CN116184550A - Optical low-pass filter and imaging device - Google Patents
Optical low-pass filter and imaging device Download PDFInfo
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- CN116184550A CN116184550A CN202111425624.4A CN202111425624A CN116184550A CN 116184550 A CN116184550 A CN 116184550A CN 202111425624 A CN202111425624 A CN 202111425624A CN 116184550 A CN116184550 A CN 116184550A
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
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- G02B5/22—Absorbing filters
- G02B5/226—Glass filters
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B1/00—Optical elements characterised by the material of which they are made; Optical coatings for optical elements
- G02B1/10—Optical coatings produced by application to, or surface treatment of, optical elements
- G02B1/11—Anti-reflection coatings
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/20—Filters
- G02B5/208—Filters for use with infrared or ultraviolet radiation, e.g. for separating visible light from infrared and/or ultraviolet radiation
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- G—PHYSICS
- G02—OPTICS
- G02B—OPTICAL ELEMENTS, SYSTEMS OR APPARATUS
- G02B5/00—Optical elements other than lenses
- G02B5/30—Polarising elements
- G02B5/3083—Birefringent or phase retarding elements
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Abstract
The embodiment of the invention provides an optical low-pass filter and an imaging device, wherein the optical low-pass filter comprises a first crystal layer, a blue glass layer and a second crystal layer which are sequentially stacked, wherein a first antireflection film is arranged on one side, far away from the blue glass layer, of the first crystal layer, and a second antireflection film is arranged on one side, far away from the blue glass layer, of the second crystal layer; the first crystal layer and the second crystal layer have double refraction and polarization characteristics, and the polarization axis of the first crystal layer is perpendicular to the polarization axis of the second crystal layer. In the embodiment of the invention, the polarization direction of the first crystal layer is perpendicular to the polarization direction of the second crystal layer, so that incident light can be trimmed to enter only vertically, and the influence of moire fringes can be greatly relieved by combining the absorption of the blue glass layer on infrared light, and the technical problem that the existing optical filter only can trim light in one direction, and the elimination of moire fringes is not thorough enough, so that the imaging effect of the photoelectric image sensor is poor is solved.
Description
Technical Field
The present invention relates to the field of photoelectric image sensing technology, and in particular, to an optical low-pass filter capable of reducing moire fringes and an imaging device.
Background
The optical low-pass filter can eliminate moire interference, so that the optical low-pass filter is widely applied to the technical field of photoelectric image sensing.
The existing optical low-pass filters are mostly a piece of blue glass plus a piece of quartz plate "two-plate" filter, wherein the blue glass is used for filtering infrared rays, and the quartz plate is used for trimming light rays. Because the existing 'two-piece' optical filter can only repair light rays in one direction, the elimination of moire fringes is not thorough enough, the imaging effect is poor, and the user experience is affected.
Accordingly, there is a need in the art for improvement.
Disclosure of Invention
The invention aims to solve the technical problems that the existing optical filter only can trim light rays in one direction, moire fringes are not thoroughly eliminated, and the imaging effect of a photoelectric image sensor is poor.
In order to solve the problems, the invention is realized by the following technical scheme:
the invention provides an optical low-pass filter, which comprises a first crystal layer, a blue glass layer and a second crystal layer which are sequentially laminated, wherein a first antireflection film is arranged on one side, far away from the blue glass layer, of the first crystal layer, and a second antireflection film is arranged on one side, far away from the blue glass layer, of the second crystal layer;
the first crystal layer and the second crystal layer have birefringence and polarization characteristics, and the polarization axis of the first crystal layer is perpendicular to the polarization axis of the second crystal layer.
Further, in the optical low-pass filter, a light incident side is arranged on one side, far away from the blue glass layer, of the first crystal layer, and an infrared cut-off film is further arranged between the blue glass layer and the second antireflection film and used for reflecting infrared light.
Further, in the optical low-pass filter, the infrared cut-off film is disposed on a side of the second crystal layer facing the blue glass layer.
