Patent Application
20110149386
The present invention relates to an optical low pass filter used in an imaging device using a charge-coupled device (CCD) or a complementary metal oxide semiconductor (CMOS) image sensor which separates incident light into four light beams and performs information processing for the four light beams.
CCD and CMOS image sensors used in imaging devices such as a video camera, a digital camera, and the like convert a quantity of brightness and darkness of light incident as an external signal to a charge quantity, i.e., perform what is called photoelectric conversion, and sequentially process the electric signal to thereby generate a digital image. In such CCD and CMOS image sensors, since distortion resulting from sampling is generated in an image having a spatial frequency smaller than a pixel pitch of incident light and a pattern (moiré) different from an original image occurs, the CCD and CMOS image sensors are constituted to include an optical low pass filter in order to prevent the occurrence of the moiré. A specific optical low pass filter has a function of cutting the vicinity of a frequency (sampling frequency) of the pixel pitch which enters the image sensor by slightly separating an incident two-dimensional image in horizontal and vertical directions, and is so devised as to prevent the occurrence of the moiré phenomenon by the function.
A description is given of a state of light in the process where incident light passes through the low pass filter 100. In
The light separated into an optical path 104a of the A-polarized light and an optical path 105a of the B-polarized light subsequently becomes incident on the wave plate 102. The wave plate 102 has a function of performing phase modulation on the light incident in a specific vibration direction such as the A-polarized light and the B-polarized light such that the A-polarized light (component) and the B-polarized light (component) have the same light quantity. The light having passed through the wave plate 102 in this manner travels straight with the A-polarized light and the B-polarized light mixed therein, and the light in correspondence to the optical path 104a of the A-polarized light is assumed to pass through the wave plate 102 and become incident on the second optical path separation birefringent plate 103 as the light of an optical path 104b, while the light in correspondence to the optical path 105a of the B-polarized light is assumed to pass through the wave plate 102 and become incident on the second optical path separation birefringent plate 103 as the light of an optical path 105b. At this point, in the second optical path separation birefringent plate 103, the A-polarized light and the B-polarized light are separated such that the A-polarized light and B-polarized light follow different courses in the same manner as in the above-described first optical path separation birefringent plate 101. At this point, the light separation direction in the first optical path separation birefringent plate 101 and the light separation direction in the second optical path separation birefringent plate 103 are made to be orthogonal to each other.
The reason why the separation directions are made to be orthogonal to each other as described above is that, since pixels of the image sensor are two-dimensionally arranged, in order to prevent the moiré with respect to two orthogonal directions of the arrangement, the light is separated in the separation directions matching with the arrangement directions of the pixels. In addition, the width of the separation (separation distance) differs according to the pitch of the pixels and the spatial frequency to be cut. Further, when a pixel shape is square, it is effective to have the same separation distance in the X direction and in the Y direction. However, in the case of the image sensor using, e.g., rectangular pixels (length of one pixel in Y direction>length of one pixel in X direction), a high priority is given to the prevention of the moiré in the X direction. Consequently, the separation distances in the X and Y directions may be different, and a quadrangle obtained by joining four separated points is not limited to a square and the quadrangle may be a parallelogram. Further, as described above, when a high priority is given to the prevention of the moiré in the X direction, there is a case where two-point separation only in the X direction is sufficient.
From each of the optical paths 104b and 105b of the incident light, the A-polarized light and the B-polarized light are separated in the second optical path separation birefringent plate, and pass therethrough. At this point, positions where the A-polarized light and the B-polarized light separated from the optical path 104b pass through the second optical path separation birefringent plate 103 are denoted by reference numerals 103a and 103b respectively, and positions where the A-polarized light and the B-polarized light separated from the optical path 105b pass through the second optical path separation birefringent plate 103 are denoted by reference numerals 103c and 103d respectively. Further, these positions are indicated in a transmission surface (X-Y plane) of the second optical path separation birefringent plate 103, as shown in
There is reported, as the wave plate 102, a wave plate having, e.g., a function of converting incident A-polarized light and B-polarized light to circularly polarized light with an ellipticity close to 1 by using crystal as a ¼ wavelength plate (hereinafter referred to as a λ/4 plate) such that each of the incident A-polarized light and B-polarized light has the equivalent light quantity (Patent Document 1).
