Chapter 1 Dissertation Organization & Introduction
1.2 General introduction
1.2.3 Review of method to achieve auto-focus function
There are several ways to vary effective focus length of image system to achieve auto-focus function. They can roughly be classified to two groups, system with moving components and system without moving components. These techniques will be introduced in this section
System with moving components is characteristic with that it has moving components to move lens or lens group back and forth to vary effective focal length.
Therefore, the system can focus on object located at difference distance from lenses module, as shown in Figure 1-9.
The principle can be illustrated by this equation [11]:
2 distance of d and whose focal length are f and f individually. According to equation
Figure 1-9 The schematic drawing of auto-focus system with a moving component.
Motor has been used for camera with auto-focus or zoom function for a long time, but with the trend to downscale of camera built-in portable device, especially the thin thickness, miniaturizing camera module becomes important. Therefore, small motor design has appeared, for instance, miniature stepping motor, piezoelectric motor, and voice and coil motor.
To downscale purpose, more and more techniques are proposed to achieve auto-focus function without any moving components. The advantage of system without moving components is that the total track length has the potential to become shorter than system with moving components since there need no space for moving component to move forth and back. The system design will become more flexible and it is easier to save space for designer. The concept of system without moving components is that tuning optical power of one or more optical elements to change system’s effective focal length. Figure 1-10 is a simple schematic of system to achieve auto-focus function without moving components. Lens 1 and Lens 3 are the fixed optical power element, and Lens 2 is tunable optical power element. Tuning optical power of Lens 2 can change effective focal length of system.
Figure 1-10 The scheme of auto-focus system without moving component.
Liquid crystals (LCs) have applications for light spatial modulators, waveguides, phase modulators, liquid crystal switches, light detectors, and displays. In recent years, it has been reported that LCs has the capability of auto focus function for lenses application. Akita University has developed a technique which is a liquid crystal lens with focal length variable from negative to positive [12-14]. The lens like phase retardation profile from positive to negative lens can be obtained by varying the two voltages appropriately, as show as Figure 1-11. Therefore, the LC cell acts as a lens whose focal length can be control by the voltages.
Another element has used to achieve auto-focus function without moving component is liquid lens. Liquid lenses developed by Varioptic [15-17]. The mechanism is based on the phenomenon of electro-wetting. Figure 1-12 is the schematic of the design of Varioptic. Two immiscible liquids, water and oil, one is used as conducting layer and the other is used as insulating layer, are contained in the two transparent
(a) (b) (c)
Figure 1-11 Lens like phase retardation when applying voltage on LC cell. [14] (a) No voltage apply. (b) result a positive optical power lens like phase retardation. (c) result a negative optical power lens like phase retardation.
oil, is determined by the balance of the surface tension produced by the two liquids.
On the other words, the focal length of the liquid lens is controlled by varying applied voltage. However, its disadvantage is that coma aberration may occur due to gravitational pull, as shown in Figure 1-13
Figure 1-12 Structure of liquid lens developed by Varioptic. [17]
Figure 1-13 The disadvantage of liquid lens is that coma aberration may occur due to gravitational pull. [16]
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