JPH01235920A - Image forming optical system for solid-state image pickup element - Google Patents

Abstract

PURPOSE:To miniaturize the title optical system by using a lens whose back- focal distance is short for the image forming optical system by providing an optical low-pass filter in front of the lens system or in its vicinity. CONSTITUTION:On an optical axis 16 in front of a front lens group 20, a diaphragm 24 is provided, and also, in its front, an optical low-pass filter 26 is provided. As for the low-pass filter 26, for instance, that which has dispersed minute grains of transparent materials whose refractive indexes are different into a plate of a transparent resin material is used. The low-pass filter 26 eliminates a component of a picture pattern having a spatial frequency being substantially higher than a spatial frequency of a segment array of a color separation filter 12 among incident light beams which are made incident on this filter 26 from a field on the optical axis 16. In such a way, a lens whose flange-focal distance is short, for instance, of a telephoto type, etc., can be used, and also, with regard to the optical low-pass filter, as well, that of a small shape can be used, therefore, the whole image forming optical system can be made small in size and light in weight.

Description

DETAILED DESCRIPTION OF THE INVENTION Technical Field The present invention relates to an imaging optical system for a camera, particularly a solid-state image sensor using a color separation filter.

BACKGROUND ART As is well known, some color solid-state imaging devices include color separation filters disposed on the surface of an imaging cell array in order to output color-separated video signals. In order to prevent false colors in the reproduced image caused by incident light representing a picture having a frequency higher than the spatial frequency of the segment array of the color separation filter, an optical low-pass filter removes such high-pass image incident light. A filter is placed in front of the image sensor.

- In conventional imaging optical systems, such as video cameras, for example, this low-pass filter is placed just in front of the image sensor. For this reason, optical lenses were designed to have a long flange pack and a low-pass filter disposed there. Therefore, the lens system had to have a large aperture and a long lens length, such as a retro type, which resulted in an increase in size.

Rule □Negative An object of the present invention is to eliminate such drawbacks of the prior art and provide a compact imaging optical system for a solid-state imaging device.

DISCLOSURE OF THE INVENTION According to the present invention, an optical low-pass filter is disposed before or in place of the lens system, thereby making it possible to use a lens with a short back focus in the imaging optical system. Therefore, in order to obtain the same brightness as before, a lens with a smaller lens diameter can be used.

According to the present invention, an imaging optical system for a solid-state imaging device that forms an image of a field on an imaging cell array of a color solid-state imaging device, in which a color separation filter is disposed on an imaging cell array, A lens is arranged on the axis and forms an image on the imaging cell array, and a lens is arranged on the optical axis opposite to the color solid-state image sensor with respect to the lens. and an optical filter that removes components of the image having a spatial frequency higher than the spatial frequency of the color filter segment array.

DESCRIPTION OF EMBODIMENTS Next, embodiments of an imaging optical system for a solid-body imaging device according to the present invention will be described in detail with reference to the accompanying drawings.

Referring to FIG. 1, an embodiment in which the present invention is applied to an imaging optical system of a camera having a color solid-state image sensor 10 is shown. The color solid-state image sensor 10 is a charge transfer device such as a COD, and its image sensor array 12
A color separation filter 14 is disposed on the surface of. The color separation filter 14 has a large number of color filter segments arranged to separate the light directed toward each imaging cell of the imaging cell array 12 into three separated colors. The figure conceptually shows these configurations in a somewhat exaggerated manner for convenience of display.

In front of the imaging cell array 12, there is a main optical axis 1 of the array 12.
In this embodiment, a zoom lens 18 having a magnification of approximately 1.5 times is disposed at 8-H. In this embodiment, the lens 18 is a concave-convex zoom lens consisting of two lens groups: a front lens group 20 and a rear lens group 22. Its basic structure
As shown in FIG. 3, the front lens group 20 is
The rear lens group 22 is movable back and forth in the direction B of the optical axis 16.
It can move forward and backward to provide a zoom function. The lens position shown in FIG. 1 is the wide-angle end state, and the lens position shown in FIG. 2 is the telephoto end state. In the following figures, elements similar to those shown in FIG. 1 are designated with the same reference numerals.

On the optical axis 16 in front of the front lens group 20, an aperture 24 is provided.
Furthermore, an optical low-pass filter 26 is arranged in front of it. The low-pass filter 26 is, for example, one in which fine particles of transparent materials having different refractive indexes are dispersed within a flat plate of a transparent resin material. The low-pass filter 28 removes image components having a spatial frequency substantially higher than the spatial frequency of the segment array of the color separation filter 12 from among the incident light incident on the optical axis 16 from the object field. In this embodiment, a filter 28 is placed in front of the zoom lens, and the frequency component to be removed changes on the imaging side as the focal length changes. However, in the case of this example,
The magnification is low and poses no practical problems. This prevents false colors from occurring in the reproduced image from the solid-state image sensor 10 due to the incident light representing such spatially high-frequency patterns.

