Optical element, lens unit and image pickup apparatus

An optical element and a lens unit each includes: a liquid container transmitting light at least in a predetermined optical axis direction, the liquid container containing an insulating liquid and an ionic liquid being mutually immiscible and optically transparent, and having different refractive indices; a first electrode contacting the ionic liquid in the liquid container; and a second electrode insulated from the ionic liquid in the liquid container. The ionic fluid is not subject to electrolysis and does not evaporate easily even at high temperature. Accordingly, the optical element, the lens unit and an image taking apparatus having the optical element can maintain high light-transmission and optical characteristics over a long period even in a high temperature environment.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical element transmitting light, a lens unit and an image pickup apparatus having subject light focused to obtain image data.

2. Description of the Related Art

There are cameras having a zoom function of freely setting a shooting field angle among electronic still cameras providing an image of a subject on a solid-state image sensing device such as a charge coupled device (CCD) and retrieving image data representing the subject as a signal and film cameras doing a photo shoot on a photographic film. Such cameras are equipped with an image taking lens of which focal length varies according to operation of a zoom switch. The image taking lens is generally a compound lens composed of a combination of multiple lens elements, where relative positions of the multiple lens elements are adjusted according to the focal length set up by the zoom switch. Such cameras have a cam mechanism. The cam mechanism conveys rotations of a motor according to the operation of the zoom switch, and the multiple lens elements thereby move back and forth in an optical axis direction respectively so as to adjust the relative positions and change the focal length.

There is a focus lens for focus adjustment among the multiple lens elements. There are the cases where a lens drive mechanism moving this focus lens is provided separately from the cam mechanism.

In recent years, there has been a proposal of a liquid lens of a variable focal length having two kinds of liquid of mutually different refractive indexes and mutually unmixable accommodated therein instead of the image taking lens having the drive mechanism (for instance refer to Non-patent Document, “Philips' Fluid Lenses”, [online], Mar. 3, 2004, Royal Philips Electronics, [searched on Mar. 31 ,2004],Internet, hereafter referred to as Non-patent document 1.)

The liquid lens proposed in Nonpatent Document 1 has two kinds of liquid of mutually different refractive indexes and mutually unmixable accommodated therein. One of the two kinds of liquid is a conductive solution having a supporting electrolyte dissolved therein, and the other liquid is insulating oil. These liquids are accommodated in a liquid container which is composed of a short glass tube having both ends thereof closed up by light-transmitting transparent end caps. Inner walls of this tube and one of the end caps are covered with a water-repellent film. In the case of the liquid lens thus configured, of the two kinds of liquid, the conductive solution repels the inner walls of the tube and one of the end caps covered with the water-repellent film. And the conductive solution stays in a semispherical form in contact with the other end cap, and so an interface portion between the conductive solution and the insulating oil functions as a concave lens. The liquid lens also has two electrodes for the sake of applying a voltage to the conductive solution, where one of the two electrodes is placed to contact the conductive solution while the other electrode is placed in the back of the water-repellent film. If the voltage is applied to such electrodes, electric charge is emitted from the electrode placed to contact the conductive solution into the conductive solution so that there occurs a phenomenon in which the emitted charge stays in the interface portion with the insulating oil in the conductive solution. The charge staying in the interface portion and the charge of reverse polarity to that charge gathered on the electrode placed in the back of the water-repellent film draw each other with a coulomb force so as to attract the charge in the conductive solution around the water-repellent film. Consequently, the conductive solution starts to wet the water-repellent film covering the inner wall of the tube, causing change of an interface form of the two kinds of liquid. To be more specific, as the higher voltage is applied to the conductive solution, a change occurs to a curvature radius of the interface portion with the insulating oil of the conductive solution initially functioning as the concave lens. For instance, the interface portion becomes completely flat or the conductive solution starts to function as the convex lens, resulting in a change in the focal length.

According to such a liquid lens, it is possible to change the focal length without moving the lens, so the zoom function and a focus function can be performed without providing the cam mechanism and lens drive mechanism. Therefore, by using the liquid lens, the apparatus can be substantially downsized, and the lens can also be applicable to a small device such as a cellular phone.

According to the liquid lens described in Nonpatent Document 1, however, the conductive solution is electrolyzed by the electric charge emitted from the electrode. Therefore, there is a problem that, if used for a long period of time, a generated gas stays in the liquid container and turns into bubbles, scattering of light occurs and light transmission lowers.

