# Patent

## Title
Exposure mechanism of optical touch system and optical touch system using the same

## Abstract

An exposure mechanism of an optical touch system, which includes a plurality of image sensors and a plurality of active light sources each irradiating corresponding to the associated image sensor, includes: capturing image frames using the image sensors with a sampling cycle to allow each of the image sensors to capture a bright image, wherein the sampling cycle includes a plurality of working modes and in each of the working modes at least one of the image sensors captures the bright image in a sampling interval; simultaneously capturing a dark image using all the image sensors in a denoising sampling interval; and calculating a differential image between the bright image and the dark image captured by each image sensor.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application is a divisional of U.S. application Ser. No. 14/052,520, filed Oct. 11, 2013, and claims the priority benefit of Taiwan Patent Application Serial Number 101139192, filed on Oct. 24, 2012, the full disclosure of which is incorporated herein by reference.


BACKGROUND
1. Field of the Disclosure
This disclosure generally relates to an optical touch system and, more particularly, to an exposure mechanism of an optical touch system and an optical touch system using the same that may shorten a total retrieval time of a bright image and a dark image spent by all image sensors.
2. Description of Related Art
An optical touch system generally utilizes a plurality of image sensors to sequentially capture a plurality of image frames containing a touch surface image and performs the object tracking according to the position of a finger image in the image frames.
For example referring to FIG. 1A, it shows a schematic diagram of the conventional optical touch system 9, which includes a touch surface 91, four image sensors 921-924 and four active light sources 931-934. When each of the image sensors 921-924 is capturing an image frame, the associated active light sources 931-934 irradiate so as to provide light for capturing the image frame.
Conventionally, it is able to calculate a differential image between a bright image and a dark image so as to eliminate the interference from ambient light.
For example referring to FIG. 1B, the image sensor 921 captures a bright image ION corresponding to the irradiation of the active light source 931 and then captures a dark image IOFF corresponding to the extinction of the active light source 931. And the image sensors 922 to 924 operate similarly. A processing unit (not shown) is configured to calculate a differential image (ION−IOFF) between the bright image ION and the dark image IOFF, and to perform the object positioning according to the differential image. It is assumed that a sampling interval of the image sensors 921 to 924 is T such that a total retrieval time for capturing a bright image ION and a dark image IOFF by all the image sensors 921-924 equals 8T. In this manner, the synchronization between the image sensors 921-924 for capturing image frames is decreased. Especially when the number of the image sensors is increased, the total retrieval time for capturing a bright image and a dark image by all the image sensors becomes longer.
Accordingly, the present disclosure further provides an exposure mechanism of an optical touch system and an optical touch system using the same that may shorten the total retrieval time of a sampling cycle thereby improving the synchronization between image sensors for capturing image frames and increasing the positioning accuracy.
SUMMARY
The present disclosure provides an exposure mechanism of an optical touch system and an optical touch system using the same that have a shorter sampling cycle period and a higher system operating frequency.
The present disclosure further provides an exposure mechanism of an optical touch system and an optical touch system using the same that may eliminate the interference from ambient light.
The present disclosure provides an exposure mechanism of an optical touch system. The optical touch system includes a plurality of image sensors and a plurality of active light sources, and each of the active light sources irradiates corresponding to the associated image sensor. The exposure mechanism includes the steps of: capturing image frames using the image sensors with a sampling cycle to allow each of the image sensors to capture a bright image, wherein the sampling cycle includes a plurality of working modes and in each of the working modes at least one of the image sensors captures the bright image in a sampling interval; simultaneously capturing a dark image using all the image sensors in a denoising sampling interval; and calculating, using a processing unit, a differential image between the bright image and the dark image captured by each of the image sensors.
