WO2011086227A1

WO2011086227A1 - Digital camera image error detection

Info

Publication number: WO2011086227A1
Application number: PCT/FI2010/050021
Authority: WO
Inventors: Samu Koskinen; Mikko Muukki
Original assignee: Nokia Corporation
Priority date: 2010-01-18
Filing date: 2010-01-18
Publication date: 2011-07-21

Abstract

A method and apparatus are disclosed for digital camera image error detection, especially for fixed pattern noise detection (312) and elimination (314). Images cast onto an image sensor through a lens system are varied using a force element when the lens system is optically connected to the image sensor so that the image sensor takes a blurred reference image. The image sensor comprises a plurality of image capture pixels, each image capture pixel being configured to produce a pixel output signal indicative of the intensity of light that falls on the image capture pixel in question at a given frequency range. Faulty image capture pixels are detected by finding pixel values that differ significantly (312) from neighboring pixel values in the blurred reference image. An indication of the faulty or defective pixels is stored in a memory (322). The blurred reference image may be formed by defocusing or by optically dithering (306) the image falling on the image sensor.

Description

DIGITAL CAMERA IMAGE ERROR DETECTION TECHNICAL FIELD The present invention generally relates to digital camera image error detection. BACKGROUND ART

Digital cameras have become ubiquitous as modern mobile telephones increasingly provide digital imaging. Initially, camera phones have had modest image quality. At present, there are yet various models with fair resolution and optics so that camera phones can produce photographic images with relatively high-quality. However, improving image quality also accents different image artifacts, as individual pixel errors stand out clearer when there is little noise in digital images.

Digital cameras have to balance between requirements of optics and semi- conduction technological challenges. For instance, the portability of the camera and sensitivity are the better the smaller the image sensor is. Unfortunately, more densely packed image sensors are more prone to electric interference coupling between adjacent signal lines (of different pixels). Further still, the smaller each pixel is, the smaller dust particle suffices to hinder proper operation of that pixel.

It is regrettable that the trend towards better optics and image sensors also increases the harm caused by defective pixels in image sensors.

SUMMARY

According to a first example aspect of the invention there is provided an apparatus comprising:

an image sensor comprising a plurality of image capture pixels, each image capture pixel being configured to produce a pixel output signal indicative of the intensity of light that falls on the image capture pixel in question at a given frequency range;

a force element configured to vary an image cast onto the image sensor through a lens system, when the lens system is optically connected to the image sensor, so that the image sensor takes a blurred reference image;

a processor configured to detect faulty image capture pixels by detecting distinct output signals from the output signals produced on taking the blurred reference image; and

a memory configured to store an indication of faulty image capture pixels.

The image sensor may be configured to take user images. The user images may be stored in the memory. The term user image may refer to an image that is desired by a user. In this context, the user may be a human being or any other party that determines timing and/or framing of the capture of the user image.

The memory may comprise two or more different memory units. The different memory units may each be selected from a group consisting of persistent memory and non-persistent memory. The force element may be chosen from a group consisting of:

a mechanical image stabilizer configured to stabilize images cast onto the image sensor through a lens system, when the lens system is optically connected to the image sensor;

a focusing actuator configured to focus images cast onto the image sensor through a lens system, when the lens system is optically connected to the image sensor; and

a vibrator configured to oscillate images cast onto the image sensor through a lens system with relation to the image sensor, when the lens system is optically connected to the image sensor.

The force element may be configured to move at least one of the image sensor and the lens system. The detection of distinct output signals may be based on detecting a difference between given output signal and average output signals that is larger than a given threshold. The average output signal may refer to average computed over image capture pixels within given area about a given image capture pixel. The given area may be smaller than the area of the blurred reference image.

According to a second example aspect there is provided a method comprising: varying images cast onto an image sensor through a lens system using a force element when the lens system is optically connected to the image sensor so that the image sensor takes a blurred reference image, the image sensor comprising a plurality of image capture pixels, each image capture pixel being configured to produce a pixel output signal indicative of the intensity of light that falls on the image capture pixel in question at a given frequency range;

detecting faulty image capture pixels by detecting distinct output signals from the output signals produced on taking the blurred reference image; and

storing in a memory an indication of the faulty image capture pixels.

