JP2010032867A - Infrared ray cutoff filter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an IR cutoff filter which reduces the dependency of light transmissivity on the light incidence angle and achieves its low profile and cost reduction. <P>SOLUTION: The IR cutoff filter has a transparent substrate 2, and a multilayer 3 made on this transparent substrate 2 by stacking a number of thin dielectric films 5 and thin silver films 6. The light transmissivity dependency on the light incidence angle is suppressed within a predetermined limit by this multilayer 3. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an IR cut filter, and more particularly to an IR cut filter suitable for exhibiting high light transmission characteristics with respect to visible light while suppressing transmission of infrared light.

  Conventionally, in a camera equipped with an image sensor such as a CCD or CMOS, the sensitivity of the near-infrared region of the image sensor (sensor) is higher than the sensitivity of the human eye, so it captures the color balance as seen by the human eye. For the purpose of reproducing on an image, an IR (Infrared) cut filter that transmits visible light with a sufficiently high transmittance while suppressing transmission of infrared light (infrared light) has been adopted.

  One of the IR cut filters of this type is a coating type IR cut filter obtained by laminating thin films on a transparent substrate (for example, a glass plate) by a coating method such as vapor deposition to form a multilayer film. There is.

Specifically, the coating type IR cut filter is formed of a thin film made of a high refractive index dielectric (eg, TiO ₂ ) and a low refractive index dielectric (eg, SiO ₂ ) on a transparent substrate. The thin films are formed by alternately laminating such that the respective optical film thicknesses are approximately ¼ of the blocking wavelength.

  Such a coating type IR cut filter suppresses transmission of infrared light by reflection of infrared light using an interference effect by a multilayer film.

Japanese Patent No. 3679268

  However, the conventional coating-type IR cut filter has a strong incident angle dependency of the light transmission characteristics.

  That is, in the conventional coating type IR cut filter, a phenomenon has occurred in which the light transmission characteristics of the filter greatly change according to the incident angle of light with respect to the filter.

  As a result, in a conventional coating-type IR cut filter, infrared light transmission is suppressed by the IR cut filter between different imaging positions on the image sensor (for example, between the optical axis and the peripheral portion). Due to the different effects, there has been a problem of uneven color.

  In addition to the coating type, the IR cut filter includes an absorption type made of glass or plastic containing metal ions. This type of absorption type IR cut filter is human in the visible light region. It exhibits a light transmission characteristic that is close to the sensitivity characteristic of the eye, has a feature that color balance adjustment after photoelectric conversion is easy, and soft tone reproduction is obtained.

  In addition, such an absorption type IR cut filter has less incident angle dependency on light transmission characteristics and less color unevenness than a coating type IR cut filter.

  However, such an absorption type IR cut filter has a drawback that a certain thickness is required to obtain a necessary light transmission characteristic, and it is difficult to reduce the thickness. It has the fault of being.

  Therefore, the present invention has been made in view of the above points, and provides an IR cut filter that can reduce the incident angle dependence of light transmission characteristics, and can be reduced in thickness and cost. It is intended to do.

  In order to achieve the object described above, the IR cut filter according to claim 1 of the present invention has a relatively low light transmission characteristic due to light reflection in the infrared region, and is relatively high in the visible light region. An IR cut filter formed to exhibit light transmission characteristics, a transparent base material, and a multilayer film formed on the transparent base material, in which a plurality of dielectric thin films and a plurality of silver thin films are laminated The incident angle dependence of the light transmission characteristics is suppressed within a predetermined limit by the multilayer film.

  According to the first aspect of the invention, the multi-layer film made of the dielectric thin film and the silver thin film suppresses the incident angle dependence of the light transmission characteristics within a predetermined limit while having a thin and inexpensive configuration. be able to.

  The IR cut filter according to claim 2 is characterized in that, in claim 1, the dielectric thin film is a metal oxide thin film.

  According to the second aspect of the present invention, the dielectric thin film can be formed of a material suitable for coating.

  According to the present invention, it is possible to reduce the incident angle dependence of the light transmission characteristics, and it is possible to reduce the thickness and reduce the cost.

  Hereinafter, embodiments of an IR cut filter according to the present invention will be described with reference to FIGS. 1 to 8.

