JPH05173093A - Optical low-pass filter and image pickup device - Google Patents

Abstract

PURPOSE:To prevent deterioration of picture quality by suppressing ring-like flares which appear on an optical low pass filter formed with a diffraction grating on the surface of a light transmitting substrate. CONSTITUTION:Thickness (t) of the substrate of the optical low pass filter 2 is determined to satisfy the relation of formula when the grating formed surface 2a of the filter 2 is arranged facing the side of a solid state image pickup element 4. In formula, (n) is the refractive index of the substrate and (x) is the longitudinal length of the pickup surface of the solid state pickup element 4. Thereby, radius of the ring-like flare increases to be larger than the area of the pickup surface, so that no ring-like flare appears on the plane, or even if the ring appears, its intensity is decreased and the ring is less observed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical low pass filter and an image pickup apparatus having the same.

[0002]

2. Description of the Related Art A color image pickup device using a single-plate type solid-state image pickup device is provided with a color filter array to obtain a color signal from an object including a high frequency component corresponding to the pitch of the color filter array. Generates a false color signal. Further, the solid-state image pickup device has pixels arranged discontinuously and regularly, and a false signal due to aliasing is generated from an object including a high frequency component corresponding to the pixel pitch. Therefore, the optical system of such a color image pickup device is provided with an optical low-pass filter for preventing generation of a false color signal and a false signal. As an optical low-pass filter, an optical low-pass filter having three or more crystal plates and one infrared cutoff filter bonded together is used, but an optical low-pass filter having a laminated structure is inferior in mass productivity and expensive. There is a problem that is. In order to solve the above problem, an optical low pass filter including a diffraction grating has been proposed. An example of an imaging device using such a diffraction grating is shown in FIG.

In FIG. 7, in the image pickup apparatus, an image pickup lens 31, an optical low-pass filter 32, and a solid-state image pickup element 34 are arranged in this order from the light incident side (subject side). The solid-state image sensor 34 is housed in a package 35, the optical low-pass filter 32 is attached to the front surface of the package 35, and is also used as a protective glass for the solid-state image sensor 4.

[0004]

The optical low-pass filter 32 is a light-transmissive substrate having a refractive index of 1.5.
2. Glass having a substrate thickness of 1 mm is used, and a diffraction grating is formed on the surface thereof. This optical low pass filter 3
When the spot light is photographed by using the image pickup device in which the distance between the grating forming surface 32a and the solid-state image pickup device 34 is 50 μm, a ring-shaped flare (hereinafter, “ Ring flare ") occurs. This ring-shaped flare occurs when, for example, a vehicle headlight or an illumination light is photographed, and becomes a factor that deteriorates image quality.

A main object of the present invention is to provide an optical low-pass filter which prevents deterioration of image quality due to the occurrence of the above-mentioned ring-shaped flare, and an image pickup apparatus having the same.

[0006]

In order to achieve the above object, the optical low pass filter of the present invention has the following structure (A) or (B).

(A) An optical system in which a diffraction grating is formed on the surface of a light-transmissive substrate, the diffraction grating is arranged between the imaging lens and the solid-state image sensor, and the grating formation surface faces the solid-state image sensor side. A low pass filter, wherein the thickness t of the substrate satisfies the following condition, where n is the refractive index of the substrate and x is the length in the longitudinal direction of the imaging surface of the solid-state imaging device. Optical low-pass filter. 4 · t · tan (Sin ⁻¹ (1 / n)) ≧ x

(B) An optical system in which a diffraction grating is formed on the surface of a light-transmissive substrate, the diffraction grating is arranged between the image pickup lens and the solid-state image pickup device, and the grating formation surface faces the solid-state image pickup device side. A low-pass filter, wherein an antireflection film composed of one or more layers is formed on the surface of the substrate opposite to the lattice formation surface, the refractive index of the substrate is n, and the solid-state imaging device captures an image. An optical low-pass filter, wherein the thickness t of the substrate satisfies the following condition, where x is the length of the surface in the longitudinal direction. 8 · t · tan (Sin ⁻¹ (1 / n)) ≧ x

Further, the image pickup apparatus of the present invention is the above (A).
Alternatively, the optical low-pass filter described in (B) is provided, and the optical low-pass filter and the solid-state imaging device are integrated with a spacer interposed therebetween.

[0010]

According to the optical low pass filter having the above structure,
As a result of the radius of the ring-shaped flare becoming large and deviating from the image pickup surface, the ring-shaped flare does not appear at all on the image pickup surface, or even if it appears, the light intensity decreases and becomes inconspicuous. Further, according to the image pickup apparatus having the above configuration, the relative positional relationship between the optical low-pass filter and the solid-state image pickup element is accurately set at the time of manufacturing, so that the effect of suppressing false color signals and false signals is ensured.

