GB2541197A

GB2541197A - An apparatus and method for calibrating cameras

Info

Publication number: GB2541197A
Application number: GB1514110.4A
Authority: GB
Inventors: Kimmel Jyrki; Roimela Kimmo
Original assignee: Nokia Technologies Oy
Current assignee: Nokia Technologies Oy
Priority date: 2015-08-11
Filing date: 2015-08-11
Publication date: 2017-02-15
Also published as: GB201514110D0

Abstract

An apparatus and method wherein the apparatus comprises: at least one first target 3 positioned to enable a camera 13 to align with an optical axis 7; at least one second target 5 positioned to enable the camera to align with a plane 9 perpendicular to the optical axis 7; wherein the apparatus is configured such that alignment with the targets enables the position of the camera to be calibrated with one or more other cameras.

Description

TITLE

An Apparatus and Method for Calibrating Cameras TECHNOLOGICAL FIELD

Examples of the present disclosure relate to an apparatus and method for calibrating cameras. In particular, they relate to an apparatus and method for calibrating the relative geometry of a camera arrangement comprising a plurality of cameras.

BACKGROUND

Camera systems comprising a plurality of cameras may be used to obtain composite images. The composite images may be obtained by combining images captured by individual cameras within the system. Such composite images could comprise three-dimensional images, stereoscopic images, panoramic images or other types of images. Such camera systems require accurate positioning of the plurality of cameras relative to each other in order to produce accurate composite images.

BRIEF SUMMARY

According to various, but not necessarily all examples of the disclosure, there may be provided an apparatus comprising: at least one first target positioned to enable a camera to align with an optical axis; at least one second target positioned to enable the camera to align with a plane perpendicular to the optical axis; wherein the apparatus is configured such that alignment with the targets enables the position of the camera to be calibrated with one or more other cameras.

In some examples the first target may comprise a retroreflector.

In some examples the second target may comprise an autocollimator.

In some examples the first target and the second target may be positioned in alignment with the optical axis. The second target may be positioned in front of the first target. A lens may be positioned between the first target and the second target.

In some examples the apparatus may be configured to enable the position of a camera to be adjusted relative to a position of a second camera.

In some examples the apparatus may be configured to enable the position of a camera to be adjusted to correct deviation from the optical axis.

In some examples the apparatus may be configured to enable the position of a camera to be adjusted to correct skew about the optical axis.

In some examples the apparatus may be configured to enable the position of a camera to be adjusted to correct deviation from a plane perpendicular to the optical axis.

In some examples the apparatus may comprise a plurality of first targets and a plurality of second targets configured to enable a plurality of cameras to be calibrated.

According to various, but not necessarily all examples of the disclosure, there may be provided a method comprising: aligning a camera with an optical axis by aligning the camera relative to a first target; aligning the camera with a plane perpendicular to the optical axis by aligning the camera relative to a second target; wherein alignment with the targets enables the position of the camera to be calibrated with one or more other cameras.

In some examples the first target may comprise a retroreflector.

In some examples the second target may comprise an autocollimator.

In some examples the method may enable the position of a camera to be adjusted relative to a position of a second camera.

In some examples the method may enable the position of a camera to be adjusted to correct deviation from the optical axis.

In some examples the method may enable the position of a camera to be adjusted to correct skew about the optical axis.

In some examples the method may enable the position of a camera to be adjusted to correct deviation from a plane perpendicular to the optical axis.

In some examples the method comprises providing a plurality of first targets and a plurality of second targets configured to enable a plurality of cameras to be calibrated.

According to various, but not necessarily all, examples of the disclosure there is provided examples as claimed in the appended claims.

BRIEF DESCRIPTION

For a better understanding of various examples that are useful for understanding the detailed description, reference will now be made by way of example only to the accompanying drawings in which:

Fig. 1 illustrates an apparatus;

Fig. 2 illustrates an apparatus;

Fig. 3 illustrates an apparatus;

Figs. 4A and 4B illustrate example images obtained by cameras using an apparatus; Fig. 5 schematically illustrates an optical structure of an apparatus;

Fig. 6 illustrates an example multiple camera system; and Fig. 7 illustrates a method.

DETAILED DESCRIPTION

The Figures illustrate an apparatus 1 comprising: at least one first target 3 positioned to enable a camera 13 to align with an optical axis 7; at least one second target 5 positioned to enable the camera 13 to align with a plane 9 perpendicular to the optical axis 7; wherein the apparatus 1 is configured such that alignment with the targets 3, 5 enables the position of the camera 13 to be calibrated with one or more other cameras 13.

