Optical device

An optical device includes: a sealed container having edge walls facing each other in a thickness direction of the container and a side wall connecting both of the edge walls; a first liquid with polarity or electrical conductivity and sealed within the container; a second liquid that is sealed within the container and does not mix with the first liquid; and a voltage applying unit for applying a voltage across the first liquid. The first liquid and the second liquid have equal specific gravity, and transmissivity of the first liquid is lower than the transmissivity of the second liquid. An interface between the first liquid and the second liquid changes shape in response to a voltage applied by the voltage applying unit. A light transmission path that passes through the edge walls and extends in a direction of the thickness of the container is formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an optical device.

2. Description of Related Art

An optical device that adjusts the amount of light to be transmitted by means of electrocapillarity (electrowetting) is proposed (For example, see Japanese Patent Application Publication No. 2001-228307).

Such an optical device10includes, as shown inFIG. 12A, a sealed container16that includes edge walls12that face each other in the direction of the thickness of the container16and a side wall14that connects both of the edge walls12, a first liquid20having polarity or electrical conductivity that is sealed within the container16, and a second liquid22that is sealed within the container16and that has a higher transmissivity than the first liquid20.

Liquids having such properties that they do not mix with each other are used as the first liquid20and the second liquid22, and further, liquids having the same specific gravity are used as the first liquid20and the second liquid22, so that when only the first liquid20and the second liquid22are sealed within the container16without getting air or the like mixed therein, the initial state in which only the first liquid20and the second liquid22were sealed within the container16is maintained even if the container16is rotated or shaken, and a state where an interface24is roughly parallel to the edge walls12is maintained.

Reference numerals28in the drawing is an electrode for applying a voltage across the first liquid20, and reference numerals30is an insulation film covering the electrode28.

By applying a voltage across the first liquid20with the above-mentioned electrode28, the shape of the interface24between the first liquid20and the second liquid22is altered between the gap shown with the solid line and the broken line inFIG. 12Adue to electrocapillarity, and thus, a light transmission path18that passes through the edge walls12and extends in the direction of the thickness of the container16is formed.

Specifically, in a state where no voltage is applied, by having the first liquid20extend, as indicated by the solid line inFIG. 12A, over the entire area in a direction that is orthogonal to the direction in which light is transmitted, transmission of light is prevented or suppressed, and as the applied voltage is increased, the transmission path18is formed by having the second liquid22come into contact with both of the edge walls12as indicated by the broken line inFIG. 12A, and the size of the transmission path18is adjusted by adjusting the applied voltage, thereby increasing or decreasing the contact area between the second liquid22and one of the edge walls12.

SUMMARY OF THE INVENTION

In such a optical device10in related art, a water-repellant film26for making the movement of the first and second liquids20and22smooth are formed on the inner side of the side wall14. The angle of contact θ formed between the first liquid20and the water-repellant films26is determined by the properties of the two, and the angle of contact θ is smaller than 90 degrees.

As shown inFIG. 12B, as the dimensions of the optical device10is reduced along the direction of light transmission (the dimension in the direction of its thickness), while it may be possible to block the light transmission path18by having the first liquid20, in a state where no voltage is applied, extend along the entire area in a direction orthogonal to the direction of light transmission, cases may arise where the light transmission path18cannot be formed since the second liquid22can only come into contact with one of the edge walls12, as shown inFIG. 12C, in a state where some voltage is applied.

Such an occurrence is due to the fact that the angle of contact θ formed between the first liquid20and the water-repellant film26is of a value smaller then 90 degrees and to the fact that the interface24forms a curved convex surface (including a spherical surface) that curves out from the first liquid20toward the second liquid22in the thickness direction.

Thus, conventionally, there is a limit in terms of the miniaturization of the dimension of the optical device10, which adjusts by means of electrocapillarity (electrowetting) the amount of light to be transmitted, in the direction in which light is transmitted (the dimension in the thickness direction).

On the other hand, miniaturization of imaging devices into which such optical devices10are incorporated is sought after, and how to achieve miniaturization of the dimension of the optical device10in the direction in which light is transmitted (the dimension in the direction of its thickness) is becoming an important issue.