Patent Document 1: Japanese Unexamined Patent Publication No. 2002-303824
For the wave plate constituting the low pass filter required for the image sensor used in the digital camera or the like described above, it is ideal that the proportion of light in correspondence to the A-polarized light or the B-polarized light with respect to the incident light quantity is 50% over the wavelength of a visible light region of the incident light. It is to be noted that the proportion of light passing through the wave plate in either one of the polarization directions is hereinafter referred to as a “transmittance”. In the wave plate constituting a low pass filter unit described in Patent Document 1, the transmittance is 50% in the vicinity of 550 nm, which is close to the ideal value. However, the transmittance is not less than 75% with respect to the light of the wavelength of, e.g., 410 nm, and the transmittance is different from the ideal transmittance by not less than 25%. That is, when the light in 410 nm becomes incident, for example, the light quantity of the A-polarized light is about 75%, while the light quantity of the B-polarized light is about 25%. Consequently, in this case, even when the optical path separation is performed, there has been a problem that it is not possible to sufficiently eliminate the moiré in a blue image.
The present invention has been achieved in order to solve the above-described problem, and there is provided a low pass filter which separates incident light into four light beams of two light beams composed of a component in an A polarization direction, and two light beams composed of a component in a B polarization direction orthogonal to the A polarization direction, including a first optical path separation birefringent plate for performing separation in a first separation direction for each of the components in the two polarization directions, a wave plate, and a second optical path separation birefringent plate for performing separation in a second separation direction intersecting the first separation direction for each of the components in the two polarization directions which are disposed therein in an order of incidence of the light, wherein the wave plate includes at least one phase plate for changing a polarization state of the incident light, when the wave plate is constituted of one phase plate, an light axis of the one phase plate is so disposed as to intersect both of the A polarization direction and the B polarization direction, when the wave plate is constituted of two or more of the phase plates, their respective optic axes of at least two of the phase plates are so disposed as to intersect each other and intersect both of the A polarization direction and the B polarization direction, and, when a light quantity of light having passed through the wave plate in the A polarization direction with respect to a total light quantity of light of a given wavelength out of light having passed through the wave plate is assumed to be Ix %, a diremption value |50−Ix| defined by using a value of the Ix which is maximally deviated from 50% with respect to light in a wavelength range of 410 to 600 nm out of the light having passed through the wave plate is not more than 20%.
In addition, there is provided a low pass filter which separates incident light into two light beams of a light beam composed of a component in an A polarization direction, and a light beam composed of a component in a B polarization direction orthogonal to the A polarization direction, including a wave plate, and an optical path separation birefringent plate which are disposed therein, wherein the wave plate includes at least one phase plate for changing a polarization state of the incident light, when the wave plate is constituted of one phase plate, an optic axis of the one phase plate is so disposed as to intersect both of the A polarization direction and the B polarization direction, when the wave plate is constituted of two or more of the phase plates, their respective optic axes of at least two of the phase plates are so disposed so to intersect each other and intersect both of the A polarization direction and the B polarization direction, and, when a light quantity of light having passed through the wave plate in the A polarization direction with respect to a total light quantity of light of a given wavelength out of light having passed through the wave plate is assumed to be Ix %, a diremption value −50−Ix| defined by using a value of the Ix which is maximally deviated from 50% with respect to light in a wavelength range of 410 to 600 nm out of the light having passed through the wave plate is not more than 20%.
With this configuration, since it is possible to suppress wavelength dependence and reduce the difference between the light quantities to be separated in a desired wavelength range when compared with conventional low pass filters, the effect is achieved that the occurrence of moiré can be suppressed at a specific level in the wavelength range of the incident light. It is to be noted that the wave plate may also be constituted of, e.g., three phase plates, and the wave plate may be adjusted appropriately such that the diremption value |50−Ix| in 410 to 600 nm is significantly reduced to, e.g., not more than 20% using two phase plates, and the diremption value is further reduced using the third phase plate.