Further, in the optical low-pass filter, the first crystal layer and the second crystal layer are crystal layers or quartz layers.
Further, in the optical low-pass filter, the infrared cut-off film is a multilayer oxide film capable of blocking infrared rays.
Further, in the optical low-pass filter, the thicknesses of the first crystal layer and the second crystal layer are the same.
Further, in the optical low-pass filter, the thicknesses of the first crystal layer and the second crystal layer are both 0.8mm.
The invention also provides an imaging device, which comprises a charge coupled device image sensor and the optical low-pass filter, wherein the second antireflection film side of the optical low-pass filter is stuck on the charge coupled device image sensor.
Further, in the imaging device, the size of the first crystal layer, the size of the blue glass layer and the size of the second crystal layer are matched with the size of the charge coupled device image sensor.
Further, when the imaging device is placed horizontally, the polarization axis direction of the first crystal layer is a vertical direction, and the polarization axis direction of the second crystal layer is a horizontal direction.
Compared with the prior art, the embodiment of the invention has the following advantages:
in the embodiment of the invention, the optical low-pass filter comprises a first crystal layer, a blue glass layer and a second crystal layer which are sequentially laminated, wherein a first antireflection film is arranged on one side of the first crystal layer far away from the blue glass layer, a second antireflection film is arranged on one side of the second crystal layer far away from the blue glass layer, the first crystal layer and the second crystal layer have double refraction and polarization characteristics, and the polarization axis of the first crystal layer is perpendicular to the polarization axis of the second crystal layer. Because the crystal has the physical polarization characteristic that the direct part of the incident light is reserved in the polarization direction and the oblique part is reflected, the first crystal layer and the second crystal layer in the embodiment of the invention have the double refraction and polarization characteristics, namely infrared light in the incident light can be refracted, and the polarization direction of the first crystal layer is perpendicular to the polarization direction of the second crystal layer, so that the incident light can be trimmed to enter only vertically, and then the influence of moire fringes can be greatly relieved by combining with the absorption of the blue glass layer to infrared light, and the technical problem that the existing optical filter can only trim light in one direction, and the moire fringes are not thoroughly removed, so that the imaging effect of the photoelectric image sensor is poor is solved.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the application.
Drawings
FIG. 1 is a schematic diagram of an optical low-pass filter according to an embodiment of the present invention;
FIG. 2 is a graph showing the birefringence effect of a crystal layer according to an embodiment of the present invention;
FIG. 3 is a spectral diagram of an optical low-pass filter provided by an embodiment of the invention after filtering;
fig. 4 is a schematic structural view of an image forming apparatus according to an embodiment of the present invention;
FIG. 5 is a schematic diagram of test pattern imaging when a "two-piece" filter is glued at a CCD image sensor;
fig. 6 is a schematic diagram of test pattern imaging when an optical low pass filter is glued at a charge coupled device image sensor.
Detailed Description
In order that the above-recited objects, features and advantages of the present invention will become more readily apparent, a more particular description of the invention will be rendered by reference to the appended drawings and appended detailed description.
Moire is a visual result of interference between two lines or objects at a constant angle and frequency, and when the human eye cannot distinguish the two lines or objects, only the pattern of interference is visible, which is the moire pattern in this optical phenomenon.
Moire fringes can result from three aspects of interference between bi-or multi-colour dots, interference between each colour dot and the screen mesh, and interference that occurs due to the nature of the object being printed.
Moire fringes have the following three characteristics:
(1) Plays a role of amplification
Since the angle θ is very small, the moire fringe distance W is much larger than the grid distance ω. If ω=0.01 mm, i.e. the lines of the grating are 100 lines per mm, this pitch cannot be resolved by the naked eye, but if the θ angle is adjusted such that w=10 mm, i.e. the magnification is W/ω=1000 times, the moire fringes 10mm wide are clearly visible.
