JP2010191471A - Optical filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical filter suitable for an imaging apparatus capable of photographing in both of a visible light band and an infrared band. <P>SOLUTION: The optical filter 10 has transparency in a visible light band, has shielding characteristics in a first wavelength band adjacent to the longer wavelength side of the visible light band and has transparency, in a second wavelength band which is a part in a first near-infrared band. Furthermore, the optical filter 10 is provided with an infrared cut filter 2 having transparency characteristics at least in a second near-infrared band and a short-wavelength pass filter (SWPF) 3 for transmitting wavelength band, up to the long wavelength end of the second wavelength band. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an optical filter, and more particularly to an optical filter disposed on the front surface of an imaging device provided in an imaging device capable of photographing in both a visible light band and an infrared band.

  2. Description of the Related Art Conventionally, in an imaging device such as a surveillance camera that captures images day and night (hereinafter simply referred to as an imaging device), infrared rays are detected during nighttime shooting. A photodiode, which is a light receiving portion of an image sensor such as a CCD sensor or a CMOS sensor, can receive light in the near-infrared wavelength band of about 1300 nm. It is possible in principle to shoot.

  Since the optical wavelength band with high human visibility is 400 nm to 700 nm, when near-infrared light is detected by the image sensor, the image looks reddish to human eyes. For this reason, at the time of shooting in a bright place in the daytime or indoors, in order to match the sensitivity of the image sensor to human visual sensitivity, an infrared cut filter for blocking light in the infrared band is provided in front of the image sensor, It is desirable to remove light having a wavelength of 700 nm or more. On the other hand, when shooting at night or in a dark place, it is necessary to perform shooting without providing an infrared cut filter.

  As such an image pickup apparatus, an image pickup apparatus for manually attaching / detaching an infrared cut filter and an image pickup apparatus for automatically inserting / removing an infrared cut filter are conventionally known. Further, Patent Document 1 discloses an imaging apparatus that eliminates the need to insert and remove the above-described infrared cut filter.

  The imaging apparatus disclosed in Patent Document 1 transmits light in the visible light band, blocks light in the first near infrared band adjacent to the visible light band, and is longer than the first near infrared band. An optical filter that transmits light in the second near-infrared band of wavelengths is provided in front of the image sensor instead of the conventional infrared cut filter. By adjusting the second near-infrared wavelength band to the output wavelength (for example, 880 nm) of infrared illumination that is used supplementarily during night photography, the second near-infrared band is excluded in the daytime. It is described that light in the infrared band is blocked and good shooting is possible, and that shooting at night is possible with infrared illumination light using the second near-infrared band that passes through the optical filter. Yes. As a result, it is possible to take images throughout the day and night without the need to insert and remove the optical filter.

JP 2004-32243 A

Patent Document 1 discloses a method for manufacturing an optical filter having the above-described characteristics. “An optical thin film is laminated on the surface of the optical filter. An optical thin film is made of an inorganic material such as TiO ₂ or SiO ₂ in a vacuum. It is formed by heating and adhering and depositing the vaporized particles generated thereby on a quartz or glass substrate.Vacuum vapor deposition is adopted for laminating such optical thin films. It is possible to stack as many layers as possible, and transmittance characteristics can be obtained by adjusting the refractive index, thickness, and number of layers of the thin film ”(from the 12th line to the 16th line from the top of page 5).

  However, when an optical filter in which an optical thin film is laminated by the manufacturing method described in Patent Document 1 described above is created, a conventionally known infrared cut filter that blocks the entire near infrared band as shown in FIG. However, it has been found that it is difficult to manufacture the optical filter described in Patent Document 1 that transmits part of the near infrared light in addition to the visible light band.

  The present invention has been made in consideration of the above-described circumstances, and an object of the present invention is to provide an optical filter suitable for an imaging apparatus capable of photographing in both the visible light band and the infrared band.

  The optical filter according to the present invention has a transmission characteristic in the visible light band, has a cutoff characteristic in the first wavelength band adjacent to the long wavelength side of the visible light band, and is in the first near-infrared band. An optical filter having a transmission characteristic in a second wavelength band as a part, wherein the optical filter has a transmission characteristic in the visible light band, has a longer wavelength than the visible light band, and A first wavelength having a cutoff characteristic in a third wavelength band that is shorter than the second wavelength band, and having a transmission characteristic in at least the second near-infrared band on the longer wavelength side than the third wavelength band. A continuous transmission characteristic in the wavelength band from the short wavelength end of the visible light band to the long wavelength end of the second wavelength band, and within the first wavelength band, A second having a cutoff characteristic in a wavelength band longer than the second wavelength band; It is obtained by superimposing a filter.