Further, there is provided the above-described low pass filter wherein the wave plate includes the one phase plate, and, when a retardation value of the one phase plate is represented by Rd [nm], an angle formed by a polarization direction of light which incident in the A polarization direction or the B polarization direction and the optic axis of the one phase plate is represented by an intersection angle θ [°], and the most acute intersection angle formed by the optic axis with respect to the A polarization direction or the B polarization direction is represented by θmin, coordinates of the Rd and the θmin (Rd, θmin) fall in a range of a region surrounded by a point (245, 17), a point (310, 30), and a point (130, 31), or a region surrounded by a point (245, −17), a point (310, −30), and a point (130, −31).
With this configuration, it is possible to reduce the difference between the light quantities to be separated in a desired wavelength range using conditions obtained by the configuration of one phase plate, and thereby suppress the occurrence of the moiré. The difference between the light quantities to be separated means a difference between two light components orthogonal to each other in the light passing through the wave plate. Particularly in this configuration, the wave plate functions as a ½ wavelength plate with respect to the light in the desired wavelength range. The wave plate used in the conventional low pass filter functions as a ¼ wavelength plate, and converts incident linearly polarized light to circularly polarized light. Actually, the linearly polarized light is not necessarily converted to the circularly polarized light (ellipticity=1) in the entire desired wavelength range, and is converted to elliptically polarized light (0<ellipticity<1) when the wavelength becomes different. On the other hand, in the ½ wavelength plate of this configuration as well, the light is not necessarily converted to the linearly polarized light (ellipticity=0) in the entire desired wavelength range, and the light is converted to the elliptically polarized light when the wavelength becomes different. However, with regard to the difference when the light is separated into the above-described two light components orthogonal to each other, since the ½ wavelength plate is lower in incident light wavelength dependence than the ¼ wavelength plate so that the difference between the two light components orthogonal to each other can be reduced in the desired wavelength range by this configuration, the wave plate is characterized in that the light quantities of the individual components can be made almost equal to each other. It is to be noted that this technological concept applies to a wave plate constituted of two phase plates described below.
Furthermore, there is provided the above-described low pass filter wherein the wave plate is constituted of the two phase plates, and, when the two phase plates are designated as a first phase plate and a second phase plate in the order of incidence of the light, a retardation value of the first phase plate is represented by Rd1, a retardation value of the second phase plate is represented by Rd2, a counterclockwise direction when viewed from an incident surface side with respect to the A polarization direction or the B polarization direction is assumed to be a plus direction, optic axes of the two phase plates are a combination of fast axes or slow axes, an angle formed by the optic axis of the first phase plate is represented by an intersection angle θ1, and an angle formed by the optic axis of the second phase plate is represented by θ2, the Rd1 is in a range of more than 0 and not more than 1200 nm, the Rd2 is in a range of 120 to 320 nm, and the θ1 and the θ2 are a combination in a range of any one of the following (1) to (4):
16≦|θ1−θ2|≦28 (1)
60≦|θ1−θ2|≦69 (2)
110≦|θ1−θ2|≦119 (3)
151≦|θ1−θ2|≦160 (4).
With this configuration, it is possible to further reduce the difference between the light quantities to be separated in the desired wavelength range using conditions obtained by the configuration of the two phase plates, and the effect is achieved that the occurrence of the moiré is further suppressed.
Moreover, there is provided the above-described low pass filter wherein the wave plate is formed of a resin material.
In addition, there is provided the above-described low pass filter wherein an IR cut layer for significantly reducing the light quantity of the wavelength in an infrared region is disposed.
The present invention has been achieved in order to solve the above-described problem of the prior art, and can provide a low pass filter including a wave plate capable of modulating incident light serving as image information such that A-polarized light and B-polarized light orthogonal to the A-polarized light are at an equivalent light quantity level over a visible light region, i.e., a transmittance takes a value close to 50%.