[0011]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows a schematic configuration of an image pickup apparatus according to the first embodiment of the present invention. In the figure, an optical low-pass filter 2 is arranged between the imaging lens 1 and the solid-state imaging device 4, and its lattice forming surface 2a is directed to the solid-state imaging device 4 side. In the case of a color imaging device, the solid-state imaging device 4
The color filter array 6 is arranged in front of the.

As shown in FIG. 2, the solid-state image pickup device 4 has a rectangular flat plate shape, and a large number of image pickup surfaces 4a are arranged along a horizontal direction H and a vertical direction V which are longitudinal directions. A light detection element 7 is provided. This solid-state image sensor 4
Are housed and protected in the package 5 of FIG. The optical low-pass filter 2 is supported by a frame (not shown) of the image pickup device.

The optical low-pass filter 2 is provided with, for example, diffraction gratings extending in two orthogonal directions, and the diffraction directions by the respective diffraction gratings are the horizontal direction H and the vertical direction V of the solid-state image pickup device 4 of FIG. 2, respectively. It is set to form an angle of 45 degrees with respect to.

The thickness of the optical low pass filter 2 is
It is set so as to have a specific relationship with the plane size of the solid-state image sensor 4. Before explaining this relationship, first, the cause of occurrence of ring-shaped flare will be described with reference to FIG.

The solid-state image sensor 4 has an S constituting the surface.
The light reflectance of the surface is high because of Al used for i and wiring. Therefore, part of the light reaching the imaging surface 4a is
The light is reflected by the imaging surface 4a, diffracted by the diffraction grating of the optical low-pass filter 2, enters the optical low-pass filter 2, reaches the transparent substrate surface 2b of the optical low-pass filter, and is transmitted and reflected. The light reflected by the substrate surface 2b reaches the imaging surface 4a after being diffracted by the grating of the optical low-pass filter 2 again.

Of the light incident on the substrate surface 2b, light having an incident angle larger than the critical angle determined by the refractive index of air and the refractive index of the substrate causes total internal reflection, so that the light is incident on the substrate surface 2b below the critical angle. The intensity of the reflected light reaching the image pickup surface 4a is significantly higher than that of the incident light. As a result, this strongly reflected light appears as a ring flare in the image. FIG. 4 shows the spot light image 11 and the ring-shaped flare 12 on the imaging surface 4a.
Indicates. The ring-shaped flare 12 appears as a circle having a radius R around the spot light image.

In FIG. 3, the optical low-pass filter 2
And the imaging surface 4a are g, the thickness of the substrate is t, the refractive index of the substrate is n, and the critical angle at the substrate surface 2b is θc. Further, the distance from the center of the spot light image 11 in FIG. 4 to the maximum intensity portion of the ring flare 12 is defined as the radius R of the ring flare. Since the refractive index of air is 1, the critical angle θc is expressed by the following equation. θc = Sin ⁻¹ (1 / n) (1)

The radius R of the ring-shaped flare 12 is expressed by the following equation. R = a1 + b + a2 = g · tan (θ1) + 2 · t · tan (θc) + g · tan (θ2) (2) where a1, b, a2, θ1 and θ2 are respectively shown in FIG.
As shown in, the horizontal distance from the reflection point 13 on the image pickup surface 4a to the incident point 14 on the grating formation surface 2a, that is, the distance measured in parallel with the solid-state image sensor 4, from the incident point 14 to the exit point 15 The horizontal distance is the horizontal distance from the exit point 15 to the entrance point 16 on the imaging surface 4a.

When the distance g is sufficiently smaller than the thickness t, a1 and a2 are sufficiently smaller than b.
The radius R of the ring flare 12 is given by the following equation. R = b = 2 · t · tan (θc) (3)

The inventors set the distance g between the grating forming surface 2a and the image pickup surface to be 50 μm, the thickness t of the substrate and the refractive index n.
Was changed and the ring-shaped flare 12 generated was observed by the image pickup apparatus using the 1/2 inch solid-state image pickup element 4.
Table 1 shows the observation results of the thickness t of the substrate and the generated ring-shaped flare 12 when the glass having the refractive index of 1.52 was used as the substrate.

[0021]

[Table 1]

Table 2 shows the substrate thickness and the observation results of the ring-shaped flare 12 when the glass having a refractive index of 1.81 was used as the substrate.