There is also a problem that, if the liquid lens described in Nonpatent Document 1 is used in a car in midsummer, the conductive solution becomes volatile in a temperature environment of approximately 100 degrees and performance of the lens is thereby deteriorated.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above circumstances and provides an optical element, a lens unit and an image pickup apparatus capable of maintaining optical performance in a high temperature environment and maintaining light transmission for a long period of time.

The optical element of the present invention is the one including:

a liquid container transmitting light at least in a predetermined optical axis direction, the liquid container containing an insulating liquid and an ionic liquid being mutually immiscible and optically transparent, and having different refractive indices;

a first electrode contacting the ionic liquid in the liquid container; and

a second electrode insulated from the ionic liquid in the liquid container.

The ionic liquid is an ionic compound composed of a cationic compound and an anionic compound, and is an organic salt which remains a liquid even at ordinary temperature. The ionic liquid is characterized by having high conductivity, not becoming electrolyzed, not evaporating even at high temperature (vapor pressure is zero) and not easily catching fire.

According to the optical element of the present invention, if a voltage is applied mutually between the first electrode and the second electrode, electric charge is emitted from the first electrode into the ionic liquid and the electric charge of reverse polarity thereto is gathered on the second electrode. Consequently, the electric charge in the ionic liquid and the electric charge gathered on the second electrode draw each other with a coulomb force so that a form of an interface between the ionic liquid and the insulating liquid is changed. In this case, a weak current passes through the ionic liquid due to the electric charge emitted from the first electrode. However, the ionic liquid is not easily electrolyzed, and so generation of bubbles is avoided and the light transmission is maintained for a long period of time. The ionic liquid is also characterized by keeping low viscosity and high conductivity, not becoming volatile and not easily catching fire even at high temperature, so that the optical performance and prolonged safety can be maintained even when applied to the image pickup apparatus for outdoor or in-car use.

As for the optical element of the present invention, it is desirable that the insulating liquid be an organic medium.

A difference in conductivity is significant between the ionic liquid and the organic medium, and so these liquids are combined to change the form of the interface between the liquids efficiently with high accuracy.

As for the optical element of the present invention, it is desirable that the insulating liquid is a hydrocarbon organic medium.

As the hydrocarbon organic medium is applied, temporal stability of the optical element is improved.

As for the optical element of the present invention, it is desirable that the ionic liquid be an imidazolium salt compound.

The imidazolium salt compound is easily available, and so manufacturing cost of the optical element can be reduced.

The lens unit of the present invention is the one including:

a liquid container transmitting light at least in a predetermined optical axis direction, the liquid container containing an insulating liquid and an ionic liquid being mutually immiscible and optically transparent, and having different refractive indices;

a first electrode contacting the ionic liquid in the liquid container; and

a second electrode insulated from the ionic liquid in the liquid container,

wherein a form of an interface between the insulating liquid and the ionic liquid varies according to a voltage applied between the first electrode and the second electrode.

According to the lens unit of the present invention, it is possible to maintain the light transmission characteristics and the optical performance for a long period of time in the high temperature environment as with the optical element of the present invention.

As for the lens unit according to the present invention, only its basic form is shown here. However, it is for the sake of avoiding duplication, and the lens unit according to the present invention includes various forms corresponding to the forms of the optical element in addition to the basic form.

The image pickup apparatus of the present invention is the one including:

a liquid container transmitting light at least in a predetermined optical axis direction, the liquid container containing an insulating liquid and an ionic liquid being mutually immiscible and optically transparent, and having different refractive indices;

a first electrode contacting the ionic liquid in the liquid container;

a second electrode insulated from the ionic liquid in the liquid container;

a control section varying a form of an interface between the insulating liquid and the ionic liquid by applying a voltage between the first electrode and the second electrode; and

an image pickup device which has subject light having passed through the insulating liquid and the ionic liquid focused on its surface to generate an image signal representing the subject light.

According to the image pickup apparatus of the present invention, it is possible to maintain the light transmission for a long period of time and maintain the optical performance in the high temperature environment as with the optical element of the present invention.

As for the image pickup apparatus according to the present invention, only its basic form is shown here. However, it is for the sake of avoiding duplication, and the image pickup apparatus according to the present invention includes various forms corresponding to the forms of the optical element in addition to the basic form.

According to the present invention, it is possible to provide the optical element, lens unit and image pickup apparatus capable of maintaining the light transmission and the optical performance for a long period of time in the high temperature environment.

