JP3125557B2 - Imaging device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an imaging device such as an electronic camera or a television camera.

2. Description of the Related Art A solid-state image sensor has sensitivity not only in the visible region but also in the near-infrared region. To obtain accurate color reproduction for visible light, it is necessary to remove infrared light reaching the image sensor. There is. For this reason, a configuration has conventionally been provided for removing infrared light in the optical path leading to the image sensor. For example, FIG. 9 shows a configuration in which a color glass 17 that absorbs infrared light is provided between the imaging optical system 1 and the image sensor 4. As shown in FIG. 13, the transmission spectral characteristics of the colored glass reduce the transmittance not only in the near-infrared region but also on the long wavelength side in the visible region, and the color reproducibility deteriorates. is there.
Therefore, as another method, there is a method using a dichroic film that reflects or transmits a specific wavelength range. In the removal of infrared light by the dichroic film, as shown in FIG. 14, the transmittance in the near infrared region can be remarkably suppressed without substantially decreasing the transmittance in the visible region. As a specific configuration, FIG. 10 shows a configuration in which a dichroic film 18 that reflects infrared light and transmits visible light is coated on a cover glass of the image sensor 4 (see Japanese Patent Application Laid-Open No. 5-207350).
FIG. 11 shows a configuration in which the same dichroic film 19 as that of FIG. 10 is combined with a transparent member 20 and arranged to be inclined with respect to the optical axis (see Japanese Utility Model Laid-Open No. 64-16771). FIG. 12 shows a configuration in which a dichroic film that transmits infrared light and reflects visible light is used as the reflecting mirror 22 to guide only visible light to the image sensor 4.

However, FIG.
In the dichroic film, a part of the light of a certain single wavelength is transmitted and the rest is reflected, as is remarkable in the vicinity of the wavelength of 670 nm in the above. Therefore, in the configuration shown in FIG. 10, a light beam once reflected by the dichroic film is reflected by another optical element such as an imaging optical system, reaches the dichroic film again, and a part of the light passes through to generate a ghost. Let me do it. Further, the spectral characteristics of the dichroic film as an interference filter have an incident angle dependency, and the half-wavelength position and the transmittance change depending on the incident angle. Therefore, in consideration of ghost prevention, FIG.
In the case of adopting the configuration as in 2, the spectral sensitivity changes in a specific direction in the imaging plane, so that a certain kind of shading occurs and the reproducibility may be reduced. This problem can be solved by using a special imaging optical system (generally called a telecentric optical system) in which the principal ray is parallel to the optical axis in the entire area of the imaging screen, but the imaging optical system is large and expensive. Not only that, it is not applicable to all types of optical systems. In addition, in order to obtain desired spectral characteristics only with a single dichroic film, it is necessary to secure a certain degree of transmittance in the visible region to the image sensor and to severely suppress the light arrival rate in the near infrared region. Yes, this makes it difficult to produce a dichroic film. Accordingly, an object of the present invention is to provide an imaging apparatus employing an optical system capable of obtaining desired spectral characteristics while preventing generation of ghost and shading.

According to the present invention, an image pickup apparatus for projecting an image on an image pickup device by an image forming optical system has a reflection spectral characteristic for removing a light beam in a predetermined wavelength range, and has an optical path. A first dichroic film that bends obliquely, and a second dichroic film that has the same reflection spectral characteristics as the first dichroic film and that obliquely bends the optical path from the first dichroic film, The angle formed between the optical axis of the optical system and the first dichroic film is equal to the angle formed between the optical axis bent by the first dichroic film and the second dichroic film. It is characterized by.

