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Abstract
In this paper, we propose a holographic image generation technique for contact lens displays. The proposed technique employs a phase-only spatial light modulator (SLM), a holographic optical element (HOE) backlight, and a polarizer. The proposed holographic technique can generate 3D images apart from the contact lens displays. Therefore, the eyes can focus on the 3D images while simultaneously observing the real scene through the phase-only SLM and the HOE backlight, which provides see-through capability. A bench-top experimental system was constructed to verify the far-distance image generation capability and see-through function.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Augmented reality (AR) technology can modify the real world by adding virtual images to real scenes, which has attracted the interest of researchers as it represents an evolutionary change in the interactions between humans and computers [1]. Since the invention of AR technology, head-mounted displays have long been used as the visual interface device. The development of contact lens displays brings AR technology to a new level because it frees users from the need to wear headsets.
Many efforts have been devoted to integrating electronic devices with the contact lens. A technique was developed to integrate a liquid crystal display device into the contact lens [2]. A method for integrating an LED and antenna into the contact lens was also developed [3–5], in which an electromagnetic wave from a remote transmitter is received by the antenna to provide electric current to the LED. The integration of solar cells [6], flexible batteries [7,8], and biofuel cells [9,10] have been proposed to supply electricity to electronic devices in contact lenses. Wireless communication devices have also been embedded in contact lenses [11–13].
To realize contact lens displays, optical imaging techniques must also be developed. With respect to image formation, the significant issue that must be resolved for contact lens displays is the inability of the eyes to focus on a display device embedded in the contact lens as the display screen is too close to the eyes. Lingley et al. [4] proposed the use of two-dimensionally aligned LEDs for image generation and combined a micro Fresnel lens with each LED to converge light on the retina. Chen et al. proposed the use of single-mode optical fibers to collimate light from LEDs on the retina [14]. These researches aim to project two-dimensional patterns produced by an LED array onto the retina using micro optical devices. Because LEDs are non-transparent current-driven devices, increasing the number of LEDs in the contact lens could decrease the transparency and increase power consumption of contact lens displays.
Unlike the contact lens display techniques mentioned above, new contact lens structures have been proposed to enable eyes to focus on display devices located in the vicinity of the eyes [15,16]. In these techniques, a small lens is embedded at the center of the contact lens to image the screen of an outer display on the retina. To provide see-through function, light from the display is transmitted through the center lens region and light from the outer scene is transmitted through the remaining region. Light from the display and outer scene are selectively transmitted through their corresponding regions by the use of wavelength selectivity [15] and polarization selectivity [16].
Here, we propose an optical imaging technique based on holography to enable the embedding of a display device in the contact lens to produce focusable images for the eyes. The proposed technique employs a phase-only spatial light modulator (SLM) and a laser backlight with a holographic optical element (HOE) to enable wavefront reconstruction. Because the phase-only SLM does not modulate the amplitude of light and the HOE backlight has high transmittance, the proposed technique provides high transmittance to the contact lens displays and see-through capability using polarization selectivity. Our preliminary experimental results obtained by this proposed technique, as reported in a conference paper [17], involved the use of a polarization beam splitter instead of an HOE backlight. In this study, we constructed an experimental system using the HOE backlight to fully investigate the proposed technique. We discuss the experimental results in detail and consider the pixel structure of the SLM.
Several holographic AR displays using HOEs have been proposed. In [18], laser light modulated by an SLM enters a waveguide using an in-coupling HOE and exits from the waveguide using an out-coupling HOE to be directed to an eye. In [19], an HOE is used to redirect laser light modulated by an SLM to enter an eye. In [20], an HOE is also used to redirect laser light modulated by an SLM; however, the redirected light is divided into plural viewpoints to enlarge the eye box. In the above-mentioned techniques, the SLM is located outside the waveguide and the HOE is used as a beam combiner so that the eyes observe the outer scene only through the HOE. In [21], a transmission-type SLM is illuminated by an HOE backlight, and the outer scene is observed through them. In this configuration, the light from the outer scene is modulated by the SLM. We use a similar configuration to realize the contact lens display in this study. To address the outer scene modulation issue by the transmission-type SLM, we used the polarization selectivity of the phase modulation characteristic of a phase-only SLM. Thus, the transmission-type phase-only SLM with a polarizer is combined with the HOE backlight that emits polarized light to be modulated. The use of the phase-only SLM increases the transmittance of light from the outer scene.