DETAILED DESCRIPTION OF THE INVENTION

Hereunder, problems of the liquid lens described in Nonpatent Document 1 will be analyzed in detail before describing embodiments of the present invention.

FIG. 1is a schematic block diagram of the liquid lens as a comparative example. Hereunder, light transmits in a direction of an arrow O, where a light incident side (upside ofFIG. 1) is designated as the upside and a light outgoing side (downside ofFIG. 1) as the downside.

As shown inFIG. 1, a liquid lens1has a clear water21to which supporting electrolyte is added and a clear oil22which is an insulating liquid without being mutually mixed in a glass container11which is composed of a glass tube11ahaving both ends thereof closed up by glass caps11band11c. As the oil22has a larger refractive index than the water21, the oil22plays a role of the lens refracting the light in the liquid lens1.

An inner surface of the tube11aof the container11and the inner surface of the cap11bclosing up an upper end of the tube11aare covered with a water-repellent film15, and the inner surface of the cap11cclosing up a lower end of the tube11ais covered with a hydrophilic film16.

An insulating film14is provided between the tube11aand the water-repellent film15. And the liquid lens1has a first electrode12contacting the water21and a second electrode13insulated from the water21by the insulating film14provided thereto.

As shown in Part (A) ofFIG. 1, in a state of having no voltage applied mutually between the first electrode12and the second electrode13, the water21repels the water-repellent film15and contacts the hydrophilic film16so that a contact portion P1between the water21and the water-repellent film15becomes smaller. For this reason, the water21stays in a semispherical form, and the oil22pushed by the water21stays in a form having a semisphere bored out of a cylindrical form. As the form of an interface between the water21and the oil22viewed from the oil22is in a concave state, the liquid lens1functions as a concave lens in Part (A) ofFIG. 1.

If a plus voltage is applied to the first electrode12and a minus voltage is applied to the second electrode13, a plus charge31ais emitted from the first electrode12to the water21, and a minus charge31bstays in the second electrode13. In this case, the plus charge31aemitted to the water21is attracted to the minus charge31bof the second electrode13by a coulomb force, and a contact-portion P2between the water21and the water-repellent film15becomes larger according to an applied voltage. In part (B) ofFIG. 1, the form of the interface between the water21and the oil22viewed from the oil22is in a convex state, and the liquid lens1functions as a convex lens. It is also possible, by adjusting the voltage applied to the first electrode12and second electrode13, to change the form of the interface between the water21and the oil22little by little.

Thus, according to the liquid lens1, it is possible to implement a zoom function and a focus function by changing the form of the interface between the water21and the oil22without providing a mechanism moving the lens.

Here, the liquid lens1has the water21electrolyzed by the plus charge31aemitted from the first electrode12to generate a gas. There is a problem that, as the glass container11does not pass the gas, the gas stays and generate bubbles in the water21and oil22over a long period of time and the bubbles cause scattering of the light, leading to deteriorated light transmission.

The liquid lens1is supposedly applied to a small device such as a cellular phone. There is a problem, however, that if the cellular phone is left in a high temperature environment such as inside of a car in midsummer, the water21evaporates and the performance of the lens is thereby deteriorated.

The present invention is based on a detailed analysis as described above.

Hereunder, the embodiments of the present invention will be described by referring to the drawings.

FIG. 2is an outline perspective view obliquely from the front of a digital camera to which an embodiment of the present invention is applied.

As shown inFIG. 2, an image taking lens101is provided at the center of the front of this digital camera100. And an optical finder objective window102and a light emitting section103are provided at the upper portion of the front of the digital camera100. Furthermore, the digital camera100has a sliding power switch104and a release switch150provided on a top surface thereof.

FIG. 3is a schematic block diagram of the digital camera100shown inFIG. 1.

As shown inFIG. 3, a breakdown of the digital camera100is roughly divided into a shooting optical system110and a signal processing section120. In addition, the digital camera100has an image display section130displaying shot images, an external recording medium140recording shot image signals, a zoom switch170having various processes for shooting performed by the digital camera100, a shooting mode switch160and a release switch150provided thereon.

First, a configuration of the shooting optical system110will be described by referring toFIG. 3.

