Ghost reducing device, imaging device including same, ghost reducing method, and imaging optical system

A ghost reducing device and an imaging device provided with it, a ghost reducing method, and an imaging optical system that can effectively reduce ghosts while having a simple and inexpensive configuration. A ghost reducing device includes a douser that has at least one opening that lets light pass through, and that totally blocks light in a symmetrical position of the opening with respect to an optical axis of an imaging optical system. The douser is disposed in the vicinity of a pupil position of the imaging optical system.

TECHNICAL FIELD

This invention relates to a ghost reducing device and an imaging device provided with it, a ghost reducing method, and an imaging optical system that can reduce ghosts caused by high-luminance subjects such as the sun.

BACKGROUND ART

It has conventionally been known that when light from a high-luminance subject such as the sun enters an imaging optical system, it is reflected by the surface of a lens, the inner surface of a barrel, etc., and becomes stray light. Because such stray light becomes a cause of forming unintended images (ghosts) within an imaging screen, technologies for preventing stray light have been proposed to reduce the ghosts.

For example, disclosed in Japanese Unexamined Patent Application No. H10-62669 is a technology to reduce ghosts by changing the operating size of an aperture according to the effective luminous flux that changes based on an aperture value (Patent Document 1). Also, proposed in Japanese Unexamined Patent Application No. H08-334725 is a technology to reduce harmful light by restricting a diaphragm within an aperture value that is smaller than its open value. Furthermore, proposed is a method to suppress ghosts by reducing reflectance by coating a lens surface (Patent Document 2).

PRIOR ART DOCUMENTS

Patent Documents

SUMMARY OF THE INVENTION

Subject(s) to be Solved by Invention

However, in such conventional ghost reduction technologies as mentioned above, there is a drawback that there is a case where stray light still cannot be totally removed, generating ghosts. Also, in the inventions according to Patent Documents 1 and 2, there is also a drawback that the device becomes more complex and costly because the aperture size and diaphragm need to be controlled.

This invention has been made in order to solve such drawbacks as these, and its objective is to offer a ghost reducing device and an imaging device provided with it, a ghost reducing method, and an imaging optical system that can effectively reduce ghosts while having a simple and inexpensive configuration.

Means to Solve the Subject(s)

A ghost reducing device of the invention comprises a douser that has at least one opening that lets light pass through, and that totally blocks light in a symmetrical position of the opening with respect to an optical axis of an imaging optical system, wherein the douser is disposed in the vicinity of a pupil position of the imaging optical system.

As another embodiment of the invention, the opening may be formed in only one position on one side of a line perpendicular to the optical axis.

As another embodiment of the invention, the opening may be formed in one circular shape, which centers on a position offset from the optical axis.

As another embodiment of the invention, it may includes an advance/retreat drive means that causes the douser to advance or retreat relative to the imaging optical system, a light detection means that detects an intensity of light, and a control means that monitors the intensity of light detected by the light detection means and controls the advance/retreat drive means so as to dispose the douser advanced toward the imaging optical system if the intensity of light is no lower than a prescribed threshold value or to retreat the douser from the imaging optical system if the intensity of light is lower than the prescribed threshold value.

An imaging device of the invention is provided with the ghost reducing device discussed above.

In a ghost reducing method of the invention, a douser having at least one opening that lets light pass through and totally blocks light in a symmetrical position of the opening with respect to an optical axis of an imaging optical system is disposed in the vicinity of a pupil position of the imaging optical system, and reflected light from an imaging element is blocked by the douser.

An imaging optical system of the invention that forms an image of light from a subject through a lens onto an imaging element includes a douser that has at least one opening that lets light pass through, and that totally blocks light in a symmetrical position of the opening with respect to an optical axis of the imaging optical system, wherein the douser is disposed in the vicinity of a pupil position of the imaging optical system.

Advantage of the Invention

This invention can effectively reduce ghosts in spite of having a simple and inexpensive configuration.

