1. Multifocal method using Liquid Crystal
Xuan Wang
Oct, 28th

2. Multifocal method using Liquid Crystal
High-speed switchable lens enables the development of a volumetric stereoscopic display. Love, Gordon D., et al. Optics express (2009):
A multi-plane optical see-through head mounted display design for augmented reality applications. Liu, Shuxin, Yikai Su et al. Journal of the Society for Information Display 24.4 (2016):
Fast-response switchable lens for 3D and wearable displays. Lee, Yun-Han, Fenglin Peng, and Shin-Tson Wu. Optics express 24.2 (2016):
Gordon : university of California Berkely
Yikai Su : Shanghai Jiao Tong University
Shin-Tson Wu : university of central florida

3. High-speed switchable lens
High-speed switchable lens enables the development of a volumetric stereoscopic display
They present a new stereoscopic display system using a high-speed, switchable lens integrated with computer display.
The lens is placed in front of the eye and is synchronized to the graphic display such that each depth region in the simulated scene is presented when the lens is in the appropriate state. In this way, they construct a temporally multiplexed image with correct focus cues.
Gordon : university of California Berkely

4. High-speed switchable lens
Key Element
1 Polarizer
2 Birefringent lenses
2 Liquid Crystal Modulators
The lens consist of ….Birefringent materials have two refractive indices (ordinary and extraordinary) depending on the polarization of the incident light, so the lens has two focal lengths that are selected with a polarization modulator. They use ferroelectric liquid-crystal modulators to switch the polarization orientation. the output polarization is rotated by either 0° or 90°. The switching between focal lengths can occur very quickly (<1ms).

5. High-speed switchable lens
Lens Performance
The four focal
states are shown. The distance:
(a) 285 mm
(b) 375 mm
(c) 590 mm
(d) 970mm
This is the performance the lens they made. The lens assembly was focused to each of its four focal lengths in the four pictures.

6. High-speed switchable lens
Setup
They constructed two display systems: one uses two CRTs and lens assemblies, a pair for each eye (Fig. a,b), and presents images simultaneously to the two eyes. The other uses two lens assemblies and one CRT and presents separate images to the two eyes in a time-sequential fashion (Fig. c,d);

7. High-speed switchable lens
Results
This is a video showing their results. Their lens system has not yet been optimized to minimize aberration, so even better quality may be able to obtain.

8. High-speed switchable lens
A multi-plane optical see-through head mounted display design for augmented reality applications
They presented a design for an optical see-through HMD utilizing multiplane display technology.
Polymer-stabilized liquid crystal scattering shutters were used in projecting slices of a 3D scene. A proof-of-concept
two-plane optical see-through HMD prototype is demonstrated.
Yikai Su : Shanghai Jiao Tong University

9. High-speed switchable lens
Polymer-stabilized liquid crystal scattering shutter
A PSLC shutter is an electronically switchable film.. Without applied voltage, a PSLC film is in scattering state, and with an appropriate voltage, it will become optically clear. It has response time in the sub millisecond range.

10. High-speed switchable lens
Setup
This is the design scheme of the optical see-through HMD system. The system includes four main components: a high speed projector, a stack of PSLC scattering shutters, an ocular lens, and an beam splitter. The projector projects a series of pictures onto the PSLC films in time sequence. By properly controlling the timing, the projection of a picture and the activation of a PSLC film are synchronized, so that at one moment, only one PSLC scattering shutter is in scattering state, while all the others are in clear state. a reconstructed 3D image will be displayed in the form of multiple 2D slices on the PSLC stacks.
And this is the experimental setup. They use an amplitude-modulated spatial light modulator (SLM) as the projector.

11. High-speed switchable lens
Results
They demonstrated a realistic augmentation of virtual letters “SJ” and “TU” with two real objects(optical components). The near object (part of a polarizer) and the letters “SJ” were at 20cm from the digital camera, while the far object (an aperture) and the letters “TU” wereat 80 cm.

12. Fast-response switchable lens
Fast-response switchable lens for 3D and wearable displays
They presented a focus-switching device for 3D head mounted displays. A twisted nematic (TN) cell liquid crystal is used as polarization switch to control the optical path length through the system. Different polarization state leads to different path length in the proposed optical system, which in turn results in different focal length.
They also proposed some configuration to cascade such device structures together to generate more different focuses for 3D displays.
Shin-Tson Wu : university of central florida

13. Fast-response switchable lens
Key Element
The LC device used here is a 90°twisted nematic (TN) cell. This is an electrically tunable achromatic half-wave (λ/2) plate.
For a linearly polarized input light, say p-wave, the output light can be converted to s-wave if the applied voltage is off (V = 0) or it remains p-wave if V>>Vth (threshold voltage).

14. Fast-response switchable lens
Experimental setup
The total path length from L1 to L2 for the red route is d1 = 43 cm.
The total path length from L1 to L2 for the blue route is d2 = 48 cm
After passing through L1, one polarization (red arrow, say s-wave) passed straight through PBS, 45°-oriented (with respect to the polarization, in the plane that was perpendicular to the propagation direction) λ/4 plate, and reached M1. Upon reflection, the beam passed through the λ/4 plate one more time and its polarization state was converted to p-wave.
On the other hand, the other polarization (blue arrow, say p-wave) was reflected first by the PBS toward the 45°-oriented λ/4 plate and M2. Upon reflection from M2, the beam passing through the λ/4 plate one more time and was converted to s-wave.

15. Fast-response switchable lens
Results
Photos on the right side are the enlarged and inverted pictures of the red dashed blocks.
As one looks through the PBS, two different inverted images can be observed upon switching-on and -off the TN cell. We can clearly see
the change in image planes from the references, which are located at 50 cm and 200 cm away from the camera, respectively.

16. Fast-response switchable lens
Features
Response time
The rise time and the decay time to be 4.3 ms and 1.0 ms, respectively. Note that the response time was not optimized due to the limited TN cells they have. (the rise time and the decay time can reach 2.8 ms and 0.6 ms theoretically)
Voltage
The power they use is an AC voltage(1kHz) of only 10V, which is safe for human.
Low chromatic aberration
PBS, TN and retardation plates are all achromatic

17. Fast-response switchable lens
Configuration
They provided some generalized device configurations for this method. To extend for multi-focus switching, they also gave two possible embodiments to cascade more than one set of the device. So this will have 8 possible focuses.

18. Thank you!