Image system and interference removing method thereof

There is provided an interference removing method of an image system including the steps of: respectively acquiring different numbers of images within two brightness intervals having different brightness variations using an image sensor having a sampling frequency; lighting a light source with a lighting frequency, which is a half of the sampling frequency, and synchronizing to the sampling frequency; and subtracting a first image, which is associated with the lighting of the light source, acquired in a later brightness interval of two adjacent brightness intervals having identical brightness variations by a second image, which is associated with a sampling time of the first image, acquired in an earlier brightness interval of the two adjacent brightness intervals. The present disclosure further provides an image system.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan Patent Application Serial Number 100102695, filed on Jan. 25, 2011, the full disclosure of which is incorporated herein by reference.

BACKGROUND

1. Field of the Disclosure

This disclosure generally relates to an image system and interference removing method thereof and, more particularly, to an image system and the interference removing method thereof capable of eliminating flicker caused by ambient light sources.

2. Description of the Related Art

In present days the image capture technology has been widely applied to various kinds of electronic products, such as the optical touch system, distance measuring system or other optical applications that perform corresponding processes according to acquired images.

Generally speaking, the optical applications mentioned above utilize an image sensor to acquire images and perform post-processing on the acquired images to calculate the variation of image content. However, since the image sensor can only detect the energy variation, the images acquired by the image sensor can be directly influenced when the brightness of ambient light sources changes with time thereby degrading the operation accuracy of the system.

Particularly, when the brightness of the ambient light sources has a complicated variation or the ambient light sources light with different brightness variation cycles, interference caused thereby can not be easily removed so that the operation accuracy of the image system can not be effectively improved.

Accordingly, it is necessary to provide an image system and interference removing method thereof that can eliminate or can at least significantly reduce the flicker problem caused by the ambient light sources mentioned above.

SUMMARY

It is an object of the present disclosure to provide an image system and an interference removing method thereof capable of eliminating flicker on an image system caused by the complicated brightness variation of ambient light sources.

The present disclosure provides an interference removing method of an image system for removing interference of ambient light having a brightness variation cycle each including two brightness intervals having different brightness variations. The interference removing method includes the steps of: respectively acquiring different numbers of images within the two brightness intervals using an image sensor having a sampling frequency; lighting a light source with a lighting frequency, which is a half of the sampling frequency, and synchronizing to the sampling frequency; and subtracting a first image, which is associated with the lighting of the light source, acquired in a later brightness interval of two adjacent brightness intervals having identical brightness variations by a second image, which is associated with a sampling time of the first image, acquired in an earlier brightness interval of the two adjacent brightness intervals.

The present disclosure further provides an image system configured to remove interference of ambient light having a brightness variation cycle each including two brightness intervals having different brightness variations. The image system includes at least one image sensor, a light source and a processing unit. The image sensors respectively acquire different numbers of images within the two brightness intervals with a sampling frequency. The light source lights with a lighting frequency, which is a half of the sampling frequency, and synchronizes to the sampling frequency. The processing unit is for subtracting a first image, which is associated with the lighting of the light source, acquired in a later brightness interval of two adjacent brightness intervals having identical brightness variations by a second image, which is associated with a sampling time of the first image, acquired in an earlier brightness interval of the two adjacent brightness intervals.

In an aspect, the sampling frequency is (2N+1) times of a reciprocal of the brightness variation cycle, and N is a positive integer larger than or equal to 1, wherein the brightness variation cycle may be ( 1/50) second or ( 1/60) second.

In an aspect, the numbers of images acquired within the two brightness intervals having different brightness variations have a difference of 1.

The present disclosure further provides an interference removing method of an image system for removing interference of ambient light having a brightness variation cycle each including two brightness intervals having different brightness variations. The interference removing method includes the steps of: acquiring an odd number of images within each brightness variation cycle using an image sensor having a sampling frequency; lighting a light source with a lighting frequency, which is a half of the sampling frequency, and synchronizing to the sampling frequency; and subtracting a first image, which is associated with the lighting of the light source, acquired in a later brightness interval of two adjacent brightness intervals having identical brightness variations by a second image, which is associated with a sampling time of the first image, acquired in an earlier brightness interval of the two adjacent brightness intervals.

