Apparatus and method for aligning an imaging lens using virtual test images

Abstract

An alignment apparatus for aligning a lens module relative to an image sensor, comprising a holographic film having a test chart pattern from which a virtual image of the test chart pattern can be generated, and a light source for illuminating the holographic film. An image sensor holder is provided to mount the image sensor, and a lens module holder is configured and positioned to mount the lens module between the holographic film and the image sensor so that the virtual image of the test chart pattern can be viewed by the image sensor through the lens module. The virtual image that can be viewed by the image sensor through the lens module is thus located at a virtual distance from the image sensor that is different from the physical location of the holographic film, thereby aligning the lens module relative to the image sensor.

Description

TECHNICAL FIELD

[0001] The present invention relates to an apparatus and method for aligning an image sensor relative to a lens module, in particular, to aligning them prior to fixing the lens module to the image sensor during assembly of an imaging camera module. BACKGROUND

[0002] Prior to combining a lens module to an image sensor, an image quality check is performed during lens alignment between the lens module and the image sensor. Typically, it is required to view a test chart with the image sensor through the lens module to align the lens module relative to the image sensor, or to test the aligned imaging module (comprising the lens module and the image sensor). The sharpness of various parts of the test chart, in particular the edges of the test chart, is evaluated at different locations of the field of view of the imaging module. However, for applications with wide angle lenses, for example, in case the lens module has a field of view of more than 60 degrees, a large test chart of several meters size is required to be placed at a distance from the lens module to cover the entire field of view of the imaging module. Furthermore, such a test chart has to be positioned at a distance corresponding to the focusing distance required for the alignment, for example, 5 or 10 meters away from the imaging module. Therefore, accommodating these large test charts requires a large machine size and footprint.

[0003] One way to reduce the machine size and footprint is to use multiple collimators to form images at various viewing angles to cover the field of view of the imaging module, for example, US patent 10,187,636 B2 entitled “Active Lens Alignment System”. When aligning the image sensor relative to the lens module prior to fixing the image sensor to the lens module, the exposure of the image sensor is turned on and the image sensor is moved relative to the lens module to different distances. At certain predetermined distances between the image sensor and the lens module, a collimator is used to illuminate a calibration pattern and two or more pictures of the calibration pattern focused through the lens module are captured with the image sensor to generate at least two pictures of the calibration pattern captured at different distances. Then the exposure of the image sensor is turned off and the pictures of the calibration pattern are analyzed to determine the alignment between the lens module and the image sensor.

[0004] Unlike conventional test charts, collimator modules comprising multiple collimators are implemented to direct calibration or chart patterns through a lens module to an image sensor. This can reduce the space required for the machine compared to using conventional test charts. Nonetheless, each collimator still needs to occupy physical space of the machine and each collimator can only form an image at a limited angle to avoid physical interference with other collimators located next to it. Therefore, the size of the active alignment machine is still relatively large when used to test imaging modules with a viewing angle exceeding 70 degrees. Furthermore, due to the physical limitations of setting up collimators mentioned above, the measurement angle available for such a machine is limited.

[0005] It would be advantageous to provide an apparatus and method for testing the alignment of imaging modules that can avoid the use of physical test charts. The use of virtual test charts can avoid the size of the test chart having to directly correspond to the field of view of the imaging module being aligned. Therefore, the size of the machine required to house such a test chart can be greatly reduced compared to the prior art. SUMMARY

[0006] It is therefore an object of the present invention to seek to provide an apparatus and method for aligning imaging modules that relies on virtual test charts to reduce the size and footprint of active alignment imaging machines.

[0007] According to a first aspect of the present invention, there is provided an alignment apparatus for aligning a lens module relative to an image sensor, the alignment apparatus comprising: a holographic film having a test chart pattern, from which a virtual image of the test chart pattern can be generated; a light source for illuminating the holographic film; an image sensor mount for mounting the image sensor; a lens module mount configured and positioned to mount the lens module between the holographic film and the image sensor such that the virtual image of the test chart pattern can be viewed by the image sensor through the lens module; wherein the virtual image viewable by the image sensor through the lens module is located at a virtual distance from the image sensor, the virtual distance being different from the physical location of the holographic film, thereby aligning the lens module relative to the image sensor.

[0008] According to a second aspect of the present application, there is provided a method for aligning a lens module relative to an image sensor, the method comprising the steps of: providing a holographic film comprising a test chart pattern; illuminating the holographic film with a light source to generate a virtual image of the test chart pattern; mounting the image sensor on an image sensor mount; mounting the lens module on a lens module mount between the holographic film and the image sensor such that the virtual image of the test chart pattern can be viewed by the image sensor through the lens module; and viewing the virtual image with the image sensor, the virtual image being located at a virtual distance from the image sensor, the virtual distance being different from the physical location of the holographic film, thereby aligning the lens module relative to the image sensor.