According to a third example aspect there is provided a computer program comprising computer executable program code configured to cause an apparatus to perform, on executing the program code, a method according to the second aspect.

According to a third example aspect there is provided a memory medium carrying the computer program of the third example aspect.

Any foregoing memory medium may comprise a digital data storage such as a data disc or diskette, optical storage, magnetic storage, holographic storage, opto- magnetic storage, phase-change memory, resistive random access memory, magnetic random access memory, solid-electrolyte memory, ferroelectric random access memory, organic memory or polymer memory. The memory medium may be formed into a device without other substantial functions than storing memory or it may be formed as part of a device with other functions, including but not limited to a memory of a computer, a chip set, and a sub assembly of an electronic device.

Different non-binding example aspects and embodiments of the present invention have been illustrated in the foregoing. The above embodiments are used merely to explain selected aspects or steps that may be utilized in implementations of the present invention. Some embodiments may be presented only with reference to certain example aspects of the invention. It should be appreciated that corresponding embodiments may apply to other example aspects as well.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described, by way of example only, with reference to the accompanying drawings, in which:

Fig. 1 shows a schematic picture of a system according to an example embodiment of the invention;

Fig. 2 shows a block diagram of an apparatus according to another example embodiment of the invention; and

Fig. 3 shows a flow chart illustrating a process according to yet another example embodiment of the invention.

DETAILED DESCRIPTION

In the following description, like numbers denote like elements.

Fig. 1 shows a schematic picture of a system 100 according to an example embodiment of the invention. The system comprises an apparatus 1 10 and a scene 120 to which the apparatus 1 10 has a view. The apparatus 1 10 may be understood as a mere digital image capture circuitry or as a larger entirety such as a camera phone in different example embodiments of the invention. In sake of simplicity of description, let us assume that the apparatus 1 10 is a camera phone with cellular functions and digital imaging capability. The apparatus 1 10 may be capable of capturing still images and video footage. It is appreciated that in case of video footage, each video image frame is processed as a still image in the following description. For instance, with video footage, a blurred reference image may not be taken between capturing sequential video image frames but instead before or after the sequence of video image frames.

Fig. 2 shows a block diagram of an apparatus 1 10 according to another example embodiment of the invention. The apparatus 1 10 of Fig. 2 comprises usual parts of a mobile telephone and also an image sensor 210. The image sensor 210 comprises a plurality of image capture pixels 220, also referred to as photo sites, each image capture pixel 220 being configured to produce a pixel output signal indicative of the intensity of light that falls on the image capture pixel 220 in question at a given frequency range. The given frequency ranges may correspond, for example, frequencies of red, green and blue colors.

The output signals may be analog signals in which case the apparatus 1 10 further comprises an analog/digital converter 230 for converting the output signals with desired bit depth. The desired bit depth may be, for instance, 8, 10, 12 or 14 bits per sample.

Depending on example embodiment of the invention, term image pixel is used to refer to the output signals as such or as digitized. In many example embodiments of the invention, the detection of faulty image pixels is performed digitally and in this case the image pixels refer to digitized values.

The apparatus 1 10 further comprises a lens system 235 and a force element 240 that is at least one of the following:

• a mechanical image stabilizer 242 configured to stabilize images cast onto the image sensor 210 through the lens system 235, when the lens system 235 is optically connected to the image sensor 210;

• a focusing actuator 244 configured to focus images cast onto the image sensor 210 through a lens system, when the lens system 235 is optically connected to the image sensor 210; and • a vibrator 246 configured to oscillate images cast onto the image sensor 210 through a lens system with relation to the image sensor 210, when the lens system 235 is optically connected to the image sensor 210. In one example embodiment of the invention, the vibrator 246 is a vibration alarm element of a mobile telephone. The lens system 235 and the image sensor

210 may be so mutually attached that the vibration caused by the vibrator 246 causes resonant vibration between the lens system 235 and the image sensor 210. In the following, there is a particularly detailed description of using the mechanical image stabilizer 242 as the force element 240. The structure and operation when the focusing actuator 244 or vibrator is used as the force element 240 will be further described at the end of the detailed description. The force element is configured in some example embodiment of the invention to vary images cast onto the image sensor 210 by changing alignment of the view on the image sensor 210.

The force element is configured in some example embodiment of the invention to vary images cast onto the image sensor 210 by changing focus of the lens system 235.

