Patent Application
20100053414
The present invention relates to a compound eye camera module for taking an image by a plurality of imaging optical lenses.
An imaging device such as a digital video camera or a digital camera forms an image of a subject on an imaging element such as a CCD, a CMOS or the like via a lens to convert the image of the subject into two-dimensional image information. Recently, cameras for obtaining a plurality of two-dimensional images of a subject using a plurality of lenses and measuring a distance to the subject based on the obtained image information have been proposed.
Patent Document 1 discloses an example of such a compound eye camera module for measuring a distance to the subject.
An image having parallax is obtained by a camera module having such a structure. Using the technique called “block matching”, a block which is most similar to an arbitrary block in a basic image 7-1 is searched for in a reference image 7-2 to calculate a parallax amount. Based on the parallax amount, a distance to the subject is calculated.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-143459
However, with the compound eye camera module disclosed in Patent Document 1, when the environmental temperature changes, the focal distance of each lens of the lens array or base line length, which is the distance between optical axes of the lenses, changes. As a result, the accuracy of the distance measurement is deteriorated. Patent Document 1 does not describe anything on how to solve this problem.
The present invention made to solve such a problem of the conventional art has an object of providing a compact and low-cost compound eye camera module which guarantees accurate distance measurement even when the environmental temperature changes.
A compound eye camera module according to the present invention includes a lens array including a plurality of lenses located on the same plane; an imaging section including a plurality of imaging areas on which a plurality of images of a subject formed by the plurality of lenses are projected in a one-to-one relationship, the imaging section converting each of the plurality of projected images into an electric signal; and an optical aperture section including a plurality of optical apertures corresponding to the plurality of lenses in a one-to-one relationship and located oppositely to the imaging section with respect to the lens array. A difference between a linear expansion coefficient of a material used to form the lens array and a linear expansion coefficient of a material used to form the optical aperture section has an absolute value of 0.7×10−5/° C. or less.
In a preferable embodiment, a difference between a linear expansion coefficient of a material used to form the lens array and a linear expansion coefficient of a material used to form the optical aperture section has an absolute value of 0.35×10−5/° C. or less.
In a preferable embodiment, a difference between a linear expansion coefficient of a material used to form the lens array and a linear expansion coefficient of a material used to form the optical aperture section has an absolute value of 0.2×10−5/° C. or less.
In a preferable embodiment, the optical aperture section includes hoods for restricting an angle of view.
In a preferable embodiment, the optical aperture section and the lens array are positioned with respect to each other in a state of contacting each other, such that the center of each of the optical apertures of the optical aperture section matches an optical axis of the corresponding lens of the lens array.
In a preferable embodiment, the optical aperture section has a structure in which positions of the plurality of optical apertures are independently adjustable.
In a preferable embodiment, the compound eye camera module further includes a lens barrel for supporting the optical aperture section and the imaging section. The lens array and the optical aperture section are fixed to each other by a first adhesive located symmetrically with respect to the center of the lens array in a plane vertical to the optical axes of the lenses. The lens barrel and the optical aperture section are fixed to each other by a second adhesive located symmetrically with respect to the center of the lens array in the plane vertical to the optical axes of the lenses.
A method for producing a compound eye camera module according to the present invention is for producing a compound eye camera module including a lens array including a plurality of lenses located on the same plane; an imaging section including a plurality of imaging areas on which a plurality of images of a subject formed by the plurality of lenses are projected in a one-to-one relationship, the imaging section converting each of the plurality of projected images into an electric signal; and an optical aperture section including a plurality of optical apertures corresponding to the plurality of lenses in a one-to-one relationship and located oppositely to the imaging section with respect to the lens array; wherein a difference between a linear expansion coefficient of a material used to form the lens array and a linear expansion coefficient of a material used to form the optical aperture section has an absolute value of 0.7×10−5/° C. or less. The method includes the step of binding together the optical aperture section and the lens module by a first adhesive in the state where a plane of the optical aperture section which is parallel to the optical axes of the lenses is in contact with a plane of the lens module which is parallel to the optical axes of the lenses such that the center of each optical aperture of the optical aperture section is located on the optical axis of the corresponding lens.