As for the digital camera100, subject light gets incident from the left ofFIG. 3and passes through an iris113adjusting a light volume of the subject light via a zoom lens115and a focus lens114so as to focus on a solid-state image sensing device111thereafter in the case where a shutter112is open. The solid-state image sensing device111is equivalent to an example of an image pickup device according to the present invention. The shooting optical system originally has multiple lenses placed therein, where at least one of the multiple lenses is significantly involved in focus adjustment while relative positions of the lenses are involved in focal length.FIG. 3schematically shows the lens involved in change in the focal length as the zoom lens115and also schematically shows the lens involved in the focus adjustment as the focus lens114.

The zoom lens115is moved by the zoom motor115ain a direction along an optical axis. As the zoom lens115is moved to a position according to a signal from the signal processing section120, the focal length changes and a shooting magnification is decided.

The focus lens114is a lens intended to implement a TTLAF (Through The Lens Auto Focus) function. In general, the TTLAF function detects a contrast of the image signal obtained by the solid-state image sensing device111with an AF/AE computing section126of the signal processing section120while moving the focus lens in the direction along an optical axis, and adjusts the focus lens114to a point of focus by setting a lens position at which the contrast hits a peak as the point of focus. It is possible, with the TTLAF function, to automatically focus on and shoot a subject of which contrast hits a peak (that is, the nearest subject). According to the present embodiment, the nearest subject is focused on by changing a lens form of the focus lens114with the focus controller114ainstead of moving the focus lens114. The configuration of the focus lens114and a method of changing the lens form will be described in detail later.

The iris113is driven based on an instruction provided from the AF/AE computing section126of the digital signal processing section120bso as to adjust the light volume of the subject light.

The configuration of the shooting optical system110is as described above.

Subsequently, the configuration of the signal processing section120will be described. A subject image provided on the solid-state image sensing device111in the shooting optical system is read as the image signal by an analog processing (A/D) section120a. An analog signal is converted to a digital signal by the analog processing (A/D) section120aso as to be supplied to the digital signal processing section120b. The digital signal processing section120bhas a system controller121placed thereon, where signal processing in the digital signal processing section120bis performed according to a program indicating a procedure of an operation in the system controller121. Data passing is performed via a bus1200between the system controller121and an image signal processing section122, an image display control section123, an image compression section124, a media controller125, the AF/AE computing section126, a key controller127, a buffer memory128, and an internal memory129. The internal memory129operates as a buffer on performing the data passing via the bus1200. The internal memory129has data turning to a variable according to progress on a handling process of each section written thereto as needed, where an adequate process is performed by referring to the data in the system controller121, image signal processing section122, image display control section123, image compression section124, media controller125, AF/AE computing section126and key controller127. To be more specific, the instruction from the system controller121is conveyed to the sections via the bus1200so as to start the handling process of each section. And the data of the internal memory129is rewritten according to the progress on the process, and is further referred to on the system controller121side to control the operations of the sections. In other words, power is turned on, and the process of each section is started according to the procedure of the program in the system controller121. For instance, if the release switch150, zoom switch170and shooting mode switch160are operated, information on that operation is conveyed to the system controller121via the key controller127. And the process according to the operation is performed according to the procedure of the program in the system controller121.

If a release operation is performed, image data read from the solid-state image sensing device is converted from the analog signal to the digital signal by the analog processing (A/D) section120a. The digitized image data is stored once in the buffer memory128in the digital signal processing section120b. An RGB signal of the digitized image data is converted to an YC signal by the image signal processing section122. Furthermore, compression called JPEG compression is performed by the image compression section124so that the image signal becomes an image file and is recorded on the external recording medium140via the media controller125. The image data recorded as the image file is reproduced by the image display section130through the image display control section123. During this process, the AF/AE computing section126performs calculations of the focus adjustment and exposure adjustment based on the RGB signal. The AF/AE computing section126detects the contrast from the RGB signal by a subject distance for the sake of the focus adjustment. Based on this detection result, the focus adjustment is performed by the focus lens114. The AF/AE computing section126also extracts a luminance signal from the RGB signal, and detects a luminance of field from it. Based on this result, the exposure adjustment is performed by the iris113so that the light volume of the subject light given to the solid-state image sensing device becomes adequate.

The digital camera100is basically configured as described above.

Here, the present invention in the digital camera100is characterized by the focus lens114. Hereunder, the focus lens114will be described in detail.

FIG. 4is a schematic block diagram of the focus lens. A description will be given by referring to the side on which the light gets incident (left side ofFIG. 4) as a front side and the side on which the light is emitted (right side ofFIG. 4) as a back side with the subject light incident from the left side ofFIG. 4to the direction of the arrow O.