As a material for forming the optical path separation birefringent plate, examples of a crystal material include crystal, yttrium.orthovanadate (YVO4) crystal, calcite (CaCO3), rutile (TiO2), and lithium niobate (LiNbO3). In addition, examples of a resin material include polyimide, polyamide imide, polyamide, polyether imide, polyether ether ketone, polyether ketone, polyketone sulfide, polyether sulfone, polysulfone, polyphenylene sulfide, polyphenylene oxide, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyacetal, polycarbonate, polyarylate, acrylic resin, polyvinyl alcohol, polypropylene, cellulose-based plastics, and polyolefin. Further, a liquid crystal material including polymeric liquid crystal from which birefringence is obtainable may also be used.
In addition, as a material for forming the phase plate constituting the wave plate 12, although a polymer film made of polycarbonate which exhibits refractive index anisotropy by drawing or the like, or polymeric liquid crystal which exhibits refractive index anisotropy by orientation treatment are preferably used, the material is not limited thereto. Examples of a crystal material include crystal, yttrium-orthovanadate (YVO4) crystal, calcite (CaCO3), rutile (TiO2), and lithium niobate (LiNbO3). Further, examples of a resin material include polyimide, polyamide imide, polyamide, polyether imide, polyether ether ketone, polyether ketone, polyketone sulfide, polyether sulfone, polysulfone, polyphenylene sulfide, polyphenylene oxide, polyethylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, polyacetal, polycarbonate, polyarylate, acrylic resin, polyvinyl alcohol, polypropylene, cellulose-based plastics, and polyolefin.
Furthermore, a configuration may also be adopted in which there is laminated, on the low pass filter, an IR cut layer, which is not shown, for reducing infrared rays in order to prevent reproduction of an image different from an image seen by a human being by photoelectrically converting light in a wavelength different from a luminous efficacy of a human being when near infrared rays become incident on the image sensor. The IR cut layer has an important function of removing light which is not necessary for a digital camera or the like, and adjusting the sensitivity of the image sensor to the luminous efficacy of a human being (the most perceptible light wavelength in visible light: the vicinity of 550 nm). In order to remove the unnecessary light, it is desirable to dispose the IR cut layer at a position as close to the incidence side as possible among optical components and, when the IR cut layer is disposed in the low pass filter of the present invention, it is appropriate to dispose the IR cut layer on the light incidence side of the first optical path separation birefringent plate 11. However, the position thereof is not limited thereto, and the IR cut layer may be disposed between the first optical path separation birefringent plate 11 and the second optical path separation birefringent plate 13, or on the light transmission side of the second optical path separation birefringent plate 13. Moreover, as the low pass filter, a configuration may be adopted in which the wave plate 12 and the IR cut layer which is not shown are stacked in layers, and the optical path separation birefringent plates 11 and 13 are not bonded to the wave plate 12 and the IR cut layer.
The IR cut layer may be formed of an optical interference multilayer film having a reflection operation, or a material having an absorption operation. Examples of the optical interference multilayer film having the reflection operation include a film in which glass or a resin film is used as a base and the optical interference multilayer film is coated on the surface of the base, and a film in which a resin film itself has an optical interference multilayer structure. On the other hand, an example of the material having the absorption operation include a material in which an element or a coloring matter which absorbs light in an infrared region is blended as compositions of glass or a resin.
When the optical interference multilayer film is used, since reflected light is trapped in a device as stray light, sufficient effects are obtained by performing optical design such that the stray light does not affect other optical components, and in consideration that IR cut characteristics change depending on the wavelength according to an incident angle. In addition, with regard to the material having the absorption operation, sufficient effects can be obtained by performing (thermal) design in consideration of an influence resulting from thermal conversion by light absorption. It is to be noted that glass is especially suitable when expansion and contraction by heat and durability are considered.
