WO2010073174A1

WO2010073174A1 - System and method for image capturing

Info

Publication number: WO2010073174A1
Application number: PCT/IB2009/055683
Authority: WO
Inventors: Rong Song
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-12-25
Filing date: 2009-12-11
Publication date: 2010-07-01

Abstract

The present invention proposes a technical solution for capturing images of at least two reflective markers (12): providing easily-confused reflective markers (12) with lights having different characteristic so as to let easily-confused reflective markers (12) reflect only one of the lights having different characteristic respectively, so that the camera (13) can distinguish the reflective markers in the captured images. With the technical solutions provided by the present invention, the problem that multiple reflective markers (12) are easily confused can be effectively overcome and the identification effect of the reflective markers (12) can be significantly improved.

Description

SYSTEM AND METHOD FOR IMAGE CAPTURING

Technical field

The present invention relates to image capturing, particularly to image capturing of reflective markers.

Background of the invention

Currently, human motion analysis is being widely used in movies, video games, arts, sports and scientific researches. Current techniques involve using different sensors to record data of human body movements, and then using the recorded data for further analysis. The most commonly used recording technique is optical recording. For example, a set of markers are attached to the human body and their images are captured by one or more cameras to record their positions.

To reduce the interference from background light and increase marker contrast, infrared cameras and retroreflective markers are often adopted. An infrared camera means that there is an infrared filter in front of the camera's lens, which allows only light in the infrared waveband to pass through. Retroreflective markers have the important property that they reflect light substantially along the incident direction. This property enables retroreflective markers to have higher luminous intensity and to be identified easier in comparison with other objects captured by the infrared camera.

However, this method still has some drawbacks. For example, when any two of multiple markers overlap in the image captured by the camera or separate after overlapping, one of the markers can very easily be erroneously identified as the other one. Especially when there is only one camera, the probability of erroneous identification will be much higher.

Summary of the invention

In view of the aforementioned problem in the prior art, according to a first aspect of the present invention, an image capturing system for capturing images of at least two reflective markers is proposed. The system comprises an illumination means, at least two reflective markers and one or more cameras. Said illumination means is configured to generate at least two lights having different characteristic,, each of said at least two reflective markers being configured to reflect only one of said at least two lights, and said one or more cameras are configured to capture lights reflected by said at least two reflective markers.

According a second aspect of the present invention, a method of capturing images of at least two reflective markers is proposed. The method comprises the following steps:

- generating, by an illumination means, at least two lights having different characteristic;

- reflecting, by each of said at least two reflective markers, respectively, only one of said at least two lights;

- capturing, by one or more cameras, lights reflected by said at least two reflective markers. With the technical solution provided by the present invention, the problem that multiple reflective markers are easily confused in the image can be effectively overcome and the identification effect of reflective markers is also significantly improved.

Brief description of the drawings The above and other objects, characteristic and merits of the present invention will become more apparent from the following detailed description considered in connection with the accompanying drawings, in which:

Fig. l(a) illustrates an image capturing system and its optical path when the illumination source 11-1 illuminates reflective markers 12 according to an embodiment of the present invention;

Fig. l(b) illustrates the optical path of the image capturing system shown in Fig. l(a) when the illumination source 11-2 illuminates reflective markers 12; Fig. 2 illustrates the given frequency of the illumination device 11 and the capturing frequency of the camera 13 according to an embodiment of the present invention;

Fig. 3 illustrates an image capturing system according to another embodiment of the present invention; Fig. 4 illustrates an image capturing system according to another embodiment of the present invention;

Fig. 5 illustrates an image capturing system according to another embodiment of the present invention;

Fig. 6 illustrates an image capturing system according to another embodiment of the present invention;

Fig. 7 illustrates an image capturing system according to another embodiment of the present invention;

Fig. 8 illustrates the flow chart of the method of capturing image according to an embodiment of the present invention; Wherein the similar reference numerals are used to denote similar step / characteristic / means (modules) throughout the figures.

Detailed description of the embodiments

Fig. l(a), Fig. l(b) and Fig. 3 to Fig. 7 respectively illustrate image capturing systems according to each embodiment of the present invention. As can be seen from Fig. l(a), Fig. l(b) and Fig. 3 to Fig. 7, each image capturing system in the embodiments of the present invention comprises an illumination means 11, at least two reflective markers 12 and one or more cameras 13. The illumination means 11 generates at least two lights having different characteristic; each reflective marker 12 is configured to reflect only one of said at least two lights respectively; and one or more cameras 13 captures lights reflected by said at least two reflective markers 12. The term "light" means "light ray", or "light beam")

Each embodiment of the image capturing system will be described in detail in conjunction with Fig. l(a) to Fig. 7 in the following paragraphs.