DETAILED DESCRIPTIONS OF PREFERRED EMBODIMENT(S)

First, as a result of their diligent research and trials and errors, inventors of this application discovered that reflection on the imaging surface of a photographic film or an image sensor was greater than reflection on a lens surface for light from a high-luminance subject such as the sun. Then, they discovered that unnecessary light reflected on the imaging surface is reflected by the lens and becomes return light, becoming a major cause to generate a ghost as shown inFIG. 11.

Then, the inventors of this application conceived an idea of effectively reducing the ghost by suppressing the unnecessary light reflected on the imaging surface so that it will not return to the imaging surface, and came to complete this invention. Explained below referring to drawings is the first embodiment of the ghost reducing device and the imaging device provided with it, the ghost reducing method, and the imaging optical system of this invention.

FIG. 1is a diagram showing an imaging device10provided with an imaging optical system2containing a ghost reducing device1A of this first embodiment. As shown inFIG. 1, the imaging device10of this first embodiment is mainly configured of the imaging optical system2for imaging a subject, and an imaging element3disposed in the focal position of this imaging optical system2. Then, by disposing the ghost reducing device1A in the vicinity of the pupil position of the imaging optical system2, return light reflected by the surface of the imaging element3is blocked, reducing ghosts. Below, their configurations are explained.

The imaging optical system2forms an image of light from a subject onto the imaging element3through a lens. In this first embodiment, as shown inFIG. 1, the imaging optical system2has a front lens group21disposed in the front of the pupil position, a diaphragm22disposed in the pupil position, the ghost reducing device1A disposed in the immediate rear of the pupil position, and a rear lens group23disposed in the rear of the pupil position.

The front lens group21and the rear lens group23are each configured of at least one lens and form an image of light from a subject onto the imaging element3. Note that although the imaging optical system2has the front lens group21and the rear lens group23in this first embodiment, this invention is not limited to this configuration but can effectively apply to the imaging optical system2having at least the front lens group21.

The diaphragm22is disposed in the pupil position of the imaging optical system2for adjusting the amount of passing light. In this first embodiment, as shown inFIG. 2, the diaphragm22is configured of an iris diaphragm that can increase or decrease the size of an aperture without changing the center position of the aperture. Note that although the imaging optical system2having the diaphragm22is used in this first embodiment, this invention is not limited to this configuration but is also applicable to the imaging optical system2having no diaphragm22.

The ghost reducing device1A is for blocking return light from the imaging element3and reducing ghosts. In this first embodiment, the ghost reducing device1A is configured of a douser11made of a black material having low reflectance. Also, as shown inFIG. 3, the douser11has an opening11athat is formed in a semicircular shape with approximately the same diameter as the aperture of the diaphragm22and lets light pass through. Then, as shown inFIG. 4, it is installed in the immediate rear of the diaphragm22so that the center of the opening11acoincides with the optical axis. Thereby, the douser11has the opening11adisposed on only one side of a line perpendicular to the optical axis to let light pass through and block light on the other side.

Note that although in this first embodiment, the ghost reducing device1A is disposed in the immediate rear of the diaphragm22because the diaphragm22is installed in the pupil position, this invention is not limited to this configuration. In the case of the imaging optical system2and the imaging device10without the diaphragm22, the ghost reducing device1A can be disposed in the pupil position of the imaging optical system2. Also, the vicinity of the pupil position in this invention is a concept that is not limited to the pupil position but includes any position, such as the immediate rear of the pupil position, that is close to the pupil position within a range where the ghost reduction effect of this invention can be performed without generating vignetting.

The imaging element3is configured of a CCD (Charge-Coupled Device) image sensor or the like, photoelectrically converts brightness degrees of portions of an image formed on a light-receiving surface into the amounts of charge, and sequentially reads and converts them into electric signals. Note that although the imaging optical system2has the imaging element3in this first embodiment, this invention is not limited to this configuration but is also applicable to an imaging optical system having no imaging element3, such as a film camera.

Next, explained are actions by the ghost reducing device1A provided with the above-mentioned configuration and the imaging device10provided with it, the ghost reducing method, and the imaging optical system2in this first embodiment.

First, in this first embodiment, as shown inFIG. 1, the douser11as the ghost reducing device1A is disposed in the vicinity of the pupil position of the imaging optical system2. Because this douser11is simply a black plate, it has an extremely simple configuration and is inexpensive.