[0023]

[Table 2]

As a result of examining these relationships, the following was found. When the radius R of the ring-shaped flare 12 in FIG. 4 is larger than the diagonal length of the solid-state image pickup device 4, the ring-shaped flare 12 does not appear on the image pickup surface 4a at all, so there is no problem (see Tables 1 and 2). ○). Also, the ring-shaped flare 12
If only one appears on the imaging surface 4a, the radius R thereof is sufficiently large to reduce the light energy per unit area of the ring-shaped flare 12, that is, the degree of concentration of light is reduced, and thus the ring-shaped flare 12 is reduced. The flare 12 becomes inconspicuous and there is no problem in practical use (marked with Δ in Table 1).

The range in which one ring-shaped flare 12 is allowed to appear is expressed by the following equation.
Here, the length of the solid-state imaging device 4 in the longitudinal direction (horizontal direction) H is x. R ≧ x / 2 That is, 4 · t · tan (Sin ⁻¹ (1 / n)) ≧ x (4) In the NTSC system, the horizontal-vertical ratio of the imaging surface 4a is 4: 3, and the horizontal direction is X is longer than the vertical length y.

Preferably, the ring flare 1 is satisfied by satisfying the following condition, that is, by making the radius R larger than the length x in the longitudinal direction H of the image pickup surface 4a.
2 can be prevented from occurring, and the deterioration of image quality can be effectively suppressed. R ≧ x 2 · t · tan (Sin ⁻¹ (1 / n)) ≧ x (5)

Further, by making the radius R larger than the length (x ² + y ² ) ^1/2 of the diagonal line of the imaging surface 4a, the appearance of the ring-shaped flare 12 can be completely prevented as described above. The deterioration of image quality can be suppressed more effectively. That is, 2 · t · tan (Sin ⁻¹ (1 / n)) ≧ (x ² + y ² ) ^1/2 (6)

Further, as shown in FIG. 2, there is a portion called "optical black" having a high reflectance of light on the outer periphery of the image pickup surface 4a, and stray light reflected by the portion of the optical black 8 also causes ring-shaped flare. It is a factor. Therefore, more desirably, it is possible to prevent the occurrence of ring-shaped flare by satisfying the following conditions. 2 · t · tan (Sin ⁻¹ (1 / n)) ≧ (p ² + q ² ) ^1/2 (7) Here, the horizontal length of the imaging surface 4a including the optical black 8 is p , And the vertical length is q.

In fact, in Tables 1 and 2, the Δ mark of the observation results indicates that the thickness t satisfies the above formula (4) or (5), and the ○ mark satisfies the formula (6) or (7). The cross mark does not satisfy any of the expressions (4) to (7). Here, the parameters used to calculate whether or not the thickness t in Tables 1 and 2 satisfies the equations (4) to (7) are as follows. x = 6.4 mm, y = 4.8 mm, (x ² + y ² ).
^1/2 = 8 mm, (p ² + q ² ) ^1/2 = 10 mm

Further, the minimum value of the thickness t satisfying each of the equations (4), (6) and (7) was calculated using the above parameters, and it was as follows. When refractive index n = 1.52, when n = 1.81 Formula (4): 1.79 mm 2.41 mm Formula (6): 4.58 mm 6.03 mm Formula (7): 5.72 mm 7.54 mm

Therefore, in the present invention, the thickness t of the optical low-pass filter 2 shown in FIG. 3 is set to the conditions (4) to (4) to the plane dimensions x, y, p, q of the image pickup surface 4a of the solid-state image pickup device 4. Set to satisfy any of (7),
The ring flare 12 is suppressed.

Further, the present inventors have found that the surface of the optical low-pass filter 2 on which the grating is not formed, that is,
It has been discovered that the strength of the ring-shaped flare is reduced by providing an antireflection film on the surface 2b opposite to the lattice formation surface 2a. As a result of studying this, in the case where the antireflection film is provided, the light of the ring flare 12 is concentrated even if the radius R becomes small to the extent that two ring flares 12 appear on the imaging surface 4a. It was found that the ring-shaped flare 12 did not pose a practical problem. This relationship is expressed as follows. 8 · t · tan (Sin ⁻¹ (1 / n)) ≧ x (8)

Therefore, in the present invention, when the above-mentioned reflective film is provided, the thickness t of the optical low-pass filter 2 in FIG. 3 is set with respect to the plane dimensions x, y, p, q of the image pickup surface 4a of the solid-state image pickup device 4. The ring flare 12 is suppressed by setting the condition (8) to be satisfied.