In the image system and interference removing method of the present disclosure, the processing unit may further calculate a displacement according to two successive differences of the first images and the second images. In the present disclosure, since the brightness of ambient light is substantially identical when the first image and the second image are being acquired, interference from the ambient light can be eliminated by calculating a difference of images acquired within two brightness intervals having identical brightness variations.

DETAILED DESCRIPTION OF THE EMBODIMENT

The present disclosure is related to the elimination of the impact on an image system from ambient light sources illuminate with a brightness variation cycle. Each brightness variation cycle of the ambient light sources outside the image system includes two brightness intervals having different brightness variations, and the different brightness variations of the two brightness intervals may be caused by a single ambient light source or by a combination of different types of ambient light sources.

Please refer toFIGS. 1 and 2,FIG. 1shows an image system1according to an embodiment of the present disclosure;FIG. 2shows the brightness variation with time of the ambient light source. Although the image system1is shown as an optical touch system herein, in other embodiments the image system1may be any system that utilizes at least one image sensor to acquire images to accordingly perform the system control and does not limited to that shown inFIG. 1.

The image system1includes a touch surface10, at least one image sensor (two image sensors11and12are shown herein), a light source13and a processing unit14. In addition, an ambient light source9is also shown inFIG. 1to represent an operational environment of the image system1. For example, the ambient light source9is shown as a set of fluorescent lamp tubes each having a first end91and a second end92. The brightness variation with time of the two ends (i.e. first end91and second end92) of the fluorescent lamp tubes is shown inFIG. 2. A reason that causes the non-uniform brightness variation of the ends of the fluorescent lamp tubes is the direction of the exciting current. For example, in the half cycle that the AC exciting current flows from the first end91to the second end92, the first end91has a higher brightness but the second end92has a lower brightness; on the contrary, in the half cycle that the AC exciting current flows from the second end92to the first end91, the first end91has a lower brightness but the second end92has a higher brightness. Therefore, respective brightness variations of the first end91and the second end92are different in different half cycles of the brightness variation cycles CLas shown inFIG. 2. The infrared light emitted by the fluorescent lamp tubes can especially have obvious non-uniform brightness variation due to different directions of the AC exciting current. The ambient light source9may illuminate in a brightness variation cycle CL, such as ( 1/60) Hz or ( 1/50) Hz, wherein a value of the brightness variation cycle CLmay be determined according to an AC frequency of the power system connected. InFIG. 2, each brightness variation cycle CLincludes a first brightness interval C1and a second brightness interval C2, wherein the first brightness interval C1and the second brightness interval C2have different brightness variations but occupy identical time intervals. It is appreciated that, although an average value of the first brightness interval C1is larger than that of the second brightness interval C2as shown inFIG. 2, respective values may be exchanged in other embodiments.

The touch surface10may be made of suitable material, and a user may use his or her finger or other touch object to approach or contact the touch surface10in order to perform various operations on the image system1, wherein the operations can be performed may be those in conventional touch systems, such as the cursor control or icon selection, but not limited thereto. Since the operation of a touch system is well known to the art, details thereof will not be repeated herein. The spirit of the present disclosure is to eliminate the negative influence on the operation accuracy of the touch system from the brightness variation of the ambient light source9shown inFIG. 2.

The image sensors11and12may be CCD image sensors, CMOS image sensors or the like. Field of views of the image sensors11and12preferably involve at least the touch surface10, and the image sensors11and12are configured to acquire images looking across the touch surface10and containing at least one touch object approaching or contacting the touch surface10. It should be mentioned that, a number of the image sensors and positions of the image sensors are not limited to those shown inFIG. 1.

The light source13is configured to provide the light needed by the image sensors11and12during acquiring images. In order to realize the interference removing method of the present disclosure (described later), the light source13lights once every two images that the image sensors11and12acquire and turns off in other time periods. In other words, if the image sensors11and12have a sampling frequency, the light source13lights with a lighting frequency, which is a half of the sampling frequency, such that the image sensors11and12may acquire a first image associated with the lighting of the light source13and acquire a second image associated with the turning-off of the light source13. It should be mentioned that, the light source13may be any arbitrary light source such as a light emitting diode (LED), but not limited thereto. In addition, the location and number of the light source13is not limited to that shown inFIG. 1, and the light source13may be disposed at any suitable location or may have any number as long as the image sensors11and12are able to acquire images of a touch object blocking the light emitted by the light source13and without any particular limitation.

