Abstract:
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.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     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.  
         [0003]     2. Description of the Related Art  
         [0004]     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.  
         [0005]     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.  
         [0006]     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&#39; Fluid Lenses”, [online], Mar. 3, 2004, Royal Philips Electronics, [searched on Mar. 31, 2004],Internet URL:http://www.dpreview.com/news/0403/04030302philipsflu idlens.asp, hereafter referred to as Non-patent document 1.)  
         [0007]     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  6 f 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.  
         [0008]     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.  
         [0009]     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.  
         [0010]     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  
       [0011]     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.  
         [0012]     The optical element of the present invention is the one including:  
         [0013]     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;  
         [0014]     a first electrode contacting the ionic liquid in the liquid container; and  
         [0015]     a second electrode insulated from the ionic liquid in the liquid container.  
         [0016]     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.  
         [0017]     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.  
         [0018]     As for the optical element of the present invention, it is desirable that the insulating liquid be an organic medium.  
         [0019]     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.  
         [0020]     As for the optical element of the present invention, it is desirable that the insulating liquid is a hydrocarbon organic medium.  
         [0021]     As the hydrocarbon organic medium is applied, temporal stability of the optical element is improved.  
         [0022]     As for the optical element of the present invention, it is desirable that the ionic liquid be an imidazolium salt compound.  
         [0023]     The imidazolium salt compound is easily available, and so manufacturing cost of the optical element can be reduced.  
         [0024]     The lens unit of the present invention is the one including:  
         [0025]     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;  
         [0026]     a first electrode contacting the ionic liquid in the liquid container; and  
         [0027]     a second electrode insulated from the ionic liquid in the liquid container,  
         [0028]     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.  
         [0029]     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.  
         [0030]     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.  
         [0031]     The image pickup apparatus of the present invention is the one including:  
         [0032]     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;  
         [0033]     a first electrode contacting the ionic liquid in the liquid container;  
         [0034]     a second electrode insulated from the ionic liquid in the liquid container;  
         [0035]     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  
         [0036]     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.  
         [0037]     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.  
         [0038]     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.  
         [0039]     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. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0040]     Preferred embodiments of the present invention will be described in detail based on the following figures, wherein:  
         [0041]      FIG. 1  is a schematic block diagram of a liquid lens as a comparative example;  
         [0042]      FIG. 2  is an outline perspective view obliquely from the front of a digital camera to which an embodiment of the present invention is applied;  
         [0043]      FIG. 3  is a schematic block diagram of the digital camera shown in  FIG. 1 ;  
         [0044]      FIG. 4  is a schematic block diagram of a focus lens;  
         [0045]      FIG. 5  is a schematic block diagram of a monitoring system to which a second embodiment of the present invention is applied; and  
         [0046]      FIG. 6  is a schematic block diagram of a printer to which a third embodiment of the present invention is applied. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0047]     Hereunder, problems of the liquid lens described in Nonpatent Document 1 will be analyzed in detail before describing embodiments of the present invention.  
         [0048]      FIG. 1  is 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 of  FIG. 1 ) is designated as the upside and a light outgoing side (downside of  FIG. 1 ) as the downside.  
         [0049]     As shown in  FIG. 1 , a liquid lens  1  has a clear water  21  to which supporting electrolyte is added and a clear oil  22  which is an insulating liquid without being mutually mixed in a glass container  11  which is composed of a glass tube  11   a  having both ends thereof closed up by glass caps  11   b  and  11   c.  As the oil  22  has a larger refractive index than the water  21 , the oil  22  plays a role of the lens refracting the light in the liquid lens  1 .  
         [0050]     An inner surface of the tube  11   a  of the container  11  and the inner surface of the cap  11   b  closing up an upper end of the tube  1   a  are covered with a water-repellent film  15 , and the inner surface of the cap  11   c  closing up a lower end of the tube  11   a  is covered with a hydrophilic film  16 .  
         [0051]     An insulating film  14  is provided between the tube  11   a  and the water-repellent film  15 . And the liquid lens  1  has a first electrode  12  contacting the water  21  and a second electrode  13  insulated from the water  21  by the insulating film  14  provided thereto.  