FIG. 5 shows a second embodiment of the present invention in which a solid-state image pickup device 4 with a color filter array 6 and an optical low-pass filter 2 installed in a package 5 have a rectangular frame type spacer 8. They are integrated with each other via the adhesive 9. The spacer 8 is formed by patterning a photosensitive resin such as polyethylene terephthalate on the surface of a base material such as glass or metal. The positional relationship between the optical low-pass filter 2 and other elements including the solid-state image sensor 4 is important in order to exert the false color signal / fake signal suppressing function which is a feature of the optical low-pass filter 2. ,
According to the second embodiment, the spacer 8 accurately sets the relative positional relationship between the optical low-pass filter 2 and the image pickup surface 4a at the time of manufacturing. Therefore, the false color signal / false signal suppressing function is achieved. Is effectively exhibited.

FIG. 6 shows a third embodiment. In this example, an antireflection film 10 is provided on the surface 2b of the optical low pass filter 2 opposite to the grating forming surface 2a. This antireflection film 10 is formed, for example, from the bottom by SiO _X film, TiO ₂ film.
It has a three-layer structure in which the film and the MgF ₂ film are stacked in this order. The spacer 8 is not a frame shape but a flat plate shape, is formed by stacking a photosensitive resin film such as polyethylene terephthalate, and is composed of the solid-state image sensor 4 with the color filter array 6 and the optical low-pass filter 2. It is inserted in between and corresponds to the entire surface of the opposing surfaces of both 4 and 2.

The solid-state image pickup device 4 and the optical low-pass filter 2 may be integrated by screwing, bonding, encapsulation, or integral molding. Unlike the frame type or the film type, the spacer 8 may be one obtained by photo-patterning a photosensitive resin on one or both of the solid-state image sensor 4 and the optical low-pass filter 2. Further, a spacer is formed by including beads, rods, etc. having a certain diameter in the adhesive, and the solid-state imaging device 4 is formed by these beads, rods, etc.
The structure may be such that the distance between the optical low-pass filter 2 and the optical low-pass filter 2 is accurately maintained.

[0037]

As described above, according to the optical low-pass filter of the present invention, the deterioration of the image quality due to the ring-shaped flare can be eliminated while effectively blocking the false signal and the false color signal. Further, according to the image pickup device of the present invention, since the relative positional relationship between the optical low-pass filter and the solid-state image pickup device is accurately set at the time of manufacturing, the generation of false color signals and false signals is effectively suppressed. It

[Brief description of drawings]

FIG. 1 is a schematic configuration diagram showing an image pickup apparatus according to a first embodiment of the present invention.

FIG. 2 is a perspective view showing a solid-state imaging device of the same embodiment.

FIG. 3 is a side view showing an optical low-pass filter and a solid-state image sensor according to the same embodiment.

FIG. 4 is a front view showing an image pickup surface of the solid-state image pickup element of the embodiment.

FIG. 5 is a schematic configuration diagram showing an imaging device according to a second embodiment of the present invention.

FIG. 6 is a schematic configuration diagram showing an image pickup apparatus according to a third embodiment of the present invention.

FIG. 7 is a schematic configuration diagram showing a conventional imaging device.

[Explanation of symbols]

1 ... Imaging lens, 2 ... Optical low-pass filter, 2a ...
Lattice forming surface, 2b ... Surface opposite to the grating forming surface, 4 ... Solid-state image sensor, 5 ... Package, 6 ... Color filter array,
8 ... Spacer, 9 ... Adhesive, 10 ... Antireflection film.

Claims (3)

[Claims] 1. An optical system in which a diffraction grating is formed on the surface of a light-transmissive substrate, the diffraction grating is arranged between an imaging lens and a solid-state image sensor, and the grating formation surface faces the solid-state image sensor side. A low-pass filter, characterized in that the thickness t of the substrate satisfies the following conditions, where n is the refractive index of the substrate and x is the length in the longitudinal direction of the imaging surface of the solid-state imaging device. Optical low-pass filter. 4 · t · tan (Sin ⁻¹ (1 / n)) ≧ x 2. An optical system in which a diffraction grating is formed on the surface of a light-transmissive substrate, the diffraction grating is arranged between an imaging lens and a solid-state image sensor, and the grating formation surface is directed to the solid-state image sensor side. A low-pass filter, wherein an antireflection film composed of one or more layers is formed on the surface of the substrate opposite to the grating formation surface, the refractive index of the substrate is n, and the imaging surface of the solid-state imaging device is An optical low-pass filter characterized in that the thickness t of the substrate satisfies the following condition, where x is the length in the longitudinal direction. 8 · t · tan (Sin ⁻¹ (1 / n)) ≧ x 3. The optical low-pass filter according to claim 1 or 2, wherein the optical low-pass filter and the solid-state imaging device are integrated with a spacer interposed therebetween. Characteristic imaging device.