JP5069267B2 - Variable focus lens - Google Patents

Description

  The present invention relates to a variable focus lens, and more particularly to a variable focus lens that can change a focal length using an optical material having an electro-optic effect.

  Conventionally, optical components such as optical lenses and prisms are optical devices such as cameras, microscopes, and telescopes, electrophotographic recording devices such as printers and copiers, optical recording devices such as DVDs, optical devices for communication, industrial use, etc. It is used for. A normal optical lens has a fixed focal length. However, a lens that can adjust the focal length according to the situation, a so-called variable focus lens may be used in the above-described devices and apparatuses. The conventional variable focus lens mechanically adjusts the focal length by combining a plurality of lenses. However, such a mechanical variable focus lens has a limit in extending the application range from the viewpoint of response speed, manufacturing cost, miniaturization, power consumption, and the like.

  Therefore, a variable focus lens in which a material capable of changing the refractive index is applied to the transparent medium constituting the optical lens, a variable focus lens that mechanically deforms the shape of the optical lens, instead of moving the position of the optical lens, etc. It was issued. As the former variable focus lens, a variable focus lens using liquid crystal as an optical lens has been proposed. This variable focus lens encloses the liquid crystal in a container made of a transparent material by sandwiching the liquid crystal between two glass plates. When the inside of the container is processed into a spherical surface and the liquid crystal is molded into a lens shape, a variable focus lens can be configured. A transparent electrode is provided inside the container, and the refractive index is controlled by applying an electric field to the liquid crystal, and the focal length is variably controlled (see, for example, Patent Document 1).

  As the latter variable focus lens, liquid is often used as the material of the deformable lens. For example, the variable focus lens described in Non-Patent Document 1 has a structure in which a liquid such as silicon oil is sealed in a space sandwiched between glass plates. The glass plate is thinly processed. By applying pressure to the glass plate with a lead zirconate titanate (PZT) piezo actuator from the outside, the lens composed of the oil and the entire glass plate is deformed, and the focal position is adjusted. Control. The operating principle of this variable focus lens is the same as that of the lens of the eyeball.

Japanese Patent Laid-Open No. 11-64817

Takashi Kaneko et al., “Long focal depth visual mechanism using variable focus lens”, Denso Technical Review, Vol. 3, no. 1, p. 52-58, 1998

  However, the conventional variable focus lens includes a variable focus lens that mechanically adjusts the focal length, a variable focus lens that controls the refractive index by applying an electric field to the liquid crystal, and a variable focus lens that deforms the lens by a PZT piezo actuator. The response speed required to change the focal length is limited, and there is a problem that it cannot be applied to a high-speed response of 1 ms or less.

  An object of the present invention is to provide a variable focus lens capable of changing the focal length at high speed.

  In order to achieve the above object, the present invention provides an electro-optic material 1 having inversion symmetry, a light incident surface of the electro-optic material 1, and a variable focus lens. An incident surface side electrode 2a, 2b and an output surface side electrode 3a, 3b formed on each of two parallel surfaces facing each other perpendicular to the exit surface, and the light is incident from the entrance surface, An optical axis is set so as to emit from the exit surface, and sides 10 and 20 of the sides of the entrance surface side electrode 2a facing the exit surface side electrode 3a are arranged in parallel to each other, and the entrance surface side electrode Among the sides of 2a, the sides 10 and 20 facing the side of the exit surface side electrode 3a and the side of the entrance surface side electrode 2b formed on the surface facing the electro-optic material 1 in between Sides 30 and 40 facing the side of the emission surface side electrode 3b are: The electric field formed between the entrance surface side electrodes 2a, 2b and the exit surface side electrodes 3a, 3b is changed in a portion where the light is transmitted around the optical axis. The focus of the light transmitted through the electro-optic material 1 is variable by changing the applied voltage between the incident surface side electrodes 2a, 2b and the output surface side electrodes 3a, 3b.

The electro-optic material 1 is preferably a perovskite single crystal material, and the electro-optic material 1 may be potassium tantalate niobate (KTa _1-x Nb _x O ₃ ). In the electro-optic material 1, the main component of the crystal is composed of a group Ia and a Va group in the periodic table, the group Ia is potassium, and the group Va includes at least one of niobium and tantalum. In addition, the additive impurities may include one or more of Group Ia or Group IIa of the periodic table excluding potassium.