         [0052]     As shown in Part (A) of  FIG. 1 , in a state of having no voltage applied mutually between the first electrode  12  and the second electrode  13 , the water  21  repels the water-repellent film  15  and contacts the hydrophilic film  16  so that a contact portion P 1  between the water  21  and the water-repellent film  15  becomes smaller. For this reason, the water  21  stays in a semispherical form, and the oil  22  pushed by the water  21  stays in a form having a semisphere bored out of a cylindrical form. As the form of an interface between the water  21  and the oil  22  viewed from the oil  22  is in a concave state, the liquid lens  1  functions as a concave lens in Part (A) of  FIG. 1 .  
         [0053]     If a plus voltage is applied to the first electrode  12  and a minus voltage is applied to the second electrode  13 , a plus charge  31   a  is emitted from the first electrode  12  to the water  21 , and a minus charge  31   b  stays in the second electrode  13 . In this case, the plus charge  31   a  emitted to the water  21  is attracted to the minus charge  31   b  of the second electrode  13  by a coulomb force, and a contact-portion P 2  between the water  21  and the water-repellent film  15  becomes larger according to an applied voltage. In part (B) of  FIG. 1 , the form of the interface between the water  21  and the oil  22  viewed from the oil  22  is in a convex state, and the liquid lens  1  functions as a convex lens. It is also possible, by adjusting the voltage applied to the first electrode  12  and second electrode  13 , to change the form of the interface between the water  21  and the oil  22  little by little.  
         [0054]     Thus, according to the liquid lens  1 , it is possible to implement a zoom function and a focus function by changing the form of the interface between the water  21  and the oil  22  without providing a mechanism moving the lens.  
         [0055]     Here, the liquid lens  1  has the water  21  electrolyzed by the plus charge  31   a  emitted from the first electrode  12  to generate a gas. There is a problem that, as the glass container  11  does not pass the gas, the gas stays and generate bubbles in the water  21  and oil  22  over a long period of time and the bubbles cause scattering of the light, leading to deteriorated light transmission.  
         [0056]     The liquid lens  1  is 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 water  21  evaporates and the performance of the lens is thereby deteriorated.  
         [0057]     The present invention is based on a detailed analysis as described above.  
         [0058]     Hereunder, the embodiments of the present invention will be described by referring to the drawings.  
         [0059]      FIG. 2  is an outline perspective view obliquely from the front of a digital camera to which an embodiment of the present invention is applied.  
         [0060]     As shown in  FIG. 2 , an image taking lens  101  is provided at the center of the front of this digital camera  100 . And an optical finder objective window  102  and a light emitting section  103  are provided at the upper portion of the front of the digital camera  100 . Furthermore, the digital camera  100  has a sliding power switch  104  and a release switch  150  provided on a top surface thereof.  
         [0061]      FIG. 3  is a schematic block diagram of the digital camera  100  shown in  FIG. 1 .  
         [0062]     As shown in  FIG. 3 , a breakdown of the digital camera  100  is roughly divided into a shooting optical system  110  and a signal processing section  120 . In addition, the digital camera  100  has an image display section  130  displaying shot images, an external recording medium  140  recording shot image signals, a zoom switch  170  having various processes for shooting performed by the digital camera  100 , a shooting mode switch  160  and a release switch  150  provided thereon.  
         [0063]     First, a configuration of the shooting optical system  110  will be described by referring to  FIG. 3 .  
         [0064]     As for the digital camera  100 , subject light gets incident from the left of  FIG. 3  and passes through an iris  113  adjusting a light volume of the subject light via a zoom lens  115  and a focus lens  114  so as to focus on a solid-state image sensing device  111  thereafter in the case where a shutter  112  is open. The solid-state image sensing device  111  is 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. 3  schematically shows the lens involved in change in the focal length as the zoom lens  115  and also schematically shows the lens involved in the focus adjustment as the focus lens  114 .  