Fig. l(a) illustrates the structure of an image capturing system according to an embodiment of the present invention. The image capturing system shown in Fig. l(a) comprises an illumination means 11, two reflective markers 12-1 and 12-2 and a camera 13, wherein the illumination means 11 generates two lights having mutually different characteristic, the reflective markers 12-1 and 12-2 respectively reflect only one of the two lights. The camera 13 captures lights reflected by the reflective markers 12-1 and 12-2.

A characteristic of a light means value of a parameter of the light by which lights can be differentiated from each other, such as center-wavelength, intensity, saturation degree, and so on. For example, the illumination means 11 generates two lights with center wavelengths X₁ and X₂ respectively. In this embodiment, the two kinds of light with center wavelengths X₁ and

X₂ correspond to at least two lights having different characteristic.

Optionally, the illumination means 11 comprises two sets of illumination sources 11-1 and 11-2, which generate light with center wavelengths Xi and X₂ respectively. The illumination means 11 can either be generated at least two lights at the same time or in sequence with a given frequency.

For example, to generate lights with center wavelengths Xi and X₂, in sequence with a given frequency means that, in a period of time it generates light with center wavelength X₁, and in another period of time it generates light with center wavelength X₂, then it repeats the above process at a given frequency.

The given frequency means the illuminating frequency of the illumination means 11, i.e. the number of illuminations within unit time. For example, generating at least two lights in sequence with a given frequency means emitting light for f_c times in a second, namely generating a kind of light every l/f_c second. In this specification, the "given frequency" should be understood in broad sense. The illumination means 11 can generate at least two lights according to a given frequency with the same time interval or with different time intervals. For example, in case of f_c = 4/s, the illumination means 11 generates light four times in a second, wherein it can generate light with center wavelength X₁ twice and light with center wavelength X₂ twice; it can also generate light with center wavelength X₁ three times and then generate light with center wavelength X₂ once; alternatively, it can firstly generate light with center wavelength Xi once, then generate light with center wavelength X₂ once, after that generate light with center wavelength Xi twice. That is to say, the given frequency of the present invention can be understood as given time and order, namely generating at least two lights with given time and order.

Each reflective marker 12 is configured to reflect only one of said at least two lights respectively means, in this embodiment, the reflective marker 12-1 only reflects light with center wavelength Xi and the reflective marker 12-2 only reflects light with center wavelength

X₂.

For the image capturing system shown in Fig. l(a), the illumination means 11 controls two illumination sources 11-1 and 11-2 to generate light with center wavelengths Xi and X₂ in sequence with the given frequency f_c. The camera 13 captures images with the capturing frequency corresponding to f_c (namely corresponding to the illuminating time and order of the illumination means), i.e. opening the shutter. It should be noted that the given frequency f_c can be fixed or vary with time, that is to say, the illuminating order and the illuminating times within unit time of the two illumination sources 11-1 and 11-2 are not confined. For example, in the first cycle, the illumination source 11-1 illuminates twice and the illumination source 11-2 illuminates once; in the second cycle, the illumination sources 11-1 and 11-2 illuminate once respectively. In addition, the capturing frequency of the camera 13 can be the same as the given frequency or different from it.

In the following text, with reference to Fig. 1 and Fig. 2, the given frequency with which the illumination means 11 generates light and the corresponding capturing frequency of the camera will be exemplarily described respectively.

For example, as shown in Fig. 2, within the time period 0-ti, the illumination source 11-1 is on and the illumination source 11-2 is off; within the time period ti-t₂, the illumination source 11-1 is off and the illumination source 11-2 is on; within the time period t₂-t3, the illumination source 11-1 is on and the illumination source 11-2 is off; within the time period t₃-t₄, the illumination source 11-1 is off and the illumination source 11-2 is on; and so on. Correspondingly, T₁, T₂, T₃ and T₄ are opening times of the camera's shutter. As can be seen in Fig. 2, as long as 0<T₁<ti, t₁<T₂<t₂, t₂<T3<t3, t3<T₄<t₄ are satisfied, the camera 13 can capture the images of the reflective markers 12-1 and 12-2 correctly. Therefore, the notion that the capturing frequency of the camera 13 corresponds to the given frequency of the illumination means 11 is not limited to the identity between the capturing frequency and the given frequency, as long as the camera 13 can capture images reflected by each reflective marker correctly.