  FIG. 1 shows an embodiment of an IR cut filter 1 according to the present invention. As shown in FIG. 1, the IR cut filter 1 according to this embodiment includes a transparent substrate 2 and the transparent substrate. 2 and the multilayer film 3 formed on the surface.

  The transparent substrate 2 may be, for example, a substrate 2 on which a multilayer film 3 such as a glass flat plate or a plastic flat plate is formed, or a surface of the substrate 2 such as a lens. A shape other than a flat surface (such as a curved surface) may be used.

  The multilayer film 3 is formed integrally with the transparent substrate 2 by laminating a plurality of dielectric thin films 5 and a plurality of silver thin films 6 on the surface of the transparent substrate 2 by a coating method such as vapor deposition. .

  In addition, the multilayer film 3 in FIG. 1 is a multilayer film 3 having a total of seven layers, in which the dielectric thin films 5 and the silver thin films 6 are alternately laminated, the dielectric thin film 5 is four layers and the silver thin film 6 is three layers. However, of course, depending on the material of the dielectric thin film 5, the number of layers may be appropriately different.

  However, in the present embodiment, as will be described later, the number of layers of the multilayer film 3 required to obtain the necessary light transmission characteristics may be smaller than the number of layers of the multilayer film in the conventional coating type IR cut filter. It has become a thing.

The dielectric thin film 5 is preferably a metal oxide thin film such as TiO ₂ , Ta ₂ O ₅ , Nb ₂ O ₅ , SiO ₂ , and Al ₂ O ₃ .

  Here, FIG. 2 is a graph showing the complex refractive index of silver. As shown in FIG. 2, silver has a characteristic that the value of the imaginary part (k) in the complex refractive index (n + ik) is large, and the value of the imaginary part increases as the wavelength used increases. The real part has a characteristic such that the value (n) is close to zero.

  In the present embodiment, the silver thin film 6 made of silver having such a complex refractive index is applied to the multilayer film 3 together with the dielectric thin film 5, so that the infrared region is relatively low due to light reflection. In addition to exhibiting light transmission characteristics (in other words, high light reflection characteristics), relatively high light transmission characteristics can be exhibited in the visible light region.

  The number of layers of the silver thin film 6 is preferably limited to about 4 from the viewpoint of reducing visible light absorption as much as possible.

<First embodiment>
In the first embodiment, as shown in Table 1, a total of 7 layers (4 of which are 4 layers) obtained by alternately laminating a TiO ₂ thin film as the dielectric thin film 5 and a silver (Ag) thin film 6 are used. TiO ₂ thin film, with three layers comprises a multi-layer film 3 of the silver thin film), on the surface of the multilayer film 3 formed surface opposite the transparent substrate 2, TiO ₂ thin film and SiO, as shown in Table 2 An IR cut filter 1 provided with an antireflection film having a four-layer structure in which _two thin films are alternately stacked was assumed.

  In addition, as the transparent base material 2, the glass flat plate which consists of BK7 material was assumed. Further, the layer numbers in Tables 1 and 2 are numbers counted from the surface side of the transparent substrate 2. Further, the refractive index in Table 1 is a refractive index under a use wavelength of 900 [nm], and the refractive index in Table 2 is a refractive index under a use wavelength of 510 [nm].

  Then, with respect to the IR cut filter 1 of this embodiment shown in Tables 1 and 2, the incident angle [°] of light to the IR cut filter 1 is set to 0 [°], that is, the IR cut filter 1 The light transmission characteristics were simulated on condition that light was incident in parallel to the surface normal of the surface (flat surface).

  As a result, a graph of light transmission characteristics as shown in FIG. 3 was obtained. The horizontal axis in FIG. 3 is the wavelength [nm] of light incident on the IR cut filter, and the vertical axis in FIG. 3 is the transmittance [%] of the IR cut filter.

Next, as a comparative example for the IR cut filter 1 of this example, as shown in Table 3, a total of 42 layers (TiO ₂ thin film, SiO ₂₎ obtained by alternately laminating TiO ₂ thin films and SiO ₂ thin films are shown. A conventional coating type IR cut filter provided with a multilayer film of 21 layers for both thin films and the antireflection film shown in Table 2 was assumed.

  In the IR cut filter of this comparative example, the transparent substrate 2 was the same as the IR cut filter 1 of this example. Moreover, the refractive index in Table 3 is a refractive index under a working wavelength of 900 [nm].