The present invention is made in view of such circumstances, and seeks to provide an optical device that is advantageous in advancing miniaturization.

According to an embodiment of the present invention, there is provided an optical device including: a sealed container that has edge walls and a side wall, the edge walls facing each other in a thickness direction of the container, the side wall connecting both of the edge walls; a first liquid that has polarity or electrical conductivity, the first liquid being sealed within the container; a second liquid that is sealed within the container and does not mix with the first liquid; and a voltage applying unit for applying a voltage across the first liquid. The first liquid and the second liquid have equal specific gravity, and transmissivity of the first liquid is lower than the transmissivity of the second liquid. Furthermore, an interface between the first liquid and the second liquid changes shape in response to a voltage applied by the voltage applying unit. Furthermore, a light transmission path that passes through the edge walls and extends in a direction of the thickness of the container is formed. Furthermore, a hydrophilic film that is formed on a portion inside the side wall corresponding to the first liquid, wettability of the hydrophilic film with respect to the first liquid being higher than wettability of the hydrophilic film with respect to the second liquid. Furthermore, a water-repellant film that is formed on a portion inside the side wall corresponding to the second liquid, wettability of the water-repellant film with respect to the second liquid being higher than wettability of the water-repellant film with respect to the first liquid.

According to the present invention, when no voltage is applied, the interface between the first and second liquids is flat. Accordingly, even if the dimension of the optical device is reduced in the direction in which light is transmitted, it is possible, unlike the optical devices in the related art, to reliably bring the second liquid into contact with both of the edge walls in a state where a voltage is applied.

Accordingly, it is possible to reliably form a light transmission path in a state where a voltage is applied, and is thus advantageous in obtaining smaller and thinner optical devices.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The issues discussed above are addressed by forming a hydrophilic film on a portion of the inner surface of the side wall of the container corresponding to the first liquid and by forming a water-repellant film on a portion of the inner surface of the side wall of the container corresponding to the second liquid.

First, the principles of electrocapillarity (electrowetting) that is made use of in the optical device of the present invention will be described.

FIGS. 2A and 2Billustrate the principles of electrocapillarity.FIG. 2Ashows a state before a voltage is applied, andFIG. 2Bshows a state after a voltage is applied.

As shown inFIG. 2A, a first electrode2is formed on the surface of a substrate1, and an insulation film3is formed on the surface of this electrode2.

On the surface of this insulation film3is located a first liquid4that possesses polarity or electrical conductivity, and a second electrode5is electrically connected to the first liquid4.

As shown inFIG. 2A, in a state where a voltage E is not applied across the first electrode2and the second electrode5, the surface of the first liquid4forms an approximately spherical shape arching upward due to surface tension. At this point, the angle θ formed between the surface of the insulation film3and the liquid surface where the first liquid4is in contact with the insulation film3, in other words the angle of contact θ, is taken to be θ0.

However, as shown inFIG. 2B, when the voltage E is applied across the first electrode2and the second electrode5, an electrical field (electrostatic force) affects the particles constituting the first liquid4as a build-up of, for example, positive charge takes place on the surface of the insulation film3. Thus, particles constituting the first liquid4are attracted, the wettability of the first liquid4with respect to the insulation film3improves, and the angle of contact θ becomes θ1, which is smaller than θ0. Further, the angle of contact θ becomes smaller as the value of voltage E increases.

This is called electrocapillarity.

Next, an optical device40of the present embodiment will be described.

FIG. 1is a sectional view indicating the configuration of the optical device40in the present embodiment.

As shown inFIG. 1, the optical device40includes a container42, a first liquid44, a second liquid46and a voltage applying unit.

The container42includes edge walls4202that face each other in the direction of the thickness of the container42, a side wall4204that connects both of the edge walls4202, and a receptacle space42A that is sealed by these edge walls4202and side wall4204.

In the present embodiment, the edge walls4202take on the form of disk-like plates, the side wall4204takes on the form of a hollow cylinder having the same outer diameter as the outer diameter of the edge walls4202, and the receptacle space42A takes on the form of a flat cylinder.