The remainder of this paper is organized as follows. In Section 2, we describe the technique proposed for constructing holographic contact lens displays. In Section 3, we discuss miniaturization of the proposed technique. Experimental verification of the performance of the proposed technique is presented in Section 4. A detailed discussion follows in Section 5, and we draw our conclusions in Section 6.
2. Theory
When a display device is embedded in a contact lens, as shown in Fig. 1(a), the lens of the eye (crystalline lens and cornea) cannot focus on the display screen due to its close proximity to the eye. To address this problem, previous techniques [4,14] have added micro optical elements to all display pixels and cancel the lens power of the eyes, as shown in Fig. 1(b). In this study, we propose the use of the holographic technique to enable the eyes to naturally focus on the produced images. Figure 1(c) shows a schematic illustrating the concept of the proposed holographic contact lens display. A display device embedded in a contact lens displays hologram patterns, which generate a wavefront emitted from three-dimensional (3D) images located far from the eyes. Thus, 3D images are generated apart from the display device based on the wavefront reconstruction. Consequently, the eyes can focus on the produced images.
Fig. 1. Concept of holographic contact lens display: (a) problem of contact lens display, (b) conventional solution using LEDs and micro optical elements, and (c) proposed technique based on holography.
Figure 2 shows an illustration of the proposed optical system of the holographic contact lens display, which consists of a phase-only SLM, a laser backlight using an HOE [19,21,22] connected to a laser diode, and a polarizer. This structure provides see-through capability to the contact lens display, while also generating focusable images for the eyes. The HOE backlight emits horizontally polarized laser light to illuminate the phase-only SLM. The phase-only SLM modulates the phase of the horizontally polarized light and does not modulate the phase of the vertically polarized light. The horizontally polarized light is modulated by the phase-only SLM for transformation into a wavefront of the 3D images. The polarizer transmits the vertically polarized light from the outer scene, which is not modulated by the SLM. Because of the wavelength selectivity of the HOE, the HOE backlight has high transmittance for light from the outer scene. The phase-only SLM also has high transmittance for light from the outer scene. Thus, see-through capability is realized.
Because the proposed technique enables both the produced images and outer scene to be shown directly to the eyes through the phase-only SLM, the system has a small form factor. However, the pixelated screen structure of the SLM in the contact lens affects the see-through function. When the LED array is used for the image formation as proposed in the previous research studies [3–5,14], the diffraction of light by the periodic structure of the LED array also affects the see-through function. When the pixel pitch and resolution of the SLM are denoted by p and N×N, respectively, the light from the outer scene is diffracted in multiple directions with an angle of mλ/p, where m is the diffraction order and λ is the wavelength of light. As multiple outer scenes are observed, the viewing angle, ϕ, of the outer scene is given by:
The viewing angle of the outer scene can be enlarged by reducing the pixel pitch. The intensity of the higher-order diffracted light is given by sinc2(m a), where a denotes the ratio of the pixel width to the pixel pitch. Figure 3(a) exhibits the computer simulation of the diffraction of the outer scene for the fill factor of the SLM with 64% (a = 0.8) and 81% (a = 0.9), when p = 1, 3, and 5 µm and λ = 0.5 µm. The simulation was performed for the outer scene with a viewing angle of 60°. When a = 0.8, the weak diffracted outer images were observed. When a = 0.9, the diffracted outer images became negligible. When the fill factor of the SLM increased, the diffracted outer scene became weaker, resulting in enlargement of the viewing angle of the outer scene. Thus, the viewing angle of the outer scene can be enlarged by reducing the pixel pitch p and increasing the ratio a.Fig. 3. Simulation of diffraction of the outer scene by the pixel structure of the SLM: (a) regular pixel structure and (b) random pixel structure.
Another approach to decrease the intensities of the diffracted outer scene is the introduction of randomness into the pixel structure of the SLM [23,24]. As there is a certain range in the pixel pitch for a random pixel structure, higher-order diffraction images are averaged to be negligible. Figure 3(b) shows a computer simulation using a random pixel structure for the fill factor of a = 0.1, 0.2, and 0.3 when p = 5 µm, with reference to [23]. The use of the random pixel structure can reduce the intensities of the diffracted outer scenes. As a becomes higher, the contrast of the images becomes lower; this is because the randomness of the pixel positions depends on the fill factor. The fill factor should be decreased to allow for the random displacement of the pixel positions.