The apparatus 1 may be for calibrating a camera system comprising a plurality of cameras. The apparatus 1 may enable the camera system to be calibrated during manufacture and/or repair. The apparatus 1 may be configured to enable the relative positions and geometry of the plurality of cameras to be measured and/or corrected so that high quality composite images can be obtained by the camera system.

Fig.1 schematically illustrates an apparatus 1 according to examples of the disclosure. The apparatus 1 comprises a first target 3 and a second target 5. In the example of Fig. 1 both the first target 3 and the second target 5 may be aligned with an axis. The axis could be an optical axis 7.

In the example of Fig. 1 the apparatus 1 is positioned adjacent to a camera support 11. The camera support 11 may be configured to hold a plurality of cameras 13 in position. In some examples the camera support 11 may enable the relative positions of the cameras 13 to be adjusted to enable calibration of the relative geometry of the plurality of cameras 13.

In the example of Fig. 1 the apparatus 1 is positioned adjacent to, but separate from the camera support 11. Having the camera support 11 separate to the apparatus 1 may enable the apparatus 1 to be used to calibrate different camera systems. In some examples the camera support 11 may be coupled to the apparatus 1 in order to reduce the tolerance in the calibrations of the cameras 13.

In the example of Fig. 1 two cameras 13 are provided within the camera support 11. It is to be appreciated that any number of cameras may be used in other examples of the disclosure. In the example of Fig, 1 the two cameras 13 are positioned adjacent to each other and facing in the same direction. For instance the cameras 13 could be positioned with spacing comparable to a person’s eyes. In such examples the spacing between the cameras 13 could be of the order of 62mm. Adjustments could be made to take into account different eye separations of different users. This example camera system may enable stereoscopic images to be obtained. It is to be appreciated that other geometries of the cameras 13 may be used in other examples of the disclosure.

In the example of Fig. 1 a camera 13 is positioned within the camera support 11 so that the first target 3 and the second target 5 of the apparatus 1 are positioned in front of the camera 13.

The first target 3 may comprise any means which may be positioned within the apparatus 1 to enable a camera 13 to be aligned with the optical axis 7. In some examples, other components in addition to the first target 3 may be used to enable the camera 13 to be aligned with the optical axis 7. In the example of Fig. 1 the first target 3 is positioned on the optical axis 7 in front of the camera 13 so that an image of the first target 3 can be captured by the camera 13. The deviation of the camera 13 relative to the optical axis 7 may be determined by determining the deviation of the image of the first target 3 relative to the optical axis 7.

The second target 5 may comprise any means which may be positioned within the apparatus 1 to enable a camera 13 to be aligned with a plane 9 perpendicular to the optical axis 7. In some examples, other components in addition to the second target 5 may be used to enable the camera 13 to be aligned with a plane 9 perpendicular the optical axis 7. In the example of Fig. 1 the second target 5 is positioned on the optical axis 7 in front of the first target 3. The second target 5 is positioned in front of the camera 13 so that an image of both the first target 3 and the second target 5 can be captured by the camera 13. The deviation of the camera 13 relative to the plane 9 perpendicular to the optical axis 7 may be determined by determining the deviation of the image of the second target 5 relative to the optical axis 7.

The cameras 13 may be configured to obtain images of the first and second targets 3, 5. The images of the first and second targets 3, 5 may enable discrepancies in the alignment of the camera 13 with the optical axis 7 and the plane 9 perpendicular to the optical axis 7 to be measured. If the discrepancies are small the camera 13 may enable the discrepancies to be corrected using image processing algorithms. If the discrepancies are large then the relative geometry of the camera system may be mechanically adjusted. The mechanical adjustment of the camera system may enable a user to physically change the position of the camera 13 relative to one or more other cameras 13 within the system.

In some examples information indicative of the discrepancies in the alignment of the camera 13 may be stored. In some examples the images of the targets 3, 5 may be stored. In some examples measurements of the discrepancies in the alignment of the camera 13 may be stored. The information may be stored within the camera 13 and/or in an external device.

Fig. 2 schematically illustrates an apparatus 1 according to another example of the disclosure. In the example of Fig. 2 only one camera 13 is illustrated. It is to be appreciated that the camera 13 could be part of a system comprising a plurality of cameras.

