A recent publication introduces and discusses the innovative use of an electronically focusable liquid lens as the means to address chromatic aberration problems experienced in prior art, time-sequential holographic display systems.
The team undertaking the research is headed by Di Wang of the School of Electronics and Information Engineering, Sichuan University (Chengdu, China). The team’s most recent publication is entitled “Color Holographic Projection Based on Liquid Lens.” It was presented at the 2015 SID International Symposium held in San Jose, CA and appears as article 30.2, on pages 435-437 in the conference proceedings.
The approach to producing a full color hologram adopted by the team is based on the use of time-sequential projection. In this approach, as a separate first step, a hologram of the object or scene is recorded (or computed) three times, one time in each of the primary colors. The configuration of the specific system used to produce the hologram is illustrated in the left hand figure below.
In this system, a Spatial Light Modulator (SLM) is used to sequentially present each of the color holograms. While the SLM is presenting the red hologram, it is illuminated by red laser light. Proceeding synchronously, the same is true of the blue and green components of the holographic image.
In prior art color holographic displays of this type, a common problem has been chromatic aberration in the projected image. This due to the fact that the focal point of the conventional, fixed focus lens was slightly different for the each of the three wavelengths of laser light.
To address this issue, the team proposed the use of an electronically focusable liquid lens. The idea being to adjust the focal length of the liquid lens synchronously with the color of the illuminating laser such that each of the three color images is focused onto the same plane. In this way, chromatic aberration can be minimized.
The structure of the liquid lens is illustrated on the right in the figure above. The lens can be thought of as a container filled with two immiscible liquids. Liquid 1 is a transparent oil. Liquid 2 is a transparent, conducting droplet. The lower substrate of the container is transparent and patterned with electrodes. Four rectangular sheets of glass form the walls of the container. These are coated first with conductive ITO and then with insulators Teflon and SU8.
Note the contact angle between the interface of the liquids and the wall. When no voltage is applied, the Liquid 1 – Liquid 2 interface bends upward and the lens is diverging. When a steady state voltage is applied to the lens device, the curvature of the interface is effected by an electrowetting effect. When the applied voltage exceeds a critical value, the interface bends downward and the lens becomes converging. In this way, a variation in the voltage applied to the electrode can be used to produce a controllable change in the focal length of the liquid lens.
Although the performance data included in the article indicates that the proposed focusable liquid lens does indeed work, it is not entirely free of problems. The first is that the response time of the liquid lens is about 80ms. This is really a bit too slow to produce a time sequential image that can be fully visually fused. The team envisions addressing this issue by reducing the thickness of the insulating layer and/or choosing an insulating layer with a higher dielectric constant. A second problem is that the aperture of the current liquid lens is quite small. This, in turn, may limit the size of the image. Problems aside, the liquid lens approach seems to have promise and it is reasonable to expect that further progress will be reported. – Arthur Berman