The varying of the images cast onto the image sensor may further comprise changing of zooming with a zoom objective.

In one example embodiment, the lens system 235 or at least a part of the lens system 235 is user replaceable while in another example embodiment, the lens system 235 is in permanent optical connection with the image sensor 210. In various example embodiments of the invention, the mechanical image stabilizer 242 is based on changing the position of the camera lens (entire lens system 235) or part of the camera lens system or position of the image sensor 210 as appropriate during image capture. The position may be changed in XY plane, but also in other dimensions. Instead of changing position the desired effect may be achieved by varying properties of the optics as appropriate. The main principles of image stabilizing according to some example embodiments are listed in the following:

1 . Lens system shift. All lenses in the lens system 235 are shifted in relation to the image sensor 210 in order to vary optical path of light through the lens system 235 to the image sensor 210.

2. Lens shift or partial lens system correction. In this case, not all lenses in the camera lens system 235 are necessarily shifted. Instead there are one or more correction lenses that are shifted in order to vary the optical path. In another option a correction lens may be such that its optical power is changed in order to vary the optical path.

3. Lens system tilt or lens tilt or variable prism. A lens or the lens system 235 is tilted in relation to the image sensor 210 in order to vary the optical path. In some example embodiments, the tilting effect is obtained by using a variable prism in the lens system 235.

4. Image sensor tilt or shift. The image sensor 210 is tilted or shifted in relation to the lens system 235 in order to vary the optical path.

5. Camera module tilt. A camera module or part of the camera module is tilted.

It shall be appreciated that the listed stabilizing techniques are applied when the mechanical image stabilizer 242 is used to stabilize a camera image as in the prior art. However, in example embodiments described in the following, the mechanical image stabilizer 242 has also a drastically different function in blurring the camera image as will become apparent to the reader from the following description. For the blurring effect, the implementation of the mechanical image stabilizer 242 only relevant in terms of implementation, i.e. to determine which part or parts are being moved and how much. On conceptual level, an ordinarily skilled reader appreciates that the mechanical image stabilizer 242 in its normal operation attempts to maintain a constant view at the image sensor 210 despite of some hand shake, for instance, while in the described blurring embodiments, the mechanical image stabilizer 242 intentionally changes the view at the image sensor 210. The apparatus 1 10 further comprises an image memory 250 configured to store a user image 252 taken with the image sensor and a buffer 260 configured to store an indication 262 of faulty image capture pixels 220. The indication may comprise, for example, a look-up table of bad image capture pixel locations or a bitmap indicating status of each image capture pixel 220.

The apparatus 1 10 further comprises control logics 270 stored in a logics memory 272 or hardwired circuitry. Further still, the apparatus 1 10 comprises a processor 280 configured to control the image stabilizer 242 according to the control logics 270 so that the image stabilizer 242 moves the image cast onto the image sensor 210 and the image sensor 210 takes a blurred reference image. The processor 280 is further configured to detect faulty image capture pixels 220 by detecting distinct output signals from the output signals produced on taking the blurred reference image.

The logics memory 272 is implemented in different example embodiments of the invention in various ways. In one example embodiment, the logics memory 272 is a non-volatile or a volatile memory, such as a read-only memory (ROM), a programmable read-only memory (PROM), erasable programmable read-only memory (EPROM), a random-access memory (RAM), a flash memory, a data disk, an optical storage, a magnetic storage, a smart card, or the like. In one example embodiments of the invention, the logics memory 272 is constructed as a part of the apparatus 1 10 or it the logics memory 272 is inserted into a slot, port, or the like of the apparatus 1 10 by a user. Moreover, in one example embodiment of the invention, the logics memory 272 is a component that is built to solely store data. In another example embodiment, the logics memory 272 is constructed as a part of a component built to serve additionally other purposes, such as processing of data.