         [0065]     The zoom lens  115 , iris  113  and shutter  112  are driven by a zoom motor  115   a,  an iris motor  113   a  and a shutter motor  112   a  to move respectively. The focus lens  114  has a focus controller  114   a  changing the form of the focus lens  114  provided thereto instead of the motor. Instructions to actuate the zoom motor  115   a,  iris motor  113   a  and shutter motor  112   a  are conveyed from a digital signal processing section  120   b  in the signal processing section  120  through a motor driver  120   c,  and an instruction to actuate the focus controller  114   a  is directly conveyed from the digital signal processing section  120   b.    
         [0066]     The zoom lens  115  is moved by the zoom motor  115   a  in a direction along an optical axis. As the zoom lens  115  is moved to a position according to a signal from the signal processing section  120 , the focal length changes and a shooting magnification is decided.  
         [0067]     The focus lens  114  is 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 device  111  with an AF/AE computing section  126  of the signal processing section  120  while moving the focus lens in the direction along an optical axis, and adjusts the focus lens  114  to 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 lens  114  with the focus controller  114   a  instead of moving the focus lens  114 . The configuration of the focus lens  114  and a method of changing the lens form will be described in detail later.  
         [0068]     The iris  113  is driven based on an instruction provided from the AF/AE computing section  126  of the digital signal processing section  120   b  so as to adjust the light volume of the subject light.  
         [0069]     The configuration of the shooting optical system  110  is as described above.  
         [0070]     Subsequently, the configuration of the signal processing section  120  will be described. A subject image provided on the solid-state image sensing device  111  in the shooting optical system is read as the image signal by an analog processing (A/D) section  120   a.  An analog signal is converted to a digital signal by the analog processing (A/D) section  120   a  so as to be supplied to the digital signal processing section  120   b.  The digital signal processing section  120   b  has a system controller  121  placed thereon, where signal processing in the digital signal processing section  120   b  is performed according to a program indicating a procedure of an operation in the system controller  121 . Data passing is performed via a bus  1200  between the system controller  121  and an image signal processing section  122 , an image display control section  123 , an image compression section  124 , a media controller  125 , the AF/AE computing section  126 , a key controller  127 , a buffer memory  128 , and an internal memory  129 . The internal memory  129  operates as a buffer on performing the data passing via the bus  1200 . The internal memory  129  has 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 controller  121 , image signal processing section  122 , image display control section  123 , image compression section  124 , media controller  125 , AF/AE computing section  126  and key controller  127 . To be more specific, the instruction from the system controller  121  is conveyed to the sections via the bus  1200  so as to start the handling process of each section. And the data of the internal memory  129  is rewritten according to the progress on the process, and is further referred to on the system controller  121  side 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 controller  121 . For instance, if the release switch  150 , zoom switch  170  and shooting mode switch  160  are operated, information on that operation is conveyed to the system controller  121  via the key controller  127 . And the process according to the operation is performed according to the procedure of the program in the system controller  121 .  
         [0071]     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) section  120   a.  The digitized image data is stored once in the buffer memory  128  in the digital signal processing section  120   b.  An RGB signal of the digitized image data is converted to an YC signal by the image signal processing section  122 . Furthermore, compression called JPEG compression is performed by the image compression section  124  so that the image signal becomes an image file and is recorded on the external recording medium  140  via the media controller  125 . The image data recorded as the image file is reproduced by the image display section  130  through the image display control section  123 . During this process, the AF/AE computing section  126  performs calculations of the focus adjustment and exposure adjustment based on the RGB signal. The AF/AE computing section  126  detects 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 lens  114 . The AF/AE computing section  126  also 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 iris  113  so that the light volume of the subject light given to the solid-state image sensing device becomes adequate.  
         [0072]     The digital camera  100  is basically configured as described above.  
         [0073]     Here, the present invention in the digital camera  100  is characterized by the focus lens  114 . Hereunder, the focus lens  114  will be described in detail.  
         [0074]      FIG. 4  is 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 of  FIG. 4 ) as a front side and the side on which the light is emitted (right side of  FIG. 4 ) as a back side with the subject light incident from the left side of  FIG. 4  to the direction of the arrow O.  