The present disclosure further provides an optical touch system including a touch surface, a plurality of reflection light bars, a first image sensor, a first active light source, a second image sensor, a second active light source and a processing unit. The reflection light bars are disposed on edges of the touch surface respectively. The first image sensor is configured to capture image frames looking across the touch surface in a first sampling interval and a denoising sampling interval to respectively obtain a first bright image and a first dark image. The first active light source is disposed adjacent to the first image sensor and configured to irradiate corresponding to the first sampling interval. The second image sensor is configured to capture image frames looking across the touch surface in a second sampling interval and the denoising sampling interval to respectively obtain a second bright image and a second dark image. The second active light source is disposed adjacent to the second image sensor and configured to irradiate corresponding to the second sampling interval. The processing unit is configured to calculate a first differential image between the first bright image and the first dark image and calculate a second differential image between the second bright image and the second dark image.
The present disclosure further provides an optical touch system including a touch surface, a plurality of irradiation light bars, a first image sensor, a second image sensor and a processing unit. The irradiation light bars are disposed at edges of the touch surface respectively. The first image sensor is configured to capture image frames looking across the touch surface in a first sampling interval and a denoising sampling interval to respectively obtain a first bright image and a first dark image, wherein the irradiation light bars within a field of view of the first image sensor irradiate corresponding to the first sampling interval. The second image sensor is configured to capture image frames looking across the touch surface in a second sampling interval and the denoising sampling interval to respectively obtain a second bright image and a second dark image, wherein the irradiation light bars within a field of view of the second image sensor irradiate corresponding to the second sampling interval. The processing unit is configured to calculate a first differential image between the first bright image and the first dark image and calculate a second differential image between the second bright image and the second dark image.
In one aspect, the sampling intervals of every working mode are identical, and the sampling intervals are identical to the denoising sampling interval.
In one aspect, the sampling intervals of every working mode are different, and the denoising sampling interval is equal to a shortest sampling interval of the sampling intervals.
In one aspect, the sampling intervals of a part of the working modes are different, and the denoising sampling interval is equal to a shortest sampling interval of the sampling intervals.
In one aspect, the processing unit may further adjust a pixel gray level of the associated dark image according to a time ratio of the sampling interval of the working mode and the denoising sampling interval, and calculate a differential image of the bright image and the adjusted dark image captured by each of the image sensors.
In one aspect, the image sensors simultaneously capture the dark image at first and then capture the bright image with the sampling cycle; or the image sensors capture the bright image with the sampling cycle at first and then simultaneously capture the dark image.
In the optical touch system and the exposure mechanism according to the embodiment of the present disclosure, as the dark image captured in the same denoising sampling interval is subtracted from the bright image captured in one sampling cycle so as to eliminate the interference from ambient light, a total retrieval time of all image sensors for capturing one bright image and one dark image is shortened. In addition, two successive sampling cycles may share the dark image captured in the same denoising sampling interval so as to further reduce the total retrieval time and increase the system operating frequency.



BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, advantages, and novel features of the present disclosure will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.
FIG. 1A shows a schematic diagram of a conventional optical touch system.
FIG. 1B shows an operational schematic diagram of the optical touch system of FIG. 1A.
FIG. 2 shows a schematic diagram of the optical touch system according to an embodiment of the present disclosure.
FIG. 3 shows a flow chart of an exposure mechanism of the optical touch system according to the embodiment of the present disclosure.
FIGS. 4A-4C show schematic diagrams of the exposure mechanism of the optical touch system of FIG. 2.
FIG. 5 shows another schematic diagram of the exposure mechanism of the optical touch system of FIG. 2.
FIGS. 6A-6B show schematic diagrams of the optical touch system according to another embodiment of the present disclosure.
FIGS. 7A-7C show schematic diagrams of another exposure mechanism of the optical touch system of FIG. 2.
FIG. 8 shows another schematic diagram of the exposure mechanism of the optical touch system of FIG. 2.



DETAILED DESCRIPTION OF THE EMBODIMENT
It should be noted that, wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.
Referring to FIG. 2, it shows a schematic diagram of the optical touch system according to an embodiment of the present disclosure. The optical touch system 1 according to the embodiment of the present disclosure includes a touch surface 11, a plurality of image sensors (e.g. S11, S12, S21, S22, S31 and S32), a plurality of light bars 121 to 124 and a processing unit 13. The image sensors may be CCD image sensors, CMOS image sensors or the like, and are configured to capture image frames looking across the touch surface 11, wherein the image sensors may be disposed at corners (e.g. 111 to 114) or edges of the touch surface 11. The processing unit 13 may be a digital signal processor (DSP) or other processors capable of processing image data, and is configured to post-process the image frames captured by the image sensors and to perform the object positioning, wherein the processing unit 13 may perform the object positioning according to conventional methods and the present disclosure is to shorten the total retrieval time for capturing a valid image by all the image sensors. Said valid image may include a bright image and a dark image herein.
The light bars 121 to 124 are respectively disposed at edges of the touch surface 11. The light bars 121 to 124 may be reflection light bars (i.e. the passive light source) or irradiation light bars (i.e. the active light source). In one embodiment, when the light bars 121 to 124 are reflection light bars, the optical touch system 1 may further include a plurality of active light sources (e.g. L11, L13, L21, L22, L31 and L32) disposed adjacent to the image sensors S11 to S32 respectively. For example, the active light sources L11 to L32 may be disposed next to or at the upper or lower rim of the image sensors S11 to S32 respectively as long as field of views of the image sensors are not blocked. In another embodiment, when the light bars 121 to 124 are irradiation light bars, the optical touch system 1 may not include the active light sources L11 to L32; or the optical touch system 1 may also include the active light sources L11 to L32 served as compensation light sources for light compensation so as to compensate the non-uniform brightness at the positions of the corners or the image sensors.
The active light sources (e.g. L11 to L32 or 121 to 124) are configured to provide light when the image sensors S11 to S32 are capturing images, and thus each of the active light sources irradiates corresponding to the associated image sensor.
Referring to FIG. 3, it shows a flow chart of the exposure mechanism of the optical touch system according to the embodiment of the present disclosure, which includes the steps of: capturing image frames with a sampling cycle to allow each image sensor to capture a bright image (Step S41); simultaneously capturing a dark image using all image sensors in a denoising sampling interval (Step S42); and calculating a differential image between the bright image and the dark image captured by each of the image sensors (Step S43).
Referring to FIGS. 2, 3 and 4A-4C, FIGS. 4A-4C, show schematic diagrams of the exposure mechanism of the optical touch system 1 of FIG. 2, wherein FIG. 4B shows an operational schematic diagram when the light bars 121 to 124 are reflection light bars and FIG. 4C shows an operational schematic diagram when the light bars 121 to 124 are irradiation light bars.
Step S41: The image sensors S11 to S32 respectively capture an image frame with a sampling cycle, wherein the sampling cycle may include a first working mode, a second working mode and a third working mode. In the first working mode, the image sensors S11, S12 respectively capture an image frame in a sampling interval T1, and as the active light sources L11, L12 associated with the image sensors S11, S12 (FIG. 4B) or the active light sources 122, 123, 124 (FIG. 4C) irradiate corresponding to the sampling interval T1, each of the image sensors S11, S12 may respectively capture a bright image I1 (e.g. including a bright image I1_11 captured by the image sensor S11 and a bright image I1_12 captured by the image sensor S12). In the second working mode, the image sensors S21, S22 respectively capture an image frame