The example apparatus 1 of Fig. 2 comprises a first target 3 and a second target 5 which are positioned as described above. In the example of Fig. 2 the first target 3 comprises a retroreflector 21 and the second target 5 comprises an autocollimator 23.

The retroreflector 21 may comprise any means which may be configured to reflect incident light back towards the camera 13. The light may be reflected in a direction which is parallel to but opposite the direction of the incident light. The retroreflector 21 may be configured so that light is reflected in a direction which is parallel to but opposite the direction of the incident light for all angles of incidence. This ensures that the light is reflected in a parallel direction even if the camera 13 is misaligned and is not parallel to the retroreflector 21.

In some examples the retroreflector 21 may comprise a corner cube retroreflector. Other types of retroreflector 21 may be used in other examples of the disclosure.

The retroreflector 21 may be aligned with the optical axis 7. The retroreflector 21 may be positioned to correspond to the ideal position of the camera 13 relative to the optical axis 7. The optical axis 7 defines the ideal position of the camera 13 in the x-y plane. The optical axis 7 may be parallel to the z axis of the coordinate system illustrated in Fig. 2.

The position of the camera 13 relative to the optical axis 7 may be calibrated by using the camera 13 to obtain an image of the retroreflector 21. Any deviation of the image of the retroreflector 21 with respect to a predetermined point may be determined to be a misalignment with the aligning the optical axis 7.

The retroreflector 21 may also enable skew of the camera 13 to be measured. The skew is the rotation of the camera 13 about the z axis. The skew may be the rotation of the camera in the x-y plane. Skew may be roll of the camera 13. Skew is indicated by angle Θ in Fig. 2. The skew may be determined by measuring rotation of the retroreflector 21 with respect to a predetermined axis in the image obtained by the camera 13.

Therefore the first target 3 enables discrepancies in the alignment of the camera 13 in three degrees of freedom to be measured. In the example of Fig. 2 the first target 3 enables the position in the x axis, the position in the y axis and the skew of the camera 13 to be determined. These discrepancies could be corrected by adjusting the position of the camera 13 or by processing the images obtained by the camera 13.

The autocollimator 23 may comprise any means which may be configured to measure the angle of the camera 13. The autocollimator 23 may be configured to measure the angular position of the camera 13 relative to a plane perpendicular to the optical axis 7. The autocollimator 23 may be configured to measure small angles with high sensitivity.

In some examples the autocollimator 23 may comprise a red dot autocollimator. The red dot autocollimator may provide a small red dot as a light source. The position of the red dot in an image obtained by the camera 13 may give a measurement of the angle of the camera 13. Other types of autocollimator 23 may be used in other examples of the disclosure.

The autocollimator 23 may be aligned with a plane 9 perpendicular to the optical axis 7. The autocollimator 23 may be positioned to correspond to the ideal position of the camera 13 relative to this plane 9. In the coordinate system illustrated in Fig. 2 the plane 9 perpendicular to the optical axis 7 is the x-y plane. The autocollimator 23 may enable rotation out of this plane to be measured.

The autocollimator 23 enables pitch of the camera 13 to be measured. The pitch is the rotation of the camera 13 about the x axis. The pitch may be the rotation of the camera in the y-z plane. Pitch is indicated by angle y in Fig. 2. The pitch may be determined by measuring the position of the red dot of the autocollimator 23 relative to a predetermined position in an image obtained by the camera 13.

The autocollimator 23 enables yaw of the camera 13 to be measured. The yaw is the rotation of the camera 13 about the y axis. The yaw may be the rotation of the camera in the x-z plane. Yaw is indicated by angle φ in Fig. 2. The yaw may also be determined by measuring the position of the red dot of the autocollimator 23 relative to a predetermined position in an image obtained by the camera 13.

Therefore the second target 5 enables discrepancies in the alignment of the camera 13 in two degrees of freedom to be measured. In the example of Fig. 2 the second target 5 enables the yaw and pitch of the camera 13 to be determined. These discrepancies could be corrected by adjusting the position of the camera 13 or by processing the images obtained by the camera 13.

Therefore the apparatus 1 provides a combination target which enables discrepancies of the camera 13 in five degrees of freedom to be measured. The sixth degree of freedom is the position of the camera 13 in the z axis. The position may be adjusted during use of the camera to enable the camera 13 to be focused. During calibration the position of the camera 13 in the z axis may be adjusted so that focused images of the first and second targets 3, 5 are obtained. This may ensure that the system is calibrated in six degrees of freedom.