In a preferable embodiment, the lens array and the optical aperture section are fixed to each other by locating the first adhesive symmetrically with respect to the center of the lens array in a plane vertical to the optical axes of the lenses.
According to the present invention, the difference in the linear expansion coefficient between the material of the lens array and the material of the optical aperture section is set to 0.7×10−5/° C. or less. Owing to this, the decentration amount between the optical axes of the lenses and the centers of the optical apertures is suppressed from changing in accordance with the environmental temperature, although such a change is difficult to be corrected merely by considering the expansion amount or shrinkage amount of the materials used to form the compound eye camera module in accordance with the environmental temperature. Since the decentration amount is suppressed from changing, the change of the parallax amount can also be suppressed. Accordingly, high distance measurement accuracy can be maintained. The distance measurement accuracy can be improved even in a compact compound eye camera module having a short base line length.
a) shows the position of an adhesive for binding the lens array and the optical aperture module;
Hereinafter, one embodiment of a compound eye camera module according to the present invention will be described with reference to the drawings.
The lens array 4 includes two lenses 4a and 4b located on the same plane, and the lenses 4a and 4b are integrally formed of resin molding or the like. The optical aperture section 1 is located on the side of a subject with respect to the lens array 4. The optical aperture section 1 includes optical apertures 2a and 2b corresponding to the lenses 4a and 4b in a one-to-one relationship. The optical apertures 2a and 2b respectively have openings for restricting the amount of light incident on the lenses 4a and 4b. The lens array 4 and the optical aperture section 1 are positioned such that centers 2ap and 2bp of the optical apertures 2a and 2b respectively match the optical axes 4ap and 4bp of the lenses 4a and 4b. The lens array 4 and the optical aperture section 1 are bound together to form a unit. The expression that “the centers 2ap and 2bp respectively match optical axes 4ap and 4bp” means that the decentration amount of the centers 2ap and 2bp with respect to the optical axes 4ap and 4bp is generally 5 μm or less, in addition to being exactly 0 μm.
As shown in these figures, the optical aperture section 1 further includes hoods 3a and 3b for preventing light from being obliquely incident on the lenses 4a and 4b. Since the optical apertures 2a and 2b and the hoods 3a and 3b of the optical aperture section 1 are integrally formed, the number of elements is decreased to reduce the cost. The optical aperture section 1 is also integrally formed by resin molding or the like. As described hereinafter in detail, the difference between the linear expansion coefficient of a material used to form the lens array 4 and the linear expansion coefficient of a material used to form the optical aperture section 1 has an absolute value of 0.7×10−5/° C. or less.
As shown in
The imaging section 6 is located with respect to the lens array 4 such that two images of the subject formed by the lenses 4a and 4b are projected on the imaging areas 6a and 6b in a one-to-one relationship. The imaging section 6 is located on the opposite side to the optical aperture section 1 with respect to the lens array 4. A light shielding wall 8 is provided between the lens array 4 and the imaging section 6, between optical paths of the lenses 4a and 4b, in order to prevent each of the two images of the subject from being incident on the imaging area 6a or 6b not corresponding thereto.
Light from the subject passes the optical apertures 2a and 2b, is formed into images separately by the lenses 4a and 4b, and is projected on the imaging areas 6a and 6b. The imaging section 6 converts each of the images formed on the imaging areas 6a and 6b into an electric signal in accordance with the light intensity thereof. In order to transmit light of only a prescribed wavelength, an optical filter 7 may be provided between the lens array 4 and the imaging section 6. In order to prevent stray light from being incident on the imaging areas 6a and 6b, a light shielding film 9 may be provided in the vicinity of the optical filter 7.