in a sampling interval T2, and as the active light sources L21, L22 associated with the image sensors S21, S22 (FIG. 4B) or the active light sources 121, 122, 124 (FIG. 4C) irradiate corresponding to the sampling interval T2, each of the image sensors S21, S22 may respectively capture a bright image I2 (e.g. including a bright image I2_21 captured by the image sensor S21 and a bright image I2_22 captured by the image sensor S22). In the third working mode, the image sensors S31, S32 respectively capture an image frame in a sampling interval T3, and as the active light sources L31, L32 associated with the image sensors S31, S32 (FIG. 4B) or the active light source 123 (FIG. 4C) irradiates corresponding to the sampling interval T3, each of the image sensors S31, S32 may respectively capture a bright image I3 (e.g. including a bright image I3_31 captured by the image sensor S31 and a bright image I3_32 captured by the image sensor S32). Therefore, when the sampling cycle finishes, each of the image sensors has captured a bright image. It should be mentioned that when the light bars 121 to 124 are irradiation light bars, the active light source associated with an image sensor is referred to the active light sources 121 to 124 within a field of view of the image sensor. When the light bars 121 to 124 are reflection light bars, the active light source associated with an image sensor is referred to the active light sources L11 to L32 (e.g. the active light source adjacent to the image sensor) that irradiate to allow the reflection light bars within a field of view of the image sensor to generate reflected light. It is appreciated that the active light sources associated with the image sensors S11 to S32 shown in FIG. 4C are only exemplary.
Step S42: Next, all the image sensors S11 to S32 respectively capture a dark image I4 simultaneously in a denoising sampling interval T4 (e.g. including a dark image I4_11 captured by the image sensor S11, a dark image I4_12 captured by the image sensor S12, a dark image I4_21 captured by the image sensor S21, a dark image I4_22 captured by the image sensor S22, a dark image I4_31 captured by the image sensor S31 and a dark image I4_32 captured by the image sensor S32); that is, in the denoising sampling interval T4, all the active light sources L11 to L32 or 121 to 124 are turned off (extinction). In other words, the image sensors S11 to S32 capture an image frame under the condition that all active light sources are turned off, and thus the image frame captured now is referred to the dark image herein.
Step S43: Finally, the processing unit 13 calculates a differential image (e.g. including I1_11-I4_11, I1_12-I4_12, I2_21-I4_21, I2_22-I4_22, I3_31-I4_31, I3_32-I4_32) between the bright images I1 to I3 and the dark image I4 captured by each of the image sensors so as to eliminate the interference from ambient light. The processing unit 13 then performs the object positioning according to the differential images.
In one embodiment, sampling intervals of every working mode are identical and the sampling intervals T1 to T3 may identical to the denoising sampling interval T4; that is, T1=T2=T3=T4.
In one embodiment, sampling intervals of different working modes may or may not be identical; that is, the sampling intervals T1 to T4 may be identical or different. For example, as distances of the image sensors S11 and S12 from the opposite active light bars are farther than distances of the image sensors S31 and S32 from the opposite active light bars, the sampling interval T1 may be longer than the sampling interval T3 to allow the bright images I1 and I3 to have substantially identical brightness, but not limited to. The value of the sampling intervals may be adjusted according to actual applications. For example, when the touch surface 11 is set perpendicular to the horizontal plane, field of views of the image sensors S21 and S22 may cover more ambient light thereby having brightness higher than that of field of views of the image sensors S11, S12, S31 and S32. Accordingly, the sampling interval T2 may be shorter than the sampling intervals T1 and T3 to allow the bright images I1 to I3 to have substantially identical brightness, but not limited to. As mentioned above values of the sampling intervals may be set according to actual applications.
In one embodiment, if the sampling intervals of every working mode are different, the denoising sampling interval may be equal to a shortest sampling interval of the sampling intervals. For example, if the second sampling interval T2 is the shortest sampling interval, the denoising sampling interval T4 is equal to the second sampling interval Meanwhile, as the first sampling interval T1 of the first working mode and the third sampling interval T3 of the third working mode are not equal to the denoising sampling interval T4, the processing unit 13 may adjust a pixel gray level of the associated dark image according to a time ratio of the sampling interval of different working modes and the denoising sampling interval before calculating the differential image; for example, calculating a time ratio (T1/T4) and adjusting pixel gray levels of the dark images I4_11, I4_12 to I4_11′, I4_12′ according to the time ratio (T1/T4); calculating a time ratio (T3/T4) and adjusting pixel gray levels of the dark images I4_31, I4_32 to I4_31′, I4_32′ according to the time ratio (T3/T4) such that the exposure condition of all dark images are substantially identical. Then, the processing unit 13 calculates a differential image between the bright image and the adjusted dark image captured by each of the image sensors, e.g. calculating I1_11-I4_11′, I1_12-I4_12′, I2_21-I4_21, I2_22-I4_22, I3_31-I4_31′, I3_32-I4_32′) and then performs the object positioning according to the differential images.