The pixel structure of the SLM also generates the higher-order reconstructed images. The higher-order reconstructed images are also generated with an angle of mλ/p and an intensity of sinc2(m a). The above-mentioned techniques to reduce the diffraction of the outer scene can also reduce the higher-order reconstructed images. The depth position of the reconstructed images is limited by the sample theorem as follows:
Recently, researchers have developed HOEs that use a photopolymer with a diffraction efficiency of ∼90% [25–27]. Because HOEs typically provide a higher diffraction efficiency in the TE mode (polarization perpendicular to the incidence plane) than in the TM mode (polarization parallel to the incidence plane), the HOE backlight is designed to illuminate the phase-only SLM by horizontally polarized light. Thus, light from the outer scene is vertically polarized by the polarizer. Because the HOE backlight utilizes the total internal reflection of light in the waveguide plate, the refractive indices of the waveguide substrate and the contact lens material, and the waveguide structure must be carefully chosen.The electro-optical power conversion efficiency of laser diodes is generally higher than that of LEDs, and the light efficiency of the HOE backlight and the transmittance of the phase-only SLM are high. Thus, the energy efficiency of the proposed holography-based system might be higher than that of previously proposed LED-based systems.
3. Miniaturization of devices
In this section, the possibility is discussed of implementing the proposed optical system into contact lenses. Particularly, the thicknesses of the optical devices are considered because typical contact lenses can have a thickness of as little as approximately 0.1 mm.
When a liquid-crystal SLM is used for the phase modulation, it comprises a liquid-crystal layer and transparent electrodes. The liquid-crystal layer for the phase modulation is several microns thick, and the transparent electrodes have a thickness of less than one micron. The implementation of the liquid-crystal display in the contact lens was demonstrated in [2]. Thus, the phase-only SLM could be implemented in contact lenses.
The photopolymer is several microns thick. The use of multiple total internal reflection inside the waveguide enables a thin backlight. Recently, waveguides used for AR glasses that are ∼1 mm thick and several tens of millimeters long have been developed. The scaling law can be applied because the total internal reflection is used. As the length of the waveguide used for the proposed optical system is several millimeters, a waveguide with a thickness of ∼0.1 mm might be possible as a result of a simple consideration.
The typical thickness of the polaroid polarizer used for liquid-crystal displays is approximately 30 µm. When the wire-grid polarizer is used, the thickness is several microns. Therefore, the polarizer can be incorporated into contact lenses.
Regardless of whether the edge-emitting laser diodes or the vertical-cavity surface-emitting lasers are used, their thicknesses can be made to be less than 0.1 mm. They can, therefore, be incorporated into contact lenses. The integration of LEDs into contact lenses was demonstrated in [3–5].
4. Experiments
Before integrating the proposed structure into contact lenses, the performance of the proposed holographic image generation technique was verified using a bench-top experimental system.
Phase-only SLMs generally employ a parallel-aligned nematic liquid crystal to modulate the phase of linearly polarized light. Unfortunately, the commercially available phase-only SLMs are reflection-type SLMs; transmission-type SLMs are not available. Thus, we used a transmission-type twisted nematic liquid-crystal SLM (TN-SLM), which is typically used for amplitude modulation, because it can modulate the phase of circularly polarized light [28,29].
Figure 4 shows a schematic of the experimental system, which consists of a polarizer, a HOE backlight, a quarter-wave plate (QWP), and a TN-SLM. The QWP was added to transform the horizontally polarized light into right-handed circularly polarized light whose phase was modulated by the TN-SLM. The QWP also transformed the light from the outer scene into left-handed circularly polarized light whose phase was not modulated by the TN-SLM.
The TN-SLM used for the experiment was an LC 2012 (HOLOEYE Photonics AG) with a resolution of 1,024 × 768 and a pixel pitch of 36.0 µm. As a light source, we used a fiber-coupled laser diode with a wavelength of 457 nm. An aspherical micro-lens was used to increase the divergence of the laser light emitted from the fiber end.