Optionally, the illumination sources 11-1 and 11-2 can be shut down immediately after the camera 13 has captured images, without waiting until times t_ls t₂ arrive. Namely, after the illumination means 11 has turned on the illumination source 11-1, the camera 13 captures images of the reflective marker 12-1, and then the illumination source 11-1 is shut down. After a period of time, the illumination source 11-2 is turned on, the camera 13 captures images of the reflective marker 12-2, and then the illumination source 11-2 is shut down. After a period of time, the illumination source 11-1 is turned on again, the cycle continues in the same way.

In the following text, with reference to Fig. 2, the working process of the image capturing systems shown in Fig. l(a) and Fig. l(b) will be exemplarily described.

Firstly, at time t=0, the illumination means 11 turns on the illumination source 11-1. At this time, the illumination source 11-2 is off. After the light with center wavelength X₁ (generated by the illumination source 11-1) has reached the reflective marker 12-1, the reflective marker 12-1 reflects the light. After the light with center wavelength X₁ (generated by the illumination source 11-1) has reached the reflective marker 12-2, the reflective marker

12-2 absorbs the light. The optical path is shown by the arrows in Fig. l(a). At time T₁, the camera 13 captures the light with center wavelength Xi that is reflected by the reflective marker 12-1.

At time t_ls the illumination means 11 shuts down the illumination source 11-1 and turns on the illumination source 11-2. After the light with center wavelength X₂ (generated by the illumination source 11-2) has reached the reflective marker 12-2, the reflective marker 12-2 reflects the light. After the light with center wavelength X₂ (generated by the illumination source 11-2) has reached the reflective marker 12-1, the reflective marker 12-1 absorbs the light. The optical path is shown by the arrows in Fig. l(b). At time T₂, the camera 13 captures the light with center wavelength λ₂ that is reflected by the reflective marker 12-2.

At time t₂, the illumination means 11 turns on the illumination source 11-1 and shuts down the illumination source 11-2. After the light with center wavelength X₁ (generated by the illumination source 11-1) has reached the reflective marker 12-1, the reflective marker 12-1 reflects the light. After the light with center wavelength X₁ (generated by the illumination source 11-1) has reached the reflective marker 12-2, the reflective marker 12-2 absorbs the light. At time T₃, the camera 13 captures the light with center wavelength Xi that is reflected by the reflective marker 12-1.

At time t3, the illumination means 11 shuts down the illumination source 11-1 and turns on the illumination source 11-2. After the light with center wavelength X₂ (generated by the illumination source 11-2) has reached the reflective marker 12-2, the reflective marker 12-2 reflects the light. After the light with center wavelength X₂ (generated by the illumination source 11-2) has reached the reflective marker 12-1, the reflective marker 12-1 absorbs the light. At time T₄, the camera 13 captures the light with center wavelength X₂ that is reflected by the reflective marker 12-2.

The cycle continues until the camera 13 doesn't need to capture images of reflective markers any more. As a variation of the above mentioned working process of capturing images of the reflective markers 12-1 and 12-2 alternately, the image capturing system can capture images of the reflective marker 12-1 two or more times and then capture images of the reflective marker 12-2 once, and vice versa.

Optionally, the image capturing system shown in Fig. l(a) can also comprise two cameras. One of them is used to capture images of the reflective marker 12-1 and the other one is used to capture images of the reflective marker 12-2. In some situations, if some markers are easily to be identified because of a special shape or special position, no need to apply the method proposed in the present invention to those markers, i.e. no need to apply lights with different characteristic specially for those markers which are easily to be identified and those markers can just use a normal reflective marker, and reflect light in a normal way, e.g those markers have no filter or have filter with a broader bandwidth.

Fig. 3 illustrates an image capturing system according to another embodiment of the present invention. In Fig. 3, the illumination means 11 comprises four sets of illumination sources 11-1, 11-2, 11-3, and 11-4 and five reflective markers 12-1, 12-2, 12-3, 12-4, and 12-5.

Suppose the relative position of the reflective markers 12-1 and 12-4 is so arranged that the camera 13 won't confuse their images. Therefore, they can reflect lights with the same center wavelength.