In another embodiment, sampling intervals of a part of the working modes may be identical but different from sampling intervals of other working modes.
For example in FIG. 4A, it is possible that the sampling intervals are arranged as T1=T3≠T2, but not limited to. As mentioned above, the processing unit 13 may also calculate a time ratio of each sampling interval and the denoising sampling interval to accordingly adjust the pixel gray level of the dark image according to the time ratio.
In addition, the image sensors may simultaneously capture the dark images at first (e.g. the dark image I4 shown by the dashed line in FIG. 4A) and then capture the bright images with the sampling cycle; or may capture the bright images with the sampling cycle at first and then simultaneously capture the dark images (e.g. the dark image I4 shown by the solid line in FIG. 4A). The present disclosure is to capture the bright images sequentially using the image sensors in the sampling cycle and to capture the dark images simultaneously in a denoising sampling interval (e.g. T4).
In addition, when the optical touch system 1 operates continuously, two sampling cycles may share the dark image captured in one denoising sampling interval. For example referring to FIG. 5, the image sensors S11 to S32 respectively capture a first bright image I1 to I3 with a first sampling cycle T1 to T3, and then simultaneously capture the dark image I4 in the denoising sampling interval T4, and then respectively capture a second bright image I1′ to I3′ with a second sampling cycle T1′ to T3′. The processing unit 13 then calculates a first differential image (e.g. including I1_11-I4_11, I1_12-I4_12, I2_21-I4_21, I2_22-I4_22, I3_31-I4_31, I3_32-I4_32) between the first bright images I1 to I3 and the dark image I4 and calculates a second differential image (e.g. including I1_11′-I4_11, I1_12′-I4_12, I2_21′-I4_21, I2_22′-I4_22, I3_31′-I4_31, I3_32′-I4_32) between the second bright images I1′ to I3′ and the dark image I4. In this manner, as the image sensors S11 to S32 do not capture the dark image between the second sampling cycle and a next first sampling cycle, the total retrieval time for capturing bright and dark images within two successive sampling cycles may further be decreased. As the frequency of capturing the bright images is increased, the system operating frequency is increased correspondingly. In addition, it should be mentioned that, for simplification, FIG. 5 only shows the operation of the image sensors S11 to S32, and the operation of the active light sources L11 to L32 or 121 to 124 are similar to FIGS. 4B-4C.
It should be mentioned that the optical touch system according to the embodiment of the present disclosure is not limited to including six image sensors S11-S32 and active light sources L11 to L32 shown in FIG. 2, the numbers thereof may be determined according to the algorithm adopted by the processing unit 13 for performing the object positioning. For example, the optical touch system 1 according to an embodiment of the present disclosure is described below with two image sensors (e.g. S11 and S12) each corresponding to a working mode respectively. The optical touch system 1 includes the touch surface 11 and a plurality of light bars 121 to 123 disposed at edges of the touch surface 11 respectively.
Referring to FIGS. 4A, 4B and 6A, when the light bars 121 to 123 are reflection light bars, the optical touch system 1′ may further include two active light sources (e.g. L11 and L12) disposed adjacent to the image sensors S11 and S12 respectively. In this embodiment, a first image sensor S11 captures image frames looking across the touch surface 11 in a first sampling interval T1 and a denoising sampling interval T4 to obtain a first bright image I1_11 and a first dark image I4_11 respectively (FIG. 4A); and a first active light source L11 irradiates corresponding to the first sampling interval T1 (FIG. 4B). A second image sensor S21 captures image frames looking across the touch surface 11 in a second sampling interval T2 and the denoising sampling interval T4 to obtain a second bright image I2_21 and a second dark image I4_21 respectively (FIG. 4A); and a second active light source L21 irradiates corresponding to the second sampling interval T2 (FIG. 4B). A processing unit 13 calculates a first differential image (I1_11-I4_11) between the first bright image I1_11 and the first dark image I4_11, and calculates a second differential image (I2_21-I4_21) between the second bright image I2_21 and the second dark image I4_21.
In this embodiment, the denoising sampling interval T4 may also be early than (e.g. I4 shown by the dashed line in FIG. 4A) or later than (e.g. I4 shown by the solid line in FIG. 4A) the first sampling interval T1 and second sampling interval T2.