The buffer and the logics memory may be implemented in part or entirely using common memory circuitries. As mentioned, the control logics 270 may be stored in hardwired circuitry. For instance, the control logics 270 may be stored as hardwired in a hardwired circuitry such as, for instance, a silicon chip. The apparatus 1 10 further comprises a user interface 290 for man-machine interfacing. The user interface may comprise, for example, a display, speaker, display, loudspeaker, touchpad, touch screen, keypad, keyboard, dedicated keys, soft keys, and tactile response production element such as a vibrator. The user interface is configured in one example embodiment of the invention to provide the processor 280 with user selections concerning one or more of the following:

• whether a blurred reference frame is formed immediately before taking the user image 252;

• whether a blurred reference frame is formed immediately after taking the user image 252; whether information of only partly faulty image capture pixels 220 should be used;

• whether a blurred reference frame should be formed instantaneously based on user command; and

• whether corrective processing should be performed by the processor 280 and/or detected information and the user image 252 should be handed over to an external device for post-processing.

The processor 280 is in one example embodiment selected from a central processing unit; a master control unit; a digital signal processor; a field effect gate array; or an application specific integrated circuit.

The processor is in one embodiment responsible for one or more other tasks. For instance, the processor 280 may control the operation of the user interface 290. Alternatively, the processor 280 may be a dedicated processor for performing processes as described in this document. Further alternatively, the processor 280 may consist of more than one processing elements disposed on one or more circuitry units. It was mentioned in the foregoing that a mechanical stabilizer 242 can be used to blur the reference image even though the mechanical image stabilizer 242 is originally invented to reduce blurring. In an embodiment of the invention, the blurred reference image is used to detect stuck image capture pixels 220 (image capture pixels stuck to given value), hot image capture pixels 220 (image capture pixels with excess brightness in comparison to surrounding image capture pixels) or dead pixels (pixels that stay blank). All the sharp image capture pixels 220 in the blurred reference image (that should be totally blurred) are detected as defected or faulty image pixels, which faulty image pixels correspond to co-aligned image capture pixels 220. It should be appreciated that according to different example embodiments of the invention, the blurred reference image is captured at largely varying times. Moreover, the present view or scenery seen by the camera makes little difference because the mechanical image stabilizer 242 can blur everything. Further still, the blurred reference image need not be flat for defected image capture pixels 220 to be detected.

In an example embodiment of the invention, green imbalance is detected in a similar manner as faulty image capture pixels 220. Green imbalance is a known phenomenon that occurs in image capture units arranged according to a Bayer pattern. In the Bayer pattern, a group of four pixels comprises one image capture pixel 220 for red color and one for blue color, and two image capture pixels 220 for green color. Between these two green image capture pixels 220, there may occur imbalance as the different image capture pixels produce more or less different analog output signals. With the blurred reference image, it is possible to compare directly the two green image capture pixels 220 with one another. In this example embodiment, however, green image capture pixels 220 of more than one recurring image capture pixel groups (such as the Bayer pattern) are combined by suitable filtering to produce a filtered output for desired area and then these filtered outputs are compared to judge about green imbalance over the desired area. The desired area may comprise, for instance, 4, 16 or 25 image capture pixel groups.

Fig. 3 shows a block diagram illustrating a process according to particular example embodiments of the invention. Fig. 3 demonstrates select embodiments, all of which are not combined in some other example embodiment of the invention. The process starts from block 300 in which the apparatus 1 10 is switched on and the camera is taken in use (e.g. by the user starting a camera application). In step 302, the user requests taking a photographic image e.g . by pressing a dedicated photo key. In response, the lens system 235 is focused using the focusing actuator 244 according to automatic focusing. Alternatively, the lens system may be manually focused, or the lens system may have a fixed focus in which case no focusing takes place.

In step 304, the user image 252 is taken with the image sensor 210 and stored in the image memory 250.

In step 304, the blurred reference image is taken. In an example embodiment of the invention, the blurred image is captured right before and/or after the image that the user wants to capture. This embodiment may particularly facilitate setting exposure time and analog gain values for the image capture pixels 220 matching with those of the image that is being processed for faulty image capture pixels 220.

It is appreciated that the appearance of hot image capture pixels 220 depend on the used exposure settings. Green imbalance on the other hand may depend on the color temperature. Generally the green balance may yet be considered as independent of the current scene.

It is thus appreciated that the blurring of the reference image sequentially with the user image 252 enables more accurate detection of faulty images than production of a blurred reference image e.g. on manufacturing of the camera. Not only can the operation of the image capture pixels 220 change, but e.g. dust may travel on the image sensors and the effect of faulty image capture pixels 220 may change depending on the image subject and illumination, as was disclosed in the foregoing. Moreover, it is appreciated that by using the image stabilizer 242 to blur the reference image, the reference image can be reliably blurred within a reasonably short time even if the camera is fixedly mounted (e.g. in a passport photo booth camera or a single-lens reflect camera fixed to a cradle).