  The electrode is preferably a material that forms a Schottky junction with the electro-optic material 1. Further, the incident surface side formed on the sides 10 and 20 facing the side of the emission surface side electrode 3a among the sides of the incident surface side electrode 2a and the surface facing the electro-optic material 1 in between. Of the sides of the electrode 2b, the sides 30 and 40 facing the side of the emission surface side electrode 3b may be arranged at positions that coincide with each other in the optical axis direction.

  In addition, a groove is formed between the incident surface side electrode 2a and the output surface side electrode 3a, and the incident surface side electrode 2b and the output surface formed on the surfaces facing each other with the electro-optic material 1 interposed therebetween. A groove can be formed between the side electrode 3b.

  According to the present invention, the electro-optic material is formed between the entrance surface side electrode and the exit surface side electrode formed on each of two opposed parallel surfaces perpendicular to the light entrance surface and the exit surface. The electric field is changed in a portion where light is transmitted around the optical axis, and the focus of the light transmitted through the electro-optical material can be changed by changing the applied voltage between the electrodes. Since the electro-optic effect can respond at high speed, this variable focus lens can achieve a high-speed response of less than 1 μs.

It is a figure which shows the structure of the variable focus lens concerning one Embodiment of this invention. It is a figure for demonstrating the principle of the variable focus lens concerning embodiment. It is a figure which shows the electric field component and refractive index distribution in the board | substrate of a variable focus lens. 1 is a diagram illustrating a configuration of a variable focus lens according to Example 1. FIG. 6 is a diagram illustrating a configuration of a variable focus lens according to Example 2. FIG. FIG. 6 is a diagram illustrating a configuration of a variable focus lens according to Example 3;

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. The variable focus lens of this embodiment is composed of an electro-optic material and an electrode attached thereto. By utilizing the electro-optic effect, a much faster response speed can be obtained as compared with a conventional variable focus lens.

  FIG. 1 shows a configuration of a variable focus lens according to an embodiment of the present invention. FIG. 1A is a front view of a variable focus lens according to an embodiment of the present invention, and FIG. 1B is a three-dimensional view of the variable focus lens according to an embodiment of the present invention. Referring to FIG. 1A, an incident surface side electrode 2a and an output surface are formed on side surfaces perpendicular to an upper surface (light incident surface) and a lower surface (light output surface) of a substrate 1 obtained by processing an electro-optic material into a plate shape. A side electrode 3a is formed. The incident surface side electrode 2b and the output surface side electrode 3b are formed on the side surface opposite to the side surface on which the incident surface side electrode 2a and the output surface side electrode 3a are formed. Hereinafter, with reference to the drawings, a side surface on which the incident surface side electrode 2a and the emission surface side electrode 3a are formed is referred to as a left side surface, and a side surface on which the incident surface side electrode 2b and the emission surface side electrode 3b are formed is referred to as a right side surface. Here, the left side surface and the right side surface are parallel to each other. The incident surface side electrode 2a and the incident surface side electrode 2b are set to the same potential, and the output surface side electrode 3a and the output surface side electrode 3b are set to the same potential. The optical axis is set in the y-axis direction so that light passes between electrode pairs having the same potential. Of the sides of the incident surface side electrode 2a, the sides 10 and 20 facing the side of the output surface side electrode 3a are formed so as to be parallel to the z axis. Here, the side of the electrode indicates a side surrounding the electrode. Referring to FIG. 1 (b), the entrance surface side electrode 2b and the exit surface side electrode 3b have the same configuration, and the positions of the opposing sides 30 and 40 are the sides of the entrance surface side electrode 2a in the y-axis direction. It coincides with the sides 10 and 20 facing the side of the emission surface side electrode 3a, that is, it coincides with the substrate 1 in between. A voltage can be applied from the entrance surface side electrode 2a to the exit surface side electrode 3a or vice versa. Similarly, a voltage can be applied from the entrance surface side electrode 2b to the exit surface side electrode 3b or vice versa.

  The electro-optic material is preferably an oxide single crystal material having inversion symmetry. The inversion symmetry will be described later in detail. Details of the electrodes will be described later.

