JP2013152378A - Optical low-pass filter, imaging apparatus, and projection device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical low-pass filter capable of sufficiently separating the polarization component of incident light and relatively easy to align the optical axis angle.
An optical low-pass filter includes a first birefringent plate, a second birefringent plate, and an optical rotatory plate. The first birefringent plate separates incident light into two linearly polarized light components and emits them. The second birefringent plate separates and emits incident light into two linearly polarized components, and the optical axis angle intersects with the first birefringent plate. The optical rotatory plate is disposed between the first birefringent plate and the second birefringent plate and rotates the polarization plane of linearly polarized light.
[Selection] Figure 1

Description

  The present invention relates to an optical low-pass filter, an imaging device, and a projection device.

  For example, in an imaging apparatus such as an electronic camera, an optical low-pass filter is disposed between a photographing lens and an imaging element in order to filter an optical pseudo signal.

  As a general optical low-pass filter configuration, a 45 ° depolarizing plate for converting linearly polarized light into circularly polarized light is disposed between two birefringent plates having orthogonal optical axis angles, or three birefringent plates. The one used is known.

JP-A-9-69967

  In the case of an optical low-pass filter using a 45 ° depolarizing plate, the conversion characteristic from linearly polarized light to circularly polarized light on the 45 ° depolarizing plate changes periodically. May not be separable.

  On the other hand, an optical low-pass filter using three birefringent plates is a second birefringent plate (optical axis angle 45 °) whose optical axis angle is inclined 45 ° with respect to the first birefringent plate (optical axis angle 0 °). ) And a third birefringent plate (optical axis angle 135 °) having an optical axis angle orthogonal to the second birefringent plate. In the case of such an optical low-pass filter, there is room for improvement in that the alignment of the optical axis angle becomes complicated as the number of birefringent plates increases.

  In view of the above circumstances, an optical low-pass filter that can separate the polarization component of incident light well and relatively easily align the optical axis angle is provided.

  An optical low-pass filter according to one aspect of the present invention includes a first birefringent plate, a second birefringent plate, and an optical rotatory plate. The first birefringent plate separates incident light into two linearly polarized light components and emits them. The second birefringent plate separates and emits incident light into two linearly polarized components, and the optical axis angle intersects with the first birefringent plate. The optical rotatory plate is disposed between the first birefringent plate and the second birefringent plate and rotates the polarization plane of linearly polarized light.

  In the above-described one aspect, the optical axis angles of the first birefringent plate and the second birefringent plate may be orthogonal to each other. The optical rotator may rotate the plane of linearly polarized light by 45 ° at the center wavelength of the incident light.

  Note that the above-described imaging device and projection device including the optical low-pass filter on one side are both effective as specific embodiments of the present invention.

  It is possible to provide an optical low-pass filter that can separate the polarization components of incident light well and relatively easily align the optical axis angle.

The figure which shows the structural example of the optical low-pass filter in one Embodiment. The figure which shows the state of the polarization in the optical low-pass filter in one Embodiment The figure which shows the change of the optical rotation in the visible light region in the optical rotation plate of wavelength 550nm and optical rotation angle 45 degrees Diagram showing the case of cutting an optical rotation plate from rough quartz Diagram showing the case where a birefringent plate is cut out from a quartz crystal Diagram showing the case where a 45 ° depolarizing plate is cut out from the quartz crystal The figure which shows the structural example of the imaging device in other embodiment. The figure which shows the structural example of the projection apparatus in other embodiment.

<Configuration example of optical low-pass filter>
FIG. 1 is a diagram illustrating a configuration example of an optical low-pass filter according to one embodiment, and FIG. 2 is a diagram illustrating a state of polarization in the optical low-pass filter according to one embodiment. The optical low-pass filter is a thin plate-like optical filter that removes a component having a high spatial frequency from incident light. For example, the optical low-pass filter is used in an optical system of an imaging device (such as an electronic camera) or a projection device (such as a projector).

  The optical low-pass filter 1 of one embodiment includes a first birefringent plate 2, an optical rotatory plate 3, and a second birefringent plate 4 in order from the incident side. The first birefringent plate 2, the optical rotatory plate 3, and the second birefringent plate 4 are bonded together with, for example, a UV adhesive (ultraviolet curable adhesive) to form an integrated optical element. In the optical low-pass filter 1, an infrared cut film or an ultraviolet cut film may be formed on the incident surface of the first birefringent plate 2 and the exit surface of the second birefringent plate 4 (in FIG. 1, a UV adhesive). (The illustration of the outer layer, the outer line cut film, and the ultraviolet cut film is omitted).