Without loss of generality, suppose that four sets of illumination sources generate light with center wavelengths X₁ , λ₂, λ₃, and X₄ respectively. The reflective marker 12-1 reflects the light with center wavelength X₁ , the reflective marker 12-2 reflects the light with center wavelength λ₂, the reflective marker 12-3 reflects the light with center wavelength λ₃, the reflective marker 12-4 reflects the light with center wavelength X₁ and the reflective marker

12-5 reflects the light with center wavelength X₄.

When the illumination means 11 controls the illumination source 11-1 to generate the light with center wavelength Xi (and all other illumination sources are off), the reflective markers 12-1 and 12-4 reflect the light with center wavelength Xi respectively and all other reflective markers absorb the light. The camera 13 captures an image of the reflective markers

12-1 and 12-4 at the same time. When the illumination means 11 controls the illumination source 11-2 to generate the light with center wavelength λ₂ (and all other illumination sources are off), the reflective marker 12-2 reflects the light with center wavelength λ₂ and all other reflective markers absorb the light. The camera 13 captures an image of the reflective marker 12-2. When the illumination means 11 controls the illumination source 11-3 to generate the light with center wavelength λ₃ (and all other illumination sources are off), the reflective marker 12-3 reflects the light with center wavelength λ₃ and all other reflective markers absorb the light. The camera 13 captures an image of the reflective marker 12-3.

When the illumination means 11 controls the illumination source 11-4 to generate the light with center wavelength X₄ (and all other illumination sources are off), the reflective marker 12-5 reflects the light with center wavelength X₄ and all other reflective markers absorb the light. The camera 13 captures an image of the reflective marker 12-5.

In detail, the illumination means 11 controls to turn on/off the illumination sources 11-1, 11-2, 11-3, and 11-4 according to a given frequency; each reflective marker reflects the corresponding light; and the camera 13 captures light of each reflective marker respectively.

The setting of the given frequency and the illuminating order of each illumination source 11-1, 11-2, 11-3, 11-4 are not limited and can be set arbitrarily depending on actual need. The camera 13 determines reflective markers in each image frame according to the illuminating order of each illumination source. Fig. 4 illustrates an image capturing system according to another embodiment of the present invention. In Fig. 4, the illumination means 11 comprises four sets of illumination sources 11-1, 11-2, 11-3, and 11-4 and two reflective markers 12-6 and 12-7.

Generally, the narrower the reflective bandwidth of the reflective marker is, the higher the requirements for the manufacture technology. In some cases, in order to reduce cost, the reflective bandwidth of the reflective marker is made slightly wider, so that light with two or more center wavelengths can be reflected. For example, in the image capturing system shown in Fig. 4, four illumination sources generate light with center wavelengths λ_ls λ₂, λ₃, and X₄ respectively. The reflective marker 12-6 reflects light with center wavelengths X₁ and X₂; the reflective marker 12-7 reflects light with center wavelengths X₃ and X₄.

When the illumination means 11 controls the illumination source 11-1 to generate light with center wavelength X₁ (and all other illumination sources are off), the reflective marker 12-6 reflects the light with center wavelength Xi and the reflective marker 12-7 absorbs the light. The camera 13 captures images of the reflective marker 12-6.

When the illumination means 11 controls the illumination source 11-2 to generate light with center wavelength X₂ (and all other illumination sources are off), the reflective marker 12-6 reflects the light with center wavelength X₂ and the reflective marker 12-7 absorbs the light. The camera 13 captures images of the reflective marker 12-6.

When the illumination means 11 controls the illumination source 11-3 to generate light with center wavelength X₃ (and all other illumination sources are off), the reflective marker 12-7 reflects the light with center wavelength X₃ and the reflective marker 12-6 absorbs the light. The camera 13 captures images of the reflective marker 12-7. When the illumination means 11 controls the illumination source 11-4 to generate light with center wavelength X₄ (and all other illumination sources are off), the reflective marker 12-7 reflects the light with center wavelength X₄ and the reflective marker 12-6 absorbs the light. The camera 13 captures images of the reflective marker 12-7.

