Rapid Liquid Lens (RLL)
Concept
Variable-focus lens technology has rapidly progressed recently. In particular, liquid variable-focus lenses based on electrowetting gathered attention. This type of lens uses an interface between two immiscible liquids as a refractive surface so that the surface is good spherical shape.
Another potential advantage of the variable-focus lens is high-speed response. Such lenses can change their focal length in very short time, since the slight change of the surface shape results in significant focal position shift. And high-speed response is important especially for optical instruments that need high-speed axial scanning such as laser scanning confocal microscopes. It's also important to realize high-speed focusing or zooming for visual inspections and security applications. The liquid variable-focus lenses based on electrowetting are, however, not enough fast for such applications. This is because this type of driving mechanism changes only the boundary condition of the interface. The interface shape change occurs only because of inherent surface tension. There are no additional forces to help the interface to change their shape.
We concentrated on speeding up focusing speed and proposed a variable-focus lens with 1-kHz bandwidth. This lens transforms its lens surface rapidly using the liquid pressure generated by a piezo stack actuator. This mechanism also includes a built-in motion amplifier with high bandwidth to compensate for the short working range of the piezo stack actuator. The lens, however, included large aberrations because the refractive surface profile was not spheric.
To solve this problem, we proposed a new structure of a variable-focus lens using the interface between liquids. The goal of this lens is to achieve both high-speed response less than 1 ms and an acceptable amount of aberrations for image forming optical systems. The initial prototype was fabricated. A photograph of the prototype is shown in below (a), and schematic figure of its focusing mechanism is shown in (b). Step response time was measured using PDMS of various kinematic viscosity (50, 500, 5000 [cSt]).
Movie
A chart was captured by a high-speed camera (5000 [frames/s]) thorough the initial prototype. This movie shows three responses with different kinematic viscosity of PDMS (50 [cSt], 500 [cSt], 5000 [cSt]). Play back speed is 0.003x.
References
- Hiromasa Oku, Masatoshi Ishikawa: Rapid Liquid Variable-Focus Lens with 2-ms Response, 19th Annual Meeting of the IEEE Lasers & Electro-Optics Society (Montreal, Canada, 2006.11.2) / Proceedings pp.947-948. [PDF (527K)]*IEEE
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