JPH05207350A - Camera provideo with infrared-ray cut filter - Google Patents

Abstract

PURPOSE:To improve the infrared-ray stray light preventing structure of a CCD camera, and to miniaturize the camera. CONSTITUTION:The CCD camera is constituted of an image forming optical system 1 including optical elements arrayed along an optical axis, and an image pickup element such as a CCD 2. An infrared-ray cut filter 7 constituted of a direct infrared ray transmission preventing dielectric multi-layer film is formed on the surface of a specific optical element, for example, a lens 6 in order to interrupt the stray light in an infrared wavelength area. The multi-layer film infrared-ray cut filter 7 can be formed on the surface of a crystal fitter 4, or the surface of a cover glass 5 instead of the lens 6. The multi-layer film infrared-ray cut filter 7 is a hyaline, and the thickness of the film is extremely thin, so that the image forming system 1 can be miniaturized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a camera having an image forming optical system composed of a plurality of lens members and an image pickup device composed of a CCD or the like. More specifically, it relates to a structure for blocking stray light in the infrared wavelength region.

[0002]

2. Description of the Related Art Conventionally, as an image pickup device of a small video camera such as a surveillance camera, for example, a CCD (Charg) is used.
e Coupled Device) is often used. The CCD image pickup device accumulates signal charges generated by photoelectric conversion for a period close to one field or one frame, and transfers the charges in a short time to read out as a video signal. CCDs have a relatively wide sensitivity and respond to light in the near infrared region in addition to light in the visible region. However, in a video camera used for ordinary photographing of an object, infrared incident light becomes stray light, which causes a problem such as a reduction in resolution, deterioration of blooming smear characteristics, or image stains or unevenness. Therefore, an infrared cut filter is inserted in the optical system of the video camera. A commonly used infrared cut filter is a colored glass plate, for example, a color glass CM-500 made by Hoya Co., Ltd. was used.

[0003]

Depending on the application of the video camera, it may be necessary to reduce the size to a considerable extent. For example, for an in-vehicle camera incorporated in the tail of an automobile or a surveillance camera for crime prevention, an ultra-small camera having a camera aperture of about 10 mm is required. Not only the camera aperture but also the optical axis length is required to be reduced. However, in the conventional stray light prevention structure, since a single infrared cut filter such as a colored glass plate was inserted into the optical system, an extra storage space was needed for that, which hindered the miniaturization of the video camera lens system. There is a problem that I was doing. In addition, when the colored plate glass is inserted, the transmittance of incident light is lost correspondingly. In order to make up for this, it is necessary to increase the amount of incident light, which has been a hindrance to miniaturization of the lens system. The thickness of the glass plate for infrared cut filter that is usually used is 1mm.
It is about 3 mm.

[0004]

SUMMARY OF THE INVENTION In view of the above-mentioned problems or problems of the prior art, an object of the present invention is to improve the structure for preventing stray light in the near infrared region and to downsize a camera using a CCD. To do. The following measures have been taken in order to achieve this purpose. That is, in a camera including an imaging optical system including optical elements arranged along the optical axis and an image pickup device such as a CCD, infrared rays are directly applied to the surface of a specific optical element in order to block stray light in the infrared wavelength region. A means of providing a permeation-preventing dielectric multilayer film was taken. As the specific optical element on which the multilayer film is formed, for example, a lens included in the image pickup optical system can be selected. It is preferable that the selected lens has a surface having a maximum incident angle with respect to the optical axis of 30 ° or less in the image forming optical system. The specific optical element may be a cover glass arranged immediately in front of the image pickup element instead of the lens. Alternatively, it may be a crystal filter arranged in the subsequent stage of the imaging optical system.

[0005]

The infrared ray transmission preventing dielectric multilayer film used in the present invention is a kind of interference filter, and it selectively reflects or absorbs only light in the infrared wavelength region by utilizing the interference of light by the thin film and is visible. Part of light is transmitted very efficiently. The dielectric multilayer film is formed by vacuum vapor deposition or the like directly on the surface of a selected optical element such as a lens. The dielectric multilayer film has a thickness of at most several μm and is colorless and transparent. Therefore, by using this, the conventional colored plate glass or infrared absorbing plate glass becomes unnecessary, and the size of the camera can be reduced. The infrared absorbing plate glass used in the past generally contained a copper compound and was colored in a pale blue color and had a thickness of about several mm.