The incident angle of the light beam not parallel to the optical axis on the first dichroic film and the incident angle on the second dichroic film are
As compared with the incident angle of the light beam parallel to the optical axis, the angle is increased or decreased by the value of the angle between the light beam and the optical axis. Thereby, the reflection spectral characteristics of the light beam not parallel to the optical axis at the first dichroic film and the reflection spectral characteristics at the second dichroic film are opposite to each other based on the reflection spectral characteristics of the light beam parallel to the optical axis. Deviate. The spectral characteristic of the light beam that reaches the image sensor is 2
Since the product is the product of the spectral characteristics at each reflection, the reflection spectral characteristics of the two light beams which are swung in the opposite direction by the same angle with respect to the optical axis become equal, and in addition, these light beams and the light beam parallel to the optical axis Are smaller than when a single dichroic film is used. Furthermore, since the spectral characteristic of the light beam that reaches the image sensor is the product of the spectral characteristics of the two reflections, the spectral characteristic is substantially the square of the reflective spectral characteristic of one dichroic film, and the light of a predetermined wavelength range is transmitted to the image sensor. The reach is greatly reduced.

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a diagram showing the configuration of the first embodiment. In the figure, 1 is an imaging optical system, 2 is a first reflecting mirror, 3 is a second reflecting mirror, and 4 is a solid-state imaging device. The first reflecting mirror 2 and the second reflecting mirror 3 are provided with a dichroic film, which have the same reflection spectral characteristics for removing infrared light. In projecting an image on the image pickup device 4 by the imaging optical system 1, the first reflecting mirror 2 bends the optical path to the image pickup device 4 obliquely, and the second reflecting mirror 3 obliquely projects the optical path from the first reflecting mirror 2. Bend to The angle 2a between the optical axis of the imaging optical system 1 and the first reflecting mirror 2 and the angle 3a between the optical axis bent by the first reflecting mirror 2 and the second reflecting mirror 3 are configured to be equal. I have. The angle dependence of the dichroic film, which is an interference filter, increases as the incident angle increases, and in consideration of the convenience of component arrangement, both the first and second reflecting mirrors of the imaging optical system have incident angles of light rays parallel to the optical axis. Is 22.5 degrees. Note that a birefringent plate or the like may be disposed between the imaging optical system 1 and the imaging device 4, but is omitted in this embodiment. The dichroic film used as the reflecting surface of the reflecting mirror in the present embodiment is an interference filter for cutting infrared light, and is generally called a cold mirror. This dichroic film is made of glass (refractive index 1.5
2) TiO2 (refractive index: 2.30) on odd-numbered layers from the side
Is a 23-layer optical multilayer film using MgF2 (refractive index: 1.38) for even-numbered layers. The optical thickness of each layer is as follows (λ is a design wavelength). 1st layer: 0.5λ / 4 2nd to 12th layers: λ / 4 13th layer: 1.19λ / 4 14th to 22nd layers: 1.38λ / 4 23rd layer: 0.69λ / 4 With the above configuration, the visible light converged by the imaging optical system 1 is reflected by the first reflecting mirror 2 and the second reflecting mirror 3 and reaches the image sensor 4. On the other hand, most of the infrared light is 5
The infrared light transmitted as shown by a and reflected by the first reflecting mirror 2 is also largely transmitted by the second reflecting mirror 3 as shown by 5b, and hardly reaches the image sensor 4. FIG. 2 is an optically developed view of the configuration of the first embodiment. 6a is a light beam parallel to the optical axis, and represents a light beam which forms an image at the center of the optical axis on the image forming plane;
Reference numerals 6b and 6c denote light beams that make an angle of 5 degrees with the optical axis, and represent light beams that are imaged at two points that are equally spaced in opposite directions from the center of the optical axis on the imaging plane. FIG. 3 is a view showing the reflection spectral characteristics of the first reflecting mirror 2 and the second reflecting mirror 3 in the first embodiment.
In the figure, 9 is an incident angle of 17.5 degrees, 10 is an incident angle of 2
2.5 degrees and 11 indicate reflection spectral characteristics at an incident angle of 27.5 degrees. FIG. 4 is a graph obtained by multiplying the reflection spectral characteristics of FIG. 3 by two, and shows a total reflection spectral characteristic in two reflections, which is a spectral characteristic of a light beam reaching the image sensor. In the figure, reference numeral 12 denotes the square of the spectral characteristic 10, 1
Reference numeral 3 denotes a product of the spectral characteristics 9 and the spectral characteristics 11. Returning to FIG. 2, the light ray 6a is incident on the first reflecting mirror 2 and the second reflecting mirror 3 at incident angles of 7a and 8a, respectively, which are both 22.5 degrees. Therefore, the first light ray 6a
The reflection spectral characteristics of the reflecting mirror 2 and the second reflecting mirror 3 are both indicated by reference numeral 10 in FIG. 3, and the total reflection spectral characteristics of these two reflecting mirrors are indicated by reference numeral 12 in FIG. . Next, regarding the light beam 6b, the first
The incident angle 7b on the reflecting mirror 2 is 27.5 degrees,