         [0075]     The focus lens  114  is formed by accommodating an ionic liquid  301  and an insulating liquid  302  unmixable with the ionic liquid  301  in a liquid container  201  composed of a tube  201   a  having both ends thereof closed up by caps  201   b  and  201   c . The liquid container  201  is composed of clear glass, and is equivalent to an example of the liquid container according to the present invention.  
         [0076]     A liquid-contacting surface (inner surface) of the cap  201   c  closing up a back-end of the tube  201   a  is covered with high-wettability film  206  which is highly wettable by the ionic liquid  301 . The inner surface of the liquid container  201  other than the portion covered with the high-wettability film  206  is covered with a low-wettability film  205  which is less wettable by the ionic liquid  301  than the insulating liquid  302 .  
         [0077]     The liquid container  201  is also equipped with a first electrode  202  contacting the liquid and placed as if sandwiching the high-wettability film  206 , an insulating film  204  sandwiched between the tube  201   a  and the low-wettability film  205 , and a second electrode  203  insulated from the liquid by the insulating film  204 . The first electrode  202  and second electrode  203  are connected to the focus controller  114   a  shown in  FIG. 3 , and the voltage is applied between these electrodes by the focus controller  114   a . The first electrode  202  is equivalent to an example of the first electrode according to the present invention, and the second electrode  203  is equivalent to an example of the second electrode according to the present invention. The focus controller  114   a  is equivalent to an example of the control section according to the present invention.  
         [0078]     The ionic liquid  301  and the insulating liquid  302  of mutually different refractive indexes are accommodated in the liquid container  201 . According to the present embodiment, 1-ethyl-3-methyl-imidazolium hexafluorophosphate (made by Tokyo Kasei) is applied as the ionic liquid  301 , and an organic solvent (made by ISOPAR: Exxon) is applied as the insulating liquid  302 . An imidazolium salt compound applied as the ionic liquid  301  is relatively easy to obtain, and so it can reduce the manufacturing cost of the focus lens  114 . The ionic liquid  301  is equivalent to an example of the ionic liquid according to the present invention, and the insulating liquid  302  is equivalent to an example of the insulating liquid according to the present invention.  
         [0079]     In the state of having no voltage applied between the first electrode  202  and the second electrode  203 , the ionic liquid  301  repels the low-wettability film  205  so that the interface between the ionic liquid  301  and the insulating liquid  302  will be in the form indicated in full line.  
         [0080]     If the focus controller  114   a  applies the voltage mutually between the first electrode  202  and the second electrode  203  according to the instruction from the signal processing section  120  shown in  FIG. 3 , electric charge is emitted from the first electrode  202  to the ionic liquid  301  and the charge of reverse polarity to the charge emitted to the ionic liquid  301  gathers on the second electrode  203 . The charge emitted to the ionic liquid  301  and the charge of the second electrode  203  draw each other with the coulomb force so that the charge in the ionic liquid  301  is attracted around the low-wettability film  205 . Consequently, the interface between the ionic liquid  301  and the insulating liquid  302  changes to the form indicated by a dotted line in  FIG. 4  for instance.  
         [0081]     The TTLAF function is implemented by the following procedure by using the focus lens  114 .  
         [0082]     First, the voltage applied to the first electrode  202  and second electrode  203  is changed little by little by the focus controller  114   a  so that the image signal is obtained by the solid-state image sensing device  111  shown in  FIG. 3  while having the form of the interface between the ionic liquid  301  and the insulating liquid  302  changed. Subsequently, the contrast of an image pickup signal is detected by the AF/AE computing section  126 , and the voltage at which the contrast hits a peak is applied to the first electrode  202  and the second electrode  203 . 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.  
         [0083]     As the electric charge is emitted from the first electrode  202  to the ionic liquid  301 , a weak current passes through the ionic liquid  301 . However, the ionic liquid  301  is not easily electrolyzed, and so even if the digital camera  100  is used for a long period of time, bubbles are not generated in the liquid container  201  and the light transmission can be maintained for a long period of time.  
         [0084]     The ionic liquid  301  does not evaporate even at high temperature so that the digital camera  100  of the present embodiment can obtain a high-quality shot image even when using the digital camera  100  outdoors or in the car.  
