Method and device for position sensing of an optical component in an imaging system

A light beam is used to illuminate a spot on a lens element which is shifted along the optical axis of an imaging system for auto-focus or optical zoom purposes. The light beam is arranged such that the reflected light beam from the lens element encounters a spot on the image sensor. The spot location on the image sensor is determined by a signal processor. As the lens element is shifted along the optical axis, the spot location changes accordingly. Based on the moving distance of the spot, the signal processor determines the shifting distance of the lens element and hence the position of the lens element. Based on the shifting distance of the lens element, a control module is used to adjust the lens position along the optical axis to achieve the desired focusing or zooming effects.

FIELD OF THE INVENTION

The present invention relates generally to optical position sensing in an imaging system and, more particularly, to position sensing for auto-focus optics and/or an optical zoom module in the imaging system.

BACKGROUND OF THE INVENTION

Auto-focus optical systems require high precision in position sensing. In general, needed accuracy is in the order to a few microns. Sensor output linearity and immunity to external disturbances is important. Furthermore, the operation mode for position sensing also requires non-contact operation to avoid mechanical wear. When considering optics for use in a small electronic device, such as mobile phone, the size and cost of the optical sensing components and the suitability to mass production are important issues.

Typically, position determination in a commercial auto-focus module is carried out by counting stepper motor steps. For that purpose, the motor can have an embedded position encoder. In order to reduce the size of the optical modules, miniature piezoelectric motors or actuators are generally used. These motors and actuators require a separate position sensor.

In fulfilling the need for an auto-focus optical system or an optical zoom system with movement in the order of a few microns, the present invention provides a simple method and device for position sensing.

SUMMARY OF THE INVENTION

The present invention uses a light beam to illuminate a spot on a lens element which is shifted along the optical axis of the imaging system for auto-focus or optical zoom purposes. The light beam is arranged such that the reflected light beam from the lens element encounters a spot on the image sensor. The spot on the image sensor is read out in a signal processor. As the lens element is shifted along the optical axis, the spot on the image sensor is moved accordingly. Based on the moving distance of the spot on the image sensor, the signal processor determines the shifting distance of the lens element and hence the position of the lens element. Based on the shifting distance of the lens element, a control module is used to adjust the lens position along the optical axis to achieve the desired focusing or zooming effects. The control module also controls the driving of the light beam such that the light beam is turned on only between image frames for lens position sensing.

It is possible that the light spot on the image sensor is distorted due to reflection and that the shape of the light spot on the image sensor changes considerably due to lens shifting. It may be desirable to carry out a calibration procedure to compare the actual lens shifting distance with that determined by the signal processor based on the shifting of the light spot. Furthermore, the light spot can be provided with a pulsed light source which is synchronized with frame capture such that information on the lens position can be acquired between frames. Moreover, when the data from the image sensor is transferred in order to determine the lens shifting, it is possible to transfer the image data from only a part of the image sensor where the light spot is expected to be located.

The present invention will become apparent upon reading the description taken in conjunction withFIGS. 2 to 6.

DETAILED DESCRIPTION OF THE INVENTION

Imaging applications such as auto-focus lens systems and optical zoom systems require high precision in position sensing. In such applications, at least one lens element is moved along the optical axis of the imaging system so as to change the focal plane of the lens or the magnification of the image formed on an image sensor. As shown inFIG. 1, the movement of the lens element is substantially along the optical axis which is parallel to the Z axis. The image sensor is located in an image plane which is substantially parallel to the XY plane. The imaging system may have one or more stationary lens elements as depicted in dotted lines.

In auto-focus or optical zoom applications, it is required to determine the position of the lens element relative to a reference point or a home position. According to the present invention, a light beam is used to illuminate a spot on the surface of the lens element such that the reflected light beam from the lens surface encounters a spot on the image sensor. As shown inFIG. 2, the imaging system1has a light source40, such as a laser or a light-emitting diode, which is used to provide a beam42to illuminate a spot A on a lens element10which is shifted along the optical axis of the imaging system for auto-focus or optical zoom purposes. The light beam is arranged such that the reflected light beam44from the lens element10encounters a spot S1on the image sensor. The light beam42may also encounter another surface of the lens element10. For example, the light beam42may hit another surface at a spot B and the reflected light beam46from the spot B may also encounter the image sensor. For position sensing purposes, it is possible to use only the spot S1produced by the reflected beam44.