At this point, as shown in
Next, by changing the angle θ formed by causing the linearly polarized light 14 (A-polarized light) in parallel with the X direction and the optic axis 15 to intersect each other (hereinafter referred to as an intersection angle) in a range of 0° to 90°, proper conditions for achieving the effect of the present invention are examined. It is to be noted that the optic axis 15 herein is assumed to be the fast axis having the direction of the ordinary light refractive index (n0). The proper conditions mentioned herein may preferably be conditions which allow the transmittance to fall within a range of 50±20% in an entire wavelength range of at least 410 to 600 nm in light incident on the image sensor, and conditions which allow the transmittance to fall within a range of 50±15% are more preferable, and conditions which allow the transmittance to fall within a range of 50±10% are further more preferable. It is to be noted that the wavelength range satisfying the transmittance within the above-described ranges is more preferably a wavelength range of 400 to 650 nm. At this point, as another parameter (variable), the retardation value Rd is changed in a range of 0 to 400 nm in addition to the intersection angle θ.
Herein, If the distribution diagram in
In addition, when values of coordinates (Rd [nm], θ [°]) are a combination falling in a range similar to a triangle corresponding to a region surrounded by a point (245, 21), a point (280, 26), and a point (180, 27), or a region surrounded by a point (245, 69), a point (280, 64), and a point (180, 63), the above-described diremption value −50−Ix| is not more than 15% with respect to the light of any wavelength in 410 to 600 nm, and the combination is more preferable.
Further, when values of coordinates (Rd [nm], θ [°]) are a combination falling in a range similar to a triangle corresponding to a region surrounded by a point (245, 23), a point (265, 25), and a point (215, 25), or a region surrounded by a point (245, 67), a point (265, 65), and a point (215, 65), the above-described diremption value |50−Ix| is not more than 10% with respect to the light of any wavelength in 410 to 600 nm, and the combination is further more preferable.
In addition, a material forming the wave plate 12 has a wavelength dispersion property as well as the birefringence. That is, the wavelength dispersion property is a property in which the value of the refractive index anisotropy Δn becomes different when the wavelength of incident light becomes different. While the polycarbonate resin is used as the material for the wave plate 12 and the calculation is performed in consideration of the wavelength dispersion property by the wavelength of the incident light in
It is to be noted that, although the intersection angle θ has been described as the angle formed by the direction of the A-polarized light and the fast axis direction of the wave plate 12 when the direction of the incident linearly polarized light is the X direction (A-polarized light), the intersection angle θ is not limited thereto. As another combination, even when the intersection angle θ is an angle formed by the direction of the A-polarized light and the slow axis, an angle formed by the B-polarized light and the fast axis, or an angle formed by the direction of the B-polarized light and the slow axis, the similar effect can be obtained. Herein, in any combination of the linearly polarized light incident on the wave plate 12 (A-polarized light and B-polarized light) and the optic axes of the wave pate 12 (fast axis and slow axis), the most acute angle (absolute value is small) of the intersection angle θ with which the effect is obtainable between 0 and 90°, or between 0 and −90° is represented by θmin.
With this arrangement, as shown in
Thus, the retardation value Rd capable of obtaining the small diremption value is in the vicinity of 250 nm and, in particular, it can be said that the retardation value Rd is in a range which provides a phase difference corresponding to about half of the luminous efficacy of a human being (wavelength of 550 [nm]), i.e., a range having a function as a ½ wavelength plate. Consequently, it can be said from the distribution diagram of
At this point, as shown in
Next, by changing each of the angle (hereinafter referred to as a first intersection angle) θ1 formed by the linearly polarized light 34 in parallel with the X direction (A-polarized light) and the optic axis 35a of the first phase plate 32a which intersect each other, and the angle (hereinafter referred to as a second intersection angle) θ2 formed by the A-polarized light and the optic axis 35b of the second phase plate 32b which intersect each other in a range of −90° to 90°, proper conditions for obtaining the effect of the present invention are examined. Herein, although the optic axes 35a and 35b are assumed to be the combination of the fast axes, similar characteristics can be obtained even with the combination of the slow axes. Further, similarly to the first embodiment, the proper conditions mentioned herein may preferably be conditions which allow the diremption value |50−Ix| of not more than 20% in an entire wavelength range of at least 410 to 600 nm of the light incident on the image sensor, conditions which allow the diremption value of not more than 15% are more preferable, conditions which allow the diremption value of not more than 10% are further more preferable, and conditions which allow the diremption value of not more than 5% are most preferable. It is to be noted that the wavelength range satisfying the transmittance in the above-described ranges is more preferably a wavelength range of 400 to 650 nm.