In one embodiment, it is assumed that the first sampling interval T1 is longer than the second sampling interval T2, and the denoising sampling interval T4 is equal to the second sampling interval T2; that is, the denoising sampling interval T4 is equal to the smaller one of the first sampling interval T1 and the second sampling interval T2. In this case, the processing unit 13 may further calculate a time ratio (T1/T4) of the first sampling interval T1 and the denoising sampling interval T4, and adjust the pixel gray level of the first dark image I4_11 according to the time ratio (T1/T4).
In another embodiment, the first sampling interval T1, the second sampling interval T2 and the denoising sampling interval T4 may all be identical.
Referring to FIG. 5, when the optical touch system 1′ operates continuously, it is assumed that the first sampling interval T1 and the second sampling interval T2 form a first sampling cycle, and the denoising sampling interval T4 is later than the first sampling cycle. The first image sensor S11 captures a bright image I1_11′ in a first sampling interval T1′ of a second sampling cycle after the denoising sampling interval T4, and the first active light source L11 irradiates corresponding to the first sampling interval T1′ of the second sampling cycle (as FIG. 4B). The second image sensor S21 captures a bright image I2_21′ in a second sampling interval T2′ of the second sampling cycle after the denoising sampling interval T4, and the second active light source L21 irradiates corresponding to the second sampling interval T2′ of the second sampling cycle (as FIG. 4B). The processing unit 13 further calculates a differential image (I1_11′-I4_11) between the bright image I1_11′ of the first sampling interval T1′ of the second sampling cycle and the first dark image I4_11, and calculates a differential image (I2_21′-I4_21) between the bright image I2_21′ of the second sampling interval T2′ of the second sampling cycle and the second dark image I4_21.
Referring to FIGS. 4A, 4C and 6B, when the light bars 121 to 123 are irradiation light bars, the optical touch system 1′′ may not further include two active light sources disposed adjacent to the image sensors S11 and S12 respectively. In this embodiment, a first image sensor S11 captures image frames looking across the touch surface 11 in a first sampling interval T1 and a denoising sampling interval T4 to obtain a first bright image I1_11 and a first dark image I4_11 respectively (FIG. 4A); and the light bar(s) (e.g. 122, 123 herein) within a field of view θ1 of the first image sensor S11 irradiate corresponding to the first sampling interval T1 (FIG. 4C). A second image sensor S21 captures image frames looking across the touch surface 11 in a second sampling interval T2 and the denoising sampling interval T4 to obtain a second bright image I2_21 and a second dark image I4_21 respectively (FIG. 4A); and the light bar(s) (e.g. 121, 122 herein) within a field of view θ2 of the second image sensor S21 irradiate corresponding to the second sampling interval T2 (FIG. 4C). A processing unit 13 calculates a first differential image (I1_11-T4_11) between the first bright image I1_11 and the first dark image I4_11, and calculates a second differential image (I2_21-I4_21) between the second bright image I2_21 and the second dark image I4_21.
Similarly, in this embodiment the denoising sampling interval T4 may also be early than (e.g. I4 shown by the dashed line in FIG. 4A) or later than (e.g. I4 shown by the solid line in FIG. 4A) the first sampling interval T1 and second sampling interval T2.
Similarly, it may be assumed that the first sampling interval T1 is longer than the second sampling interval T2, and the denoising sampling interval T4 is equal to the second sampling interval T2. The processing unit 13 may further calculate a time ratio (T1/T4) of the first sampling interval T1 and the denoising sampling interval T4, and adjust the pixel gray level of the first dark image I4_11 according to the time ratio (T1/T4).
Similarly, the first sampling interval T1, the second sampling interval T2 and the denoising sampling interval T4 may all be identical.
Referring to FIG. 5, when the optical touch system 1′′ operates continuously, it is assumed that the first sampling interval T1 and the second sampling interval T2 form a first sampling cycle, and the denoising sampling interval T4 is later than the first sampling cycle. The first image sensor S11 captures a bright image I1_11′ in a first sampling interval T1′ of a second sampling cycle after the denoising sampling interval T4, and the light bars 122, 123 within the field of view θ1 of the first image sensor S11 irradiate corresponding to the first sampling interval T1′ of the second sampling cycle (as FIG. 4C). The second image sensor S21 captures a bright image I2_21′ in a second sampling interval T2′ of the second sampling cycle after the denoising sampling interval T4, and the light bars 121, 122 within the field of view θ2 of the second image sensor S21 irradiate corresponding to the second sampling interval T2′ of the second sampling cycle (as FIG. 4C). The processing unit 13 further calculates a differential image (I1_11′-I4_11) between the bright image I1_11′ of the first sampling interval of the second sampling cycle and the first dark image I4_11, and calculates a differential image (I2_21′-I4_21) between the bright image I2_21′ of the second sampling interval T2′ of the second sampling cycle and the second dark image I4_21.