Fig. 3 schematically illustrates another apparatus 1 according to another example of the disclosure. The example apparatus 1 of Fig. 3 is similar to the apparatus 1 of Fig. 2 in that the first target 3 also comprises a retroreflector 21 and the second target 5 also comprises an autocollimator 23. Corresponding reference numerals are used for corresponding features.

In the example of Fig. 3 the apparatus 1 also comprise a lens 31. The lens may comprise any means which may enable the camera 13 to obtain a focussed image of the retroreflector 21. The lens 31 may enable the camera 13 to obtain an image of the retroreflector 21 focussed at infinity. In the example apparatus 1 of Fig. 3 the lens 31 is positioned in front of the retroreflector 21. The lens 31 is positioned between the retroreflector 21 and the autocollimator 23. It is to be appreciated that other means of adjusting the focus of the camera 31 may be used in other examples of the disclosure.

Figs. 4A and 4B illustrate examples of images of the first target 3 and second target 5 which may be obtained by a camera 13 positioned adjacent to the apparatus 1. The example images of Figs. 4A and 4B may comprise images which may be obtained by the camera 13 in the examples of Fig. 3.

In the example of Fig. 4A the camera 13 is correctly aligned so that the camera 13 is ideally positioned relative to the optical axis 7 of the apparatus 1 and the plane 9 perpendicular to the optical axis 7.

The image 41 comprises an image 43 of the retroreflector 21 and an image 45 of the lens 31 in the retroreflector 21. When the camera 13 is in the ideal position these images 43, 45 are centrally positioned within the image 41. The centre of the image 43 of the retroreflector 21 and the image 45 of the lens 31 in the retroreflector 21 coincide with the centre of the sensor array of the camera 13. The centre of the images 43, 45 coincides with the centre of the image 41 in the vertical direction as indicated by the line 48 and the centre of the image 41 in the horizontal direction as indicated by the line 49.

The skew of the camera 13 may also be determined from the image 41. In the ideal position there is no skew and the image 43 of the retroreflector 21 is aligned with the vertical axis of the image 41.

The image 41 in Fig. 4A also comprises an image 47 of the red dot of the autocollimator 23. When the camera 13 is in the ideal position this image 47 is also centrally positioned within the image 41.

Fig. 4B shows a corresponding image 41 which is obtained when the camera 13 is not correctly aligned so that the camera 13 is not positioned in the ideal position. The example image 41 of Fig. 4B also comprises an image 43 of the retroreflector 21, an image 45 of the lens 31 in the retroreflector 21 and image 47 of the red dot of the autocollimator 23. However as the camera 13 is not in the ideal position these images 43, 45, 47 are provided in different positions in Fig. 4B compared to the image 41 of Fig. 4A.

In Fig. 4B the optical axis of the camera 13 is not aligned with the optical axis 7 of the apparatus 1. The ideal position for optical axis 7 of the apparatus 7 may correspond to the centre of the image sensor of the camera 13. In the example of Fig. 4B the image 43 of the retroreflector 21 and the image 45 of the lens 31 are shifted in both the x and y directions. The discrepancy between the ideal x position of the camera 13 and the actual x position is indicated, at least in part, by Δχ in Fig. 4B. Similarly the discrepancy between the ideal y position of the camera 13 and the actual y position is indicated, at least in part, by Ay in Fig. 4B. It is to be appreciated that Δχ and Ay may also be affected by rotation about the x and/or y axis. However this rotation can be measured using the autocollimator 23.

In the example of Fig. 4B the camera 13 is also rotated about the optical axis 7 of the apparatus 1. The image of the retroreflector 21 is rotated about the z axis. The discrepancy between the ideal angular position of the camera 13 and the actual angular position is indicated by ΔΘ in Fig. 4B.

In the example of Fig. 4B the camera 13 is also tilted relative to the plane 9 perpendicular to the optical axis 7 of the apparatus 1. The image 47 of the red dot of the autocollimator 23 is shifted in both the x and y directions. The shift in the x and y directions corresponds to a shift in yaw and pitch respectively. The discrepancy between the ideal yaw of the camera 13 and the actual yaw is indicated by Δφ in Fig. 4B. Similarly the discrepancy between the ideal pitch of the camera 13 and the actual pitch is indicated by Δ/ in Fig. 4B.