The processing unit14receives the images acquired by the image sensors11and12and performs post-processing. For example, the processing unit14calculates a displacement or other operating parameters according to an image variation of the touch object in the images acquired so as to accordingly control the software being executed by a host15, wherein the method that the processing unit14calculates the operating parameters is well known to the art, e.g. calculating the displacement by means of the correlation between images, and thus details thereof will not be repeated herein.

Since the field of views of the image sensors11and12involve the ambient light source9, the image of ambient light source9may be contained in the images acquired by the image sensors11and12. When the brightness of the ambient light source9various asFIG. 2, error-control due to the interference from the ambient light source9may happen if the processing unit14directly calculates the displacement or control parameters using the images acquired by the image sensors11and12. Therefore, the processing unit14can eliminate the impact of the ambient light source9on the image system1by using the interference removing method of the present disclosure as described hereinafter.

Please refer toFIGS. 1 to 3,FIG. 3shows a schematic diagram of the interference removing method according to an embodiment of the present disclosure. Under the brightness variation of the ambient light source9, the image sensors11and12acquire images with a sampling frequency that is (2N+1) times of a reciprocal of the brightness variation cycle CL, e.g. 60 Hz×(2N+1) or 50 Hz×(2N+1), wherein N is a positive integer larger than or equal to 1, e.g. N=2 inFIG. 3. It should be mentioned that, in order to realize the interference removing method of the present disclosure, the image sensors11and12acquire different numbers of images respectively within the first brightness interval C1and the second brightness interval C2, and preferably the numbers of images acquired within the two different brightness intervals have a difference of 1. For example inFIG. 3, the image sensors11and12acquire three images within the first brightness interval C1and acquire two images within the second brightness interval C2. In other words, the image sensors11and12acquire an odd number of images in each brightness variation cycle CL. Since the sampling frequency of the image sensor and the brightness variation cycle CLare fixed, the image sensors11and12may acquire images at identical sampling times within the brightness intervals having identical brightness variations. The light source13lights once every two images acquired by the image sensors11and12such that the images sensors11and12may alternatively acquire a bright image I1(e.g. hollow rectangles, which are associated with the lighting of the light source13, shown inFIG. 3) and dark images I2(solid rectangles, which are associated with the turning-off of the light source13, shown inFIG. 3). It is appreciated that, a lighting interval TLof the light source13is preferably equal to or a little larger than a sampling interval Ts of the image sensors11and12such that the light source13may provide enough light when the image sensors11and12are acquiring images.

Next, the processing unit14receives and processes the images acquired by the image sensors11and12, e.g. subtracting a first image, which is associated with the lighting of the light source, acquired in a later brightness interval of two adjacent brightness intervals having identical brightness variations by a second image, which is associated with a sampling time of the first image, acquired in an earlier brightness interval of the two adjacent brightness intervals. For example inFIG. 3, in two adjacent first brightness intervals C1, the processing unit14subtracts a bright image I1acquired at a sampling time t1by a dark image I2acquired at a sampling time t1′; subtracts a bright image I1acquired at a sampling time t2by a dark image I2acquired at a sampling time t2′; and subtracts a bright image I1acquired at a sampling time t4by a dark image I2acquired at a sampling time t4′; wherein the light source13turns on at the sampling times t1, t2and t4but turns off at the sampling times t1′, t2′ and t4′. In addition, in two adjacent second brightness intervals C2, the processing unit14subtracts a bright image I1acquired at a sampling time t3by a dark image I2acquired at a sampling time t3′; wherein the light source13turns on at the sampling time t3but turns off at the sampling time t3′. The processing unit14may obtain an object image after the subtraction, and then calculates a displacement or a control parameter according to an image variation between two object images. It is appreciated that, when the image sensors11and12acquire images at different sampling times within the brightness variation cycle CL, two images may be acquired within the first brightness interval C1and three images may be acquired in the second brightness interval C2.

In another embodiment, when an image acquired by the image sensors11and12is at a connection time Tc of the first brightness interval C1and the second brightness interval C2, the image may be defined to be acquired within the first brightness interval C1or within the second brightness interval C2. Therefore, the processing unit14may still calculate the displacement or control parameter according to the interference removing method of the present disclosure.