Next, in the imaging optical system2of this first embodiment, if a high-luminance subject such as the sun exists in its field of view, as shown inFIG. 1, the ghost reducing device1A in the vicinity of the pupil position restricts an incident luminous flux to light passing through the opening11aformed on the upper half of a line perpendicular to the optical axis. At this time, in this first embodiment, only one opening11ais formed on one side of a line perpendicular to the optical axis. Also, because the area of the opening11ais the maximum under the condition that the symmetrical position of the opening11awith respect to the optical axis is totally blocked, a sufficient amount of light can be secured.

Light that passed through the ghost reducing device1A forms an image on the surface of the imaging element3through the rear lens group23, and most of it is absorbed into the imaging element3. However, as shown inFIG. 1, part of the light is specularly reflected on the surface of the imaging element3and becomes return light. When it comes back to the vicinity of the pupil position again, as shown inFIG. 1, this return light reaches the symmetrical position to the position where it entered with respect to the optical axis.

That is, in this first embodiment, because the return light is derived from light that passed through the opening11ainstalled on the upper half of the douser11, all of it comes back to the lower half of the douser11in the vicinity of the pupil position. Thereby, the douser11totally blocks the return light in the vicinity of the pupil position and never returns it forward. Therefore, the ghost reducing device1A of this first embodiment effectively suppresses such a phenomenon as shown inFIG. 11that return light reaches the front lens group21and is reflected again by the front group lens21, generating a ghost image on the surface of the imaging element3.

Note that if the rear lens group23has many lenses, the probability that return light is reflected before reaching the douser11and returns to the imaging element3becomes high, therefore the number of removed ghosts is relatively small. However, if the front lens group21has many lenses, many ghosts (return light) that are targets of removal are generated, therefore the effect of the douser11that can block them is extremely great.

According to this first embodiment mentioned above, the following effects are achieved.

1. Ghosts can be effectively reduced in spite of its simple and inexpensive configuration.

2. Because ghosts are effectively reduced, easy-to-see images can be offered in surveillance cameras, automobile-mounted cameras, etc. that are demanded to photograph a certain region constantly.

3. Especially when the front lens group21has many lenses, a great ghost reduction effect is performed.

4. If an image recognition technology is applied to an image taken by the imaging device10as in a machine vision system, ghosts that become noise can be reduced, therefore the recognition accuracy can be improved.

Note that although in this first embodiment mentioned above, the douser11has one semicircular opening11aon only one side relative to a line perpendicular to the optical axis, this invention is not limited to this configuration. That is, the douser11only needs to have at least one opening11athat lets light pass through and block light totally in the symmetrical position to the opening11awith respect to the optical axis of the imaging optical system2.

For example, as shown inFIG. 5, three openings11a, each of which is formed in an arc shape with a central angle of 60 degrees, can be installed with 60-degree intervals around the optical axis. In this manner, even with the douser11having multiple openings11aon not only one side but also both sides of a line perpendicular to the optical axis, light is totally blocked in the symmetrical positions of the openings11awith respect to the optical axis of the imaging optical system2.

Also, as shown inFIG. 6, an opening11athat resembles one half of the yin-yang symbol can be formed on the douser11. In this manner, even if the douser11has one opening11aacross both sides of a line perpendicular to the optical axis, light is totally blocked in the symmetrical position to the opening11awith respect to the optical axis of the imaging optical system2.

Furthermore, as shown inFIG. 7, the douser11can have one opening11aformed in a circular shape centering on a position off the optical axis. By this kind of douser11, a natural blurred image in a circular shape can be obtained.

Also, as shown inFIG. 8, the douser11can have one opening11aformed in an elliptical shape centering on a position off the optical axis. By this kind of douser11, a sufficient amount of light can be secured compared with the opening11ainFIG. 7.

Next, explained is the second embodiment of the ghost reducing device and the imaging device provided with it, the ghost reducing method, and the imaging optical system of this invention. Note that components in the second embodiment that are identical or correspond to those in the first embodiment mentioned above are given the same reference numerals, and their repeated explanations are omitted.