It is appreciated that in some example embodiments of the invention it is not necessary to store fault indication 262 of faulty image capture pixels 220 in a nonvolatile memory.

The blurred reference image may be taken at a range of different cases depending on example embodiment of the invention. For instance, the blurred reference image is taken in particular example embodiments of the invention in following cases: when the apparatus 1 10 is started, when a camera application is started in the apparatus 1 10, on request by user, and/or when closing or shutting down the apparatus 1 10 or the camera application. It is also appreciated that there is a practical minimum lower limit for the time in which image can be blurred to a particular extent. For instance, the degree of blurring depends on the extent to which the optical path of the image travels with relation to the image sensor 210. This travelling depends on the speed of the mechanical image stabilizer 242 and the time of travel. In other words, on taking the blurred reference image, the exposure may be restricted 304 on lower end to allow the mechanical image stabilizer 242 to move sufficiently for desired extent of blurring in the reference image.

In one example embodiment of the invention, the blurring is performed 306 with motion of the optical path in different directions so as to avoid or m itigate producing of clear directional lines. In an example embodiment of the invention, the mechanical image stabilizer 242 is configured to move the optical path from one extreme end to another extreme end from top to bottom and from left to right. In another example embodiment of the invention, the mechanical image stabilizer 242 is configured to move the optical path along a circular, oval or curved path. In yet another example embodiment of the invention, the mechanical image stabilizer 242 is configured to move the optical path along an arbitrary path with deviations in both X- and Y-direction so as to blur the image in different directions. In one example embodiment of the invention, the optical path is moved in at least two non-parallel directions, i.e. in directions that have a mutual angle other than 0 or 180 degrees. In an example embodiment of the invention, the mechanical image stabilizer 242 comprises a gyroscope configured to produce motion information for stabilizing the image on image capture. Such a gyroscope is known from the field of image stabilizing. In this embodiment, the motion information is further used to avoid or mitigate the chance that the user accidentally cancels the effect of the mechanical image stabilizer 242 by moving the camera in opposite directions. I n th is embodiment, the motion information is used 308 to account for user-induced motion on moving the optical path from the image to the image sensor 210 along a desired path. In an example embodiment of the invention, on particularly high exposure times set for the user image 252, the blurring is performed 310 over a period of time that is shorter than the exposure time in order to reduce the time over which the image sensor 210 is occupied for taking of the blurred reference image. In this embodiment, detection of faulty image capture pixels 220 may apply lower fault detection threshold than when full exposure time can be applied.

In an example embodiment of the invention, faulty image capture pixels and correspondingly faulty image pixels are detected 312 by determining individual sharp or distinct image pixels. The determination of individual sharp pixels is performed in one example embodiment of the invention by calculating an average over the entire blurred reference image or over given sub-part of the blurred reference image and then comparing individual image pixels to the average pixel. A difference larger than a predetermined threshold is interpreted as indication of a faulty image capture pixel 220. In an example embodiment of the invention, the average pixel is computed using one of the following averaging methods: average, mean, median, and mode. In one example embodiment of the invention, faulty image pixels of the user image 252 are replaced 314 with estimated image pixels for concealing of errors in the user image 252. The estimated image pixels are calculated by the processor 280 using a suitable method. The suitable method may comprise, for example, one or more of the following: taking an average of the values of the surrounding image pixels; taking a mean of the values of the surrounding image pixels; taking a weighed average of the values of the surrounding image pixels; taking a weighed mean of the values of the surrounding image pixels; taking a running average over surrounding pixels; detecting surrounding contours in the user image 252 and extrapolating detected contours to determine the estimated image pixel; by replacing the pixel by one of the surrounding image pixels; and any one of said methods.

It is appreciated that the hot image pixels on the other hand often contain some valid information that may be usable. In an example embodiment of the invention, the information of the hot image pixels is used 316 to produce an enhanced estimated image pixel. The hot pixels are detected and the difference in sensitivity with surrounding image pixels is detected and stored in the buffer 260. The detected difference in sensitivity is compensated to counter-balance the detected difference. This embodiment may help to mitigate loss of resolution.