(2) The movement of moire fringes is proportional to the pitch
When the scale grating moves, the moire fringes move along the direction perpendicular to the grating movement direction, and each time the grating moves by one grating pitch omega, the moire fringes accurately move by one grating pitch W, so that the number of the grating pitches moved by the grating can be known as long as the number of the moire fringes is sensed by the photoelectric element, and the grating pitch is determined when the grating is manufactured, and therefore, the moving distance of the workbench can be calculated.
And when the stage moving direction is changed, the moving direction of the moire fringes is also changed regularly: the scale grating is fixed, the indication grating is rotated by an angle theta in the anticlockwise direction, and then when the indication grating moves leftwards, the moire fringes move downwards; conversely, when the indication grating is shifted right, the fringes are shifted upward. If the index grating is rotated in a clockwise direction through an angle θ, the situation is reversed as described above.
From the above, it can be seen that if two sets of photo-electric elements with a distance difference of W/4 are mounted along the moire direction, the moving distance and direction of the grating can be measured.
(3) Acting as an average error
Since moire fringes are composed of a plurality of grating lines, if the length (i.e. the pitch) of the received photoelectric element is 10mm, and when the pitch ω=0.01 mm, the signal received by the photoelectric element is composed of 1000 lines, thus manufacturing defects, such as few lines intermittently affect the strength of the photoelectric sensing signal by a few thousandths. Thus, with moire patterns, the accuracy is determined by the average effect of a set of lines, and the accuracy, especially the repetition accuracy, is higher.
Moire is a potential problem with halftone screen printing. Halftone printing is a method of representing gradation by decomposing a continuously-tone original document into dots of different sizes by photography or other methods. The dark call is performed by printing larger dots, the bright call is performed by printing smaller dots, and interference can occur among the dots with the same color, particularly among the dots of various color plates in multicolor printing or four-color printing to form moire fringes.
Moire fringes formed between dots are a common problem for all layers of screen printing. Dots and screens can also form another form of moire patterns whose distribution on the screen can produce patterns that are indistinguishable from the original.
The purpose of using a moire protection system is to predict moire from the selected number of meshes, number of screening lines, number of print colors and screening angle.
Since a CCD or Complementary Metal Oxide Semiconductor (CMOS) solid-state image sensor is a discrete pixel photo-imaging device. According to the nyquist theorem, the highest spatial frequency that an image sensor can resolve is equal to half of its spatial sampling frequency, which is called the nyquist limit frequency. When the sampled image exceeds the nyquist limit frequency of the system when the target image information is acquired with a CCD camera, high frequency components will be reflected into the fundamental frequency band on the image sensor, causing a so-called ripple effect or moire effect, causing the image to produce a periodic spectrum overlap aliasing or a so-called beat phenomenon. Assuming that the sampling frequency of the CCD is 15MHZ, when the image signal is 10MHZ, the aliasing frequency component is 15MHZ-10 mhz=5 MHZ, and when the image signal is 9MHZ, the aliasing frequency component is 15MHZ-9 mhz=6 MHZ, both of which are not filtered out after low-pass filtering by the circuit, and are output as useful image signals, such as 5MHZ and 6MHZ signal components superimposed at 9MHZ and 10MHZ bands in the observed waveform. There is a significant low frequency beat present on the 7MHZ signal, with a beat frequency of about 1MHZ. These aliased signals will affect the image sharpness and even appear as color streak disturbances.
As shown in fig. 1, an optical low-pass filter 10 provided by the embodiment of the invention includes a first crystal layer 11, a blue glass layer 12 and a second crystal layer 13, which are sequentially stacked, wherein a first antireflection film 14 is disposed on a side of the first crystal layer 11 away from the blue glass layer 12, and a second antireflection film 15 is disposed on a side of the second crystal layer 13 away from the blue glass layer 12; the first crystal layer 11 and the second crystal layer 13 have birefringence and polarization characteristics, and the polarization axis of the first crystal layer 11 is perpendicular to the polarization axis of the second crystal layer 13.