  Then, the incident angle [°] is set for the IR cut filter 1 of this embodiment assumed as shown in Tables 1 and 2 and the conventional IR cut filter assumed as shown in Tables 3 and 2, respectively. The light transmission characteristics were simulated in the case of 0 [deg.] Incidence with 0 [[deg.] And 30 [deg.] Incidence with an incidence angle [[deg.]] Of 30 [[deg.].

  As a result, four graphs of light transmission characteristics numbered (1) to (4) in FIG. 4 were obtained.

  The conditions on the horizontal axis and the vertical axis in FIG. 4 are the same as those in FIG.

  Also, in the graph of FIG. 4, the graph showing the simulation result for the IR cut filter 1 of this embodiment in the case of 0 ° incidence (the graph with (1) attached) is exactly the same as the graph shown in FIG. belongs to.

  As shown in FIG. 4 and Tables 1 and 3, according to the IR cut filter 1 of the present example, the transmittance [%] in the infrared region, while significantly reducing the number of layers of the multilayer film as compared with the prior art. Shows sufficiently low light transmission characteristics and can suppress infrared light transmission, and in the visible light region, it shows light transmission characteristics with sufficiently high transmittance [%] and sufficiently transmits visible light. be able to.

  Furthermore, in the conventional IR cut filter, the light transmission characteristic at 0 ° incidence (graph with (3)) and the light transmission characteristic at 30 ° incidence (graph with (4)) On the other hand, in the IR cut filter 1 of the present embodiment, the change in the light transmission characteristic due to the change in the incident angle is greatly reduced as compared with the conventional IR cut filter. Yes. Incidentally, in the IR cut filter 1 of this embodiment, the maximum value of the wavelength shift indicating the incident angle dependency of the light transmission characteristic is approximately 10 [nm] / 30 [°]. The wavelength shift means a change in wavelength accompanying a change in incident angle when the transmittance is constant. In the present embodiment, it can be said that the incident angle dependence of the light transmission characteristic is suppressed within 10 [nm] / 30 [°] as an allowable limit.

  This indicates that the IR cut filter 1 according to the present embodiment has less incident angle dependency of the light transmission characteristics than the conventional coating type IR cut filter.

  Next, as another comparative example for the IR cut filter 1 of the present embodiment, an absorption type IR cut filter is assumed, and for this absorption type IR cut filter and the IR cut filter 1 of the present embodiment, Simulation of light transmission characteristics in the case of 0 ° incidence and 30 ° incidence was performed, respectively, and results as shown in the graph of FIG. 5 were obtained.

  As shown in FIG. 5, in the IR cut filter 1 in the present embodiment, the peak value of the transmittance in the visible light region is lower than that of the absorption type. Good light transmission characteristics similar to the type of IR cut filter can be exhibited.

<Second embodiment>
In the second example, as shown in Table 4, a total of 11 layers (of which 5 are included) obtained by laminating the TiO ₂ thin film and the SiO ₂ thin film as the dielectric thin film 5 and the silver (Ag) thin film 6 are stacked. On the surface of the transparent substrate 2 opposite to the surface on which the multilayer film 3 is formed, a multilayer film 3 having a layer of TiO ₂ thin film, four layers of SiO ₂ thin film, and two layers of silver thin film) An IR cut filter 1 having an antireflection film as shown in FIG.

  The conditions for the transparent substrate 2, the wavelength used and the layer number corresponding to the refractive index are the same as in the first embodiment.

  And the simulation of the light transmission characteristic in the case of 0 degree incidence was performed with respect to the IR cut filter 1 of the present Example shown in Tables 4 and 2.

  As a result, a graph of light transmission characteristics as shown in FIG. 6 was obtained. Note that the horizontal and vertical axes in FIG. 6 are the same as those in FIG.

  The incident angle [°] is set to 0 [°] with respect to the IR cut filter 1 of the present embodiment assumed as shown in Tables 4 and 2 and the conventional IR cut filter shown in Tables 3 and 2, respectively. The light transmission characteristics were simulated when the incident angle [°] was 30 ° and when the incident angle [°] was 30 °.

  As a result, four graphs of light transmission characteristics numbered (1) to (4) in FIG. 7 were obtained.