The focus lens114is formed by accommodating an ionic liquid301and an insulating liquid302unmixable with the ionic liquid301in a liquid container201composed of a tube201ahaving both ends thereof closed up by caps201band201c. The liquid container201is composed of clear glass, and is equivalent to an example of the liquid container according to the present invention.

A liquid-contacting surface (inner surface) of the cap201cclosing up a back-end of the tube201ais covered with high-wettability film206which is highly wettable by the ionic liquid301. The inner surface of the liquid container201other than the portion covered with the high-wettability film206is covered with a low-wettability film205which is less wettable by the ionic liquid301than the insulating liquid302.

The liquid container201is also equipped with a first electrode202contacting the liquid and placed as if sandwiching the high-wettability film206, an insulating film204sandwiched between the tube201aand the low-wettability film205, and a second electrode203insulated from the liquid by the insulating film204. The first electrode202and second electrode203are connected to the focus controller114ashown inFIG. 3, and the voltage is applied between these electrodes by the focus controller114a. The first electrode202is equivalent to an example of the first electrode according to the present invention, and the second electrode203is equivalent to an example of the second electrode according to the present invention. The focus controller114ais equivalent to an example of the control section according to the present invention.

The ionic liquid301and the insulating liquid302of mutually different refractive indexes are accommodated in the liquid container201. According to the present embodiment, 1-ethyl-3-methyl-imidazolium hexafluorophosphate (made by Tokyo Kasei) is applied as the ionic liquid301, and an organic solvent (made by ISOPAR: Exxon) is applied as the insulating liquid302. An imidazolium salt compound applied as the ionic liquid301is relatively easy to obtain, and so it can reduce the manufacturing cost of the focus lens114. The ionic liquid301is equivalent to an example of the ionic liquid according to the present invention, and the insulating liquid302is equivalent to an example of the insulating liquid according to the present invention.

In the state of having no voltage applied between the first electrode202and the second electrode203, the ionic liquid301repels the low-wettability film205so that the interface between the ionic liquid301and the insulating liquid302will be in the form indicated in full line.

If the focus controller114aapplies the voltage mutually between the first electrode202and the second electrode203according to the instruction from the signal processing section120shown inFIG. 3, electric charge is emitted from the first electrode202to the ionic liquid301and the charge of reverse polarity to the charge emitted to the ionic liquid301gathers on the second electrode203. The charge emitted to the ionic liquid301and the charge of the second electrode203draw each other with the coulomb force so that the charge in the ionic liquid301is attracted around the low-wettability film205. Consequently, the interface between the ionic liquid301and the insulating liquid302changes to the form indicated by a dotted line inFIG. 4for instance.

The TTLAF function is implemented by the following procedure by using the focus lens114.

First, the voltage applied to the first electrode202and second electrode203is changed little by little by the focus controller114aso that the image signal is obtained by the solid-state image sensing device111shown inFIG. 3while having the form of the interface between the ionic liquid301and the insulating liquid302changed. Subsequently, the contrast of an image pickup signal is detected by the AF/AE computing section126, and the voltage at which the contrast hits a peak is applied to the first electrode202and the second electrode203. Thus, it is possible, by doing a photo shoot in a state of having the lens form determined, to focus on the subject at the nearest position.

As the electric charge is emitted from the first electrode202to the ionic liquid301, a weak current passes through the ionic liquid301. However, the ionic liquid301is not easily electrolyzed, and so even if the digital camera100is used for a long period of time, bubbles are not generated in the liquid container201and the light transmission can be maintained for a long period of time.

The ionic liquid301does not evaporate even at high temperature so that the digital camera100of the present embodiment can obtain a high-quality shot image even when using the digital camera100outdoors or in the car.

This concludes the description of the first embodiment of the present invention.

Here, the optical element of the present invention has an advantage that its optical performance is maintained even in the high temperature environment. A description will be given as to second and third embodiments exploiting this advantage hereunder.

First, a description will be given as to the second embodiment in which the optical element of the present invention is applied to a monitoring system having a photographic device installed outdoors. As the first embodiment and the second embodiment have approximately the same device configuration, attention is paid to a difference from the first embodiment, and the same elements are given the same symbols to omit a description thereof.

FIG. 5is a schematic block diagram of the monitoring system to which the second embodiment of the present invention is applied.