In addition, the edge walls4202and the side wall4204are made of insulative materials, and the edge walls4202are made of a transparent material that allows the transmission of light.

As materials for the edge walls4202, for example, synthetic resin materials that are transparent and have insulative properties, or transparent glass materials may be used.

On the inside of the side wall4204is formed in the shape of a hollow cylinder a first electrode48(negative electrode) that extends along the entire circumference of the side wall4204, and on the entire circumference of the inside of the first electrode48is formed in the shape of a hollow cylinder an insulation film50so as to cover all of the first electrode48.

On a place on the inner surface of one of the two edge walls4204and toward its outer circumference is formed a second electrode52(positive electrode) that extends in the shape of a ring that is concentric with this edge wall4204. The second electrode52exposes part of its inner circumference in the receptacle space42A, and the second electrode52is insulated from the first electrode48by the insulation film50.

On a place on the inner surface of one of the two edge walls4204and over the entire area within the second electrode50is formed a transparent hydrophilic film54that allows transmission of light. The hydrophilic film54is so formed that its wettability with respect to the first liquid44is higher than its wettability with respect to the second liquid46.

A power source56with a variable output voltage is provided on the outside of the container42. The negative voltage output terminal of the power source56is electrically connected to the first electrode48, and the positive voltage output terminal of the power source56is electrically connected to the second electrode52.

In the present embodiment, the above-mentioned voltage applying unit may include the first electrode48, the second electrode52and the power source56.

The first liquid44has polarity or electrical conductivity, and is sealed within the container42.

The second liquid46does not mix with the first liquid44and is sealed within the container42.

In addition, the first liquid44and the second liquid46have equal specific gravity, and the first liquid44is such that its transmissivity is lower than the transmissivity of the second liquid46.

The first liquid44and the second liquid46will be described in detail later.

On a portion on the inside of the side wall4204corresponding to the first liquid44is formed a hydrophilic film58, and on a portion on the inside of the side wall4204corresponding to the second liquid46is formed a water-repellant film60.

The hydrophilic film58is so configured that its wettability with respect to the first liquid44is higher than its wettability with respect to the second liquid46. In other words, the hydrophilic film58is so configured that the angle of contact of the first liquid44in relation to the hydrophilic film58would be smaller than the angle of contact of the second liquid46in relation to the hydrophilic film58.

The hydrophilic film58may be formed by, for example, applying a hydrophilic polymer or a surfactant on the inner surface of the side wall4204, and various known materials may be used to this end.

The water-repellant film60is so configured that its wettability with respect to the second liquid46is higher than its wettability with respect to the first liquid44. In other words, the water-repellant film60is so configured that the angle of contact of the second liquid46in relation to the water-repellant film60would be smaller than the angle of contact of the first liquid44in relation to the water-repellant film60.

The water-repellant film60may be formed by applying, for example, a water-repellant agent of fluoride compounds and the like on the inner surface of the side wall4204, and various known materials may be used to this end.

First, the second liquid46is injected into the receptacle space42A of the container42and onto the edge wall4202on the side on which the water-repellant film60is provided, so that its fluid level is at the upper edge of the water-repellant film60. Then, the first liquid44is injected thereonto, and the second liquid46and the first liquid44are sealed within the receptacle space42A by taking out the air inside.

Thus, the entire area of the first liquid44located at the entire outer circumference of the inner surface of the edge wall4202where the first liquid44is located becomes electrically connected to the second electrode52by coming into contact therewith, and further, the entire area of the first liquid44located at the entire outer circumference of the receptacle space42A faces the first electrode48with the insulation film50, the hydrophilic film58and the water-repellant film60in-between.

Therefore, when a voltage is applied across the first electrode48and the second electrode52by the power source56, a voltage is applied across the first liquid44.

Next, operations of the optical device40will be described.

FIG. 3illustrates a state where no voltage is applied to the optical device40,FIG. 4illustrates a state where a first voltage E1is applied to the optical device40,FIG. 5illustrates a state where a second voltage E2of a value greater than the first voltage E1is applied to the optical device40, andFIG. 6illustrates a state where a third voltage E3of a value greater than the second voltage E2is applied to the optical device40.