Optionally, each reflective marker can comprise a filter (not shown in the figure), wherein the filter allows only one of said at least two lights having different characteristic to pass through. For example, the function that reflective markers reflect light with different center wavelengths respectively can be realized by adding different filters to general reflective markers reflecting all lights. For the scenario shown in Fig. l(a), the filter of the reflective marker 12-1 can allow only light with center wavelength Xi to pass through and the filter of the reflective marker 12-2 can allow only light with center wavelength X₂ to pass through. For the scenario shown in Fig. 4, the filter of the reflective marker 12-6 can allow only light with center wavelengths Xi and X₂ to pass through. It should be noted that the bandwidth of the filter of a reflective marker is not limited. For the filter of a certain reflective marker, its reflective bandwidth can be any value as long as it doesn't cover the reflective wavelength region of other reflective markers that need to be distinguished from said certain reflective marker. For example, for the situation shown in Fig. l(a), when the illumination means 11 generates a wide spectrum of infrared light and a wide spectrum of ultraviolet light, the reflective marker 12-1 reflects a wide spectrum or narrow spectrum of infrared light and the reflective marker 12-2 reflects a wide spectrum or narrow spectrum of ultraviolet light. At this time, the filter of the reflective marker 12-1 can be a wide-spectrum infrared filter and the filter of the reflective marker 12-2 can be a wide-spectrum ultraviolet filter as long as one light can only be reflected by the reflective marker 12-1 and the other light can only be reflected by the reflective marker 12-2.

Optionally, the function that reflective markers reflect only one of at least two lights having different characteristic generated by the illumination means 11 can be realized by replacing filters with special reflective materials. For example, reflective markers made of different special reflective materials reflect light with different center wavelengths respectively.

Optionally, reflective markers made of retroreflective materials can also be utilized to improve the reflective capability and facilitate the identification.

Under the circumstances that different reflective markers are sensitive to light with different luminous intensities, different luminous intensities can also be deployed to distinguish those easily-confused reflective markers. Fig. 5 shows a schematic view of such an image capturing system.

The system in Fig. 5 comprises an illumination means 11, three reflective markers 12-8,

12-9, and 12-10, and a camera 13. Wherein the illumination means 11 can generate at least two lights with different intensities in sequence with a given frequency. The intensity ranges of the light reflected by the reflective markers 12-8, 12-9, and 12-10 are not completely overlapping. For example, the reflective markers reflect light with different intensities respectively.

Suppose the illumination means 11 generates light with intensities (SI1+SIL), (SI2+SIL), and (SI3+SIL) in sequence with a given frequency. The reflective marker 12-8 reflects light of which the incident luminous intensity is not less than SIl, the reflective marker 12-9 reflects light of which the incident luminous intensity is not less than SI2, and the reflective marker 12-10 reflects light of which the incident luminous intensity is not less than SB; wherein SI1<SI2<SI3 and SIL is the path loss of the luminous intensity from the illumination means 11 to each reflective marker (without loss of generality, suppose the path loss from the illumination means 11 to each reflective marker is approximately identical). The camera 13 respectively captures light reflected by each reflective marker with a capturing frequency corresponding to the given frequency. Hereinafter, the process will be explained in detail. The given frequency, the capturing frequency, and their relationship are similar to the description about Fig. l(a), thus no further details will be given herein.

At first, at time t_ls the illumination means 11 generates light with intensity (SI1+SIL), the reflective marker 12-8 reflects the light, and the camera 13 captures the light reflected by the reflective marker 12-8 to obtain an image of the reflective marker 12-8.

Then, at time t₂, the illumination means 11 generates light with intensity (SI2+SIL), the reflective markers 12-8 and 12-9 reflect the light, and the camera 13 captures the light reflected by the two reflective markers simultaneously to obtain a common image of the reflective markers 12-8 and 12-9.

After that, at time t₃, the illumination means 11 generates light with intensity (SI3+SIL), the reflective markers 12-8, 12-9, and 12-10 all reflect the light, and the camera 13 captures the light reflected by the three reflective markers simultaneously to obtain a common image of the reflective markers 12-8, 12-9, and 12-10. Afterwards, the image capturing system can repeat the aforementioned three processes until there is no need to capture images of each reflective marker.

The camera 13 determines the reflective markers in each image frame according to the illuminating order of each illumination source.

Since the position of the same reflective marker in two neighboring images within a very short time interval can be regarded as approximately unchanged, the image capturing system can distinguish the images of the reflective marker 12-8 and those of the reflective marker 12-9 from their common images according to the images of the reflective marker 12-8.

Based on the images of the reflective markers 12-8 and 12-9, the images of the reflective markers 12-8 and 12-9 and those of the reflective marker 12-10 can be distinguished from the common images of the three reflective markers.