The electric signals output from the imaging section 6 are subjected to image processing by means of various types of signal processing. For example, the parallax amount may be found using two images formed on the imaging areas 6a and 6b and to measure the distance to the subject. Such processing may be performed using a digital signal processor (not shown) or the like.
Now, with reference to
An image on the imaging area 6a is defined as a basic reference. The image on the imaging area 6a is divided into a plurality of pixel blocks, each including 32×32 pixels. An area correlated to one pixel block of the imaging area 6a is searched for and specified in the image on the imaging area 6b, which is a reference image. This is the so-called “block matching” technique. Based on the parallax between the one pixel block and the specified pixel block, the distance to the subject is calculated.
The distance from each of the lenses 4a and 4b to the subject is defined as L[mm]. It is assumed that the lenses 4a and 4b have the same optical characteristics, and the focal distance thereof is f[mm]. The base line length, which is the distance between the lenses 4a and 4b (the distance between the optical axes), is defined as D[mm]. The parallax amount, which is the relative deviation between the pixel block in the basic image and the pixel block calculated by block matching is defined as z[pixels]. The pixel pitch of the imaging element is defined as p[mm/pixel]. The distance L to the subject can be found by the following expression 1.
By using expression 1 as described above, the distance to the subject can be measured based on a pair of images taken.
According to the present invention, in order to maintain a high distance measuring accuracy even when the environmental temperature changes, the absolute value of the difference between the linear expansion coefficient of the material used to form the lens array 4 and the linear expansion coefficient of the material used to form the optical aperture section 1 is set to 0.7×10−5/° C. or less. Hereinafter, the reason for this will be described.
Regarding the compound eye camera module having the structure shown in
The base line length D or the like changes due to the change of the environmental temperature. The true base line length D after expanding or shrinking by the change of the environmental temperature can be estimated by detecting the environmental temperature as long as the linear expansion coefficient of the resin used to form the lens array 4 is known. Thus, an accurate distance to the subject corrected in consideration of the influence by the change of the environmental temperature can be easily calculated.
For example, in the case where the compound eye camera module is mounted on a vehicle, the environmental temperature is rarely constant and changes moment by moment. In order to accurately measure the distance to the subject in such a situation, it is important to correct the distance in accordance with the change of the environmental temperature as described above in order to accurately measure the distance to the subject.
Against an error of the distance caused by the volume change of the lens array 4 or the like, the distance can be corrected by detecting the change of the environmental temperature as described above. However, in the compound eye camera module, the influence of the change of the environmental temperature is not exerted only on the lens array 4. As a result of a detailed investigation of the present inventors, it was found that the deviation between the centers 2ap and 2bp of the optical apertures 2a and 2b and the optical axes 4ap and 4bp of the lenses 4a and 4b, namely, the decentration, increases the error included in the measured distance.
However, the decentration is not easily correctable merely by detecting the environmental temperature, for the following reason. When the decentration occurs between each center 2ap, 2bp of the optical aperture 2a, 2b and the optical axis 4ap, 4bp of the corresponding lens 4a, 4b, the parallax amount changes in accordance with the image height of the subject, and this change is not linear to the image height. Therefore, it is very difficult to correct the parallax amount in accordance with the image height. In addition, when the decentration amount changes between each center 2ap, 2bp of the optical aperture 2a, 2b and the optical axis 4ap, 4bp of the corresponding lens 4a, 4b due the change of the environmental temperature, the parallax amount further changes. This makes it more difficult to correct the parallax amount in accordance with the environmental temperature or the image height.
Hereinafter, the result of investigation on how the deviation between the center of the optical aperture and the optical axis of the lens influences the image height and the parallax amount will be described.
As shown by the dashed line (condition 1) in
By contrast, in the case where, as shown by the solid line (condition 2) in
The decentration in condition 2 is assumed to be the decentration between the centers of the optical apertures and the optical axes of the lenses in an initial period of assembly. More specifically, the decentration in condition 2 is assumed to occur immediately after the compound eye camera module is assembled at room temperature, due to the deviation of the pitch between the optical apertures 2a and 2b of the optical aperture section 1 or the deviation of the pitch between the lenses 4a and 4b of the lens array 1.