  The first birefringent plate 2 is made of single crystal quartz, and is arranged so that the optical axis extends in the x direction (left-right direction) in FIG. 2 (optical axis angle 0 °). The first birefringent plate 2 separates the incident light into two linearly polarized components whose vibration directions are orthogonal to each other and emits the incident light. For example, when natural light including polarization components in all directions is incident on the first birefringent plate 2, the first birefringent plate 2 separates the incident light into ordinary light and extraordinary light, and the extraordinary light in the y direction. A predetermined amount is moved and emitted. In the first birefringent plate 2, ordinary light is linearly polarized light that oscillates in the x direction in FIG. 2, and extraordinary light is linearly polarized light that oscillates in the y direction (vertical direction) in FIG.

  The optical rotatory plate 3 is made of single crystal quartz, and rotates the polarization plane of linearly polarized light by 45 ° clockwise when the central wavelength of incident light is 550 mn. The ordinary light and the extraordinary light emitted from the first birefringent plate 2 are skewed in the direction of vibration of linearly polarized light by the action of the optical rotator 3. Thereby, the linearly polarized light emitted from the optical rotatory plate 3 is in a state including vibration components in the x direction and the y direction.

Here, when the optical rotation plate is formed of single crystal quartz, the optical rotation angle θ (deg) of the optical rotation plate can be obtained by the following equation (1).
θ = 7.19 × 10 ⁶ × λ ² / (λ ² −92.6 ² ) × T (1)
In the above formula (1), “λ” represents the center wavelength (nm × 10 ⁴ ) of incident light, and “T” represents the thickness (mm) of the optical rotation plate. According to the above formula (1), when the center wavelength of the incident light is 550 nm, the optical rotation angle θ of the optical rotation plate having a thickness of 1 mm is 25.18 °. In order to make θ = 45 ° with respect to incident light having a center wavelength of 550 mn, the thickness of the optical rotation plate 3 may be set to 1.78 mm.

  FIG. 3 is a diagram showing a change in optical rotation in the visible light region in an optical rotation plate having a wavelength of 550 nm and an optical rotation angle of 45 °. The vertical axis in FIG. 3 indicates the optical rotation angle, and the horizontal axis in FIG. 3 indicates the wavelength. According to the optical rotation plate in one embodiment, the optical rotation continuously changes with a gentle curve in the visible light region. Therefore, it can be seen that the optical rotation plate rotates linearly polarized light without causing periodicity over the entire visible light range.

  The second birefringent plate 4 is formed of single crystal quartz, and is arranged so that the optical axis extends in the y direction in FIG. 2 (optical axis angle 90 °). The second birefringent plate 4 separates the incident light into two linearly polarized light components whose vibration directions are orthogonal to each other and emits the incident light. For example, when linearly polarized light rotated by the optical rotator 3 is incident on the second birefringent plate 4, the second birefringent plate 4 separates the incident light into ordinary light and extraordinary light, and places the extraordinary light in the x direction. A fixed amount is moved and emitted. In the second birefringent plate 4, ordinary light is linearly polarized light that vibrates in the y direction in FIG. 2, and extraordinary light is linearly polarized light that vibrates in the x direction in FIG.

  Hereinafter, the function and effect of the optical low-pass filter of one embodiment will be described.

  When a natural light beam (center wavelength 550 nm) enters from the incident surface side of the optical low-pass filter 1, the first birefringent plate 2 causes linearly polarized light (ordinary light) oscillating in the x direction and linearly polarized light (abnormal light) oscillating in the y direction. ) And incident light is separated. The linearly polarized light separated by the first birefringent plate 2 is polarized 45 in the clockwise direction on the optical rotation plate 3 disposed between the first birefringent plate 2 and the second birefringent plate 4. Rotate. Thereafter, each linearly polarized light rotated by the optical rotator 3 oscillates in the y direction in the second birefringent plate 4 whose optical axis angle is orthogonal to the first birefringent plate 2 and in the x direction. It is further separated into linearly polarized light (abnormal light). As a result, a light beam of a four-point separated image of incident light is emitted from the exit surface side of the optical low-pass filter 1. The optical low-pass filter 1 removes a component having a high spatial frequency of incident light.

  In the optical low-pass filter 1 of one embodiment, the optical rotatory plate 3 is disposed between the first birefringent plate 2 and the second birefringent plate 4 whose optical axis angles are orthogonal to each other. Since the optical rotatory plate 3 rotates linearly polarized light without causing periodicity over the entire visible light region, the optical low-pass filter 1 of one embodiment is compared with an optical low-pass filter using a 45 ° depolarizing plate. Each wavelength of visible light can be separated relatively well by the second birefringent plate 4.

  Further, the optical low-pass filter 1 according to the embodiment is easy to manufacture because the alignment of the optical axis angle of the birefringent plate is easier than the optical low-pass filter using three birefringent plates.