As with hot image pixels, in one example embodiment of the invention deviations in green balance are rectified 318 based on distinctions in the blurred reference frame. In an example embodiment of the invention, image pixels with green imbalance are detected and the difference in green balance with surrounding pixels is detected and stored in the buffer 260. The detected difference in green balance is compensated to counter-balance the detected difference.

In one example embodiment of the invention, the process further comprises storing 320 in the image memory 250 the user image 252 after performing of corrections of image pixel faults. The stored user image 252 may replace the originally taken user image 252. In one example embodiment of the invention, the process further comprises storing 322 in the image memory 250 a raw image (i.e. the user image 252 without any image pixel correction and possibly also without compression). Also fault information comprising indication of detected image pixel faults is stored in this example embodiment.The stored raw image and fault information is then used in one example embodiment of the invention for post-processing. The postprocessing may be performed by the apparatus 1 1 0 or in external computing equipment. With post-processing, more complex and efficient algorithms may be employed than during the process of taking the user image 252, as during normal image capture delays should be kept to the minimum. In post-processing, instead, substantially greater time spending is normally acceptable and greater computational resources may be available.

It is appreciated that particular example embodiment of the invention may enable run time calibrated defect pixel detection in which only defected pixels (very near in time to the capture moment) are detected and corrected. It is also appreciated that detecting and correcting image pixels only at defected image pixels may avoid causing such image artifacts that are commonly produced by known user image estimation based image capture pixel defect correction methods.

Moreover, it is appreciated that by configuring a camera to detect and compensate for pixel defects during use may enable abstaining from running time consuming and complex calibration processes on camera production. This may be particularly useful on mass production of camera modules to, for example, mobile phones.

It is further appreciated that some described example embodiment of the invention may enable detection and correction of different types of defects of the image capture pixels 220, e.g. stuck pixels, hot pixels and/or pixels with green imbalance. Yet further, it is appreciated that by performing image capture pixel 220 fault detection according to some example embodiment of the invention, it is possible to detect single defects, couplets, triplets etc.. Moreover, surrounding image pixels of the user image 252 have no bearing on detection of the faulty pixels so that these embodiments may be operable under vastly varying conditions and with drastically different image subjects.

Even still further, it is appreciated that on producing the estimated image pixel, even simple algorithms can be used. Hence, even a simple algorithm may produce good image quality and thus computational cost of implementing the image pixel estimation may be minute in comparison to some prior known methods.

By taking a blurred reference picture with blurring invoked by the image stabilizer 242, good blurred reference pictures may also be produced even without a mechanical shutter. This may help to reduce thickness of the camera. It is recalled that lower thickness is particularly desired in camera phones.

In the foregoing, the image stabilizer 242 has been used as an example of the force element 240 with which the blurred reference image is made. An ordinarily skilled person will appreciate how various implementation-related details can be adapted to use a focusing actuator 244 or the vibrator 246. In any case, following observations are expressly provided:

1 . When the focus of the lens system is rapidly changed during exposing the reference image with the image sensor 210, the reference image becomes blurred. To enhance the blurring, possible auto-focus circuitry is over-ridden in one example embodiment of the invention. In another example embodiment of the invention, the auto-focus circuitry is configured to operate in a reverse mode, in which the auto-focus circuitry also aims at producing an un-focused or blurred reference image. Moreover, the focusing actuator 244 is, in one example embodiment of the invention, configured to have two operation modes, i.e. a normal mode in which the motion is smooth and controlled, and a blurring mode, in which the motion is rapid and less controlled.

2. The vibrator 246 may be mounted in the proximity of the image sensor 210 and/or the lens system 235. The remaining one of the image sensor 210 and the lens system 235 may be elastically supported so that rapid vibrations cause in such a floating part resonance that effectively varies the incidence of image on the image sensor 210. Such a construction may employ vibration alarm unit of a mobile phone, in which case addition blurred reference images may be formed without any actuatable parts. In this case, the camera may even be implemented with a fixed focus, without a mechanical shutter and without a mechanical image stabilizer.