In another embodiment, the optical touch system 1 may include three image sensors (e.g. S11, S21 and S31) corresponding to one working mode of a sampling cycle respectively, and details thereof are similar to FIGS. 2 and 4A-4C by only considering the image sensors S11, S21 and S31, and the active light sources L11, L21 and L31 or 121 to 123. For example when the light bars are reflection light bars, a third image sensor S31 captures image frames looking across the touch surface 11 in a third sampling interval T3 and the denoising sampling interval T4 to obtain a third bright image I3_31 and a third dark image I4_31 respectively; and a third active light source L31 is disposed adjacent to the third image sensor S31 to irradiate corresponding to the third sampling interval T3. For example when the light bars are irradiation light bars, the third image sensor S31 captures image frames looking across the touch surface 11 in the third sampling interval T3 and the denoising sampling interval T4 to obtain a third bright image I3_31 and a third dark image I4_31 respectively; and the light bar 123 within a field of view of the third image sensor S31 irradiates corresponding to the third sampling interval T3.
In addition, it should be mentioned that in one sampling cycle of the optical touch system according to the embodiment of the present disclosure, it is not limited to capturing image frames simultaneously using two image sensors in every working mode as shown in FIG. 4A. In other embodiments, it may use at least one image sensor to capture the bright image in every working mode.
For example referring to FIGS. 7A-7C, they show schematic diagrams of another exposure mechanism of the optical touch system 1 of FIG. 2. In this embodiment, one sampling cycle includes two working modes. In a first working mode, the image sensors S11, S12, S31 and S32 capture an image frame in a sampling interval T3 respectively and as the active light sources L11, L12, L31 and L32 associated with the image sensors S11, S12, S31 and S32 (FIG. 7B) or the active light sources 122, 123 and 124 (FIG. C) irradiate corresponding to the sampling interval T5, the image sensors S11, S12, S31 and S32 may capture a bright image I5 respectively. In a second working mode, the image sensors S21 and S22 capture an image frame in a sampling interval T6 respectively and as the active light sources L21 and L22 associated with the image sensors S21 and S22 (FIG. 7B) or the active light sources 121, 122 and 124 (FIG. C) irradiate corresponding to the sampling interval T6, the image sensors S21 and S22 may capture a bright image I6 respectively. Accordingly, when the sampling cycle finishes, each of the image sensors has captured a bright image similar to the step S41 of FIG. 3.
Next, all image sensors S11 to S32 simultaneously capture a dark image respectively in a denoising sampling interval T7 similar to the step S42 of FIG. 3.
Finally, the processing unit 13 calculates a differential image (e.g. including I5_11-I7_11, I5_12-I7_12, I6_21-I7_21, I6_22-I7_22, I5_31-I7_31, I5_32-I7_32) between the bright images I5 to I6 (e.g. including I5_11, I5_12, I6_21, I6_22, I5_31, I5_32) and the dark image I7 (e.g. including I7_11, I7_12, I7_21, I7_22, I7_31, I7_32) captured by each of the image sensors so as to eliminate the interference from ambient light. The processing unit 13 then performs the object positioning according to the differential images similar to the step S43 of FIG. 3.
Similarly, when the optical touch system 1 operates continuously, two sampling cycles may share the dark image captured in one denoising sampling interval. For example referring to FIG. 8, the image sensors S11 to S32 respectively capture a first bright image I5 to I6 with a first sampling cycle T5 to T6, and then simultaneously capture the dark image I7 in the denoising sampling interval T7, and then respectively capture a second bright image I5′ to I6′ with a second sampling cycle T5′ to T6′. The processing unit 13 then calculates a first differential image (e.g. including I5_11-I7_11, I5_12I7_12, I6_21-I7_21, I6_22-I7_22, I5_31-I7_31, I5_32-I7_32) between the first bright images I5 to I6 and the dark image I7 and calculates a second differential image (e.g. including I5_11′-I7_11, I5_12′-I7_12, I6_21′-I7_21, I6_22′-I7_22, I5_31′-I7_31, I5_32′-I7_32) between the second bright images I5′ to I6′ and the dark image I7. In this manner, the total retrieval time for capturing the bright and dark images within two successive sampling cycles may further be decreased. Similarly, for simplification, FIG. 8 only shows the operation of the image sensors S11 to S32, and the operation of the active light sources L11 to L32 or 121 to 124 are similar to FIGS. 7B-7C.
As mentioned above, the conventional optical touch system may sacrifice the synchronization between image frames and the positioning accuracy in order to eliminate the interference from ambient light. Therefore, the present disclosure further provides an exposure mechanism of an optical touch system (FIGS. 4A-4C, 5, 7A-7C and 8) and an optical touch system (FIGS. 2 and 6A-6B) that may eliminate the interference form ambient light and increase the synchronization between image frames captured by different image sensors and the system operating frequency.
Although the disclosure has been explained in relation to its preferred embodiment, it is not used to limit the disclosure. It is to be understood that many other possible modifications and variations can be made by those skilled in the art without departing from the spirit and scope of the disclosure as hereinafter claimed.