When the transmittance under these two arbitrary conditions (θ1, θ2) to be changed with the retardation values fixed as described above is represented by Ix [%], the diremption value |50−Ix| may be not more than 20 [%], preferably not more than 15 [%], and further more preferably not more than 10 [%]. By setting the first and second intersection angles θ1 and θ2 satisfying these proper conditions, the light quantity of the A-polarized light and that of the B-polarized light can be separated without large deflection so that it is possible to appropriately reduce the moiré phenomenon generated by the image sensor, and suppress degradation of an image quality caused by the light quantity deflection of the separated light beam. In addition, in the present embodiment in which two phase plates are stacked in layers, the configuration for obtaining the most preferable effect which allows the diremption value |50−Ix| of not more than 5 [%] can be found.
Herein, the distribution diagram of
16≦|θ1−θ2|≦28 (1)
60≦|θ1−θ2|≦69 (2)
110≦|θ1−θ2|≦119 (3)
151≦|θ1−θ2|≦160 (4).
It is to be noted that a measure used above is [°], and all of the following Expressions concerning the intersection angle are represented in [°].
In addition, in Expression (1), a condition for θ1>θ2 is represented by the following Expression:
16≦(θ1−θ2)≦28 (5a)
and, under conditions satisfying the combination of Expression (5a) and any one of the following Expressions (5b) to (5e), the effect which allows the diremption value within the range of 20 [%] can be obtained:
19≦θ1≦38 (5b)
−71≦θ1≦−52 (5c)
52≦θ2≦71 (5d)
−38≦θ2≦−19 (5e).
Further, in Expression (1), a condition for θ1<θ2 is represented by the following Expression:
−28≦(θ1−θ2)≦−16 (6a)
and, under conditions satisfying the combination of Expression (6a) and any one of the following Expressions (6b) to (6e) , the effect which allows the diremption value within the range of 20 [%] can be obtained:
−38≦θ1≦−19 (6b)
52≦θ1≦71 (6c)
−71≦θ2≦−52 (6d)
19≦θ2≦38 (6e).
Herein, with regard to the two arbitrary conditions (θ1, θ2), the design may be performed appropriately such that ranges of θ1 and θ2, which satisfy particularly the combination of Expression (5a) and any one of Expressions (5b) to (5e) or the combination of Expression (6a) and Expressions (6b) to (6e), become more effective ranges. For example, a combination which allows the diremption value of not more than 10 [%] includes the following Expression:
20≦(θ1−θ2)≦25 (7a)
and any one of the following Expressions (7b) to (7e):
25≦θ1≦33 (7b)
−65≦θ1≦−57 (7c)
57≦θ2≦65 (7d)
−33≦θ2≦−25 (7e)
and, conditions satisfying the above-described combination may be adopted appropriately.
Similarly, a combination which allows the diremption value of not more than 10 [%] includes the following Expression:
−25≦(θ1−θ2)≦−20 (8a)
and any one of the following Expressions (8b) to (8e):
−33≦θ1≦−25 (8b)
57≦θ1≦65 (8c)
−65≦θ2≦−57 (8d)
25≦θ2≦33 (8e)
and, conditions satisfying the above-described combination may be adopted appropriately.
In addition, a combination which allows the diremption value of not more than 5 [%] includes the following Expression:
21≦(θ1−θ2)≦23 (9a)
and any one of the following Expressions (9b) to (9e):
27≦θ1≦31 (9b)
−63≦θ1≦−59 (9c)
59≦θ2≦63 (9d)
−31≦θ2≦−27 (9e)
and, conditions satisfying the above-described combination may be adopted appropriately.
Similarly, a combination which allows the diremption value of not more than 5 [%] includes the following Expression:
−23(θ1−θ2)≦−21 (10a)
and any one of the following Expressions (10b) to (10e):
−31≦θ1≦−27 (10b)
59≦θ1≦63 (10c)
−63≦θ2≦−59 (10d)
27≦θ2≦31 (10e)
and, conditions satisfying the above-described combination may be adopted appropriately.