  The principle of the variable focus lens according to the present embodiment will be described with reference to FIG. In the variable focus lens shown in FIG. 1, a positive voltage is applied to the incident surface side electrode 2a and the incident surface side electrode 2b, and a negative voltage is applied to the output surface side electrode 3a and the output surface side electrode 3b. At this time, an electric field is generated from the incident surface side electrode 2a toward the output surface side electrode 3a. In addition, the electric field is generated from the incident surface side electrode 2b toward the output surface side electrode 3b. The electric field is generated not only between the entrance surface side electrode 2a and the exit surface side electrode 3a and between the entrance surface side electrode 2b and the exit surface side electrode 3b, but also around the light transmitting portion. appear. The arrows shown in FIG. 2 are examples of electric lines of force. Since the electric field has a spatial dependence in the portion where light is transmitted, an electro-optic effect is generated in the substrate 1 which is an electro-optic material, and the refractive index of the portion where the light is transmitted is modulated.

  The electric field distribution and refractive index modulation of the portion through which light is transmitted will be described. The electro-optic material generally has a relative dielectric constant sufficiently larger than 1. For this reason, the electric field lines of the electric field inside the substrate 1 are close to being parallel to the substrate surface near the surface. The lines of electric force traveling rightward from the incident surface side electrode 2a travel almost parallel to the upper surface of the substrate 1. The lines of electric force traveling in the left direction from the incident surface side electrode 2b also travel substantially parallel to the upper surface of the substrate 1. Since these two lines of electric force collide at the center between the left side surface and the right side surface, the direction of the electric force changes greatly from there and proceeds in the y-axis direction. Then, these two lines of electric force reach the lower surface, change their directions greatly, proceed in opposite directions to each other, and proceed to the emission surface side electrodes 3a and 3b, respectively. As described above, the electric lines of force traveling near the surface inside the substrate 1 are bent sharply in the gap between the pair of electrodes having the same potential, so that the electric field greatly changes in the bent portion. That is, the refractive index is modulated by changing the electric field around the optical axis where light is transmitted.

FIG. 3 shows the electric field component and the refractive index distribution inside the substrate. 3 (a) shows an x-axis direction of the distribution of the electric field component E _x in the position y = y ₀ shown in FIG. The horizontal axis represents the position in the x-axis direction of the portion where light between the pair of electrodes having the same potential is transmitted. Since the direction of the lines of electric force differs 180 degrees between the left side and the right side, with the central portion between the left side and the right side as such, this distribution is obtained. FIG. 3B shows the distribution in the x-axis direction of the electric field component E _{y at} the same position y = y ₀ . The electric field component E _y has the same sign, but its absolute value is small at the center and increases as it approaches the electrode. Such an electric field distribution modulates the refractive index in the x-axis direction.

FIG. 3 (c), potassium tantalate niobate electro-optic material _{(KTa 1-x Nb x O} 3, hereinafter referred to as KTN) using a refraction when the direction of the optical electric field incident light in the z-direction Indicates rate modulation. Near the central portion of the substrate 1, that is, near the optical axis, the refractive index is lower than the portion near the electrode pair away from the central portion in the x-axis direction. The faster the part, the slower the speed of light. For this reason, the wavefront of the light transmitted through the substrate 1 is delayed in a portion closer to the electrode pair than in the vicinity of the central portion, and functions as a concave lens. Considering the portion through which light is transmitted as a lens, it is possible to realize a lens having a strong condensing or diverging effect. In the configuration of FIGS. 1 and 2, light condensing or diverging occurs only in the x-axis direction, and no diverging occurs in the z direction, so that it functions as a so-called cylindrical lens instead of a general spherical lens.

  Another set of variable focus lenses having the configuration shown in FIG. 1 and FIG. 2 is prepared, and the optical axes of the portions through which light is transmitted are aligned with each other. Two variable focus lenses are arranged so that one variable focus lens rotates 90 degrees with respect to the other variable focus lens with respect to the optical axis, and condensing or diverging in two directions is equivalent to a spherical lens. Function can be realized.