  The conditions on the horizontal axis and the vertical axis in FIG. 7 are the same as those in FIG.

  In addition, among the graphs in FIG. 7, the graph showing the simulation results for the IR cut filter 1 of the present example in the case of 0 ° incidence (the graph to which (1) is attached) is completely the same as the graph shown in FIG. 6. Are the same. Moreover, the simulation results (graphs with (3) and (4)) for the conventional IR cut filter are exactly the same as those shown in FIG.

  As shown in FIG. 7 and Tables 4 and 3, also in the IR cut filter 1 of this example, the transmittance [%] is sufficiently low in the infrared region while significantly reducing the number of layers of the multilayer film as compared with the prior art. Shows light transmission characteristics and can suppress transmission of infrared light, and shows light transmission characteristics with sufficiently high transmittance [%] in the visible light region, and allows visible light to be transmitted with sufficiently high transmittance. it can.

  Furthermore, the change in the light transmission characteristic between the case of 0 ° incidence and the case of 30 ° incidence can be suppressed as compared with the conventional case, and the incident angle dependency of the light transmission property can be reduced.

  Next, as another comparative example for the IR cut filter 1 of the present embodiment, the above-described absorption type IR cut filter is assumed, and the absorption type IR cut filter and the IR cut filter 1 of the present embodiment are compared. Then, the simulation of the light transmission characteristics in the case of 0 ° incidence and 30 ° incidence was performed, respectively, and the results as shown in the graph of FIG. 8 were obtained.

  As shown in FIG. 8, the IR cut filter 1 in this example also has a transmittance peak value in the visible light region that is lower than that of the absorption type. Good light transmission characteristics similar to the type of IR cut filter can be exhibited.

  That is, according to the present embodiment, the thickness can be made thinner than the conventional one while maintaining the original function of the IR cut filter that suppresses the transmission of infrared light and sufficiently transmits visible light. At the same time, the incident angle dependence of the light transmission characteristics can be relaxed.

  In addition, this invention is not limited to embodiment mentioned above, A various change is possible as needed.

The block diagram which shows embodiment of IR cut filter which concerns on this invention The graph which shows the characteristic of the complex refractive index of silver in embodiment of the IR cut filter which concerns on this invention In the embodiment of the IR cut filter according to the present invention, a graph showing the light transmission characteristics at 0 ° incidence in the IR cut filter of the first example In the embodiment of the IR cut filter according to the present invention, the light transmission characteristics at 0 ° incidence and 30 ° incidence in the IR cut filter of the first example, and at 0 ° incidence and 30 in the conventional coating type IR cut filter. ° Graph showing light transmission characteristics at incidence In the embodiment of the IR cut filter according to the present invention, the light transmission characteristics at 0 ° incidence and 30 ° incidence in the IR cut filter of the first example and at 0 ° incidence in the conventional absorption type IR cut filter and 30 ° Graph showing light transmission characteristics at incidence In the embodiment of the IR cut filter according to the present invention, a graph showing the light transmission characteristics at 0 ° incidence in the IR cut filter of the second example In the embodiment of the IR cut filter according to the present invention, the light transmission characteristics at 0 ° incidence and 30 ° incidence in the IR cut filter of the second example, and at 0 ° incidence in the conventional coating type IR cut filter and 30 ° Graph showing light transmission characteristics at incidence In the embodiment of the IR cut filter according to the present invention, the light transmission characteristics at 0 ° incidence and 30 ° incidence in the IR cut filter of the second example, and at 0 ° incidence in the conventional absorption type IR cut filter and 30 ° Graph showing light transmission characteristics at incidence

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 IR cut filter 2 Transparent base material 3 Multilayer film 5 Dielectric thin film 6 Silver thin film

Claims (2)

In the infrared region, an IR cut filter formed to exhibit a relatively low light transmission characteristic by light reflection, and in the visible light region to exhibit a relatively high light transmission characteristic,
A transparent substrate;
A multilayer film formed on the transparent substrate and formed by laminating a plurality of dielectric thin films and a plurality of silver thin films;
The IR cut filter characterized in that the multilayer film suppresses the incident angle dependence of light transmission characteristics within a predetermined limit. The IR cut filter according to claim 1, wherein the dielectric thin film is a metal oxide thin film.

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