         [0085]     This concludes the description of the first embodiment of the present invention.  
         [0086]     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.  
         [0087]     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.  
         [0088]      FIG. 5  is a schematic block diagram of the monitoring system to which the second embodiment of the present invention is applied.  
         [0089]     A monitoring system  400  shown in  FIG. 5  is configured by connecting a photographic device  410  installed in an outdoor parking lot and so on to a monitoring device  420  installed in an observation room and so on by a network.  
         [0090]     The photographic device  410  obtains various instructions from the monitoring device  420  via a motor driver  120   d,  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 electrode  202  and the second electrode  203  shown in  FIG. 4  is controlled by the focus controller  114   a,  and the form of the interface between the ionic liquid  301  and the insulating liquid  302  is adjusted to focus on the subject. The generated shot image data is sent to the monitoring device  420 .  
         [0091]     The monitoring device  420  performs image processing to the shot image data generated by the photographic device  410 , and displays the shot image represented by the shot image data on a monitor  131 . The monitoring device  420  has various instructions inputted thereto by an operator having checked the shot image displayed on the monitor  131 . The inputted various instructions are conveyed to the photographic device  410 .  
         [0092]     As the photographic device  410  is installed outdoors, it is required to operate normally in a considerably high-temperature environment in midsummer. The focus lens  114  of the photographic device  410  is downsized since no transfer mechanism is necessary, and the ionic liquid  301  not evaporating at atmospheric pressure is used as a conductive liquid so as to maintain high optical performance in the high temperature environment. The ionic liquid  301  has the advantages that it does not easily catch fire and is not easily electrolyzed. Therefore, the monitoring system  400  of the present embodiment can improve safety against a fire and device life.  
         [0093]     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.  
         [0094]     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 lens  114  of the first embodiment (refer to  FIG. 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 using  FIG. 4  in the description of the third embodiment.  
         [0095]      FIG. 6  is a schematic block diagram of the printer to which the third embodiment of the present invention is applied.  
         [0096]     A printer  500  has a control section  510  controlling various elements of the printer  500 , an image processing section  520  generating the image data representing an output image, a laser emitting section  530  emitting the laser beam based on the image data, a prism  540  refracting the laser beam, and a lens  550  condensing the laser beam on paper  600 .  
         [0097]     For instance, the image data obtained by reading an original image is obtained by the image processing section  520 , and undergoes predetermined image processing therein. The image data after the image processing is sent to the laser emitting section  530 .  
         [0098]     The laser emitting section  530  emits the laser beam based on the image data sent from the image processing section  520 . The emitted laser beam is refracted by the prism  540  and gets incident on the lens  550 .  
         [0099]     The lens  550  has the same configuration as the focus lens  114  of the first embodiment shown in  FIG. 4 . The control section  510  controls the voltage applied to the first electrode  202  and the second electrode  203  of the lens  550  and adjusts the form of the interface between the ionic liquid  301  and the insulating liquid  302  of the lens  550  so as to condense the laser beam on the paper  600 . The control section  510  is equivalent to an example of the control section according to the present invention.  
         [0100]     The paper  600  has a sensitive material applied on its surface. If the laser beam is irradiated by the lens  550 , the image according to the laser beam is formed on the paper  600 .  
         [0101]     Here, when forming the image, the inside of the lens  550  is heated at considerably high temperature by the laser beam. However, the lens  550  has the ionic liquid  301  applied thereto so as to maintain the high optical performance even in the high temperature environment. For this reason, the printer  500  can form the high-quality image for a long period of time.  
         [0102]     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.  
         [0103]     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.  
         [0104]     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.  
         [0105]     Subsequently, a note will be appended as to various forms adoptable to component parts configuring the present invention.  
         [0000]     &lt;Ionic Liquid&gt; 
         [0106]     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 aphosphonium 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.  
         [0107]     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.  
         [0108]     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.  
         [0000]     &lt;Insulating Liquid&gt; 
         [0109]     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).  
         [0110]     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.  
         [0111]     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.  
         [0112]     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.  
         [0113]     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.  
         [0114]     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.  
         [0115]     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.  
         [0116]     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.