Therefore the image obtained by the camera 13 enables the position of a camera 13 to be calibrated relative to an ideal position. Discrepancies in the positions can be accounted for by processing of images obtained by the camera or by adjusting the position of the camera 13 within the camera system.

Fig. 5 schematically illustrates an optical arrangement which may be used in apparatus 1 in examples of the disclosure. In the example of Fig. 5 the apparatus comprises a first target 3 comprising a retroreflector 21 and second target 5 comprising an autocollimator 23 which may be arranged within an apparatus 1 as described above.

In the example of Fig. 5 the autocollimator 23 comprises a semitransparent reflector 51 and a light source 53. The semitransparent reflector 51 may be curved so that light 57 from the light source 53 which is reflected by the reflector 51 is focused at infinity. The light which is reflected by the reflector 51 is parallel to the optical axis 7 of the apparatus 1. This may enable a camera 13 positioned adjacent to the apparatus 1 to obtain an image of a dot focused at infinity. The curved reflector 51 may have any suitable shape. In some examples the shape of the curved reflector 51 may comprise a parabola.

The reflector 51 may be semitransparent to enable illumination of the retroreflector 21 by the light source 53. The retroreflector 21 is positioned behind the semitransparent reflector 51. In other examples a separate light source could be provided for the retroreflector 21.

In the example of Fig. 5 the retroreflector 21 comprises a corner cube mirror. The optical axis 7 of the apparatus 1 is aligned with the apex 55 of the corner of the corner cube mirror. Other types of retroreflectors 21 may be used in other examples of the disclosure.

Fig. 6 illustrates a calibration system 61 which may be used to calibrate a multicamera system 63.

The calibration system 61 comprises a plurality of cameras 13. In the example of Fig. 6 the multi-camera system 63 comprises eight cameras 13 which are equally spaced around the circumference of a circle. The multi-camera system 63 could be configured to enable panoramic views extending for 360 degrees to be obtained or any other suitable use.

The calibration system 61 comprises an annulus 65. The multi-camera system 63 is positioned within the annulus 65 to enable calibration of the multi-camera system 63.

The calibration system comprises a plurality of apparatus 1. The apparatus 1 may comprise targets 3, 5 as described above. The plurality of apparatus 1 are positioned at intervals around the annulus.

The example calibration system 61 of Fig. 6 comprises five apparatus 1. Four of the apparatus 1 are provided at right angles to each other. A fifth apparatus 1 is provided mid way between two other apparatus 1. In the example of Fig. 6 the multicamera system 63 may be calibrated in a first angular position. The multi-camera system 63 may then be rotated through 45° so that the multi-camera system 63 can be calibrated in a second angular position. The rotation of the multi-camera system 63 may be repeated until all of the cameras 13 within the multi-camera system 63 have been calibrated relative to each other.

It is to be appreciated that other numbers of apparatus 1 could be provided in other calibration systems 61. The number of apparatus 1 within the calibration system 61 may depend on the multi-camera system 63 which is to be calibrated. For instance in some examples the calibration system 61 could comprise one apparatus 1 corresponding to each camera 13 within the multi-camera system 63. In other examples the number of apparatus 1 may be less than the number of cameras 13 so that rotation or other movement of the multi-camera system is needed to calibrate all of the cameras.

Fig. 7 illustrates an example method. The method of Fig. 7 could be implemented using apparatus 1 as described above. It is to be appreciated that the blocks of the method may be carried out in any order.

The method comprises, at block 71, aligning a camera 13 with an optical axis 7 by aligning the camera 13 relative to a first target 3. The method also comprises, at block 73, aligning the camera 13 with a plane 9 perpendicular to the optical axis 7 by aligning the camera 17 relative to a second target 5. The alignment with the targets 3, 5 enables the position of the camera 13 to be calibrated with one or more other cameras 13.

Examples of the disclosure provide an apparatus 1 which may be used to calibrate a multi-camera system 63. The calibration may be carried out during manufacture and/or repair of the multi-camera system 63.

The calibration can be easily achieved by positioning the multi-camera system 63 adjacent to one or more apparatus 1 and obtaining images of the apparatus 1. The discrepancies in the alignments can be determined form the obtained images. The correct alignment of the cameras could be achieved by processing of the images obtained by the cameras or by adjusting the relative positions of the cameras 13. This provides a simple and accurate method for calibrating multi-camera system 63 and ensures that high quality composite images may be obtained by the multicamera system 63.