A monitoring system400shown inFIG. 5is configured by connecting a photographic device410installed in an outdoor parking lot and so on to a monitoring device420installed in an observation room and so on by a network.

The photographic device410obtains various instructions from the monitoring device420via a motor driver120d, and the subject is shot according to the instructions so as to generate digital shot image data. When the shot image data is generated, the voltage applied to the first electrode202and the second electrode203shown inFIG. 4is controlled by the focus controller114a, and the form of the interface between the ionic liquid301and the insulating liquid302is adjusted to focus on the subject. The generated shot image data is sent to the monitoring device420.

The monitoring device420performs image processing to the shot image data generated by the photographic device410, and displays the shot image represented by the shot image data on a monitor131. The monitoring device420has various instructions inputted thereto by an operator having checked the shot image displayed on the monitor131. The inputted various instructions are conveyed to the photographic device410.

As the photographic device410is installed outdoors, it is required to operate normally in a considerably high-temperature environment in midsummer. The focus lens114of the photographic device410is downsized since no transfer mechanism is necessary, and the ionic liquid301not evaporating at atmospheric pressure is used as a conductive liquid so as to maintain high optical performance in the high temperature environment. The ionic liquid301has the advantages that it does not easily catch fire and is not easily electrolyzed. Therefore, the monitoring system400of the present embodiment can improve safety against a fire and device life.

Thus, even if used outdoors, the liquid lens using the ionic liquid as the conductive liquid has the optical performance maintained for a long period of time and prolongs the device life.

This concludes the description of the second embodiment, and a description will be given as to a third embodiment wherein the optical element of the present invention is applied to a printer having the image formed on paper with a laser beam. As the focus lens114of the first embodiment (refer toFIG. 4) and the lens applied to the third embodiment have approximately the same configuration, only the difference from the first embodiment will be described by usingFIG. 4in the description of the third embodiment.

FIG. 6is a schematic block diagram of the printer to which the third embodiment of the present invention is applied.

A printer500has a control section510controlling various elements of the printer500, an image processing section520generating the image data representing an output image, a laser emitting section530emitting the laser beam based on the image data, a prism540refracting the laser beam, and a lens550condensing the laser beam on paper600.

For instance, the image data obtained by reading an original image is obtained by the image processing section520, and undergoes predetermined image processing therein. The image data after the image processing is sent to the laser emitting section530.

The laser emitting section530emits the laser beam based on the image data sent from the image processing section520. The emitted laser beam is refracted by the prism540and gets incident on the lens550.

The lens550has the same configuration as the focus lens114of the first embodiment shown inFIG. 4. The control section510controls the voltage applied to the first electrode202and the second electrode203of the lens550and adjusts the form of the interface between the ionic liquid301and the insulating liquid302of the lens550so as to condense the laser beam on the paper600. The control section510is equivalent to an example of the control section according to the present invention.

The paper600has a sensitive material applied on its surface. If the laser beam is irradiated by the lens550, the image according to the laser beam is formed on the paper600.

Here, when forming the image, the inside of the lens550is heated at considerably high temperature by the laser beam. However, the lens550has the ionic liquid301applied thereto so as to maintain the high optical performance even in the high temperature environment. For this reason, the printer500can form the high-quality image for a long period of time.

Thus, the liquid lens using the ionic liquid as the conductive liquid is also applicable to the apparatus generating high heat such as the laser beam.

Here, the description is given as to the example of the case where the two kinds of liquid of the ionic liquid and insulating liquid are accommodated in the liquid container. However, the liquid container according to the present invention may accommodate three or more kinds of liquid.

The description is also given as to the example of the case where the optical element of the present invention is applied to the focus lens. However, the optical element of the present invention may also be applied to the zoom lens.

Subsequently, a note will be appended as to various forms adoptable to component parts configuring the present invention.

The ionic liquid according to the present invention may be anything. However, it should preferably be an imidazolium salt compound, a pyridinium salt compound, ammonium salt compound or a phosphonium salt compound. To be more precise, it may be 1-ethyl-3-methyl-imidazolium chloride, 1-ethyl-3-methyl-imidazolium bromide, 1-ethyl-3-methyl-imidazolium iodide, 1-ethyl-3-methyl-imidazolium trifluoro-methane sulfonate, 1-ethyl-3-methyl-imidazolium tetrafluoroborate, 1-ethyl-3-methyl-imidazolium hexafluorophosphate, 1-n-butyl-3-methyl-imidazolium chloride, 1-n-butyl-3-methyl-imidazolium bromide, 1-n-butyl-3-methyl-imidazolium tetrafluoroborate, octyl-triphenyl-phosphonium bromide, methyl-tributyl-ammonium tosylate, methyl-tributyl-ammonium chloride or methyl-tributyl-ammonium bromide. It should desirably be the imidazolium salt compound, and preferably a 1-ethyl-3-methyl-imidazolium salt compound or a 1-n-butyl-3-methyl-imidazolium salt compound.