  By configuring in this way, an optical filter having a characteristic of transmitting light in the visible light band and part of the light in the near infrared band can be obtained. Therefore, in both the visible light band and the infrared band described above. It is possible to provide an optical filter suitable for an imaging device capable of photographing.

  The second wavelength band is preferably configured to have a transmission characteristic with a single peak.

  The short wavelength end of the first wavelength band may be 700 nm or more, and the long wavelength end of the first wavelength band may be 1300 nm or less. By configuring in this way, light in the visible light band with high visibility can be sufficiently transmitted, and light in a band of 1300 nm or less that can be received by the photodiode of the image sensor can be sufficiently blocked. The image quality of the imaging apparatus to which the filter is applied can be improved.

  Furthermore, the bandwidth of the second wavelength band may be 100 nm or less, and the center wavelength of the second wavelength band may be located within a range of 850 nm to 1000 nm. Since the main emission center wavelength of the infrared light emitting diode of infrared illumination used for night photography is between 850 nm and 1000 nm and the half width is about 40 nm, the infrared illumination wavelength can be effectively reduced by configuring in this way. It is possible to prevent unnecessary near-infrared light from being taken into the image sensor.

  Furthermore, it is preferable that the first filter and the second filter are formed on the surface of one optical substrate. Thereby, an increase in aberration due to the overlapping of the optical substrates can be suppressed.

  Furthermore, it is desirable that the first filter and the second filter are continuously formed on the same surface of the optical substrate. Thereby, since the first filter and the second filter can be formed continuously, the production can be performed efficiently.

  The first filter and the second filter may be formed of a dielectric multilayer filter.

  According to the present invention, it is possible to provide an optical filter suitable for an imaging device capable of photographing in both the visible light band and the infrared band.

Embodiment 1 of the Invention
FIG. 1 is a schematic diagram showing a cross section of an optical filter 10 according to the present embodiment. In the configuration shown in FIG. 1, an infrared cut filter 2 is first formed on the surface of the optical substrate 1, and a SWPF (Short-wave-pass-filter) 3 is continuously formed on the infrared cut filter 2. It is set as the structure to form. Note that FIG. 1 is a schematic diagram for ease of understanding of the present invention, and therefore the size and the like are different from actual ones.

  The infrared cut filter 2 is obtained by alternately laminating a high refractive index film 2H and a low refractive index film 2L on a parallel flat optical substrate 1, and laminating each layer so as to satisfy a desired optical characteristic condition. The number and the film thickness are adjusted. Here, the desired optical characteristic condition for the infrared cut filter 2 is that the light in the visible light band (400 nm to 700 nm) having high human visibility is transmitted, and among the light in the near infrared band, the above-described optical characteristic condition is described above. It has a transmittance characteristic that blocks light in a wavelength band shorter than a light emission wavelength of infrared illumination for night photography (for example, a center wavelength of 950 nm and a spectrum half width of 40 nm). FIG. 3 shows an example of the transmittance characteristic that matches this desired characteristic.

  Similarly to the infrared cut filter 2, the SWPF 3 is formed by alternately laminating the high refractive index films 3H and the low refractive index films 3L on the parallel flat optical substrate 1 so as to satisfy the desired optical characteristic conditions. The number of layers and the film thickness of each layer are adjusted. The desired optical characteristics for SWPF3 transmit from the visible light band (400 nm to 700 nm) with high human visibility to the emission wavelength of infrared illumination for night photography described above (for example, center wavelength 950 nm, spectral half width 40 nm). In addition, it has a so-called edge filter-like transmittance characteristic that blocks light in the near-infrared band up to approximately 1300 nm that can be received by the photodiode of the imaging device on the longer wavelength side. FIG. 4 shows an example of transmittance characteristics that match the desired characteristics.