(Electro-optic material)
The electro-optic effect includes several different-order electro-optic effects, but in general, the first-order electro-optic effect (hereinafter referred to as Pockels effect) and the second-order electro-optic effect (hereinafter referred to as Kerr effect). Is used). However, among the electro-optic effects, a material having a secondary electro-optic effect (Kerr effect) in which refractive index modulation proportional to the square of the electric field occurs is preferable. For Kerr effect, as shown in FIG. 3, the refractive index distribution Δn is because it does not depend on the sign of the electric field component E _x, become suitable symmetrical shape as a lens. On the other hand, in the case of the Pockels effect, refractive index modulation is proportional to the first power of the electric field, the refractive index change due to the electric field component E _x is not symmetrical, it does not work well as a lens.

  A single crystal having inversion symmetry refers to a crystal that has the same arrangement as the original arrangement of atoms when the arrangement of atoms is reversed in the x, y, z coordinate system around a certain origin. Note that when a material having spontaneous polarization is inverted on the coordinate axis, the direction of spontaneous polarization is inverted, and thus such a crystal material does not have inversion symmetry. On the other hand, a single crystal having inversion symmetry has no Pockels effect, and the Kerr effect is the lowest order electro-optic effect. Therefore, among crystal materials having an electro-optic effect, a single crystal having inversion symmetry is desirable.

  The magnitude of the electric field inside the crystal is proportional to the voltage applied to the electrode. Also, since the refractive index modulation is proportional to the square of the electric field, the magnitude of the refractive index modulation is proportional to the square of the voltage. Thereby, the focal length of the concave lens can be controlled by the voltage. Although it has been described here that it functions as a concave lens, the sign of the electro-optic coefficient differs depending on the material and light polarization, so that a convex lens can also be realized.

The electro-optic material is preferably a single crystal material having a perovskite crystal structure. This is because the perovskite single crystal material has a cubic phase having reversal symmetry in the use state if the use temperature is appropriately selected, and does not have the Pockels effect in this cubic phase. For example, even the most well-known barium titanate (BaTiO ₃ , hereinafter referred to as BT) may exceed the temperature at which the phase transition from the tetragonal phase to the cubic phase (hereinafter referred to as the phase transition temperature) occurs at around 120 ° C. For example, it becomes a cubic phase and exhibits the Kerr effect.

  Furthermore, the single crystal material mainly composed of KTN has more preferable characteristics. BT has a predetermined phase transition temperature, whereas KTN can select a phase transition temperature depending on the composition ratio of tantalum and niobium. Thereby, the phase transition temperature can be set near room temperature. KTN has a cubic phase at a temperature higher than the phase transition temperature, has inversion symmetry, and has a large Kerr effect. Even in the same cubic phase, the Kerr effect becomes overwhelmingly closer to the phase transition temperature. For this reason, setting the phase transition temperature around room temperature is very important for easily realizing a large Kerr effect.

As a single crystal material having reversal symmetry, the main component of the crystal is composed of groups Ia and Va in the periodic table, group Ia is potassium, and group Va contains at least one of niobium and tantalum. Can be used. Further, the single crystal material having inversion symmetry may include one or more members of Group Ia or Group IIa of the periodic table excluding potassium as an additive impurity. For example, a cubic phase KLTN (K _1-y Li _y Ta _1-x Nb _x O ₃ , 0 <x <1, 0 <y <1) crystal having a large Kerr effect may be used.

  In KTN, when the operating temperature is brought close to the phase transition temperature, the dielectric constant increases rapidly, and the electro-optic effect increases. For example, when the relative dielectric constant of KTN exceeds 10,000 and the voltage applied to the KTN substrate exceeds 500 V, the focal length becomes 1 m or less, and practically effective characteristics can be obtained.

Note that the refractive index modulation of KTN varies depending on the relationship between the direction of the applied electric field and the direction of the optical electric field, as in other electro-optic crystals. In the configuration of FIG. 2, there are two types of polarized light, when the direction of the optical electric field is the x-axis direction and when the direction is the z-axis direction. In each case, the refractive index modulation [Delta] n _x and [Delta] n _z where light feel,

It becomes different. Here, n ₀ is the refractive index before modulation.

Further, s ₁₁ and s ₁₂ are electro-optic coefficients, but s ₁₁ is positive, whereas s ₁₂ has a negative value, and the absolute value is larger in s ₁₁ . Because of this feature, the function changes completely depending on the polarization state of the incident light, a convex lens when the direction of the optical electric field is the x direction, and a concave lens when the direction of the optical field is the z direction.