In a state where no voltage is applied across the first electrode48and the second electrode52from the power source56(E=0V), as shown inFIG. 3, the entire area of the first liquid44located at the entire outer circumference of the receptacle space42A is in contact with the surface of the hydrophilic film58, the angle of contact thereof is 90 degrees, the entire area of the second liquid46located at the entire outer circumference of the receptacle space42A is in contact with the surface of the water-repellant film60, and the angle of contact thereof is 90 degrees.

Therefore, an interface62formed between the first liquid44and the second liquid46is flat.

At this point, since the first liquid44extends across an entire area in a direction that is orthogonal to the direction in which light is transmitted, light that travels in the direction of the thickness of the container42is blocked.

When the first voltage E1is applied across the first electrode48and the second electrode52from the power source56(where E1>0V), as shown inFIG. 4, due to electrocapillarity, the interface62changes its shape into a convex curved surface (spherical surface) that arches outward from the second liquid46toward the first liquid44such that the center of the interface62is now closer to one of the edge walls4202. In other words, the thickness of the first liquid44is smallest (thinnest) at the center, and its thickness becomes greater (thicker) the further away it moves from the center toward the outer circumference of the receptacle space42A.

At this point, the angle of contact of the first liquid44with respect to the water-repellant film60is smaller than 90 degrees, and at the side wall4204(the water-repellant film60), the first liquid44enters the second liquid46along the side wall4204.

When the second voltage E2of a value greater than the first voltage E1is applied across the first electrode48and the second electrode52from the power source56(where E2>E1), as shown inFIG. 5, the gradient of the convex curved surface (spherical surface) of the interface62becomes greater, and the center of the interface62touches one of the edge walls4202(the hydrophilic film54).

As a result, the first liquid44ceases to be present on the edge wall4202(the hydrophilic film54) where the interface62is in contact with, an area64where only the second liquid46is present is formed in the center of the receptacle area42A (the center of both of the edge walls4202), and a light transmission path66that passes through the edge walls4202and extends in the direction of the thickness of the container42is formed by way of this area64.

When the third voltage E3of a value greater than the second voltage E2is applied across the first electrode48and the second electrode52from the power source56(where E3>E2), as shown inFIG. 6, the gradient of the convex curved surface (spherical surface) of the interface62becomes even greater.

The diameter of the area64formed in the center of the receptacle space42A (the center of both of the edge walls4202) where only the second liquid46is present is enlarged, and the diameter of the light transmission path66is enlarged.

Thus, by adjusting the voltage applied across the first electrode48and the second electrode52from the power source56, it is possible to enlarge or reduce the diameter of the area64where only the second liquid46is present, and it is possible to perform aperture operations whereby the diameter of the light transmission path66is enlarged or reduced.

According to the present embodiment, when no voltage is applied, the angle of contact θ of the first liquid44with respect to the hydrophilic film58and to the water-repellant film60is 90 degrees, the angle of contact of the second liquid46with respect to the hydrophilic film46and to the water-repellant film58is 90 degrees, and the interface62is flat. Therefore, even if the dimension of the optical device40in the direction in which light is transmitted (the dimension in the direction of its thickness) is reduced, unlike conventional optical devices, it is possible to bring the second liquid46into contact with both of the edge walls4202reliably in a state where a voltage is applied.

Therefore, the light transmission path66can be formed reliably in a state where a voltage is applied, and it is advantageous in obtaining thinner devices.

If, as is conventional, the interface62between the first and second liquids44and46takes on the form of a concave curved surface where the first liquid44curves out toward the second liquid46(seeFIG. 12A), a situation arises where the second liquid46exists between the first liquid44and the first electrode48, and therefore, since the voltage applied via the first electrode48is obstructed by the second liquid46, it becomes more difficult to apply a voltage across the first liquid44, electrocapillarity in the first liquid44cannot be brought about reliably, and it is disadvantageous in stabilizing aperture operations.