Since the infrared ray transmission preventing dielectric multilayer film performs a filtering function by utilizing the multiple interference between layers, it has a dependency on the incident angle. Although it has a substantially perfect blocking efficiency for vertically incident light, the blocking efficiency becomes worse as it is inclined from the optical axis. Therefore, when applied to wide-angle cameras such as surveillance cameras, in order to suppress the incident angle dependence as much as possible, the infrared ray transmission prevention dielectric multilayer film is formed on the surface of a specific lens having a surface with a maximum incident angle of 30 ° or less. It is preferably formed. In other words, in the imaging optical system, it is preferable to consider in the design so that at least one lens surface having a maximum light incident angle of 30 ° or less is provided.

The infrared ray transmission preventing dielectric multilayer film formed on the surface of a specific optical element such as a lens is 700 to 700% of the incident light.
Selectively reflects near infrared light having a wavelength of 1000 nm. On the contrary, visible light having a wavelength of 400 to 600 nm is transmitted by about 90% on average. Therefore, by using such an optical element, it is possible to effectively prevent the stray light to the CCD. As the optical element, the same effect can be obtained by selecting a cover glass or a crystal filter in addition to the lens.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described in detail below with reference to the drawings. FIG. 1 is a schematic partial sectional view showing an embodiment of a camera with an infrared cut filter according to the present invention. This example is an ultra-small and wide-angle surveillance camera used for vehicle installation or crime prevention monitoring. The camera diameter is about 10 mm, the optical axis length is about several tens of mm, and the photographing viewing angle is 120 to 130 °. As shown in the figure, the surveillance camera is composed of an imaging optical system 1 including optical elements arranged along the optical axis, and an image pickup device composed of a CCD 2. The imaging optical system 1 includes a lens system including a condenser lens, a correction lens, an objective lens, etc., a field stop 3, a crystal filter 4, a cover glass 5, and the like.

In this embodiment, an infrared cut filter 7 made of an infrared transmission preventing dielectric multilayer film is formed on the surface of a particular selected lens 6. The multilayer infrared cut filter 7 is colorless and transparent and has a thickness of about several μm. Therefore, the axial length of the imaging optical system 1 is not substantially affected. In addition, it does not affect the camera aperture size.

FIG. 2 is a geometrical optical diagram of the surveillance camera shown in FIG. Incident light 9 parallel to the optical axis 8 is condensed through an image forming optical system and forms an image at the center of the image forming surface 10 of the CCD. On the other hand, the incident light 11 having a maximum inclination angle of about 60 ° with respect to the optical axis 8 is also condensed by the imaging optical system and forms an image at the end of the imaging surface 10. At this time, the maximum inclined incident light 11 passes through the specific lens 6 provided with the multilayer infrared cut filter 7 with an inclination of 30 ° or less with respect to the optical axis 8. Therefore, the incident angle dependence of the multilayer infrared cut filter 7 can be suppressed to a considerable extent. Thus, it is preferable to form the multilayer infrared cut filter 7 by selecting a lens having a surface with a small maximum incident angle.

FIG. 3 shows a modification of the surveillance camera shown in FIG. To facilitate understanding, corresponding components have corresponding reference numbers. In this example, the multilayer infrared cut filter 7 is directly formed on the surface of the crystal filter 4. The crystal filter 4 is arranged in the subsequent stage of the imaging optical system 1, and the width of the incident angle with respect to this surface is 30 ° or less. The crystal filter 4 is required in the case of a color video camera, and cancels Moire fringes and performs focusing by the birefringent action of the crystal. The crystal filter 4 generally has a laminated structure in which a plurality of crystal plates are stacked. The plate thickness, the number of sheets, and the arrangement are appropriately selected according to the design conditions of the optical system 1. In this example, three crystal plates are used, and their polarization axes intersect each other at equal intervals of 60 °.

FIG. 4 shows another modification of the surveillance camera shown in FIG. In this example, the multilayer infrared cut filter 7 is formed on the surface of the cover glass 5. The cover glass 5 is arranged immediately before the CCD 2 and protects it. The width of the incident angle with respect to the cover glass 5 is also 30 ° or less, which is a portion suitable for reducing the incident angle dependence of the multilayer infrared cut filter 7. In order to facilitate understanding, parts corresponding to those in FIG. 1 are designated by corresponding reference numbers.

Next, a specific configuration example of the multilayer infrared cut filter 7 shown in FIG. 1 will be described with reference to FIG. In this example, the specific lens 6 is used as a substrate (refractive index n = 1.6
85) On top of that, a dielectric is laminated in multiple layers by using ordinary vacuum deposition. In this example, TiO is used as the high refractive index film material.
₂ was used and SiO ₂ was used as the low refractive index film material. Regarding the structure of the film, SiO ₂ was vacuum-deposited on the first layer as the adjusting part A at a film thickness d = 184.8 nm counting from the substrate surface. Next, as the alternate portion B from the second layer to the eleventh layer, the film thickness d
= 85.1 nm TiO ₂ and film thickness d = 136.9 nm Si
Alternately vacuum deposited with O ₂ . Further, a TiO ₂ film having a film thickness d = 93.6 nm is formed as a bonding layer C on the 12th and 13th layers.
And SiO ₂ with a film thickness d = 150.6 nm were vapor deposited. Further, a film thickness d is formed as an alternate layer D over the 14th to 24th layers.
= 105.5 nm TiO ₂ and film thickness d = 169.8 nm S
Alternately deposited iO ₂ . SiO ₂ was vacuum-deposited as a control layer E on the final 25th layer with a film thickness d = 82.1 nm. The outer diameter of the lens member 6 is 6 mm.