The coordinates of the spot S1on the image sensor20can be determined by a signal processor50. As shown inFIG. 2, the image sensor20is disposed on a substrate30, and the light source40is also disposed on the substrate30near the image sensor20. As shown, the imaging system1also comprises a movement mechanism60to shift the lens element10along the optical axis and a light source driver80to drive the light source40. A control module70is used to control the position of lens element10through the movement mechanism60, based on the reading of the spot location on the image sensor. The control module70also controls the on/off time of the light source40through the light source driver80. For example, if optical zooming is carried out while a series of image frames are acquired, the light source40is turned on only during the time period between two consecutive image frames, and position of the light spot on the image sensor is read out by the signal processor50accordingly.

When the lens element10is shifted along the optical axis in a direction away from the image sensor20, the light beam42hits a new spot C on the lens element and a new reflected light beam45occurs. The reflected light beam45encounters the image sensor20at a new spot S2, as shown inFIG. 3. Through calibration, it is possible to determine the shifting distance d of the lens element10based on the moving distance D of the spot on the image sensor20.

An approximate relationship between the moving distance, D, of the light spot on the image sensor and the shifting distance, d, of the lens element is shown inFIG. 4. As shown inFIG. 4, the length L in the triangle is approximately equal to the shifting distance d. If the lens shifting distance d is small, the spot moving distance D is approximately equal to L. If the lens shifting distance is 0.5 mm, for example, then D is also in the order of 0.5 mm. With an image sensor having 2.8 μm pixel resolution, the number of pixels between S1and S2is approximately 178. If the mechanical movement of the lens element can be made in steps in the neighborhood of 2.8 μm and the light beam42is well collimated, the position of the lens element can be determined by the signal processor with the same precision. In general, a good auto-focus performance requires approximately 50 steps to shift the lens element over a distance of 0.5 mm. The present invention provides a method that can yield over 3× improvement in accuracy.

While it is preferable to dispose the light source40on the substrate30along with the image sensor20, the light source40can be disposed at an appropriate location within the imaging system1. For example, the light source40can be separated from the substrate30, as shown inFIG. 5. Furthermore, the light beam42can be reflected from a surface14before it encounters the lens element10and the reflected beam44can be reflected from a surface12before it encounters the image sensor20.

Depending on the shape of the light beam, the spot of the lens surface at which the illuminating beam42is reflected, and other intervening optical components, the light spot on the image sensor may be distorted. Also, the shape of the spot on the image sensor may change significantly due to the lens shifting. Thus, it may be desirable to carry out a calibration procedure in order to compare the actual lens shifting distance with that determined by the signal processor based on the shifting of the light spot on the image sensor. The calibration data can be stored in a memory device54as shown inFIG. 5, for example.

The light source40can be a single collimated light emitter or a combined system of a light source and beam shaping optics. While it is preferred that the light beam42is collimated, it is possible to shape the light beam such that a clear spot can be observed on the image sensor so as to allow the signal processor to determine the lens shifting distance with a desired precision.

Moreover, while it is preferable to use the reflected light beam from the first surface encountering the light beam42, the light beams from other surfaces of the lens element can also be used. Furthermore, the light source40can be a pulsed light source which is synchronized with frame capture such that information on the lens position can be acquired between image frames. Also, the amount of data transferred from the image sensor20to the signal processor50can be smaller when information on the lens position is acquired than when an image frame is captured. For example, when the data from the image sensor20is transferred in order to determine the lens shifting, it is possible to transfer the image data from only a part of the image sensor where the light spot is expected to be located. It is also possible to skip a plurality of pixels in such data transfer.

The position sensing, according to various embodiments of the present invention, can be illustrated in a flowchart as shown inFIG. 6. As shown in the flowchart100, after a start signal is used to start the process of capturing one or more image frames, a light spot is provided to illuminate a lens element for achieving a reflected light spot on the image sensor and the lens position on the image sensor is determined at step110, based on the light spot position on the image sensor. If calibration data is available, then the lens position is adjusted based on the calibration data at step120. One or more image frames are captured at step130. If one or more further frames to be captured, as determined at step140, then the process loops back to step110. Otherwise, the process is caused to end.

Thus, although the invention has been described with respect to one or more embodiments thereof, it will be understood by those skilled in the art that the foregoing and various other changes, omissions and deviations in the form and detail thereof may be made without departing from the scope of this invention.