Various embodiments have been presented. It should be appreciated that in this document, words comprise, include and contain are each used as open-ended expressions with no intended exclusivity. The foregoing description has provided by way of non-limiting examples of particular implementations and embodiments of the invention a full and informative description of the best mode presently contemplated by the inventors for carrying out the invention. It is however clear to a person skilled in the art that the invention is not restricted to details of the embodiments presented above, but that it can be implemented in other embodiments using equivalent means or in different combinations of embodiments without deviating from the characteristics of the invention.

Furthermore, some of the features of the above-disclosed embodiments of this invention may be used to advantage without the corresponding use of other features. As such, the foregoing description shall be considered as merely illustrative of the principles of the present invention, and not in limitation thereof. Hence, the scope of the invention is only restricted by the appended patent claims.

Claims

ims:

An apparatus comprising:

a memory configured to store an indication of faulty image capture pixels.

The apparatus of claim 1 , wherein the force element is a mechanical image stabilizer configured to stabilize images cast onto the image sensor through a lens system, when the lens system is optically connected to the image sensor.

The apparatus of claim 1 , wherein the force element is a focusing actuator configured to focus images cast onto the image sensor through a lens system, when the lens system is optically connected to the image sensor.

The apparatus of claim 1 , wherein the force element is a vibrator configured to oscillate images cast onto the image sensor through a lens system with relation to the image sensor, when the lens system is optically connected to the image sensor.

The apparatus of claim 2, wherein the force element is configured to change mutual orientation of the image sensor and the lens system so that the image cast by the lens system onto the image sensor moves in at least two non- parallel directions.

6. The apparatus of claim 2, wherein the force element is configured to change mutual orientation of the image sensor and the lens system so that the image cast by the lens system onto the image sensor moves along a curved path. 7. The apparatus of any one of the preceding claims, wherein the image sensor is further configured to take a user image and to take the blurred reference image substantially on taking the user image.

8. The apparatus of any one of the preceding claims, wherein the processor is further configured to conceal errors in user images taken with the image sensor by estimating image pixels at the locations of the detected faulty image capture pixels.

9. The apparatus of any one of the preceding claims, wherein the processor is further configured to enhance user images taken with the image sensor by using pixel output signals corresponding to some faulty image capture pixels.

10. The apparatus of any one of the preceding claims, wherein the detection of distinct output signals is based on detecting a difference between given output signal and average output signals that is larger than a given threshold.

1 1 .A method comprising:

varying images cast onto an image sensor through a lens system using a force element when the lens system is optically connected to the image sensor so that the image sensor takes a blurred reference image, the image sensor comprising a plurality of image capture pixels, each image capture pixel being configured to produce a pixel output signal indicative of the intensity of light that falls on the image capture pixel in question at a given frequency range; detecting faulty image capture pixels by detecting distinct output signals from the output signals produced on taking the blurred reference image; and storing in a memory an indication of the faulty image capture pixels.

12. The method of claim 1 1 , wherein the force element is a mechanical image stabilizer, the method further comprising stabilizing with the mechanical image stabilizer images cast onto the image sensor through a lens system, when the lens system is optically connected to the image sensor.

13. The method of claim 1 1 , wherein the force element is a focusing actuator, the method further comprising focusing by the focusing actuator images cast onto the image sensor through a lens system, when the lens system is optically connected to the image sensor.

14. The method of claim 1 1 , wherein the force element is a vibrator, the method further comprising oscillating by the vibrator images cast onto the image sensor through a lens system with relation to the image sensor, when the lens system is optically connected to the image sensor.

15. The method of claim 12, the method further comprising changing by the force element mutual orientation of the image sensor and the lens system so that the image cast by the lens system onto the image sensor moves in at least two non-parallel directions.

16. The method of claim 12, the method further comprising changing by the force element mutual orientation of the image sensor and the lens system so that the image cast by the lens system onto the image sensor moves along a curved path.

17. The method of any one of claims 1 1 to 16, further comprising taking a user image, wherein the blurred reference image is taken substantially on taking the user image.

18. The method of any one of claims 1 1 to 17, further comprising concealing by the processor errors in user images taken with the image sensor by estimating image pixels at locations of the detected faulty image capture pixels.

19. The method of any one of claims 1 1 to 18, further comprising detecting distinct output signals based on detecting a difference between given output signal and average output signals that is larger than a given threshold.

20. A computer program comprising computer executable program code configured to cause an apparatus, when executing the program code, to perform the method of any one of claims 1 1 to 19.