Above, several embodiments of the present invention have been elucidated in detail by taking examples that different reflective markers reflect light with different intensities or different center wavelengths. Those skilled in the art should understand that the present invention is not limited to the afore-mentioned two characteristics, any characteristic which is able to distinguish two kinds of light can be applicable for the present invention, such as the saturation degree of the light, and so on. Optionally, the image capturing system, for example shown in Fig. l(a) or Fig. 3 to Fig.

5, can further comprise a broadband filter placed in front of the lens of the camera 13. The broadband filter allows only light reflected by all the reflective markers to pass through. For the scenario shown in Fig. l(a), the broadband filter allows only lights with center wavelengths X₁ and λ₂, reflected by the reflective markers 12-1 and 12-2, to pass through. By using the broadband filter, some interference background light can be filtered, such as visible light and so on.

Those skilled in the art should understand that the broadband filter in front of the lens of the camera 13 is merely an optional device. In a scenario where, except for the illumination means 11 , there is no other illumination source and no other light coming in (for example, in a dark room), the interference of the background light is not apparent, hence no broadband filter is needed.

It should be noted that the illumination means 11 can be placed at any suitable position, as long as the light generated by the illumination means 11 can be captured by the camera 13 after being reflected by each reflective marker. Optionally, the illumination means 11 can be placed around the lens of the camera 13 to achieve integration with the camera. In this way, said at least two lights are generated around the lens of the camera. For example, for the scenario shown in Fig. l(a), the illumination sources 11-1 and 11-2 of the illumination means

11 comprise multiple identical illumination sources respectively, distributed evenly or unevenly around the camera 13. This kind of integration device is very advantageous for the user operation. The camera 13 and the control unit of the illumination means 11 can also be integrated. The illumination means 11 can consist of multiple sets of illumination sources, wherein each set generates light having one characteristic. For example, the illumination sources 11-1, 11-2, 11-3, and 11-4 of the illumination means 11 shown in Fig. 3 and Fig. 4 generate light with center wavelengths X₁ , λ₂, λ₃, and λ₄ respectively, wherein each illumination source may comprise only one illuminating component, such as an LED, or comprise multiple same illuminating components, such as multiple LEDs.

The illumination means 11 can also comprise only a single illumination source. This single illumination source can generate at least two lights having different characteristic, such as the illumination means shown in Fig. 5. Similarly, this single illumination source may consist of multiple different illuminating components, each of which can generate light having one characteristic. It may also consist of multiple same illuminating components, each of which can generate at least two lights having different characteristic. This single illumination source can also consist of only one illuminating component which can generate at least two lights having different characteristic.

The light generated by the illumination means 11 can be invisible light such as infrared light, ultraviolet light and so on, and can also be visible light.

Above, several embodiments of generating at least two lights having different characteristic in sequence by the illumination means 11 have been elucidated in detail. In the following part, under the circumstances that the illumination means 11 generates at least two lights having different characteristic simultaneously, the working process of the image capturing system will be elucidated in detail.

Under the circumstances that the illumination means 11 generates at least two lights having different characteristic simultaneously, and at least two reflective markers only reflect one of said at least two lights respectively. At this time, the image capturing system comprises multiple filters placed in sequence in front of the lens of one or more cameras. Each filter is configured to allow light reflected by at least one reflective marker of said multiple reflective markers to pass through. If the camera 13 has multiple image sensors, such as CCD image sensors or CMOS image sensors, a filter can also be placed in front of each image sensor. The pass band of each filter corresponds to center wavelengths of the light reflected by each reflective marker, so that the camera 13 can capture light with various center wavelengths simultaneously. Alternatively, if each image capturing system comprises multiple cameras, under the circumstances that the illumination means 11 generates light with multiple center wavelengths simultaneously, each camera can capture images of one reflective marker or of multiple reflective markers simultaneously.

Without loss of generality, taking characteristic being center wavelengths as example, under the circumstances that the illumination means 11 generates light with multiple center wavelengths simultaneously, various filters are placed in front of image sensors or lens of the camera 13 in sequence. Each filter allows light with different center wavelengths to pass through. The camera 13 captures light with different center wavelengths in sequence and determines the reflective markers in each image according to the placing order of the filters.