In an embodiment of the invention, an optical low pass filter 10 (Optical low pass filter, OLPF) is placed in front of a charge coupled device (Charge Coupled Device, CCD) image sensor. The light beam of the target image is subjected to birefringence after passing through the crystal layer in the optical low-pass filter 10, and is split into an ordinary ray o-beam and an extraordinary ray e-beam as shown in fig. 2. The sampling cut-off frequency can be calculated according to the size of the CCD pixel size and the total photosensitive area, and the separation distance d of o light and e light can be calculated. The target frequency of the difference frequency formed by the incident light beam is changed, so that the aim of weakening or eliminating low-frequency interference fringes, in particular to the aim of pseudo-color interference fringes of a color CCD (charge coupled device), is fulfilled.
Wherein, because light rays can generate partial refraction and reflection when passing through different media, the side of the first crystal layer 11 far away from the blue glass layer 12 is provided with a first antireflection film 14, and the side of the second crystal layer 13 far away from the blue glass layer 12 is provided with a second antireflection film 15, so that the optical low-pass filter 10 can reach 98-99% of transmittance, otherwise, only 90-95% of transmittance.
In the optical low-pass filter 10, since the crystal has the physical polarization characteristic of retaining the direct portion of the incident light and reflecting the oblique portion, the first crystal layer 11 and the second crystal layer 13 in the embodiment of the invention have the birefringence and polarization characteristic, so that the infrared light in the incident light can be refracted out, and the polarization direction of the first crystal layer 11 is perpendicular to the polarization direction of the second crystal layer 13, so that the incident light can be trimmed to enter only perpendicularly; meanwhile, since the blue glass can well absorb the infrared light and the blue glass layer 12 is positioned between the first crystal layer 11 and the second crystal layer 13, most of the infrared light can be removed after the incident light passes through the optical low-pass filter 10, thereby greatly relieving the influence of moire fringes.
In the optical low-pass filter 10 of the embodiment of the invention, the thicknesses and other dimensions of the first crystal layer 11 and the second crystal layer 13 are the same, so that the infrared light can be better matched and eliminated, and the incident light is trimmed to be perpendicular to the optical low-pass filter 10, thereby avoiding light interference with adjacent photoreceptors on the CCD image sensor.
Optionally, the first crystal layer 11 and the second crystal layer 13 are crystal layers or quartz layers, that is, the first crystal layer 11 and the second crystal layer 13 are made of crystal or quartz, so that the infrared light can be refracted and separated from the incident light in the form of e light, and infrared light in the emergent light is reduced.
Optionally, the thicknesses of the first crystal layer 11 and the second crystal layer 13 are 0.65-0.98 mm, and the crystal layer and the quartz layer within the thickness range have good birefringence characteristics and polarization characteristics, so that infrared light can be well refracted and separated from incident light in the form of e light, and infrared light in emergent light is reduced. Preferably, the thickness of the first crystal layer 11 and the second crystal layer 13 is 0.79 to 0.81mm, for example, 0.8mm, and the birefringent property and the polarizing property are particularly good, so that the infrared light can be well refracted and separated from the incident light in the form of e-light.
Optionally, in one embodiment, in the optical low-pass filter 10, a light entering side is a side of the first crystal layer 11 away from the blue glass layer 12, and an infrared cut-off film (IR-Coating) 16 is further disposed between the blue glass layer 12 and the second anti-reflection film 15, and the IR cut-off film 16 is used for reflecting infrared light. The infrared cut film 16 can remove part of infrared light in incident light in a reflective manner, and can further alleviate the occurrence of moire fringes.
Specifically, the infrared cut-off film 16 is an oxide film that can block infrared rays, specifically, infrared rays in incident light are removed in a reflective manner, such as a mixed oxide of silicon dioxide, titanium dioxide, selenium dioxide, or such as zinc aluminum oxide and/or indium tin oxide.
Alternatively, in one crystal embodiment, the infrared cut film 16 is disposed on the side of the second crystal layer 13 facing the blue glass layer 12, and after the light beam absorbs the infrared light through the blue glass layer 12, the residual infrared light can be removed by using the reflection mode of the infrared cut film, so as to further alleviate the moire effect.