Condition 3 (two-dot chain line) corresponds to a case where the linear expansion coefficient of the lens array 4 is different from the linear expansion coefficient of the optical aperture section 1, and the decentration amount increases by 7.3 μm in the base direction from the state of condition 2 by the change of the environmental temperature. Namely, this corresponds to a case where a decentration of 12.3 μm occurs from the state with zero decentration. The decentration of 7.3 μm corresponds to the decentration which occurs when the lens array 4 is formed of a cycloolefin polymer-based material having a linear expansion coefficient of 7.0×10−5/° C., the optical aperture section is formed of aluminum having a linear expansion coefficient of 2.3×10−5/° C., and the temperature changes by 60° C. As is clear from
Condition 4 (one-dot chain line) corresponds to a case where the linear expansion coefficient of the lens array 4 is different from the linear expansion coefficient of the optical aperture section 1, and the decentration amount increases by 7.3 μm in the base direction from the state of condition 1 by the change of the environmental temperature.
As shown by the two-dot chain line in
As shown by the one-dot chain line in
For the compound eye camera module in this embodiment, the linear expansion coefficient of the material of the optical aperture section 1 is generally the same as the linear expansion coefficient of the material of the lens array 4, in order not to increase the decentration amount between the centers of the optical apertures and the optical axes of the lenses even when the environmental temperature changes. Namely, in order to maintain a necessary accuracy of the measured distance, the compound eye camera module is structured such that the decentration amount between the optical axes of the lenses and the centers of the optical apertures is within a certain range even when the environmental temperature changes, instead of being structured to estimate the decentration amount with respect to the change of the environmental temperature and correct the measured distance.
Regarding the specific materials of the lens array 4 and the optical aperture section 1, for example, where a cycloolefin-based resin is used for the lens array, the linear expansion coefficient thereof is 7×10−5/° C., and where polycarbonate is used for the optical aperture section 1, the linear expansion coefficient thereof is 6.8×10−5/° C. The linear expansion coefficients of these two materials are substantially the same. Any other appropriate combination of materials than this is selectable. For example, the linear expansion coefficients can be adjusted by dispersing glass in ABS resin.
As is clear from comparing
As understood from
With the compound eye camera module in this embodiment, as described above, the difference in the linear expansion coefficient between the material of the lens array and the material of the optical aperture section is set to 0.7×10−5/° C. or less. Owing to this, the decentration amount between the optical axes of the lenses and the centers of the optical apertures is suppressed from changing in accordance with the environmental temperature, although such a change is difficult to be corrected merely by considering the expansion amount or shrinkage amount of the materials used to form the compound eye camera module in accordance with the environmental temperature. Since the decentration amount is suppressed from changing, the change of the parallax amount can also be suppressed. Accordingly, the distance measurement accuracy can be remarkably improved.
As understood from the graph of
By setting the absolute value of the difference in the linear expansion coefficient between the lens array 4 and the optical aperture section 1 to a prescribed value or less, the influence of the measuring error caused by the decentration, i.e., the deviation, between the optical axis 4ap, 4bp of the lens 4a, 4b and the center 2ap, 2bp of the corresponding optical aperture 2a, 2b can be minimum regardless of the environmental temperature. However, this cannot suppress the change of the base line length D caused by the change of the environmental temperature. Accordingly, it is preferable to find the change amount of the base line length D caused by the change of the environmental temperature using the linear expansion coefficient of the material used to form the lens array 4 and to correct the parallax amount based on the change amount of the base line length D as described above. This makes it possible to perform highly accurate measurement regardless of the environmental temperature.