The HOE backlight is explained as follows: Fig. 5(a) illustrates the structure of the HOE backlight. A spherical wave entering the slanted side of the waveguide is totally reflected twice in the waveguide so that its beam diameter increases. Then, the HOE reflects the spherical wave to transform it into a plane wave that proceeds as normal to the HOE, exiting from the waveguide and illuminating the phase-only SLM in a normal direction. The HOE was fabricated using recording fringes generated by interfering a spherical wave with a plane wave. Figure 5(b) shows the recording process of the HOE. A photopolymer (Bayfol HX 200, Covestro AG) was used as the HOE material. A DPSS laser with a wavelength of 457.0 nm was used as the coherent light source. Two prisms were used to interfere the spherical wave with the plane wave with a total reflection angle. Then, the HOE was peeled from the prism. This was followed by the fixing process using ultra-violet light. The measured diffraction efficiency of the HOE was 35.0%. The fabricated HOE was pasted on an acrylic waveguide. The measured light efficiency of the HOE backlight was 26.8%, and the measured transmittance of the HOE backlight was 88.9%.
Fig. 5. The HOE backlight used for the proposed structure of holographic contact lens display: (a) structure of the HOE backlight and (b) fabrication process of the HOE.
Figure 6 shows a photograph of the constructed experimental system, through which the measured transmittance of light from the outer scene was 20.0%.
First, we measured the phase modulation of the TN-SLM. The Mach-Zehnder interferometer was used and the shifts of the interference fringes were measured to calculate the phase modulation. Figure 7 shows the measured phase modulation characteristics of left- and right-handed circularly polarized light. The phase of the right-handed circularly polarized light was modulated, whereas that of the left-handed circularly polarized light was not. The maximum phase modulation was approximately 1.5π. The phase distributions of the phase-only holograms were calculated using the Gerchberg–Saxton (GS) algorithm [30]. The computer simulation showed that the brightness of the reconstructed images obtained with the continuous phase modulation from 0 to 1.5π was quite low. Thus, we quantized the phase distribution with four phase levels (0, π/2, π, and 3π/2). Because the Fresnel-type computer-generated holograms were displayed on the phase-only SLM to produce the reconstructed images at specific distances, the quadratic phase distribution corresponding to the specific distance was added to the phase distribution obtained by the GS algorithm. The phase change of the quadratic phase distribution rapidly increases from the center toward the periphery. Considering the sampling theorem, the central area of the SLM (324 × 324 pixels) was used for displaying the hologram patterns, and the remaining area was covered by a mask. Thus, from Eq. (2), the depth of the images should be z ≥ 0.92 m.
Rather than using human eyes, retinal images were captured using a video camera with a pupil diameter of 5 mm (the average pupil diameter in humans). The lens of the camera was positioned to make contact with the TN-SLM. Figure 8(a) shows the retinal image obtained when the reconstructed image (characters “ar”) was produced at a distance 1,500 mm from the TN-SLM, and Fig. 8(b) shows the retinal image obtained when the reconstructed image (characters “AR”) was produced at a distance of 2,000 mm. Two real objects, a toy deer and toy car, were placed at distances of 1,500 mm and 2,000 mm, respectively, and the reconstructed images and real objects could be observed simultaneously. The two objects were observed using the experimental system. In Fig. 8(a), the focus of the camera was at a distance of 1,500 mm, so both the characters “ar” and the toy deer were observed without blur. In Fig. 8(a), the focus of the camera was at a distance of 1,500 mm, so both the characters “ar” and the toy deer were observed without blur. In Fig. 8(b), the focus of the camera was at a distance of 2,000 mm, so the characters “AR” and the toy car were observed without blur. The image quality of the reconstructed images was low because they contained background light. Then, we replaced the HOE backlight with a polarizing beam splitter that reflected the horizontally polarized light and transmitted the vertically polarized light. Figure 9 shows the experimental results of which the image quality is better than those shown in Fig. 8. Hence, the low image quality of the reconstructed images shown in Fig. 8 resulted, in the main, from the imperfection of the HOE backlight. In Fig. 9, the accommodation effects on the reconstructed images and the real objects can be more clearly seen.
Fig. 9. Captured retinal images when the HOE backlight was replaced with a polarizing beam splitter: focus at (a) 1,500 mm and (b) 2,000 mm.