Next, the material and manufacturing method of each part of the optical filter 10 will be described. As the optical substrate 1, amorphous glass, crystallized glass, or another optical substrate can be used. Specifically, oxide single crystals such as LiNbO ₃ , LiTaO ₃ , TiO ₂ , SrTiO ₃ , Al ₂ O ₃ and MgO, polycrystalline substrates, fluoride single crystal substrates such as CaF ₂ , MgF ₂ BaF ₂ and LiF, Polycrystalline substrates, chlorides such as NaCl, KBr, KCl, bromide single crystals, polycrystalline substrates, and the like can be applied. The optical substrate 1 preferably has a high transmittance in the near-infrared band including the wavelength of infrared light used for night photography from the visible light band. For example, the optical substrate 1 has a transmittance of 99.8% or more. It is desirable.

As materials of the high refractive index films 2H and 3H forming the infrared cut filter 2 and SWPF3, Ta ₂ O ₅ , TiO ₂ , ZnS, ZnSe, GaP, InP, Si ₃ N ₄ , SiC, ZrO ₂ , Nb ₂ At least one kind is selected from O ₅ , Y ₂ O ₃ , WO ₃ , In ₂ O ₃ , SnO ₂ , CeO ₂ , HfO ₂ , and the like, and a mixture thereof. The low refractive index films 2L and 3L are made of at least one material selected from SiO ₂ , MgF ₂ , Al ₂ O ₃ , CaF ₂ , LiF, Na ₂ AlF 6, SiO, ThO ₂ , MgO, etc., and a mixture thereof. Is selected. Moreover, you may shift | deviate somewhat from said stoichiometric composition.

  The infrared cut filter 2 and the SWPF 3 formed of a dielectric multilayer film can be manufactured by, for example, a vacuum film forming method. As the vacuum film forming method, various film forming methods such as a vacuum deposition method, a sputtering method, a chemical vapor deposition method, and a laser brazing method can be used. When using vacuum deposition, ion deposition is performed by ionizing a part of the vapor stream to improve the film quality and applying a bias to the optical substrate, the cluster ion beam method, or ions are applied to the substrate using an ion gun. It is effective to use an ion-assisted deposition method for irradiation. As the sputtering method, a DC reactive sputtering method, an RF sputtering method, an ion beam sputtering method, or the like can be used. Further, as the chemical vapor phase method, a plasma polymerization method, a light-assisted vapor phase method, a thermal decomposition method, a metal organic chemical vapor phase method, or the like can be used. In addition, the film thickness of each thin film can be made into a desired film thickness by changing the vapor deposition time etc. at the time of film formation.

Hereinafter, the manufacturing procedure and optical characteristics of the optical filter 10 will be described with specific examples. In the present embodiment, an infrared cut filter 2 having the characteristics shown in FIG. 3 is formed on one surface of the optical substrate 1 using a vacuum deposition apparatus with ion assist, and a SWPF 3 having the characteristics shown in FIG. Formed. For the optical substrate 1, glass BK7 (refractive index: 1.5187@546 nm) manufactured by SCHOTT GLAS, Germany was used. Titanium oxide (TiO ₂ ) was used as the material for the high refractive index films 2H and 3H, and silicon dioxide (SiO ₂ ) was used as the material for the low refractive index films 2L and 3L.

Formation of the dielectric multilayer film (infrared cut filter 2 and SWPF 3) on the surface of the optical substrate 1 was performed as follows. First, the optical substrate 1 is mounted on a vacuum deposition apparatus and evacuated, and after setting to a predetermined substrate temperature, a mixed gas of argon and oxygen is introduced into an ion gun to create a plasma flow, which is used as an optical substrate. The substrate was cleaned by irradiation. Thereafter, TiO ₂ and SiO ₂ were alternately formed to form a multilayer film on the optical substrate. Note that an electron gun heating method was used for vapor deposition of TiO ₂ and SiO ₂ , and the plasma flow irradiation was continuously performed even during film formation.

The optical filter 10 in which the infrared cut filter 2 and the SWPF 3 were continuously formed on one side of the optical substrate was obtained by the manufacturing procedure described above. The number of layers of the optical filter 10 is 36, and the film thickness ratio of each layer is as shown in Tables 1 and 2. Table 1 shows the configuration of the infrared cut filter 2, and Table 2 shows the configuration of the SWPF 3. In addition, the value of the film thickness in the table is indicated by the ratio of the reference wavelength to the 1/4 wavelength film thickness (optical film thickness) with the reference wavelength being 800 nm. The refractive indexes of TiO ₂ and SiO ₂ at the reference wavelength of 800 nm are 2.25 and 1.453, respectively. These refractive indexes were obtained by preparing single layer films of TiO ₂ and SiO ₂ and measuring them using a wavelength dispersion type ellipsometer.