The incident angle 8b on the reflecting mirror 3 is 17.5 degrees. Therefore, the reflection spectral characteristics of the first reflecting mirror 2 and the second reflecting mirror 3 with respect to the light beam 6b are as shown by 11 and 9 in FIG. 3, respectively. Is indicated by 13. Further, regarding the light ray 6c, the incident angle 7c at the first reflecting mirror 2 is 17.5 degrees, and the incident angle 8c at the second reflecting mirror 3 is 27.5 degrees. Therefore, ray 6
The reflection spectral characteristics of the first reflecting mirror 2 and the second reflecting mirror 3 with respect to c are as shown by 9 and 11 in FIG. 3, respectively, and the total reflection spectral characteristic by these two reflecting mirrors is 13 in FIG. As shown, it is equal to the case of the light ray 6b. Reflection spectral characteristics 12 and 1 in FIG.
3 is much smaller than the difference between the reflection spectral characteristics 9 and 10 or the difference between the reflection spectral characteristics 10 and 11 in FIG. That is, not only does the spectral sensitivity of the light beam 6b and the light beam 6c having a certain angle in the opposite direction to the optical axis become equal, but also the spectral sensitivity of the light beam 6a which is a light beam parallel to the optical axis. Are also reduced compared to using a single dichroic membrane. Further, reflection spectral characteristics 12 and 13
In any of the above, the reflectance in the near-infrared region is much lower than the reflection spectral characteristics 9, 10, and 11, and the arrival rate of infrared light to the image sensor is lower than that of a single dichroic film. Since the reflecting mirrors in this embodiment are all flat and the imaging optical system is axially symmetric, the angle of incidence of the light beam on the reflecting mirror is also 0 ° in the direction perpendicular to the direction shown here. You will touch 5 degrees, but from 17.5 degrees 2
Since the incident angle itself is smaller than the 7.5 degree deflection, the influence of the angle dependence is negligibly small. FIG. 5 is a diagram showing the configuration of the second embodiment. The second embodiment differs from the first embodiment in that both the first reflecting mirror 2 and the second reflecting mirror 3 are arranged closer to the subject than the imaging optical system 1. The other points are the same as in the first embodiment. This second embodiment is advantageous when the distance from the imaging optical system 1 to the imaging plane cannot be maintained. FIG. 6 is a diagram showing the configuration of the third embodiment. In the third embodiment, the imaging optical system 1 is divided into two lens systems 1a and 1b, and the first reflecting mirror 2
The second embodiment is different from the first embodiment in that both the second reflecting mirror 3 and the second reflecting mirror 3 are disposed between the lens system 1a and the lens system 1b.
The other points are the same as in the first embodiment. This third embodiment is advantageous when there is an air gap between the lens groups of the imaging optical system, and contributes to the overall miniaturization. The first
When an optical element having a refraction function such as a lens is inserted between the reflecting mirror 2 and the second reflecting mirror 3, the light beam 6b in the first embodiment is obtained.
Since the relationship between the incident angle of the light beam 6c and each of the reflecting mirrors is broken, a sufficient effect may not be obtained. Therefore, it is preferable not to dispose an optical element having a refracting action such as a lens between the first reflecting mirror 2 and the second reflecting mirror 3. FIG. 7 is a diagram showing the configuration of the fourth embodiment. Infrared light absorbing members 14a and 14b for absorbing infrared light are provided on the back surfaces of the first reflecting mirror 2 and the second reflecting mirror 3. The other points are the same as in the first embodiment. These members 14
The members 14a and 14b may be provided directly on the back surface of the reflecting mirror or provided with an anti-reflection film on the back surface of the reflecting mirror.
May be provided separately from the reflecting mirror. By these infrared light absorbing members 14a and 14b, it is possible to prevent the infrared light from being reflected on the back surface of the reflecting mirror or from being reflected by other components in the image pickup device and becoming harmful light. FIG. 8 is a diagram showing the configuration of the fifth embodiment. In the fifth embodiment, the first
Instead of the first reflecting mirror 2 and the second reflecting mirror 3 in the embodiment, a prism 15 made of a transparent material such as optical glass or optical plastic is used. A dichroic film similar to that of the first embodiment is provided on the reflecting surfaces 16a and 16b of the prism 15. According to the fifth embodiment, the optical position accuracy of the two reflecting surfaces can be increased, and the optical length can be longer than the actual size. Note that the third
Providing an infrared light absorbing member on the back side of the reflecting mirror as in the embodiment or using a prism instead of the reflecting mirror as in the fifth embodiment is different from the other embodiments other than the first embodiment. May be applied. In each of the above embodiments, the dichroic film that cuts the infrared light is used. However, the wavelength range removed by the dichroic film is not limited to this, and can be freely selected depending on the application. In this case, a dichroic film having reflection spectral characteristics for removing light in a predetermined wavelength range may be used. Further, a two-dimensional image sensor such as a CCD area sensor is used as an image sensor in each of the above embodiments. However, a one-dimensional image sensor such as a CCD line sensor is used, and the image sensor itself is moved in a scanning manner or in an optical path. A configuration in which a scanning member is provided to perform image scanning may be employed.

