the projected angles vary with the changes in the variable-focus ble-focus lens to meet the two Bragg diffraction conditions of 2D and 3D displays. lens to meet the two Bragg diffraction conditions of 2D and 3D displays. Therefore, our Therefore, our approach does not require polarization devices and reduces the systematic approach does not require polarization devices and reduces the systematic complexity, complexity,enhances the enhances effective utilizationthe effective of MHOE utilization and maintains of MHOE optical and see-through maintains properties optical see-throughfor both 2D andproperties 3D displays, for both all 2D of whichand 3D are displays, highly beneficialall of which for are AR highly applications. beneficial for AR applications. 2. Structure and Principle 2. StructureFigure1 and shows Principle the structure of our proposed optical see-through 2D/3D compatible displayFigure system. 1 shows It is the mainly structure composed of our of propos a MHOE,ed optical a variable-focus see-through lens 2D/3D and compatible a projector. displayEssentially, system. the MHOEIt is mainly is a volumetric composed holographicof a MHOE, grating.a variable-focus The scattering lens and wavefront a projector. of a Essentially,diffuser and the the MHOE spherical is a wavefrontvolumetric array holograp of MLAhic grating. are recorded The scattering on the single wavefront volumetric of a diffuserholographic and the grating spherical by two-step wavefront recording array of with MLA angular are recorded multiplexing on the single technology. volumetric The holographicvariable-focus grating lens has by twotwo-step optical recording states of diopterwith angularj1 and multiplexingj2, and it modulates technology. the lightThe variable-focustransmitted by lens the has projector two optical respectively states of to diopter meet the φ1 two and Bragg φ2, and diffraction it modulates conditions the light of transmittedMHOE. by the projector respectively to meet the two Bragg diffraction conditions of MHOE. (a) (b) Figure 1. Schematic diagrams of system structure: (a) 3D image reconstruction when the probe beam satisfies the Bragg diffraction condition 1; (b) 2D image display when the probe beam satisfies the Bragg diffraction condition 2.