The infrared cut film 16 may be vacuum coated or electroless coated on the surface of the second crystal layer 13. Wherein, the chemical plating is to immerse the second crystal layer 13 in a solvent for electroplating, and the vacuum plating is to uniformly vapor-deposit an infrared multi-layer film on the second crystal layer 13 by a vacuum vapor deposition method.
The total thickness of the optical low-pass filter 10 is controlled to be 2.69-3.02 mm, the length is 8.8+/-0.1 mm, and the width is 8.2+/-0.1 mm.
The optical low-pass filter 10 provided by the embodiment of the invention does not contain an inclined plane or a chamfer. Alternatively, the blue glass layer 12 may be CM500 or NF50D; the first crystal layer 11 has a directional light incidence Angle (Orientation Angle) of 45 ° ± 1 °, a rotational light incidence Angle (rotation Angle) of 45 ° ± 1 °, and the second crystal layer 13 has a directional light incidence Angle of 45 ° ± 1 °, a rotational light incidence Angle of ±1°, with reference to the optical axis; of course, the rotation light incident angle of the first crystal layer 11 may be in the range of-45 ° ± 1 °.
In practical applications, the spectrum of the optical low-pass filter 10 provided by the embodiment of the present invention is shown in fig. 3, and it can be seen that the optical low-pass filter 10 provided by the embodiment of the present invention can effectively filter most of infrared light in light.
The invention also provides an imaging device, as shown in fig. 4, comprising a charge coupled device image sensor 20 and the optical low-pass filter 10, wherein the second antireflection film 15 side of the optical low-pass filter 10 is glued on the charge coupled device image sensor 20, and particularly can be glued on the charge coupled device image sensor 20 by adopting UV glue or double sided glue.
Alternatively, in the above imaging apparatus, the size of the first crystal layer 11, the size of the blue glass layer 12, and the size of the second crystal layer 13 are matched with the size of the ccd image sensor 20.
Since the camera only considers the horizontal resolution, that is, needs to perform horizontal trimming on the light, in one embodiment of the optical low-pass filter 10 provided by the embodiment of the present invention, the polarization axis direction of the first crystal layer 11 is set to be the vertical direction, and correspondingly, the polarization axis direction of the second crystal layer 13 is set to be the horizontal direction, so that the first crystal layer 11 can be used to perform horizontal trimming on the light, and the second crystal layer 13 can be used to perform vertical trimming on the light, so as to finally trim the light to a direct state.
Alternatively, in another embodiment of the optical low-pass filter 10 provided by the embodiment of the present invention, the polarization axis direction of the first crystal layer 11 is set to be the horizontal direction, and correspondingly, the polarization axis direction of the second crystal layer 13 is set to be the vertical direction, so that the first crystal layer 11 can be used to implement vertical trimming of light, and the second crystal layer 13 can be used to implement horizontal trimming of light, and finally, the light is trimmed to be in a direct state.
The CCD image sensor 20 comprises CCD sensor protection glass 21, a CCD pixel micro-lens array 22, a color CCD coding film array 23, a CCD photosensitive original pixel array 24 and a CCD driving processing circuit 25 which are sequentially stacked; the optical low-pass filter 10 is adhered to the CCD sensor-protecting glass 21.
The imaging apparatus further includes an objective lens 30 disposed in front of the optical low-pass filter 10 and a monitor 40 connected to the charge-coupled device image sensor 20; the monitor 40 is used to display imaging at the ccd image sensor 20.
In practical application, the test pattern 50 is placed in front of the objective lens 30, and when a "two-piece" filter in the prior art is attached to the ccd image sensor 20, the display effect on the monitor 40 is as shown in fig. 5, and there is obvious moire in the vertical direction; in the case where the ccd image sensor 20 is attached to the optical low-pass filter 10 according to the embodiment of the present invention, the display effect on the monitor 40 is as shown in fig. 6, and no obvious moire is generated in both the horizontal direction and the vertical direction.