Next, an ellipticity of light passing through the wave plate 32 under conditions which allow the desired diremption value is considered. First, an ellipticity κ and an azimuth angle Ψ when θ1=8 [°] and θ2=30 [°] as conditions for obtaining the diremption value of not more than 5% are satisfied are shown in Table 1. It is to be noted that the values of Rd1 and Rd2 at this point are set to 260 nm. It is to be noted that the azimuth angle Ψ denotes an azimuth angle of the light passing through the wave plate 32 with respect to the direction of the linearly polarized light incident on the wave plate 32. For example, when the light passes through the wave plate 32 as the linearly polarized light, the azimuth angle Ψ denotes an angle formed with the direction of the linearly polarized light and, in the case of elliptically polarized light, the azimuth angle Ψ denotes an angle formed with the direction of the longer axis of the ellipse. It is to be noted that, with regard to a sign, as described in
In addition, Tables 2(a) to 2(d) show the ellipticities κ and the azimuth angles Ψ under conditions satisfying Expression (7a) and Expression (7b), and conditions satisfying Expression (7a) and Expression (7c) which allow the diremption value of not more than 10%. For example, in Table 2(a), the lower limit value of θ1 (=25 [°]) in Expression (7b) is substituted for θ1 in Expression (7a), and the value of θ2 (=5 [°]) is determined such that the value of θ1−θ2 takes the lower limit value of 20. Similarly, in Table 2(b), the upper limit value of θ1 (=33 [°]) is substituted for θ1 in Expression (7a), and the value of θ2 (=8 [°]) is determined such that the value of θ1−θ2 takes the upper limit value of 25.
Thus, in the wavelength range of 400 to 650 nm under the conditions of θ1 and θ2 which allow the diremption value of not more than 5% shown in Table 1, the ellipticity κ is not more than about 0.2, and it can be said that the polarization state of the light passing through the wave plate 12 is linearly polarized light. In addition, under the conditions of θ1 and θ2 which allow the diremption value of not more than 10% shown in Table 2, the ellipticity K of the luminous efficacy of a human being (wavelength of 550 [nm]) is not more than about 0.16, which is the smallest value. Thus, it can be seen that the configuration of the wave plate is a configuration for converting the light to the linearly polarized light in the wavelength range of the visible light.
Further, in the wavelength range of 400 to 650 nm under the conditions of θ1 and θ2 which allow the diremption value of not more than 5% shown in Table 1, the azimuth angle Ψ indicates values in the vicinity of ±45 [°]. That is, since an angle of about 45 [°] is formed with respect to the A polarization direction and the B polarization direction, the light components of the individual directions become equivalent to each other. Similarly, under the conditions of θ1 and θ2 which allow the diremption value of not more than 10% shown in Tables 2(a) to 2(d), since the azimuth angle Ψ does not take a value largely deviated from ±45 [°], the ratio between the component of the A polarization direction and that of the B polarization direction falls in a specific range close to 1:1. Thus, it can be said that, in a desired wavelength range, the resultant diremption value can be reduced by providing conditions which cause the value of the azimuth angle Ψ to approach ±45 [°].
Next, a description is given of characteristics when the retardation value Rd1 of the first phase plate and the retardation value Rd2 of the second phase plate are changed.
Herein, the distribution diagram of
Herein, when the two phase plates are formed of the same birefringent material, and have the same thickness (d1=d2) and the same retardation value (Rd1=Rd2), there are many preferable points in terms of a product quality and productivity obtained by a reduction in distortion resulting from thermal expansion caused by a change in temperature, and a description is given of a margin of error between Rd1 and Rd2 when the phase plates are designed such that Rd1=Rd2 is satisfied. First, according to the distribution diagram of
In addition, when the margin of error between the values of Rd1 and Rd2 is considered, even in a case where Rd2 is in the range of 160 to 290 [nm] and Rd1 becomes different from Rd2, as long as the margin includes a range represented by −45≦Rd2−Rd1≦45, it is possible to achieve the diremption value −50−Ix| of not more than 10 [%]. Further, even in a case where Rd2 is in a range of 180 to 280 [nm] and Rd1 becomes different from Rd2, as long as the margin includes a range represented by −15≦Rd2−Rd1≦20, it is possible to achieve the diremption value |50−Ix| of not more than 5 [%]. Consequently, even when the difference occurs between the retardation values of the two phase plates, by satisfying the above-described conditions, it is possible to obtain a specific effect.