As shown in Figs. 8 and 9, weak diffracted images of the outer scene could be observed. As explained in Sec. 2, the angle pitch of the diffracted outer scenes provides the viewing angle of the outer scene. The calculated viewing angle of the outer scene was ϕ = 0.73° from Eq. (1), and the measured one was 0.73°. The viewing angle was small because the pixel pitch of the experimental SLM was large (36.0 µm).
5. Discussion
The experimental results obtained by the video camera indicated that eyes might be able to simultaneously focus on holographic images and real objects. Therefore, the proposed holographic image generation technique for contact lens displays is suitable for AR applications. Human eyes might be able to naturally focus on images superimposed on a real scene. This capability of providing focusable images is the advantage of our proposed technique, which differs significantly from previous techniques [4,14] that aim to provide images whose blurs do not depend on the focus of eyes.
The light transmittance of the experimental system from the outer scene was 20.0%, but the ideal transmittance of the proposed technique is 50%. This shortfall occurred because the fill factor of the SLM was 58.0% and the measured transmittance of the polarizer was 43.0%. To increase the transmittance of the holographic contact lens displays, an SLM with a high fill factor and a polarizer with low absorption are required.
As shown in Fig. 8, the holographic images contain many speckles. This is mainly because we used the GS algorithm to calculate phase-only holograms, which provides reconstructed images with random phase distributions. The use of LED light to illuminate the SLM might reduce the speckles, but this might decrease the diffraction efficiency of the HOE. Because the number of phase levels of the holograms was only four, we considered the effects of the number of phase levels on the reconstructed images. Figure 10 shows the computer simulation results for several numbers of phase levels. The computer simulation was performed with a resolution of 324 × 324 pixels, this being equal to the resolution of the phase-only holograms. From the computer simulation, it can be seen that, as the number of phase levels increases, the image quality improves.
Fig. 10. Computer simulation of reconstructed images when the number of phase levels of holograms is (a) 4, (b) 8, (c) 16, and (d) 256.
In Fig. 8, higher-order diffraction images are observed in addition to a reconstructed image. The intensities of the higher-order diffraction images depend on the fill factor of the pixel structure of the SLM. When we assume a = 0.762 from the fill factor of 58.0% of the TN-SLM, the theoretical diffraction efficiency of the first-order diffraction image is 8.07%. To decrease the intensities of the higher-order diffraction images, the fill factor must be increased. When the fill factor approaches 100%, the intensity of the first-order diffraction image approaches zero. However, the fill factor of SLMs generally become lower when their resolution is increased to produce high-resolution reconstructed images. Another approach for decreasing the intensity of higher-order diffraction images is to introduce randomness into the pixel structure of the SLM [23,24]. Figure 11(a) shows a computer simulation of a reconstructed image using a random pixel structure when the fill factor was set to a = 0.2, with reference to [23]. The computer simulation was performed with a resolution of 6,480 × 6,480 pixels, and the central area with a resolution of 1,500 × 1,500 pixels is shown. One pixel of the SLM was divided into 20 × 20 pixels to enable the representation of the fill factor of the SLM. For comparison, Fig. 11(b) shows a reconstructed image using the regular pixel structure with a fill factor of 100% (a = 1.0), and Fig. 11(c) shows that when a = 0.75, which corresponds to the experimental system.
Fig. 11. Computer simulations of retinal images using (a) a random pixel structure (a = 0.2) and regular pixel structures with a fill factor of (b) 100% (a = 1.0) and (c) 58.0% (a = 0.762).
6. Conclusion
In this paper, we proposed a holographic image generation technique that enables contact lens displays to produce focusable images for eyes. The combination of a phase-only SLM and an HOE backlight enables the generation of images at appropriate distances for eyes to focus on and see-through capability. An experimental system was constructed using a transmission-type TN-SLM and a photopolymer. Both the generation of images at distances of 1.5 m and 2.0 m and the see-through function were demonstrated. The transmittance of the experimental system was 20.0% because of the light absorbed by the polarizer and the low fill factor of the TN-SLM (58.0%). The elimination of higher-order diffraction images was discussed by the use of a high fill-factor pixel structure and a random pixel structure of the phase-only SLM.
Funding
Japan Society for the Promotion of Science (KAKENHI Grant Number 19H02189, KAKENHI Grant Number JP17K18872).
Acknowledgments
The authors acknowledge the support of Covestro AG for its provision of the photopolymer Bayfol HX film used for recording the HOEs.
Disclosures
The authors declare no conflicts of interest.
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