In summary, in the embodiment, the optical low-pass filter provided includes a first crystal layer, a blue glass layer and a second crystal layer that are sequentially stacked, wherein a first antireflection film is disposed on a side of the first crystal layer away from the blue glass layer, a second antireflection film is disposed on a side of the second crystal layer away from the blue glass layer, the first crystal layer and the second crystal layer have birefringence and polarization characteristics, and a polarization axis of the first crystal layer is perpendicular to a polarization axis of the second crystal layer. Because the crystal has the physical polarization characteristic that the direct part of the incident light is reserved in the polarization direction and the oblique part is reflected, the first crystal layer and the second crystal layer in the embodiment of the invention have the double refraction and polarization characteristics, namely infrared light in the incident light can be refracted, and the polarization direction of the first crystal layer is perpendicular to the polarization direction of the second crystal layer, so that the incident light can be trimmed to enter only vertically, and then the influence of moire fringes can be greatly relieved by combining with the absorption of the blue glass layer to infrared light, and the technical problem that the existing optical filter can only trim light in one direction, and the moire fringes are not thoroughly removed, so that the imaging effect of the photoelectric image sensor is poor is solved.
While preferred embodiments of the present invention have been described, additional variations and modifications in those embodiments may occur to those skilled in the art once they learn of the basic inventive concepts. Therefore, it is intended that the appended claims be interpreted as including the preferred embodiment and all such alterations and modifications as fall within the scope of the embodiments of the invention.
The foregoing has described in detail an optical low-pass filter and an imaging device according to the present invention, and specific examples have been used herein to illustrate the principles and embodiments of the present invention, the above examples being provided only to assist in understanding the method of the present invention and its core ideas; meanwhile, as those skilled in the art will have variations in the specific embodiments and application scope in accordance with the ideas of the present invention, the present description should not be construed as limiting the present invention in view of the above.
Claims (10)
1. The optical low-pass filter is characterized by comprising a first crystal layer, a blue glass layer and a second crystal layer which are sequentially laminated, wherein a first antireflection film is arranged on one side, far away from the blue glass layer, of the first crystal layer, and a second antireflection film is arranged on one side, far away from the blue glass layer, of the second crystal layer;
the first crystal layer and the second crystal layer have birefringence and polarization characteristics, and the polarization axis of the first crystal layer is perpendicular to the polarization axis of the second crystal layer.
2. The optical low-pass filter according to claim 1, wherein a side of the first crystal layer away from the blue glass layer is a light incident side, and an infrared cut-off film is further disposed between the blue glass layer and the second antireflection film, and the infrared cut-off film is used for reflecting infrared light.
3. The optical low-pass filter according to claim 2, wherein the infrared cut film is provided on a side of the second crystal layer facing the blue glass layer.
4. The optical low-pass filter according to claim 2, wherein the infrared cut-off film is an oxide film that blocks infrared rays.
5. The optical low pass filter of claim 1, wherein the first crystal layer and the second crystal layer are crystal layers or quartz layers.
6. The optical low pass filter of claim 5, wherein the first and second crystal layers have the same thickness.
7. The optical low-pass filter according to claim 6, wherein the thickness of the first crystal layer and the second crystal layer is 0.65-0.98 mm.
8. An imaging device, comprising a charge coupled device image sensor and the optical low-pass filter according to any one of claims 1 to 7, wherein the second antireflection film side of the optical low-pass filter is glued to the charge coupled device image sensor.
9. The imaging apparatus of claim 8, wherein a size of the first crystal layer, a size of the blue glass layer, and a size of the second crystal layer are all matched to a size of a charge coupled device image sensor.
10. The imaging device of claim 8, wherein the polarization axis direction of the first crystal layer is a vertical direction and the polarization axis direction of the second crystal layer is a horizontal direction when the imaging device is placed horizontally.
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| Application Number | Priority Date | Filing Date | Title |
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| CN202111425624.4A CN116184550A (en) | 2021-11-26 | 2021-11-26 | Optical low-pass filter and imaging device |
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| CN202111425624.4A CN116184550A (en) | 2021-11-26 | 2021-11-26 | Optical low-pass filter and imaging device |
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