The ionic liquid according to the present invention should desirably have a configuration with a fluorine-substituted anion pair from a viewpoint of low viscosity and high speed of response. For instance, it may be trifluoro-methane sulfonate salt, pentafluoro-ethane sulfonate salt, hexafluorophosphate salt or tetrafluoroborate salt.

The ionic liquid according to the present invention may also be a mixture of two or more kinds of ionic liquid. It is also possible to add an additive to the ionic liquid for the sake of adjusting the refractive index, viscosity and surface tension of the ionic liquid. The additive may be anything, for example, a low-molecular organic compound, a polymer inorganic compound or a metallic salt compound for instance.

The insulating liquid according to the present invention should desirably be an organic solvent. Furthermore, it should preferably be a hydrocarbon (hexane, heptane, pentane, octane or isopar and so on), a hydrocarbon aroma compound (benzene, toluene, xylene, mesitylene and so on), a halogen hydrocarbon (difluoropropane, dichloroethane, chloroethane, promoethane and so on), a halogen hydrocarbon aroma compound (chlorobenzene and soon) or an ether compound (dibutyl ether, anisole, diphenyl ether and so on).

A combination of the ionic liquid and insulating liquid applied to the present invention should desirably have the mutual difference in the refractive index becoming larger. A desirable difference in the refractive index is between 0.05 and 0.5, and preferably between 0.1 and 0.2.

Here, the description is given as to the basic embodiments for the sake of realizing the concept of the present invention. To put the optical element adopted by the present invention in practical use, it should preferably be designed to prevent a problem that dirt and droplets adhere onto a light path and deteriorate the lens performance.

For instance, it is desirable to provide the water-repellent film on an outer surface (hereafter, this surface is referred to as a light transmitting surface) intersecting with the light path of the container accommodating the liquids. It is possible, by providing water-repellency to the light transmitting surface, to prevent adherence of the dirt and droplets so as to maintain high light transmission of the optical element. The material composing the water-repellent film should desirably be a silicone resin, an organopolysiloxane block copolymer, a fluorinated polymer or a polytetrafluoroethane.

It is desirable to provide the hydrophilic film on the light transmitting surface of the container configuring the optical element. It is also possible to prevent the adherence of the dust by providing hydrophilic oil-repellency to the light transmitting surface. The hydrophilic film should desirably be the one composed of an acrylate polymer or the one having a surface-active agent such as a non-ionic organo-silicone surface-active agent applied thereto. As for the method of making the hydrophilic film, plasma polymerization of a silane monomer or an ion beam process is applicable.

It is also desirable to provide a photo catalyst such as a titanium oxide on the light transmitting surface of the container configuring the optical element. It is possible to decompose a smear with the photo catalyst reacting to the light and thus to keep the light transmitting surface clean.

It is also desirable to provide an antistatic film on the light transmitting surface of the container configuring the optical element. If the light transmitting surface of the container has static electricity accumulated thereon or becomes charged with the electrode, there is a possibility that the dirt and dust may adhere to the light transmitting surface. It is possible, by providing the antistatic film to the light transmitting surface, to prevent adherence of such undesired substances and thus to maintain the light transmission of the optical element. It is desirable that the antistatic film be composed of a polymer alloy material. And it is especially desirable that the polymer alloy be the one having a polyether, a polyether-ester-amide or a cation group or Reo Mix (product name, Dai-ichi Kogyo Seiyaku Co., Ltd.). It is also desirable that the antistatic film be made by a mist method.

It is also possible to apply an antifouling material to the container configuring the optical element. A fluorocarbon resin is desirable as the antifouling material. To be more precise, a fluorine-containing alkyl-alkoxysilane compound, a fluorine-containing alkyl group-containing polymer or an oligomer. And the one having a functional group bridgeable with the thermosetting resin is especially desirable. An added amount of the antifouling material should desirably be a minimum amount necessary to implement an antifouling property.