Fig. 6 illustrates the above mentioned image capturing system. In Fig. 6, the illumination means 11 generates light with center wavelengths X₁ and X₂ simultaneously and the reflective markers 12-1 and 12-2 reflect light with center wavelengths X₁ and X₂ respectively. In front of the image sensor or lens of the camera 13, different filters 14-1 and 14-2 are placed in sequence alternately, wherein filter 14-1 allows only light with center wavelength X₁ , reflected by the reflective marker 12-1, to pass through, and filter 14-2 allows only light with center wavelength λ₂, reflected by the reflective marker 12-2, to pass through. Then, the camera 13 captures light with center wavelengths X₁ and λ₂ in sequence alternately. It should be noted that the bandwidth of the filters 14-1 and 14-2 is not limited, as long as unwanted light can be filtered. For the filter 14-1, its bandwidth can be any value, as long as it doesn't allow light reflected by the reflective marker 12-2 to pass through. For the filter 14-2, its bandwidth can be any value, as long as it doesn't allow light reflected by the reflective marker 12-1 to pass through. For example, under the circumstances that the illumination means 11 generates wide spectrum infrared and wide spectrum ultraviolet simultaneously, the reflective marker 12-1 reflects wide-spectrum or narrow- spectrum infrared light, the reflective marker 12-2 reflects wide-spectrum or narrow- spectrum ultraviolet light. At this time, the filter 14-1 can be a wide-spectrum infrared filter and the filter 14-2 can be a wide-spectrum ultraviolet filter. Fig. 7 illustrates a variation of the image capturing system shown in Fig. 6. In Fig. 7, there are two cameras 13-1 and 13-2, which are employed to capture images of the reflective markers 12-1 and 12-2 respectively. The filter 14-1 is placed in front of the lens of the camera 13-1 and the filter 14-2 is placed in front of the lens of the camera 13-2.

It should be noted that the image capturing system in each embodiment of the present invention can also comprise one or more general reflective markers reflecting all lights. For example, under the circumstances that one reflective marker is far away enough from other reflective markers and the camera 13 can distinguish its images from images of other reflective markers, this reflective marker can also adopt a general reflective marker.

Optionally, the image capturing systems shown in Fig. l(a) or Fig. 3 to Fig. 7 can also comprise an analysis unit for analyzing the images captured by the camera 13 to obtain the position of each reflective marker.

Optionally, the image capturing systems shown in Fig. l(a) or Fig. 3 to Fig. 7 can also comprise a calculation unit for performing interpolation operation of multiple positions of each reflective marker to obtain a trajectory of each reflective marker.

The function of the analysis unit and/or the calculation unit can be fulfilled by a microcontroller or a digital signal processor. For example, multiple reflective markers can be attached to a human body. With any image capturing system shown in Fig. l(a), Fig. l(b) or Fig. 3 to Fig. 7, the camera 13 captures multiple sets of images of multiple reflective markers. The analysis unit analyzes the multiple sets of images to obtain multiple sets of positions of the reflective markers. The calculation unit performs interpolation operation of the multiple positions to obtain a traj ectory of the human body.

For another example, the image capturing systems shown in Fig. l(a), Fig. l(b) or Fig. 3 to Fig. 7 can also be utilized in the process of a surgery operation, particularly under the circumstances that the operation is performed on the part which is invisible to human eyes or the surgical knife is operated by a manipulator. With multiple reflective markers attached to the surgical knife and human body, a doctor can get positions of each reflective marker by means of the analysis of the analysis unit so as to control the moving direction of the surgical knife.

Fig. 8 illustrates a flow chart of the method of capturing images of multiple reflective markers according to an embodiment of the present invention. In step S801, the illumination means 11 generates at least two lights having different characteristic. Optionally, the illumination means 11 can generate at least two lights simultaneously or in sequence with a given frequency.

In step S802, each reflective marker reflects only one of said at least two lights having different characteristic. In step S803, one or more cameras capture light reflected by each reflective marker.

Optionally, under the circumstances that the illumination means 11 generates lights having different characteristic in sequence with a given frequency, the camera captures light reflected by each reflective marker with a capturing frequency corresponding to the given frequency. Under the circumstances that the illumination means 11 generates lights having different characteristic simultaneously, multiple filters are placed in sequence in front of the lens of one or more cameras, wherein each filter is configured to only allow one of said at least two lights reflected by at least one reflective marker to pass through, then the one or more cameras capture images of each reflective marker respectively.

Steps S801 to S803 are described from the point of view of the operation of each device. Under the circumstances that the illumination means 11 generates at least two lights having different characteristic in sequence with a given frequency, steps S801, S802, and S803 can also be executed alternately. Hereinafter, the alternate execution process of steps S801, S802, and S803 will be explained in detail by taking the image capturing system shown in Fig. 3 as an example.