## Claims

1. An optical touch system, comprising:
a touch surface;
a plurality of reflection light bars disposed on edges of the touch surface, respectively;
a first image sensor configured to capture image frames looking across the touch surface in a first sampling interval and a denoising sampling interval to respectively obtain a first bright image and a first dark image;
a first active light source configured to irradiate corresponding to the first sampling interval;
a second image sensor configured to capture image frames looking across the touch surface in a second sampling interval and the denoising sampling interval to respectively obtain a second bright image and a second dark image, wherein the first sampling interval is longer than the second sampling interval;
a second active light source configured to irradiate corresponding to the second sampling interval; and
a processing unit configured to
calculate a first differential image between the first bright image and the first dark image, and
calculate a second differential image between the second bright image and the second dark image.

2. The optical touch system as claimed in claim 1, wherein the denoising sampling interval is early than or later than the first sampling interval and the second sampling interval.

3. The optical touch system as claimed in claim 1, wherein the denoising sampling interval is equal to the second sampling interval.

4. The optical touch system as claimed in claim 3, wherein the processing unit is further configured to
calculate a time ratio of the first sampling interval and the denoising sampling interval, and
adjust a pixel gray level of the first dark image according to the time ratio.

5. The optical touch system as claimed in claim 1, wherein the first sampling interval and the second sampling interval form a first sampling cycle, and the denoising sampling interval is later than the first sampling cycle;
the first image sensor is further configured to capture a bright image in a first sampling interval of a second sampling cycle after the denoising sampling interval, and the first active light source irradiates corresponding to the first sampling interval of the second sampling cycle;
the second image sensor is further configured to capture a bright image in a second sampling interval of the second sampling cycle after the denoising sampling interval, and the second active light source irradiates corresponding to the second sampling interval of the second sampling cycle; and
the processing unit is further configured to
calculate a differential image between the bright image of the first sampling interval of the second sampling cycle and the first dark image, and
calculate a differential image between the bright image of the second sampling interval of the second sampling cycle and the second dark image.

6. The optical touch system as claimed in claim 1, further comprising:
a third image sensor configured to capture image frames looking across the touch surface in a third sampling interval and the denoising sampling interval to respectively obtain a third bright image and a third dark image; and
a third active light source configured to irradiate corresponding to the third sampling interval.

7. The optical touch system as claimed in claim 6, wherein the third active light source is disposed adjacent to the third image sensor.

8. The optical touch system as claimed in claim 1, wherein the first active light source is disposed adjacent to the first image sensor.

9. The optical touch system as claimed in claim 1, wherein the second active light source is disposed adjacent to the second image sensor.

10. The optical touch system as claimed in claim 1, wherein the processing unit is further configured to perform an object positioning according to the first and second differential images.

11. An optical touch system, comprising:
a touch surface;
a plurality of reflection light bars disposed on edges of the touch surface, respectively;
a first image sensor configured to capture image frames looking across the touch surface in a first sampling interval and a denoising sampling interval to respectively obtain a first bright image and a first dark image;
a first active light source configured to irradiate corresponding to the first sampling interval;
a second image sensor configured to capture image frames looking across the touch surface in a second sampling interval and the denoising sampling interval to respectively obtain a second bright image and a second dark image;
a second active light source configured to irradiate corresponding to the second sampling interval; and
a processing unit configured to
calculate a first differential image between the first bright image and the first dark image, and
calculate a second differential image between the second bright image and the second dark image,

wherein the first sampling interval and the second sampling interval form a first sampling cycle, and the denoising sampling interval is later than the first sampling cycle,
the first image sensor is further configured to capture a bright image in a first sampling interval of a second sampling cycle after the denoising sampling interval, and the first active light source irradiates corresponding to the first sampling interval of the second sampling cycle,
the second image sensor is further configured to capture a bright image in a second sampling interval of the second sampling cycle after the denoising sampling interval, and the second active light source irradiates corresponding to the second sampling interval of the second sampling cycle, and
the processing unit is further configured to calculate a differential image between the bright image of the first sampling interval of the second sampling cycle and the first dark image, and calculate a differential image between the bright image of the second sampling interval of the second sampling cycle and the second dark image.

12. The optical touch system as claimed in claim 11, wherein the first sampling interval, the second sampling interval and the denoising sampling interval are identical.

13. The optical touch system as claimed in claim 11, wherein the first active light source is disposed adjacent to the first image sensor.

14. The optical touch system as claimed in claim 11, wherein the second active light source is disposed adjacent to the second image sensor.

15. The optical touch system as claimed in claim 11, wherein the processing unit is further configured to perform an object positioning according to the first and second differential images.