  The characteristic of the lens is expressed by optical path length modulation that the light receives by passing through the substrate 1. The optical path length modulation Δs is obtained by integrating the refractive index modulation Δn over the path through the electro-optic material. Since the refractive index modulation is a function of x and y, this is assumed to be Δn (x, y). The refractive index modulation Δn does not depend on z. Since the variable focus lens according to the present embodiment propagates light in the y-axis direction, the optical path length modulation Δs is

Thus, it is a function of only x without depending on y. That is, it changes only in the x-axis direction where light is condensed, and does not change in the z-axis direction.

(Electrode material)
When a high voltage is applied to the electro-optic material, charges are injected from the electrodes, and space charges can be generated in the crystal. This space charge causes an electric field to be tilted in the direction of voltage application, so that the refractive index is also tilted.

  Therefore, in order to prevent a desired refractive index distribution for causing the electro-optic material to function as a lens or to prevent light transmitted through the electro-optic material from being deflected, when a voltage is applied to the substrate 1, It is better that no space charge is formed inside the substrate 1. Since the amount of space charge depends on the carrier injection efficiency, the carrier injection efficiency injected from the electrode should be small. As the work function of the electrode material increases, the Schottky junction approaches between the electrode and the substrate, and the carrier injection efficiency decreases. When the carrier that contributes to electric conduction in the electro-optic crystal is an electron, the work function of the electrode material is preferably 5.0 eV or more. For example, as an electrode material having a work function of 5.0 eV or more, Co (5.0), Ge (5.0), Au (5.1), Pd (5.12), Ni (5.15), Ir (5.27), Pt (5.65), Se (5.9) can be used. Figures in parentheses indicate work functions (eV).

  On the other hand, when the carriers contributing to electrical conduction in the electro-optic crystal are holes, the work function of the electrode material is preferably less than 5.0 eV in order to suppress the injection of holes. For example, Ti (3.84) or the like can be used as an electrode material having a work function of 5.0 eV or more. Since the Ti single-layer electrode is oxidized and becomes high resistance, generally, the Ti layer and the electro-optic crystal are bonded using an electrode in which Ti / Pt / Au is laminated. Further, a transparent electrode such as ITO (Indium Tin Oxide) or ZnO can be used.

Example 1
FIG. 4 illustrates a configuration of the variable focus lens according to the first example. 4A is a front view, and FIG. 4B is a three-dimensional view. The substrate 1 was cut out from a KTN single crystal and formed into a shape of 6 mm × 6 mm × 6 mm. All six surfaces of the substrate 1 are parallel to the (100) plane of the crystal and optical polishing is performed. An entrance surface side electrode 2a and an exit surface side electrode 3a are formed on the left side surface of the substrate 1 of the electro-optic material shown in FIG. Similarly, an entrance surface side electrode 2b and an exit surface side electrode 3b are formed on the right side surface. Since this KTN single crystal had a phase transition temperature of 35 ° C., it was decided to use it at 40 ° C., which is slightly higher than this. The relative dielectric constant at this temperature is 20,000.

  Each of the entrance surface side electrode 2a, the exit surface side electrode 3a, the entrance surface side electrode 2b, and the exit surface side electrode 3b is a 0.6 mm × 5 mm square, and is formed by depositing platinum (Pt) to a thickness of about 2000 mm. Has been. The distance between the entrance surface side electrode 2a and the exit surface side electrode 3a is 4 mm. Similarly, the interval between the entrance surface side electrode 2b and the exit surface side electrode 3b is 4 mm.

  The collimated laser light is incident from the upper surface in a state where the temperature of the variable focus lens of Example 1 is controlled at 40 ° C. The polarization of light is a straight line, and the direction of the oscillating electric field is the z-axis direction. When a voltage of 500 V is applied between the incident surface side electrode 2a and the output surface side electrode 3a, and similarly, a voltage of 500 V is applied between the incident surface side electrode 2b and the output surface side electrode 3b, the light is emitted from the lower surface. The spreading light spreads in the x-axis direction and functions as a cylindrical concave lens. The focal length is 25 cm. Here, when the applied voltage is 250 V, the spread becomes small and the focal length becomes about 1 m. That is, the focal length can be changed by the applied voltage. Since changing the focal length only changes the applied voltage, the response time is 1 μs or less, which is an improvement of three orders of magnitude or more compared to the response time of the conventional variable focus lens.