Please refer toFIG. 4, it shows a schematic diagram of the interference removing method according to another embodiment of the present disclosure. Herein the image sensors11and12acquire images with a higher sampling frequency and the light source13also lights with a higher lighting frequency, e.g. N=3 herein. In this embodiment, the image sensors11and12acquire four images within the first brightness intervals C1and acquire three images within the second brightness intervals C2. The numbers of images being acquired may be exchanged in another embodiment as long as an odd number of images are acquired in each brightness variation cycle CL. The light source13also lights once every two images acquired by the image sensors11and12such that the image sensors11and12may alternatively acquire bright images I1and dark images I2. In two adjacent first brightness intervals C1, the processing unit14subtracts a bright image I1acquired at a sampling time t1by a dark image I2acquired at a sampling time t1′; subtracts a bright image I1acquired at a sampling time t2by a dark image I2acquired at a sampling time t2′; subtracts a bright image I1acquired at a sampling time t5by a dark image I2acquired at a sampling time t5′; and subtracts a bright image I1acquired at a sampling time t6by a dark image I2acquired at a sampling time t6′. In addition, in two adjacent second brightness intervals C2, the processing unit14subtracts a bright image I1acquired at a sampling time t3by a dark image I2acquired at a sampling time t3′; and subtracts a bright image I1acquired at a sampling time t4by a dark image I2acquired at a sampling time t4′. Similarly, the processing unit14may obtain an object image after the subtraction, and then calculates a displacement or a control parameter according an image variation between two object images.

In addition, the image system1may further include a storage unit (not shown) for temporarily storing a part of the images acquired in a brightness variation cycle previous to the brightness variation cycle CLthat a current image is acquired by the image sensors11and12. For example, when the image sensors11and12acquire images at the sampling time t1, the storage unit stores at least the dark images acquired within a brightness variation cycle (including the first brightness interval C1and the second brightness interval C2) previous to the brightness variation cycle CLassociated with the sampling time t1; e.g. 3 images inFIG. 3and four images inFIG. 4are stored in the storage unit. But in other embodiments, the storage unit may store all images acquired within a brightness variation cycle previous to the brightness variation cycle CLassociated with the sampling time t1. The storage unit may be a frame buffer disposed in or outside the processing unit14.

Please refer toFIG. 5, it shows a flow chart of the interference removing method according to an embodiment of the present disclosure including the steps of: respectively acquiring different numbers of images within two brightness intervals having different brightness variations using an image sensor having a sampling frequency (Step S11); lighting a light source with a lighting frequency, which is a half of the sampling frequency, and synchronizing to the sampling frequency (Step S12); subtracting a first image, which is associated with the lighting of the light source, acquired in a later brightness interval of two adjacent brightness intervals having identical brightness variations by a second image, which is associated with a sampling time of the first image, acquired in an earlier brightness interval of the two adjacent brightness intervals (Step S13). In addition, the interference removing method of the present disclosure further includes the steps of: outputting an object image according to a difference of the first image and the second image; and calculating a displacement according to two successively outputted object images. Details of the interference removing method of the present disclosure have been described inFIGS. 3 and 4and associated descriptions, and thus will not be repeated herein.

Please refer toFIG. 6, it shows a flow chart of the interference removing method according to another embodiment of the present disclosure including the steps of: acquiring an odd number of images within each brightness variation cycle using an image sensor having a sampling frequency (Step S21); lighting a light source with a lighting frequency, which is a half of the sampling frequency, and synchronizing to the sampling frequency (Step S12); and subtracting a first image, which is associated with the lighting of the light source, acquired in a later brightness interval of two adjacent brightness intervals having identical brightness variations by a second image, which is associated with a sampling time of the first image, acquired in an earlier brightness interval of the two adjacent brightness intervals (Step S13). Details of the present embodiment are similar to those ofFIG. 5, and thus details thereof will not be repeated herein. Compared to the previous embodiment, even an image is acquired at a connection time Tc of the first brightness interval C1and the second brightness interval C2by the image sensors11and12as shown inFIG. 3, the processing unit14may still calculate the displacement or control parameter according to the acquired images.

As mentioned above, conventional image systems are not able to totally eliminate the negative influence caused by ambient light sources, especially when the ambient light sources have a complicated brightness variation. Therefore, the present disclosure further provides an image system (FIG. 1) and the interference removing method thereof (FIGS. 3 to 6) that are able to effectively eliminate the negative influence caused by the ambient light sources including two brightness intervals having different brightness variations thereby increasing the operation accuracy of the image system.