[0009] According to a third aspect of the present application, there is provided a method for manufacturing a camera module, the method comprising the steps of: providing a holographic film comprising a test chart pattern; illuminating the holographic film with a light source to generate a virtual image of the test chart pattern; mounting an image sensor on an image sensor mount; mounting a lens module on a lens module mount between the holographic film and the image sensor such that the virtual image of the test chart pattern can be viewed by the image sensor through the lens module; viewing the virtual image with the image sensor, the virtual image being located at a virtual distance from the image sensor, the virtual distance being different from the physical location of the holographic film, thereby aligning the lens module relative to the image sensor; and then after the lens module and the image sensor have been aligned, fixing the lens module and the image sensor to each other to form a camera module.

[0010] These and other features, aspects, and advantages will become better understood with reference to the following description, appended claims, and accompanying drawings. BRIEF DESCRIPTION OF DRAWINGS

[0011] Embodiments of the present application will now be described, by way of example only, with reference to the following drawings in which:

[0012] Figure 1 The application of the basic lens formula for relating image position to physical position of a laser beam is shown, which is applicable to embodiments of the present application;

[0013] Figure 2 is a side view of an alignment apparatus according to a preferred embodiment of the present application, in which an image sensor can view a virtually reconstructed test chart image on a holographic film illuminated by a laser source;

[0014] Figure 3 is Figure 2an isometric view of the alignment apparatus showing an exemplary virtual test chart reconstructed from a holographic film viewable by the image sensor;

[0015] Figure 4 a flow chart showing a method for aligning a lens module relative to an image sensor according to an embodiment of the present application; and

[0016] Figure 5A is an example of a virtual generated image of a test chart viewed by the image sensor prior to alignment of the imaging module, and Figure 5B is an example of the test chart viewed by the image sensor after the imaging module has been aligned.

[0017] In the drawings, like reference numerals refer to like elements throughout. DETAILED DESCRIPTION

[0018] Computer-generated holography (CGH) is a method of digitally generating holographic interferograms. For example, a holographic image can be generated by digitally computing a holographic interferogram and printing it onto a film to be subsequently illuminated by a suitable coherent light source. The advantage of CGH is that the object to be displayed does not need to have any physical entity at all and can be completely computer-synthetic generated to be imprinted on the holographic film.

[0019] Figure 1 application of the basic lens formula for relating the image position to the physical position of the coherent light source (e.g., laser beam) is shown, which is applicable to embodiments of the present application.

[0020] The relationship between the focal length (f) of the lens, the image position (v), and the physical position of the object (e.g., laser spot) (u) can be expressed according to the lens formula as follows:

[0021] 1 / v + 1 / u = 1 / f

[0022] v = 1 / (1 / f - 1 / u)

[0023] Thus, if the lens is a collimating lens 10, by knowing the focal length (f) of the collimating lens 10 and its physical position, as well as the physical position of the laser spot (u) after passing through the collimating lens 10, the image position (v) (which in this application is the virtual image of the laser spot) can be determined.

[0024] When u = f (the physical position of the laser spot equals the focal length of the lens), a virtual image of the laser spot will be formed at infinity because this setup generates collimated light rays. On the other hand, different v values are generated by changing u (e.g., by physically moving the position of the laser spot relative to the collimating lens 10) or by changing f (e.g., by replacing the lens or employing a variable focal length lens, such as a liquid lens, to change the focal length of the optical lens).

[0025] Figure 2 is a side view of an alignment apparatus according to a preferred embodiment of the present application, in which an image sensor 26 can view a virtual reconstructed image on a holographic film 22 (e.g., a Fourier holographic film) illuminated by a laser source, which can be in the form of a coherent light source (e.g., a laser source 20). The laser source 20 emits a laser beam toward a collimating lens 10, and light rays from the laser beam pass through the collimating lens 10. When the distance (u) between the laser source 20 and the collimating lens 10 equals the focal length (f) of the collimating lens 10, the coherent light rays from the laser beam are collimated.

[0026] The collimated light rays 21 then pass through the holographic film 22 to illuminate the holographic film 22 and are focused by a lens module 24 onto an image sensor 26. It is necessary to actively align the lens module 24 relative to the image sensor 26 so that once the two components are properly aligned, the lens module 24 can be attached to the image sensor 26 to form an imaging camera module 32 to acquire the focused image received by the image sensor 26. The lens module 24 is held by a lens module holder 28, which is operable to adjust the lens module 24 in six degrees of freedom. In addition, the image sensor 26 is held by a sensor holder 30, which is operable to adjust the imaging sensor 26 in another six degrees of freedom.