Owing to the above-described structure, the change of the decentration amount caused by the change of the environmental temperature can be suppressed. However, in order to decrease the initial value itself of the decentration amount, it is important to reduce the decentration amount at the time of assembly to a minimum possible value in addition to making the linear expansion coefficients of the lens array 4 and the optical aperture section 1 substantially the same with each other. Therefore, for the compound eye camera module in this embodiment, the optical aperture section 1 and the lens array 4 are positioned with respect to each other in a state of contacting each other, such that that centers of the optical apertures of the optical aperture sections 1 and the optical axes of the lenses match each other, and then bound together. Hereinafter, a method for producing the compound eye camera module will be described including this point.
As shown in
For producing the compound eye camera module, the optical aperture section 1, the lens array 4, the lens barrel 5 and the imaging section 6 each processed to have a prescribed shape are first prepared. Next, the optical aperture section 1 and the lens array 4 are bound together to form a unit. At this point, as shown in
Then, as shown in
Next, the unit is bound with the lens barrel 5. As shown in
By setting the application area and amount of the adhesive symmetrical with respect to the center C1 or C2 as described above, the stress caused by the expansion or shrinkage of the adhesive due to the change of the environmental temperature is applied on the lens array 4, the optical aperture section 1 and the lens barrel 5 symmetrically in the up-down direction and the left-right direction. Accordingly, the assembly of the lens array 4, the optical aperture section 1 and the lens barrel 5 is expanded or shrunk with respect to the center of the elements. Owing to this, the positional change of the optical axis of each optical system can be estimated highly accurately, and so highly accurate compensation for the temperature change is realized.
In this embodiment, the optical aperture section 1 includes the optical apertures 2a and 2b integrally. In the case where the optical apertures 2a and 2b are formed in the optical aperture section 1 highly accurately, such an integral structure is advantageous in that only one element needs to be positionally aligned to the lens array 4 and the assembly is simplified. However, in the case where the centers of the optical apertures 2a and 2b are not distanced from each other at a prescribed accuracy, or in the case where the optical apertures 2a and 2b are formed in the optical aperture section 1 highly accurately but the positional accuracy of the lenses 4a and 4b in the lens array 4 is not high, the optical aperture section 1 may have a structure in which the positions of the optical apertures 2a and 2b are independently adjustable such that the optical axes of the lenses 4a and 4b respectively match the centers of the optical apertures 2a and 2b.
Similarly, the optical aperture section 1b may be translated or rotated to be positioned with respect to the lens 4b of the lens array 4, such that the optical axis 4bp of the lens 4b matches the center 2bp of the optical aperture 2b. Preferably, in the state where the optical axis 4bp of the lens 4b matches the center 2bp of the optical aperture 2b, a plane 4bf of the lens array 4 which is parallel to the optical axis of the lens 4b and a plane 1bf of the second optical aperture section 1b which is parallel to the optical axis of the lens 4a are positioned with respect to each other in a state of contacting each other.
The lens array 4, and the first optical aperture sections 1a and the second optical aperture section 1b may be bound together by an adhesive in a state of being positioned in this manner. Owing to this, adjustments can be made in order to reduce the decentration amount of the center of each optical aperture with respect to the optical axis of the corresponding lens. As a result, even in a lens array including a plurality of lenses integrally formed, the decentration between the optical axis of each lens and the center of the corresponding optical aperture can be made infinitely close to zero, and thus accurate distance measurement can be guaranteed.
In this embodiment, the lens array 4 includes two lenses 4a and 4b. Substantially the same effect is provided where the lens array 4 includes three or more lenses.
In this embodiment, the optical filter 7 is located in the vicinity of the lens array 4. Alternatively, the optical filter 7 may be located for each pixel on the imaging section 6.
Needless to say, the resin material for used to form the optical aperture section 1 needs to be light shielding. The light shielding property may be obtained by adding 3% or more of carbon to the resin material used to form the optical aperture section 1.
A compound eye camera module according to the present invention is useful for a vehicle-mountable distance measuring device or for an imaging device of three-dimensional images.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2008-004002 | Jan 2008 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2009/000067 | 1/9/2009 | WO | 00 | 10/29/2009 |