FIG. 6 shows the transmission characteristics of the multilayer infrared cut filter 7 shown in FIG. In this graph, the horizontal axis shows the wavelength (unit: nm), and the vertical axis shows the transmittance (unit:%). As is clear from the graph, this filter has a transmittance of 90% or more for visible light with a wavelength of 700 nm as a boundary, and has an extremely excellent blocking property for near infrared rays. The adjustment layers A and E shown in FIG. 5 are for suppressing the first ripple 12, and the bonding layer C is for flattening other ripples 13.

In the multilayer infrared cut filter shown in FIG. 5, TiO ₂ is used as the high refractive index film material and SiO ₂ is used as the low refractive index film material by the ordinary vacuum deposition.
However, the present invention is not limited to the infrared cut filter obtained by such a material, structure or manufacturing method. For example, a combination of Ta ₂ O ₅ for the high refractive index film material and SiO _{2 for} the low refractive index film material may be used to perform the assisted deposition by ion plating. At this time,
The refractive index dispersion on the short wavelength side is smaller and the wavelength dependence of the transmittance, that is, the wavelength shift, is smaller than in the ordinary vacuum deposition using the combination of TiO ₂ and SiO ₂ shown in FIG.

In the above-described embodiment, for example, the multilayer infrared cut filter is formed on the lens member and incorporated in the camera. However, the basic concept of the present invention can be applied not only to a lens incorporated in a camera but also to a wide range of general lenses. Therefore, FIG. 7 shows an example in which a multilayer infrared cut filter is applied to a single convex lens.
A multilayer infrared cut filter 15 is provided on one surface side of the lens member 14. Further, the AR film 16 is provided on the facing surface side.
Has been applied. The AR film 16 functions as a transparent film or an antireflection film.

[0017]

As described above, according to the present invention, by forming a multilayer infrared cut filter directly on the surface of a specific optical element, for example, a lens included in an image forming optical system of a camera, a CCD or the like is formed. There is an effect that stray light to the image pickup device can be suppressed. Since this multilayer infrared cut filter is colorless and transparent and has an extremely thin film thickness, it has an effect that the imaging optical system 1 can be downsized. In addition,
The lens member on which the multilayer infrared cut filter is directly formed can be applied not only to the camera but also to various optical devices in place of the conventional colored infrared absorbing glass plate.

[Brief description of drawings]

FIG. 1 is a schematic sectional view showing an example of a camera with an infrared cut filter according to the present invention.

FIG. 2 is a geometrical optical diagram of the camera shown in FIG.

FIG. 3 is a schematic cross-sectional view showing a modified example of the camera shown in FIG.

FIG. 4 is a schematic sectional view showing another modification of the same.

5 is a schematic diagram showing a layer structure of the multilayer infrared cut filter shown in FIG.

6 is a graph showing transmission characteristics of the multilayer infrared cut filter shown in FIG.

FIG. 7 is a schematic view showing an example in which a multilayer infrared cut filter is directly applied to a lens surface.

[Explanation of symbols]

 1 Imaging Optical System 2 CCD 3 Field Stop 4 Crystal Filter 5 Cover Glass 6 Lens 7 Multilayer Infrared Cut Filter

Claims (6)

[Claims] 1. In a camera including an imaging optical system including optical elements arranged along an optical axis and an image pickup element, infrared rays are directly applied to a surface of a specific optical element in order to block stray light in an infrared wavelength region. A camera with an infrared cut filter, which is provided with a transmission-preventing dielectric multilayer film. 2. The camera with an infrared cut filter according to claim 1, wherein the specific optical element is a lens. 3. The camera with an infrared cut filter according to claim 2, wherein the lens has a surface having a maximum incident angle of 30 ° or less with respect to the optical axis in the imaging optical system. 4. The camera with an infrared cut filter according to claim 1, wherein the specific optical element is a cover glass arranged immediately before the image pickup element. 5. The camera with an infrared cut filter according to claim 1, wherein the specific optical element is a crystal filter arranged in a subsequent stage of the imaging optical system. 6. A lens coated with an infrared ray transmission preventing dielectric multilayer film.