A characteristic of this second embodiment is that a ghost reducing device1B is let function only when a high-luminance subject such as the sun enters its photographing field. Specifically, as shown inFIG. 9, the ghost reducing device1B of this second embodiment has an advance/retreat drive means12that lets the douser11advance or retreat relative to the imaging optical system2, a light detection means13that detects the intensity of light, and a control means14that controls the advance/retreat drive means12based on the detection result of this light detection means13.

The advance/retreat drive means12lets the douser11advance or retreat relative to the imaging optical system2. In this embodiment, the advance/retreat drive means12is configured of a solenoid actuator that lets the douser11move sliding along a direction perpendicular to the optical axis. However, not being limited to this configuration, it can be any mechanism that can let the douser11advance or retreat relative to the imaging optical system2.

The light detection means13detects the intensity of light entering the imaging optical system2. In this embodiment, the imaging element10is also used as the light detection means13. However, not being limited to this configuration, any light sensor such as one having built-in cadmium sulfide (CdS) cells can be adopted as the light detection means13as long as it can detect the intensity of light entering the imaging optical system2.

The control means14is configured of a CPU (Central Processing Unit) etc. and controls the advance/retreat drive means12based on the detection result of the light detection means13. In this second embodiment, if the intensity of light is no lower than a prescribed threshold value, the control means14disposes the douser11advanced toward the imaging optical system2. On the other hand, if the intensity of light is lower than the prescribed threshold value, the control means14controls the advance/retreat means12so as to let the douser11retreat from the imaging optical system2.

In the above configuration, the intensity of light that makes ghosts easy to occur is obtained in advance, and that intensity of light is set as the above-mentioned threshold value. Thereby, once a subject having such a high luminance as to generate ghosts enters the photographing field, the douser11advances toward the imaging optical system2and reduces the ghosts. On the other hand, if there is no subject having such a high luminance as to generate ghosts within the photographing field, the douser11retreats and secures a sufficient amount of light.

By the ghost reducing device1B and the imaging device10provided with it, the ghost reducing method, and the imaging optical system2of this second embodiment mentioned above, in addition to achieving the same actions and effects as the above-mentioned first embodiment, under the photographing condition where no ghost is generated, the douser11can automatically retreat to secure a sufficient amount of light.

Next, explained is a specific example of the ghost reducing device and the imaging device provided with it, the ghost reducing method, and the imaging optical system of this invention.

In this Example 1, it was presumed that the ghost reducing device1A, the imaging device10, and the imaging optical system2of the above-mentioned first embodiment were mounted on an automobile.

Specifically, as shown inFIG. 10, a vehicle-mounted camera as the imaging device10was installed facing the front in the vicinity of the upper end of the windshield. In this case, light from the sun that is a high-luminance subject would enter depending on time of the day and the orientation of the automobile.

In the above configuration, the ghost reducing device1A restricted the incident luminous flux of sunlight entering the imaging optical system2to only light that passes through the opening11aof the douser11. Also, the douser11blocked return light that was reflected by the surface of the imaging element3and returned to the vicinity of the pupil position. Therefore, even when the sun was within the photographing field, ghost occurrences were effectively suppressed in images of the vehicle-mounted camera.

According to this Example 1 above, it has been demonstrated that even in a vehicle-mounted camera that is demanded to photograph a certain region constantly, ghosts can be effectively reduced, and easy-to-see images can be offered.

Note that the ghost reducing device and the imaging device provided with it, the ghost reducing method, and the imaging optical system of this invention are not limited to the above-mentioned embodiments or example but can be changed as appropriate.

For example, in the above-mentioned embodiments, the imaging device10of this invention is applied to a digital video camera, and the imaging optical system2of this invention is applied to an imaging optical system built in the digital video camera. However, the scope of application of this invention is not limited to the above, but this invention can be widely applied to imaging optical systems built in various kinds of cameras such as digital still cameras, film cameras, and cameras built in smartphones and tablets, and imaging devices provided with these imaging optical systems.

DESCRIPTION OF REFERENCE NUMERALS