  Further, the measurement is performed by rotating the polarized light by 90 degrees while keeping the traveling direction of the light. That is, the direction of the oscillating electric field of light is the x-axis direction. In this case, it functions as a convex lens. When the applied voltage is 500 V, the focal length is 19 cm, and the focal length can be changed by the applied voltage.

  In the first embodiment, the sides 10 and 20 that face the side of the exit surface side electrode 3a among the sides of the entrance surface side electrode 2a, and the side that faces the exit surface side electrode 3b among the sides of the entrance surface side electrode 2b. The sides 30 and 40 that coincide with each other in the y-axis direction with the substrate 1 interposed therebetween need not be completely coincident with each other, and may be parallel to each other.

(Example 2)
FIG. 5 illustrates a configuration of a variable focus lens according to the second embodiment. Grooves were formed between the electrodes on the left side surface and the right side surface in the same configuration as in Example 1. The groove shape was 0.5 mm in depth, 1 mm to 2 mm in width, and 5 mm in length. By forming the groove, the electric field distribution changed and the lens performance improved.

(Example 3)
FIG. 6 illustrates a configuration of the variable focus lens according to the third example. In this embodiment, another set of variable focus lenses having the configuration of the first embodiment is prepared and arranged so that the optical axes of the portions through which light is transmitted coincide with each other. Two variable focus lenses are arranged such that one variable focus lens rotates 90 degrees with respect to the other variable focus lens with respect to the optical axis. Similar to the first embodiment, collimated laser light is incident from the upper surface. By condensing or diverging in two directions, a function equivalent to a spherical lens can be realized.

DESCRIPTION OF SYMBOLS 1 Substrate 2a Incident surface side electrode 2b Incident surface side electrode 3a Outgoing surface side electrode 3b Outgoing surface side electrode 10 Side facing the emitting surface side electrode 3a 20 Side facing the incident surface side electrode 2a 30 Opposing to the emitting surface side electrode 3b Side 40 The side facing the incident surface side electrode 2b

Claims (8)

An electro-optic material having inversion symmetry, and an entrance surface-side electrode and an exit surface-side electrode formed on each of two parallel surfaces facing each other perpendicular to the light entrance surface and the exit surface of the electro-optic material With
The optical axis is set so that the light enters from the incident surface and exits from the exit surface,
Of the sides of the incident surface side electrode, sides facing the side of the output surface side electrode are arranged in parallel to each other,
Of the sides of the entrance surface side electrode, the exit surface of the sides of the entrance surface side electrode formed on the surface facing the side of the exit surface side electrode and the surface facing the electro-optic material The sides facing the sides of the side electrodes are each arranged in parallel,
An electric field formed between the incident surface side electrode and the output surface side electrode is changed in a portion where the light is transmitted around the optical axis,
A variable focus lens, wherein a focal point of light transmitted through the electro-optic material is variable by changing an applied voltage between the entrance surface side electrode and the exit surface side electrode.   The variable focus lens according to claim 1, wherein the electro-optic material is a perovskite single crystal material. The electro-optical material, a variable focus lens according to claim 2, characterized in that the potassium tantalate niobate _{(KTa 1-x Nb x O} 3).   The electro-optic material is characterized in that the main component of the crystal is composed of groups Ia and Va in the periodic table, group Ia is potassium, and group Va includes at least one of niobium and tantalum. The variable focus lens according to claim 2.   5. The variable focus lens according to claim 4, wherein the electro-optic material further includes one or more of Group Ia or Group IIa of the periodic table excluding potassium as an additive impurity.   6. The variable focus lens according to claim 1, wherein the electrode is a material that forms a Schottky junction with the electro-optic material.   Of the sides of the entrance surface side electrode, the exit surface of the sides of the entrance surface side electrode formed on the surface facing the side of the exit surface side electrode and the surface facing the electro-optic material The variable focus lens according to claim 6, wherein the sides facing the sides of the side electrode are respectively arranged at positions that coincide with each other in the optical axis direction.   A groove is formed between the entrance surface side electrode and the exit surface side electrode, and between the entrance surface side electrode and the exit surface side electrode formed on a surface facing each other with the electro-optic material interposed therebetween. The variable focus lens according to claim 6 or 7, wherein a groove is formed.

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