In this disclosure the term coupled means operationally coupled and any number or combination of intervening elements can exist (including no intervening elements).

The term “comprise” is used in this document with an inclusive not an exclusive meaning. That is any reference to X comprising Y indicates that X may comprise only one Y or may comprise more than one Y. If it is intended to use “comprise” with an exclusive meaning then it will be made clear in the context by referring to “comprising only one...” or by using “consisting”.

In this detailed description, reference has been made to various examples. The description of features or functions in relation to an example indicates that those features or functions are present in that example. The use of the term “example” or “for example” or “may” in the text denotes, whether explicitly stated or not, that such features or functions are present in at least the described example, whether described as an example or not, and that they can be, but are not necessarily, present in some of or all other examples. Thus “example”, “for example” or “may” refers to a particular instance in a class of examples. A property of the instance can be a property of only that instance or a property of the class or a property of a sub class of the class that includes some but not all of the instances in the class. It is therefore implicitly disclosed that a features described with reference to one example but not with reference to another example, can where possible be used in that other example but does not necessarily have to be used in that other example.

Although embodiments of the present invention have been described in the preceding paragraphs with reference to various examples, it should be appreciated that modifications to the examples given can be made without departing from the scope of the invention as claimed.

Features described in the preceding description may be used in combinations other than the combinations explicitly described.

Although functions have been described with reference to certain features, those functions may be performable by other features whether described or not.

Although features have been described with reference to certain embodiments, those features may also be present in other embodiments whether described or not.

Whilst endeavoring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims

1. An apparatus comprising: at least one first target positioned to enable a camera to align with an optical axis; at least one second target positioned to enable the camera to align with a plane perpendicular to the optical axis; wherein the apparatus is configured such that alignment with the targets enables the position of the camera to be calibrated with one or more other cameras.

2. An apparatus as claimed in any preceding claim wherein the first target comprises a retroreflector.

3. An apparatus as claimed in any preceding claim wherein the second target comprises an autocollimator.

4. An apparatus as claimed in any preceding claim wherein the first target and the second target are positioned in alignment with the optical axis.

5. An apparatus as claimed in claim 4 wherein the second target is positioned in front of the first target.

6. An apparatus as claimed in claim 5 comprising a lens positioned between the first target and the second target.

7. An apparatus as claimed in any preceding claim wherein the apparatus is configured to enable the position of a camera to be adjusted relative to a position of a second camera.

8. An apparatus as claimed in any preceding claim wherein the apparatus is configured to enable the position of a camera to be adjusted to correct deviation from the optical axis.

9. An apparatus as claimed in any preceding claim wherein the apparatus is configured to enable the position of a camera to be adjusted to correct skew about the optical axis.

10. An apparatus as claimed in any preceding claim wherein the apparatus is configured to enable the position of a camera to be adjusted to correct deviation from a plane perpendicular to the optical axis.

11. An apparatus as claimed in any preceding claim wherein the apparatus comprises a plurality of first targets and a plurality of second targets configured to enable a plurality of cameras to be calibrated.

12. A method comprising: aligning a camera with an optical axis by aligning the camera relative to a first target; aligning the camera with a plane perpendicular to the optical axis by aligning the camera relative to a second target; wherein alignment with the targets enables the position of the camera to be calibrated with one or more other cameras.

13. A method as claimed in claim 12 wherein the first target comprises a retroreflector.

14. A method as claimed in any of claims 12 to 13 wherein the second target comprises an autocollimator.

15. A method as claimed in any of claims 12 to 14 wherein the first target and the second target are positioned in alignment with the optical axis.

16. A method as claimed in claim 15 wherein the second target is positioned in front of the first target.

17. A method as claimed in claim 16 wherein a lens is positioned between the first target and the second target.

18. A method as claimed in any of claims 12 to 17 wherein the method enables the position of a camera to be adjusted relative to a position of a second camera.

19. A method as claimed in any of claims 12 to 18 wherein the method enables the position of a camera to be adjusted to correct deviation from the optical axis.

20. A method as claimed in any of claims 12 to 19 wherein the method enables the position of a camera to be adjusted to correct skew about the optical axis.

21. A method as claimed in any of claims 12 to 20 wherein the method enables the position of a camera to be adjusted to correct deviation from a plane perpendicular to the optical axis.

22. A method as claimed in any of claims 12 to 21 comprising providing a plurality of first targets and a plurality of second targets configured to enable a plurality of cameras to be calibrated.