  FIG. 5 shows the transmittance characteristics of the optical filter 10 obtained by the manufacturing procedure described above. As shown in FIG. 5, the near-infrared band (700 nm to 900 nm and 1000 nm to 1300 nm) in which the photodiodes in the other imaging elements can receive light while transmitting light in the visible light band and near infrared band near 950 nm. A transmittance characteristic that sufficiently cuts off the wavelength was obtained. Note that the wavelength of 950 nm was selected because it is a typical emission wavelength of a light-emitting diode that can be used as infrared illumination in night photography. When it is desired to transmit other wavelength bands in the near infrared band, the first peak position on the short wavelength side in the near infrared band of the infrared cut filter 2 shown in FIG. 3 appears in the desired wavelength band, In addition, the infrared cut filter 2 and the SWPF 3 may be designed so that the first peak is included in the transmission wavelength band of the SWPF 3 shown in FIG.

  The transmission characteristics of SWPF3 can be obtained by other types of filters such as a resin filter to which a near-infrared absorbing dye (metal complex or the like) is added instead of the dielectric film filter, but in the visible light band. As a result, there is a problem in characteristics that the transmittance is reduced by the absorption of light, and the light is not sufficiently blocked in the near infrared region, and the advantage of continuous formation with the infrared cut fill 2 cannot be obtained. For this reason, it is desirable to form both the infrared cut filter 2 and the SWPF 3 by dielectric film filters.

  It is necessary to insert and remove the infrared cut filter by disposing the optical filter 10 of the present embodiment on the front surface, that is, the incident side of the image pickup device included in the image pickup device capable of photographing in both the visible light band and the infrared band. However, it is possible to provide an imaging apparatus that can shoot continuously day and night.

  In the present embodiment, the configuration in which the infrared cut filter 2 is first formed on the optical substrate 1 is shown. However, since there is no difference in optical characteristics depending on the order in which the infrared cut filter 2 and the SWPF 3 are formed, this Instead of the configuration, the SWPF 3 may be formed first.

  Further, the near-infrared light transmission band of the optical filter 10 shown in the present embodiment is formed by one peak as shown in FIG. Depending on the characteristics, the characteristics may include a plurality of peaks. However, since the emission spectrum of the light-emitting diode has a peak shape that peaks at the emission center wavelength, the transmittance characteristic of the optical filter 10 efficiently transmits the light of the light-emitting diode and transmits other near-infrared light. It is desirable to have characteristics as shown in FIG.

Embodiment 2 of the Invention
FIG. 2 is a schematic diagram showing a cross section of the optical filter 20 according to the present embodiment. In the configuration shown in FIG. 2, the infrared cut filter 2 and the SWPF 3 are formed on different surfaces of the optical substrate 1. Note that FIG. 2 is a schematic diagram for easy understanding of the present invention, and therefore the size and the like are different from the actual ones.

A manufacturing procedure and optical characteristics of the optical filter 20 will be described with reference to specific examples. In the present embodiment, an infrared cut filter 2 having the characteristics shown in FIG. 3 is formed on one side of the optical substrate 1 by using a vacuum deposition apparatus with ion assist, and the optical substrate 1 is taken out once and the orientation of the optical substrate 1 is changed. By reversing, SWPF3 having the characteristics shown in FIG. 4 was formed on the surface opposite to the surface on which the infrared cut filter 2 was formed. In addition, the glass BK7 (refractive index: 1.5187@546nm) made from SCHOTT GLAS, Germany was used for the optical board | substrate 1 like the specific example shown in Embodiment 1 of invention. Titanium oxide (TiO ₂ ) was used as the material for the high refractive index films 2H and 3H, and silicon dioxide (SiO ₂ ) was used as the material for the low refractive index films 2L and 3L.

  The number of laminated infrared cut filters 2 and SWPFs 3 in the optical filter 20 obtained by the manufacturing procedure described above and the film thickness ratio of each layer are the same as those shown in the first embodiment of the present invention.