In contrast, in the present embodiment, since the interface62between the first and second liquids44and46is flat, the second liquid46never exists between the first liquid44and the first electrode48. Therefore, the voltage applied via the first electrode48is applied across the first liquid44without being obstructed by the second liquid46, and thus, electrocapillarity in the first liquid44can be brought about reliably, and it is advantageous in stabilizing aperture operations.

In addition, since the water-repellant film60is formed on the portion of the side wall4204corresponding to the second liquid46, if the first liquid44comes to where the water-repellant film60is, the surface of the first liquid44moves smoothly over the water-repellant film60, and it is advantageous in achieving faster aperture operations.

In addition, since the hydrophilic film54is formed on the edge wall4202on the side of the first liquid44, the hydrophilic film54is very wettable with respect to the first liquid44. Therefore, when the second liquid46moves away from the edge wall4202on the side of the first liquid44after having been in contact with that edge wall4202, it is easier for the second liquid46to detach from the hydrophilic film54, and it is advantageous in achieving faster aperture operations.

Next, the first liquid44and the second liquid46used in the embodiment above will be described.

The first liquid44is obtained by mixing three kinds of liquids each having a specific gravity and refractive index that are different from those of one another, and the present inventor discovered the fact that the specific gravity and refractive index of the first liquid44can be changed over a large range by changing the mixing ratio of these three kinds of liquids.

As an example, a case where the first liquid44is obtained by mixing two kinds of liquids will first be described.

The first liquid44will be obtained by mixing pure water and ethanol as the two kinds of liquids, and the mixing ratio thereof will be varied.

As shown inFIG. 7, as the mixing ratio of these liquids is varied, the specific gravity and refractive index of the first liquid44changes linearly or in a curve.

In addition, the first liquid44will be obtained by mixing pure water and ethylene glycol as the two kinds of liquids, and the mixing ratio thereof will be altered.

As shown inFIG. 8, as the mixing ratio of these liquids is varied, the specific gravity and refractive index of the first liquid44changes linearly or in a curve.

It is noted that the specific gravity and refractive index of pure water are 1.0 and 1.333, respectively, that the specific gravity and refractive index of ethanol are 0.789 and 1.361, respectively, and that the specific gravity and refractive index of ethylene glycol are 1.113 and 1.430, respectively.

In contrast to the examples above, the first liquid44is next obtained by mixing three kinds of liquids, and the mixing ratio thereof is varied.

As an example, the first liquid44is obtained using pure water, ethanol and ethylene glycol as the three kinds of liquids, and the mixing ratio thereof is varied.

As shown inFIG. 9, by varying the mixing ratio of pure water, ethanol and ethylene glycol, it is possible to alter the specific gravity and refractive index of the first liquid44over a large range R that is obtained by joining the three coordinates for pure water, ethanol and ethylene glycol.

On the other hand, inFIG. 9, coordinates of the specific gravity and refractive index of various silicone oils that are commercially available are indicated.

Therefore, a commercially available silicone oil that falls within the triangular area R may be used as the second liquid46, and the first liquid44, which is obtained by mixing pure water, ethanol and ethylene glycol and whose specific gravity and refractive index are made equal to those of the silicone oil above, may be used.

In the present embodiment, the first liquid44is formed by dissolving carbon black in a mixture of pure water, ethanol and ethylene glycol, has a black color, is so formed that it can block light with a thickness of approximately 0.1 mm, and is advantageous in obtaining thinner optical devices.

By making the refractive index of the first liquid44and the refractive index of the second liquid46equal, occurrences of a lens effect at the interface62can be prevented, and it is advantageous in improving the reliability of aperture operations.

In addition, by forming the first liquid44by mixing ethanol in water, its freezing-point (melting-point) can be lowered, freezing in cold climates can be prevented, and the use of the optical device40in cold climates becomes possible.

In the present embodiment, the freezing-point of ethanol is −114 degrees Celsius, the freezing-point of ethylene glycol is −13 degrees Celsius, and it is possible to keep the freezing-point of the first liquid44at 40 degrees Celsius or below.