As described above, according to the present invention, the occurrence of shading due to the angle dependency of the dichroic film is suppressed while the occurrence of ghost is prevented, and the arrival rate of light rays in a predetermined wavelength range to the image sensor is reduced. It can be greatly reduced. In addition, it is possible to relax the requirements for the characteristics of the dichroic film. As a result, an inexpensive imaging device with good reproducibility is provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing a configuration of a first embodiment of the present invention.

FIG. 2 is an optically developed view of the configuration of the first embodiment.

FIG. 3 is a diagram showing reflection spectral characteristics of a dichroic film in the first embodiment.

FIG. 4 is a diagram showing a total reflection spectral characteristic by two reflections in the first embodiment.

FIG. 5 is a diagram showing a configuration of a second embodiment of the present invention.

FIG. 6 is a diagram showing a configuration of a third embodiment of the present invention.

FIG. 7 is a diagram showing a configuration of a fourth embodiment of the present invention.

FIG. 8 is a diagram showing a configuration of a fifth embodiment of the present invention.

FIG. 9 is a diagram showing a configuration of a first related art.

FIG. 10 is a diagram showing a configuration of a second conventional technique.

FIG. 11 is a diagram showing a configuration of a third conventional technique.

FIG. 12 is a diagram showing a configuration of a fourth conventional technique.

FIG. 13 is a diagram showing transmission spectral characteristics of a colored glass according to the first conventional technique.

FIG. 14 is a diagram showing transmission spectral characteristics of dichroic films in the second and third conventional techniques.

[Explanation of symbols]

 REFERENCE SIGNS LIST 1 imaging optical system 2 first reflecting mirror 3 second reflecting mirror 4 image sensor 14 a, 14 b infrared light absorbing member

Claims (2)

(57) [Claims] 1. An image pickup apparatus for projecting an image on an image pickup device by an image forming optical system, comprising: a first dichroic film having a reflection spectral characteristic for removing a light beam in a predetermined wavelength range and bending an optical path obliquely. And a second dichroic film having reflection spectral characteristics equal to those of the first dichroic film and obliquely bending an optical path from the first dichroic film; and an optical axis of the imaging optical system and the second dichroic film. An imaging apparatus, wherein an angle formed by the first dichroic film is equal to an angle formed by the optical axis bent by the first dichroic film and the second dichroic film. 2. The device according to claim 1, wherein a member that absorbs light transmitted through the first and second dichroic films is disposed on the back side of the first and second dichroic films. An imaging device according to any one of the preceding claims.