  The transmittance characteristics of the optical filter 20 according to the present embodiment obtained in this way are the same as those in FIG. 5 shown in the first embodiment of the invention, and also by the optical filter 20 according to the present embodiment, It is possible to provide an imaging device that can continuously shoot day and night without requiring the infrared cut filter to be inserted and removed.

Other embodiments.
In the above-described two embodiments, the filter configuration in which both the infrared cut filter 2 and the SWPF 3 are formed on one side or both sides of one optical substrate 1 has been described. However, the infrared cut filter 2 and the SWPF 3 are separately provided. After film formation, these may be used in layers. Further, the formed multilayer film may be peeled off from the optical substrate and used alone. Even with this configuration. Desired transmittance characteristics as shown in FIG. 5 can be obtained. However, an increase in the number of optical substrates causes an increase in aberrations, and it is difficult to handle the used multilayer film by peeling it from the optical substrate, so that it is difficult to handle at the time of use. The configuration shown is desirable.

  In the above-described embodiment, the infrared cut filter 2 is configured by a multilayer filter in which high-refractive index films and low-refractive index films are alternately accumulated. Instead, it is configured by a rugate filter. It is good as well. The rugate filter has a structure in which the refractive index is continuously distributed along the film thickness direction and periodically changes. As shown in FIG. The transmittance characteristic of the infrared cut filter 2 can be realized.

  Further, the SWPF 4 has an increased transmittance in a band longer than 1300 nm. If it is necessary to propose the transmittance in the longer wavelength band, the visible light AR having the transmittance characteristics as shown in FIG. An (Anti-Reflection) filter may be provided. For example, in the optical filter 10, a visible light AR filter as shown in FIG. 7 may be laminated on the SWPF 2 formed thereon. The transmittance characteristic of the visible light AR filter shown in FIG. 7 can be easily realized by a multilayer filter or a rugate filter.

  In the above-described embodiment, specific configurations are described and described, and technically preferable limitations such as the number of layers and a film thickness ratio are given. However, the scope of the present invention is limited to these configurations. Is not to be done.

It is a block diagram of the optical filter concerning this invention. It is a block diagram of the optical filter concerning this invention. It is the transmittance | permeability characteristic graph of the infrared cut filter concerning this invention. It is a transmittance | permeability characteristic graph of SWPF concerning this invention. It is a transmittance | permeability characteristic graph of the optical filter concerning this invention. It is the transmittance | permeability characteristic graph of the conventional infrared cut filter. It is a transmittance | permeability characteristic graph of visible light AR filter.

10, 20 Optical filter 1 Optical substrate 2 Infrared cut filter 3 SWPF
2H, 3H High refractive index film 2L, 3L Low refractive index film

Claims (8)

It has a transmission characteristic in the visible light band, a cutoff characteristic in the first wavelength band adjacent to the long wavelength side of the visible light band, and is transmitted through the second wavelength band that is a part of the first wavelength band An optical filter having characteristics,
A transmission characteristic in the visible light band, a longer wavelength than the visible light band, and a cutoff characteristic in a third wavelength band that is shorter than the second wavelength band; On the longer wavelength side than the wavelength band, a first filter having transmission characteristics in at least the second wavelength band;
The wavelength band from the short wavelength end of the visible light band to the long wavelength end of the second wavelength band has transmission characteristics continuously, is within the first wavelength band, and the second wavelength A second filter having a cutoff characteristic in a wavelength band longer than the band;
Optical filter with   The optical filter according to claim 1, wherein the second wavelength band has a transmission characteristic having a single peak.   The optical filter according to claim 1 or 2, wherein a short wavelength end of the first wavelength band is 700 nm or more, and a long wavelength end of the first wavelength band is 1300 nm or less.   4. The optical filter according to claim 1, wherein a bandwidth of the second wavelength band is 100 nm or less, and a center wavelength of the second wavelength band is located within a range of 850 nm to 1000 nm.   The optical filter according to claim 1, wherein the first filter and the second filter are formed on a surface of one optical substrate.   The optical filter according to claim 1, wherein the first filter and the second filter are continuously formed on the same surface of the optical substrate.   The optical filter according to claim 1, wherein the first filter and the second filter are dielectric multilayer filters.   It has a transmission characteristic in the visible light band, a cutoff characteristic in the first wavelength band adjacent to the long wavelength side of the visible light band, and is transmitted through the second wavelength band that is a part of the first wavelength band An optical filter having characteristics.

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