[0027] The lens module holder 28 is configured and positioned to mount the lens module 24 between the holographic film 22 and the image sensor 26 so that the test chart pattern imprinted in the holographic film 22 can be viewed by the image sensor 26 through the lens module 24.

[0028] As mentioned above, when u = f, the virtual image of the laser spot generated by the laser source 20 is formed at infinity. This can allow the image of the test chart formed on the holographic film 22 to be aligned at a single predetermined distance between the test chart and the image sensor 26. However, with the apparatus according to the present application, it is also possible to generate a virtual test chart that appears to be at a different distance (v) with respect to the image sensor 26 by changing the position (u) of the laser source 20 or changing the focal length (f) of the collimating lens 10. In particular, when the distance between the laser source 20 and the collimating lens 10 is different from the focal length (f) of the collimating lens 10, it is possible to obtain a virtual test chart generated at a different distance (v) with respect to the image sensor 26 such that the light rays from the laser source 20 are not collimated after passing through the collimating lens 10.

[0029] Figure 3 is Figure 2 an isometric view of the alignment apparatus showing an exemplary virtual test chart 34 reconstructed from the holographic film 22 that can be viewed by the image sensor 26 to align the lens module 24 with respect to the image sensor 26. The phase of the light rays 21 reaching the holographic film 22 is changed, for example, by moving the laser source 20 with respect to the collimating lens 10. The holographic film 22 includes a test chart pattern from which a virtual image of the test chart pattern can be generated. For the purpose of aligning the lens module 24 with respect to the image sensor 26, the virtual image that can be viewed by the image sensor 26 through the lens module 24 is located at a virtual distance from the image sensor 26 that is different from the physical location of the holographic film 22.

[0030] As a result, the virtual test chart 34 including the test chart pattern can be viewed at a virtual distance that is not limited by the focal length (f) of the collimating lens 10, such that the virtual test chart 34 appears much farther away than the actual distance of the laser source 20 and the holographic film 22 to the image sensor 26.

[0031] To move the laser source 20 with respect to the collimating lens 10, a positioning mechanism 31 (see Figure 2 ) can be coupled to the laser source 20 to adjust the position of the laser source 20 and change the separation distance between the laser source 20 and the holographic film 22. On the other hand, to facilitate changing the focal length of the collimating lens 10, the collimating lens 10 can take the form of a liquid lens with a variable focal point.

[0032] The above method has the benefit that the collimating lens 10 is able to form a virtual image for active alignment at distances far exceeding the size of the active alignment machine without having to increase the field of view that the image sensor 26 can view. The lens module 24 is operable to focus the image of the virtual test chart 34 onto the image sensor 26 at different virtual distances of the virtual test chart 34 from the image sensor 26 during the relative alignment between the lens module 24 and the image sensor 26. Accordingly, the lens module holder 28 and the image sensor holder 30 are able to adjust the position and orientation of the lens module 24 and the image sensor 26, respectively, until the image sensor 26 obtains a focused image of the virtual test chart 34. Thus, the physical limitations faced by prior art active alignment machines (e.g., active alignment machines using multiple collimators) can be avoided, and the size of the active alignment machine required to accommodate the physical test chart can be greatly reduced compared to the prior art.

[0033] Figure 4 A flowchart of a method for aligning the lens module 24 with respect to the image sensor 26 according to an embodiment of the present application is shown. First, the holographic film 22 including the test chart pattern is set (step 38). The holographic film 22 is illuminated with the laser source 20 to generate a virtual image of the test chart pattern (step 40). The image sensor 26 is mounted on the image sensor holder 30 (step 42), and the lens module 24 is mounted on the lens module holder 28 (step 44). When the virtual image is viewed by the image sensor 26 through the lens module 24, the image sensor holder 30 and the lens module holder 28 manipulate the image sensor 26 and the lens module 24, respectively, so as to align the lens module 24 with respect to the image sensor 26 (step 46) to obtain a focused image of the test chart pattern. After the lens module 24 and the image sensor 26 have been aligned, the lens module 24 is fixed to the image sensor 26 using an adhesive such as glue so as to generate the camera module 32 (step 48).

[0034] Figure 5A is an example of a virtual generated image 34a of a test chart viewed by the image sensor 26 prior to imaging module alignment for alignment at a virtual generated distance. The test chart can include a center shape 50 and corner shapes 52 at four respective corners of the virtual generated image 34a. In Figure 5A In this case, the corner shapes 52 are apparently out of focus. Accordingly, the lens module holder 28 and the sensor holder 30 will re-orient the lens module 24 and the image sensor 26 with respect to each other according to the image obtained by the image sensor 26.