In addition, in the embodiment above, since three kinds of existing liquids with different values of specific gravity were mixed and used as the first liquid44, as indicated by the area R inFIG. 9, variations over a wide range are possible.

In other words, when two kinds of liquids with different values of specific gravity are mixed, the specific gravity of the first liquid44that can be obtained by varying the mixing ratio of those two kinds of liquids can only be varied, as shown inFIG. 9, within the range of the line that joins the coordinates of those liquids.

In contrast, when three kinds of liquids are mixed, it becomes possible to vary the specific gravity of the first liquid44within the larger triangular area R that is obtained by joining the three coordinates for pure water, ethanol and ethylene glycol.

Therefore, it is easier to make the specific gravity of the first liquid44and the specific gravity of the second liquid46equal, and it is easier to obtain the optical device40with the desired properties.

Further, as shown inFIG. 9, since the first liquid44is obtained by mixing at least three kinds of liquids, for example, pure water, ethanol and ethylene glycol, that have not only differing values of specific gravity but differing refractive indices as well, while it is easier to make the specific gravity of the first liquid44and the specific gravity of the second liquid46equal, it is also easier to make the refractive index of the first liquid44and the refractive index of the second liquid46equal, and it is therefore advantageous in preventing the occurrence of a lens effect.

In addition, in the embodiment above, a case where the first liquid44is obtained by mixing pure water, ethanol and ethylene glycol as the several kinds of liquids is described, however, the several kinds of liquids to be used are not limited to pure water, ethanol and ethylene glycol, and various kinds of other existing liquids may also be chosen instead.

A description will be given with reference toFIG. 10andFIG. 11.

FIG. 10is a graph indicating the specific gravity and refractive index of various kinds of liquids, andFIG. 11is a graph indicating the values of specific gravity and refractive index of the various liquids to be used.

For example, as shown inFIG. 10, as liquids to be used, those that belong to group A, group B, group C and group D may be considered, and the actual names of liquids to be used in groups A to D are shown inFIG. 1.

As indicated with a triangular area R1inFIG. 10, it is possible to vary the specific gravity and refractive index by varying, within the large triangular area R1that is obtained by joining the coordinates of one liquid chosen from group A, another from group B, and another from group C as the three kinds of liquids, the mixing ratio of those liquids.

In addition, as shown with a triangular area R2inFIG. 10, it is possible to vary the specific gravity and refractive index by varying, within the large triangular area R2that is obtained by joining the coordinates of one liquid chosen from group B, another from group C, and another from group D as the three kinds of liquids, the mixing ratio of those liquids.

In other words, by choosing various known liquids and changing the mixing ratio thereof, it is possible to vary the specific gravity and refractive index with ease.

It is to be noted that the number of liquids to be used for the first liquid is not limited to three, and four or more kinds of liquids may be also be used.

In addition, in the embodiment above, a case where the first liquid44is so formed to be equal in specific gravity with the second liquid46by mixing several kinds of liquids, each having a different specific gravity and refractive index, is described, however, it is also possible to form the second liquid46by mixing several kinds of liquids, each having a different specific gravity and refractive index, so that its specific gravity equals that of the first liquid44.

Further, in the embodiment above, a case where a single silicone oil is used as the second liquid46is described, however, several silicone oils that have differing properties, such as refractive index and specific gravity, are available, and while it is possible to choose one kind of silicone oil that has the desired properties and use it as the second liquid46, it is also possible to select several kinds of silicone oils with differing properties, vary their mixing ratio, and use them as the second liquid46with the desired refractive index and specific gravity.

In addition, in the embodiment above, a case where electrocapillarity is brought about by applying a DC, voltage across the first liquid44is described, however, the voltage to be applied across the first liquid44is not limited to a DC voltage, and any kind of voltage, such as an AC voltage, pulse voltage, a voltage that fluctuates in steps, may be used so long as electrocapillarity can be caused in the first liquid44.

The present document claims priority to Japanese Priority Document JP 2005-063324, filed in the Japanese Patent Office on Mar. 8, 2005, the entire contents of which are incorporated herein by reference to the extent permitted by law.