  Furthermore, since the optical low-pass filter 1 of the embodiment uses the optical rotatory plate 3, the manufacturing cost is much lower than that of an optical low-pass filter using a 45 ° depolarizing plate or an optical low-pass filter using three birefringent plates. Can be made smaller. The reason will be described below.

  4 is a diagram showing a case where an optical rotatory plate is cut out from the quartz raw stone 5, FIG. 5 is a diagram showing a case where a birefringent plate is cut out from the quartz raw stone 5, and FIG. 6 shows a quartz raw stone 5 as a comparative example. It is a figure which shows the case where 45 degree depolarization board is cut out from.

  4 to 6 grows in the direction of the optical axis indicated by the arrows. And the longer the quartz crystal 5 is in the optical axis direction, the longer it takes to grow, so the cost of the quartz crystal 5 increases.

  The birefringent plate shown in FIG. 5 is generated by cutting a surface inclined by 45 ° with respect to the optical axis direction of the quartz raw stone 5 into a square. If the optical axis angle of the birefringent plate is 0 °, the cut-out crystal plate may be used as it is. If the optical axis angle of the birefringent plate is 90 °, the cut-out crystal plate is rotated by 90 ° and used. do it. When the optical axis angle of the birefringent plate is 45 ° or 135 °, it is necessary to incline the cut crystal plate by 45 ° or 135 ° and further cut it into a square. For this reason, a birefringent plate having an optical axis angle of 45 ° or 135 ° requires a larger quartz plate at the time of manufacture than a birefringent plate having an optical axis angle of 0 ° or 90 °, and is expensive.

  In addition, the 45 ° depolarizing plate shown in FIG. 6 is generated by cutting the surface of the quartz raw stone 5 along the optical axis direction into a thin plate, and then cutting the cut quartz plate (wafer) by tilting it by 45 °. (See FIG. 6B). Therefore, in order to cut out the depolarizing plate, the quartz crystal 5 must be grown very large, which increases the cost.

  On the other hand, the optical rotation plate shown in FIG. 4 is generated by cutting a surface perpendicular to the optical axis direction of the quartz crystal 5 into a thin plate shape. Although the optical rotation angle of the optical rotatory plate is determined by the thickness of the optical rotatory plate, the optical rotatory plate can be cut out from the quartz raw stone 5 having a shorter growth period than the birefringent plate or the depolarizing plate, so that the manufacturing cost can be suppressed. In addition, since the optical rotatory plate is different from the birefringent plate and the optical rotation canceling plate in the cutting direction of the quartz crystal 5, it is easy to manufacture a large size plate.

<Configuration example of imaging device>
FIG. 7 is a diagram illustrating a configuration example of an imaging apparatus according to another embodiment. An imaging apparatus according to another embodiment is an electronic camera including the optical low-pass filter 1 according to one embodiment.

  The electronic camera 11 includes a photographing optical system 12, the optical low-pass filter 1 according to the embodiment, an image sensor 13, a signal processing unit 14, an image processing engine 15, a memory 16, a recording I / F 17, and a user. And an operation unit 18 for receiving the operation. Here, the signal processing unit 14, the memory 16, the recording I / F 17, and the operation unit 18 are each connected to the image processing engine 15.

  In another embodiment, the optical low-pass filter 1 is disposed between the imaging optical system 12 and the image sensor 13 for filtering the optical pseudo signal. The optical low-pass filter 1 may be integrally attached to the light receiving surface of the image sensor 13.

  The imaging element 13 is a device that captures (captures) an image of a subject formed by the imaging optical system 12. The image output of the image sensor 13 is connected to the signal processing unit 14. The signal processing unit 14 includes an analog front-end circuit that performs analog signal processing and a digital front-end circuit that performs A / D conversion and digital signal processing.

  The image processing engine 15 is a processor that comprehensively controls the operation of the electronic camera 11. The image processing engine 15 performs various types of image processing (color interpolation processing, gradation conversion processing, white balance adjustment, contour enhancement processing, color conversion processing, etc.) on image data for one frame. The memory 16 temporarily holds data before and after image processing by the image processing engine 15.

  The recording I / F 17 has a connector for connecting a nonvolatile storage medium 19. The recording I / F 17 writes / reads image data to / from the storage medium 19 connected to the connector. The storage medium 19 is a memory card incorporating a semiconductor memory, for example.

  According to the imaging apparatus of the other embodiment, since the optical pseudo signal is removed by the optical low-pass filter 1 of the embodiment, the moire of the image can be suppressed.

<Configuration example of projection device>
FIG. 8 is a diagram illustrating a configuration example of a projection apparatus according to still another embodiment. The projection apparatus according to another embodiment is a projector including the optical low-pass filter 1 according to one embodiment.