In this embodiment, the low-pass filter 26 is disposed in front of the lens 18 in view of the fact that in the front part of the lens 18, the incident light from the object field is substantially parallel light. As a result, the lens 18 is equipped with a low-pass filter 26 as in the conventional case.
Therefore, it is possible to use a lens 18 with a short flange back.

Therefore, in order to obtain the same brightness as before, a lens with a smaller lens diameter can be used.

The low-pass filter 26 originally needs to be precisely aligned in the rotational direction with respect to the optical axis 16 with respect to the array of imaging cells of the imaging device 10. However, in this embodiment, since the low-pass filter 26 is disposed in front of the lens 18, its adjustment work is easy to perform, and high-precision mounting does not require any equipment. For example, this work can be performed while reproducing the video signal output from the image sensor 10 on a video monitor and viewing the reproduced image.

For comparison, an example of a conventional imaging optical system is shown in FIG. In this conventional example, a low-pass filter 26,
The aperture 24 and the zoom lens 50 are arranged in this order. The lens 50 has a zoom function of approximately 1.5 times, and thereafter the lens group 52 is movable back and forth in the direction A of the optical axis 16, thereby realizing the zoom function. In this conventional example, since the low-pass filter 26 is placed just in front of the image sensor 10, the lens 50 is designed to have a long flange pack. Therefore, the lens 50 must have a large aperture and a long focal length, which increases the size of the entire optical system.

In the embodiment of the present invention shown in FIG. 1, the design value example when realizing the same specifications of brightness and zoom magnification is shown in the first example.
Shown in the table. In this example, the brightness F is 3.5. Wide-angle end focal length fW is 19.895mm, telephoto end focal pressure @fT is 28
．． As shown in FIG.
It consists of a surface. The aperture 24 is arranged between the second and third surfaces or between the eighth and ninth surfaces. Aberration diagrams at the finite distance are shown in FIGS. 4 and 5. FIG. 4 shows a case where the lens position is at the wide-angle end, and FIG. 5 shows a case where the lens position is at the telephoto end.

As can be seen from the above, in this embodiment, the optical low-pass filter 26 is disposed in the afocal part of the lens system 20, for example, in the front, so that a lens with a short flange back can be used in the imaging optical system. . Therefore, with the same brightness as before, it is quite small.
．． '85 -18.447 0.34 8 8.8!37'1.3El 1.8051B
25. '57 15.974 1.82 8 -9.328 ]', 3? 1.72
43.79 -37.458 7. E144-2
82 10 -10.2,49 1.77 1.58144
40.811 -7.899 1.06 12 -64.103 0.80 +, 80500
．． 38.6E'' According to the present invention, the optical low-pass filter is disposed in the afocal part of the lens system, so a short flange puncture lens such as a telephoto lens can be used in the imaging optical system. It is also possible to use a small-sized optical low-pass filter. In addition, the exit pupil is longer and the influence of color shading is smaller. Therefore, the entire imaging optical system can be made smaller and lighter than before.

[Brief explanation of the drawing]

1 and 2 are explanatory diagrams showing an embodiment in which the present invention is applied to an imaging optical system of a camera having a color solid-state image sensor. Figure 2 shows the lens position at the telephoto end.
゛Figure 3 is an explanatory diagram showing the basic configuration of the embodiment shown in Figures 1 and 2. Figures 4 and 5 are the same brightness and zoom magnification specifications as the conventional example shown in Figure 6. 4 shows aberration diagrams at a finite distance of a design example when realized by applying the present invention, FIG. 4 shows the case where the lens position is at the wide-angle end state, and FIG. 5 shows the case where the lens position is at the telephoto end state, FIG. 6 is a diagram similar to FIG. 1, showing an example of an imaging optical system of a conventional solid-state image sensor camera. Explanation of symbols for main parts 10, Color solid-state image sensor 12, Imaging cell array 1B, Lens 2B, Optical low-pass filter Patent applicant Fuji Photo Film Co., Ltd. Fuji Photo Koki Co., Ltd. Number of persons Takao Sakiyama Takao

Claims (1)

[Scope of Claims] An imaging optical system for a solid-state imaging device that forms an image of a field on an imaging cell array of a color solid-state imaging device in which a color separation filter is disposed on the imaging cell array. The optical system includes a lens that is disposed on the optical axis of the imaging cell array and forms the image on the imaging cell array, and a lens that is disposed on the optical axis opposite to the color solid-state image sensor with respect to the lens. and an optical filter that removes a component of the image having a spatial frequency higher than a spatial frequency of a color filter segment array of the color separation filter from among the light incident from the object field. Imaging optical system for use.