[0035] Figure 5Bis an example of a virtual generated image 34b of the test chart viewed by the image sensor 26 after the imaging module has been aligned. The lens module 24 and image sensor 26 have been adjusted in relation so that both the center shape 50 and the corner shape 52 are in focus to obtain the best focused image. After this active alignment, the lens module 24 can then be fixedly attached to the image sensor 26 in the best relative orientation by using an adhesive such as glue to form the camera module 32.

[0036] It will be appreciated that by employing the holographic film 22 according to the embodiments described herein, the configuration of a single holographic film 22 is much simpler than using a physical test chart or multiple collimators, and the size of the lens alignment apparatus can be significantly reduced. Furthermore, changing the test distance of the imaging module requires minimal conversion work, and the size of the pattern generated from the holographic film 22 can remain constant at various focal distances. Since the apparatus is capable of generating the same test pattern at any location within the field of view of the imaging module, the complete image of the test pattern within the field of view of the imaging module can be conveniently evaluated.

[0037] While the application has been described in some detail with reference to certain embodiments thereof, other embodiments are possible.

[0038] Therefore, the spirit and scope of the appended claims should not be limited to the description of the embodiments contained herein.

Claims

1. An alignment apparatus for aligning a lens module relative to an image sensor, the alignment apparatus comprising: a holographic film comprising a test chart pattern from which a virtual image of the test chart pattern can be generated; a light source for illuminating the holographic film; an image sensor mount for mounting the image sensor; and a lens module mount configured and positioned to mount the lens module between the holographic film and the image sensor so that the virtual image of the test chart pattern can be viewed by the image sensor through the lens module; wherein the virtual image as viewed by the image sensor through the lens module is located at a virtual distance from the image sensor that is different from the physical location of the holographic film, thereby aligning the lens module relative to the image sensor. The light source comprises a coherent light source.

2. The alignment apparatus of claim 1, wherein, The apparatus further comprises a collimating lens and the coherent light source is operable to generate coherent light rays that pass through the collimating lens prior to illuminating the holographic film.

3. The alignment apparatus of claim 2, wherein, The collimating lens has a variable focal point.

4. The alignment apparatus of claim 3, wherein, The collimating lens comprises a liquid lens.

5. The alignment apparatus of claim 4, wherein, The distance between the light source and the collimating lens is different from the focal length of the collimating lens such that light rays from the light source are not collimated after passing through the collimating lens.

6. The alignment apparatus of claim 3, wherein, 7. The alignment apparatus of claim 1, further comprising a positioning mechanism coupled to the light source for adjusting the position of the light source and varying the separation distance between the light source and the holographic film. The holographic film is a Fourier holographic film.

8. The alignment apparatus of claim 1, wherein, During relative alignment of the lens module and the image sensor, the lens module is operable to focus the virtual image onto the image sensor.

9. The alignment apparatus of claim 1, wherein, The lens module mount and the image sensor mount are operable to adjust the position and orientation of the lens module and the image sensor, respectively, until the image sensor acquires a focused image of the virtual image.

10. The alignment apparatus of claim 9, wherein, 11. A method for aligning a lens module relative to an image sensor, the method comprising the steps of: providing a holographic film comprising a test chart pattern; illuminating the holographic film with a light source to generate a virtual image of the test chart pattern; mounting the image sensor on an image sensor mount; mounting the lens module on a lens module mount between the holographic film and the image sensor so that the virtual image of the test chart pattern can be viewed by the image sensor through the lens module; and viewing the virtual image with the image sensor, the virtual image being located at a virtual distance from the image sensor that is different from the physical location of the holographic film, thereby aligning the lens module relative to the image sensor.

12. The method of claim 11, further comprising the step of, after the lens module and the image sensor have been aligned, securing the lens module and the image sensor to one another to form a camera module.

13. A method for manufacturing a camera module, the method comprising the steps of: ​ ​ providing a holographic film including a test chart pattern; illuminating the holographic film with a light source to generate a virtual image of the test chart pattern; mounting an image sensor on an image sensor mount; mounting a lens module on a lens module mount between the holographic film and the image sensor such that the virtual image of the test chart pattern can be viewed by the image sensor through the lens module; viewing the virtual image with the image sensor, the virtual image being at a virtual distance from the image sensor, the virtual distance being different from the physical location of the holographic film, thereby aligning the lens module with respect to the image sensor; then after the lens module and the image sensor have been aligned, securing the lens module and the image sensor to each other to form a camera module.

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