  The projector 21 projects a projection image, an LED 22 that is a light source for image projection, a condenser lens 23, a polarization beam splitter 24, an LCOS 25 (reflection type liquid crystal element), the optical low-pass filter 1 according to one embodiment. And a projection lens 26 for the purpose. In FIG. 8, the left surface 24a of the polarization beam splitter 24 is subjected to non-reflection processing such as black processing.

  The condenser lens 23 makes light emitted from the LED 22 substantially parallel. The polarization beam splitter 24 separates incident light into P-polarized component light and S-polarized component light. The LCOS 25 is a device that displays a projection image to be projected to the outside. In FIG. 8, the optical low-pass filter 1 of one embodiment is disposed between the polarization beam splitter 24 and the projection optical system 26.

  In the projector 21, the light that has been emitted from the LED 22 and turned into substantially parallel light by the condenser lens 23 enters the polarization beam splitter 24. Of the light incident on the polarization beam splitter 24, the S-polarized component light is reflected to the left in FIG. 8 and travels toward the surface 24 a on which the non-reflection treatment of the polarization beam splitter 24 has been performed. On the other hand, the P-polarized component light passes through the polarization beam splitter 24 and enters the LCOS 25. The P-polarized component light incident on the LCOS 25 travels through the liquid crystal layer of the LCOS 25 and is reflected by a reflective film (not shown), and then travels in the reverse direction through the liquid crystal layer of the LCOS 25 and is emitted from the LCOS 25. The light emitted from the LCOS 25 is incident on the polarization beam splitter 24 again. When a voltage is applied, the liquid crystal layer of the LCOS 25 functions as a phase plate, and converts P-polarized component light into S-polarized component light. Therefore, the light incident on the polarization beam splitter 24 again becomes a mixed light of the S-polarized component and the P-polarized component. The polarization beam splitter 24 reflects only the S-polarized light component of the re-incident light. The reflected S-polarized light component is emitted from the projection window of the projector 21 via the optical low-pass filter 1 and the projection lens 26. As a result, the image displayed on the LCOS 25 is read out as a projection image and projected onto the projection area in front of the projector 21.

  According to the projection apparatus of another embodiment, since the high-frequency component of the projection image is removed by the optical low-pass filter 1 of one embodiment, the pixel grid of the LOCOS 25 is clearly reflected in the projection image and the image quality is deteriorated. Can be prevented.

<Supplementary items of the embodiment>
(A) In the above embodiment, an example of an optical low-pass filter having a center wavelength of 550 nm has been described. However, the center wavelength in the optical low-pass filter of the present invention is not limited to the above example. For example, for a three-plate electronic camera equipped with a color separation prism, the low-pass filter of one embodiment having a red wavelength region, a green wavelength region, and a blue wavelength region as the center wavelengths is arranged in front of each color image sensor. May be.

  (B) In the above embodiment, the configuration of the electronic camera has been described as an example of the imaging apparatus. However, the imaging apparatus of the present invention may be, for example, a solid-state imaging device that integrally includes the low-pass filter of one embodiment.

  (C) In the above embodiment, the configuration of the projector has been described as an example of the projection apparatus. However, the projection apparatus of the present invention may be a display device such as a head mounted display.

  From the above detailed description, features and advantages of the embodiments will become apparent. It is intended that the scope of the claims extend to the features and advantages of the embodiments as described above without departing from the spirit and scope of the right. Further, any person having ordinary knowledge in the technical field should be able to easily come up with any improvements and modifications, and there is no intention to limit the scope of the embodiments having the invention to those described above. It is also possible to use appropriate improvements and equivalents within the scope disclosed in.

DESCRIPTION OF SYMBOLS 1 ... Optical low pass filter, 2 ... 1st birefringent plate, 3 ... Optical rotatory plate, 4 ... 2nd birefringent plate, 11 ... Electronic camera, 21 ... Projector

Claims (4)

A first birefringent plate that divides incident light into two linearly polarized light components and emits the light;
Incident light is separated into two linearly polarized light components and emitted, and a second birefringent plate whose optical axis angle intersects the first birefringent plate;
An optical rotator arranged between the first birefringent plate and the second birefringent plate and rotating a polarization plane of linearly polarized light;
An optical low-pass filter comprising: The optical low-pass filter according to claim 1,
The optical axis angles of the first birefringent plate and the second birefringent plate are orthogonal,
The optical rotation plate is an optical low-pass filter that rotates the polarization plane of linearly polarized light by 45 ° at the center wavelength of incident light. The optical low-pass filter according to claim 1 or 2,
An imaging unit for imaging an image formed by a light beam that has passed through the optical low-pass filter;
An imaging apparatus comprising: The optical low-pass filter according to claim 1 or 2,
A projection unit for emitting a light beam for forming an optical image to the outside via the optical low-pass filter;
A projection apparatus comprising:

Images