## 2 Monocular System  
Holographic VR is a true 3D display, which is different from that based on stereoscopic 3D technology. Stereoscopic displays can cause visual fatigue, which is not suitable for most people to watch for a long time. In holographic 3D VR, the image has a certain depth of field, which is the distance between the nearest and the furthest objects that are in acceptably-sharp focus in an image [22], and the maximum diffraction angle depends on the pixel pitch of SLM, while the viewing angle of the reconstructed images, \(\theta\), is twice the maximum diffraction angle, which is the maximum angle at which an image can be viewed with acceptable visual performance. Equation (1) shows the relationship between the pixel pitch pand the viewing angle \(\theta\).  
\[\theta=2\arcsin\frac{\lambda}{2p} \tag{1}\]  
Figure 1: Stereoscopic display. PD: pupil distance.  
where \(\lambda\) is the wavelength of reading light. For instance, here, the wavelength of reading light is 532.8 nm. When the pixel pitch of SLMs is 8.1 \(\upmu\)m and 3.74 \(\upmu\)m, respectively, obtained by calculation, the viewing angle should be 3.81\({}^{\circ}\) (\(\theta_{1}\)) and 8.2\({}^{\circ}\) (\(\theta_{2}\)), respectively, as shown in Figure 2a. It is obvious that the smaller pixel pitch of SLM leads to a larger viewing angle, as shown in Figure 2b.  
In Figure 3a, \(a\) is the distance between image and eyes and \(b\) is the width in the pupil plane. The image plane is the hologram plane, and it is a Fresnel hologram. Equation (2) shows the relationship between \(a\), \(b\), and \(\theta\).  
\[a=b/(2\tan(\theta/2)) \tag{2}\]  
The pupillary distance is the distance between a human's two pupils, which is 58-64 mm for adults, and 52-64 mm for children. It is different for different humans. The difference in the range of pupillary distance between children and adults is 12 mm. We compare distances between displayed images and eyes based on SLM with a pixel pitch of 3.74 \(\upmu\)m and 8.1 \(\upmu\)m, respectively. Setting the pupillary distance of d and the difference in the range of pupillary distance of \(\Delta\)d, \(\Delta\)b = (\(\Delta\)d)/2 = 6 mm, because the two eyes are symmetrical, obtained by calculation; a\({}_{1}\) is 42.1 mm, and a\({}_{2}\) is 90.2 mm. Obviously, the distance between the displayed image and eyes in the system with a 3.74-\(\upmu\)m pixel pitch SLM can be much smaller than that with the 8.- \(\upmu\)m pixel pitch SLM. Figure 3b shows the dependence of the distance between the image and eyes on the pixel pitch of SLM. Figure 4 shows the schematic layout of the  
Figure 2: **(a)** Viewing angles of spatial light modulators (SLMs). **(b)** Dependence of the viewing angle on the pixel pitch of SLM.  
monocular system, which consists of two SLMs (SLM1 and SLM2) for two eyes, two beam splitters (BS1 and BS2), a half wave plate, etc. Some mirrors were used to make the whole system more compact.  
In Figure 5a, the image plane and convergence plane of the stereoscopic virtual reality HMD are shown. If they are in different planes, once all the elements of the HMD are fixed, there will be difficulty in making them be in the same plane. The image plane and convergence plane of the holographic virtual reality HMD can easily be in the same plane, as shown in Figure 5b, because the depth of field of the holographic 3D display can be set in the process of digital hologram computation (we changed some parameters in our MATLAB program of the computer-generated holog  
Figure 4: Schematic layout of the monocular system, BS: beam splitter, BE: beam expander, \(\lambda/2\): half wave plate, M: mirror.  
Figure 3: (**a**) Schematic diagram of the viewing angle and pupillary distance range difference. (**b**) Dependence of the distance between the image and eyes on the pixel pitch of SLM.  
of the holographic reconstruction image). The computer-generated holograms are displayed in SLMs. This can improve the interaction and can eliminate the limitation of stereoscopic VR HMD in Figure 5a.