Conventionally, optical components such as optical lenses and prisms have been used for optical devices such as a camera, a microscope, or a telescope, electrophotographic printing apparatuses such as a printer or a copier, optical recording apparatuses such as DVD, and optical devices for communication and industrial applications for example. A general optical lens has a fixed focal length. However, some of the above-described devices and apparatuses may use a so-called variable focusing lens. The variable focusing lens is a lens having a focal length that can be adjusted depending on the situation. A conventional variable focusing lens mechanically adjusts the focal length by combining a plurality of lenses. In the case of such a mechanical variable focusing lens however, the applicable scope was limited because of factors such as a response speed a manufacture cost, downsizing, and power consumption.
To solve this, such a variable focusing lens was considered for example that has an optical lens composed by a transparent medium using a substance having a variable refractive index and another variable focusing lens for which the shape of the optical lens is deformed instead of changing the position of the optical lens. As the former variable focusing lens, such a variable focusing lens has been suggested in which an optical lens is composed of liquid crystal. This variable focusing lens is structured so that liquid crystal is sealed by a container made of transparent substance (e.g., a glass plate). The inner side of the container is machined to have a spherical surface and liquid crystal is formed to have a lens-like shape. The inner side of the container also has a transparent electrode. By changing the voltage applied to this electrode, the electric field applied to the liquid crystal can be controlled. This can consequently control the refractive index of the liquid crystal by the voltage, thus achieving the variable control of the focal length (see Patent Publication 1 for example).
As the latter variable focusing lens, the lens having a deformable shape is frequently made of liquid. For example, the variable focusing lens disclosed in Non-patent Publication 1 is structured so that a space sandwiched between glass plates is filled with liquid such as silicon oil. The glass plates are machined to have a thin thickness. By externally applying a pressure to the glass plates by a lead zirconate titanate (PZT) piezo actuator, the lens entirely constituted by oil and the glass plates is deformed to thereby control the focal position. This variable focusing lens has the same operating principle as that of a lens of an eyeball.
Among the above-descried optical devices, a microscope is a device that is expected for practical application by the introduction of a variable focusing lens. A microscope has a very shallow depth of field because the microscope uses an objective lens having a high numerical aperture (NA). Thus, if a microscope observes a three-dimensional object as a measuring object, the microscope can simultaneously observe only a partial region of the three-dimensional object covered by the focal point height. Thus, in order to obtain the total image of the three-dimensional object, the three-dimensional object must be observed while gradually moving the lens system of a stage having thereon the measuring object in the up-and-down direction. Another technique also has been nearly established according to which the image is photographed at every fixed height while moving the stage and a plurality of photographed images are processed to synthesize a stereoscopic image.
In recent years, among microscopes, confocal microscopes have been used in a wider range of applications. With reference to FIG. 1, the principle of the confocal microscope will be described. In this system, light emitted from a measuring object 1 is converted by a lens 3 (generally referred to as an objective lens) to parallel beam that is subsequently collected by a lens 4. At a position of the light collection point, a pinhole 5 having a similar diameter to that of the spot diameter is placed and the power of light having passed through the pinhole 5 is detected by a light detector 6. A case will be considered where a measuring object 2 is placed immediately below the measuring object 1. As shown by the broken line of FIG. 1, the light emitted from the measuring object 2 passes through the lenses 3 and 4 and is subsequently collected at a position lower than the position of the pinhole 5. The light emitted from the measuring object 2 expands again when reaching the height of the pinhole, thus causing a remarkably-reduced amount of light passing through the pinhole 5. Specifically, this system can detect only an optical signal emitted from the position of the measuring object 1.
When a measuring object has different objects from which light is emitted, it is difficult for a general microscope to extract only the information regarding the measuring object because the light from these objects is superimposed as noise on the light from the measuring object. A confocal microscope on the other hand can extract only the information regarding the measuring object by adjusting the layout of the optical systems. However, only the information regarding an object at the position of the measuring object 1 of FIG. 1 can be simultaneously extracted. Thus, in order to obtain the total image of the object, three-dimensional data must be collected by gradually moving the object in the up-and-down and left-and-right directions. The object can be moved, without moving the measuring object itself, in the left-and-right direction by an apparatus that uses a component such as a galvano mirror for rapidly deflecting the direction of light beam (light deflector). However, the measuring object was generally moved in the up-and-down direction by mechanically moving the measuring object.
However, it took a long time to acquire the entire data regarding a measuring object if a conventional microscope including a confocal microscope was used to perform the series of measurements by mechanically moving in the up-and-down direction a stage having thereon the measuring object. To solve this, if the focal point can be electrically controlled by a variable focusing lens instead of moving the stage, the measurement accuracy can be improved and an improved scanning speed also can be expected.
Conventional variable focusing lenses include: a variable focusing lens for mechanically adjusting the focal length; a variable focusing lens for controlling the refractive index by applying an electric field to liquid crystal; and a variable focusing lens whose shape is deformed by a PZT piezo actuator for example. However, any of these conventional variable focusing lenses had a limited response speed required to change the focal length and thus could not be used for a high-speed response of 1 ms or shorter, thus failing to catch a high-speed phenomenon.
It is an objective of the present invention to provide a variable focusing lens that can change the focal length at a high speed and a microscope that can use the variable focusing lens to measure a three-dimensional object including information in the height direction at a high speed.