Thus far, although θ1 and θ2 have been fixed to satisfy θ1=8° and θ2=30°, and Rd1 and Rd2 have been changed using
As the wave plate 12 of the low pass filter 10 according to the first embodiment, a polycarbonate film is subjected to a drawing process, and a film exhibiting birefringence is thereby obtained. At this point, the retardation value with respect to light of the wavelength of 550 nm is set to 245 nm. The film is cut into predetermined size, and is bonded to an IR cut glass surface for reducing the light quantity in an infrared region using a transparent adhesive. Further, two optical path separation birefringent plates formed of crystal are bonded to the film so as to sandwich the film using the transparent adhesive such that separation directions of optical paths are orthogonal to each other, and a low pass filter is thereby fabricated.
Light as linearly polarized light is caused to become incident on the low pass filter, and the low pass filter is adjusted such that an intersection angle (=θ) as an angle formed by the direction of the linearly polarized light and the fast axis of the wave plate 12 is 24°. At this point, the direction of the linearly polarized light is assumed to be A polarization and, when the light of the A polarization in 410 to 600 nm becomes incident, the light quantity of the light of the A polarization passing through the low pass filter and the light quantity of B polarization orthogonal to the A polarization with respect to the light quantity of the incident light are in a range of 44 to 54% in any wavelength described above, and it can be seen that the light quantity of the A polarization and that of the B polarization are almost equivalent to each other.
As the wave plate 32 of the low pass filter 30 according to the second embodiment, a polycarbonate film is subjected to a drawing process, and a film exhibiting birefringence is thereby obtained. At this point, the retardation value with respect to light of the wavelength of 550 nm is set to 260 nm. The film is cut into predetermined size, and two film pieces are stacked on each other such that their respective fast axes form an angle of 22° and bonded together using a transparent adhesive, and the bonded film pieces are bonded to an IR cut glass surface using the transparent adhesive. Further, two optical path separation birefringent plates formed of crystal are bonded to the film pieces so as to sandwich the film pieces using the transparent adhesive such that separation directions of optical paths are orthogonal to each other, and a low pass filter is thereby fabricated.
Light as linearly polarized light is caused to become incident on the low pass filter, and the low pass filter is adjusted such that an intersection angle (=θ1) formed by the direction of the linearly polarized light and the fast axis of the first phase plate 32a of the wave plate 32 is 8°, and an angle (=θ2) formed by the direction of the linearly polarized light and the fast axis of the second phase plate 32b is 30°. At this point, the direction of the linearly polarized light is assumed to be A polarization and, when the light of the A polarization in 410 to 600 nm becomes incident, the light quantity of the light of the A polarization passing through the low pass filter and the light quantity of B polarization orthogonal to the A polarization with respect to the light quantity of the incident light are in a range of 48 to 52% in any wavelength described above, and it can be seen that the light quantity of the A polarization and that of the B polarization are almost equivalent to each other.
While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing the spirit and scope thereof.
The present application is based on Japanese Patent Application No. 2008-220884 filed on Aug. 29, 2008, and the contents thereof are incorporated herein by reference.
Thus, it is possible to constitute a wave plate capable of separating incident light as linearly polarized light which is incident light information in a wavelength range of 410 to 600 nm as a visible light region such that light quantities of two orthogonal light components are equivalent to each other. With this, it is possible to appropriately reduce a moiré phenomenon generated by an image sensor constituting an optical low pass filter including this wave plate, and suppress degradation of an image quality caused by light quantity deflection of a separated light beam, and the wave plate is therefore useful.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-220884 | Aug 2008 | JP | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/JP09/64734 | Aug 2009 | US |
| Child | 13034729 | US |