At first, in step S801, the illumination means 11 controls the illumination source 11-1 to emit light with center wavelength X₁ (and all other illumination sources are off). Then, in step S802, reflective markers 12-1 and 12-4 reflect light with center wavelength X₁ respectively and all other reflective markers absorb the light.

After that, in step S803, the camera 13 captures an image of the reflective markers 12-1 and 12-4 simultaneously.

After a period of time or immediately back to step S801, the illumination means 11 controls the illumination source 11-2 to emit light with center wavelength X₂ (and all other illumination sources are off).

In step S802, the reflective marker 12-2 reflects light with center wavelength X₂ and all other reflective markers absorb the light.

In step S803, the camera 13 captures an image of the reflective marker 12-2. Then, after a period of time or immediately back to step S801, the illumination means

11 controls the illumination source 11-3 to emit light with center wavelength X₃ (and all other illumination sources are off). In step S802, the reflective marker 12-3 reflects light with center wavelength λ₃ and all other reflective markers absorb the light.

In step S803, the camera 13 captures an image of the reflective marker 12-3.

Then, after a period of time or immediately back to step S801, the illumination means 11 controls the illumination source 11-4 to emit light with center wavelength λ₄ (and all other illumination sources are off).

In step S802, the reflective marker 12-5 reflects light with center wavelength X₄ and all other reflective markers absorb the light.

In step S803, the camera 13 captures an image of the reflective marker 12-5. Then, back to step S801 in which the illumination means 11 controls the illumination source 11-1 to emit light with center wavelength X₁. Such cycle can continue until the camera 13 doesn't need to capture images of each reflective marker.

Optionally, after all the steps shown in Fig. 8, the process further comprises an analysis step in which the images captured by the camera 13 are analyzed to obtain positions of each reflective marker. Optionally, after this step, the process further comprises a calculation step in which interpolation operations of multiple positions of each reflective marker are performed to obtain a trajectory of each reflective marker.

Above, embodiments of the present invention have been described. It should be noted that aforesaid embodiments can be carried out alone or in conjunction with each other. For example, an image capturing system can comprise a broadband filter placed in front of the lens of the camera and can also comprise reflective markers consisting of general reflective markers with filters and so on. Furthermore, it is to be understood that the description above has been given by way of example only and that it is very illustrative. The example cannot be deemed to be a restriction of the present invention. Those skilled in the art can make various variations or modifications within the scope of the appended claims.

There are numerous ways of implementing functions by means of items of hardware or software, or both. In this respect, the drawings are also very illustrative, each representing only one possible embodiment of the invention.

The remarks made hereinbefore demonstrate that the detailed description with reference to the drawings illustrates rather than limits the invention. There are numerous alternatives which fall within the scope of the appended claims. Any reference sign in a claim should not be construed as limiting the claim. Use of the verb "comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. Use of the indefinite article "a" or "an" preceding an element or step does not exclude the presence of a plurality of such elements or steps.

Claims

What is claimed is:

1. A system for capturing images, comprising: an illumination means (11), for generating at least two lights having different characteristic; at least two reflective markers (12), each of which being configured to reflect only one of said at least two lights respectively; a camera (13), for capturing lights reflected by said at least two reflective markers (12).

2. A system according to claim 1, wherein said illumination means (11) is configured to generate said at least two lights in sequence with a given frequency; and said camera (13) is configured to capture lights with a capturing frequency corresponding to said given frequency.

3. A system according to claim 1, wherein said illumination means (11) is configured to generate said at least two lights simultaneously; said system further comprises at least two filters (14), each of which being placed in front of the lens of said camera (13) in sequence and allowing only one of said at least two lights to pass through respectively.

4. A system according to any one of claims 1 to 3, wherein said at least two lights have different center wavelength.

5. A system according to claim 4, wherein each of said at least two reflective markers (12) comprises a filter, each of the filters allowing only one of said at least two lights to pass through respectively.

6. A system according to claim 4, wherein the reflective material of each of said at least two reflective markers (12) is configured to reflect only one of said at least two lights.

7. A system according to claim 2, wherein said at least two lights generated in sequence has different intensities.

8. A system according to claim 1, further comprising: a broadband filter placed in front of the lens of said camera (13) for allowing only light reflected by said at least two reflective markers (12) to pass through.

9. A system according to claim 1, wherein said illumination means (11) is configured to generate said at least two lights around the lens of said one or more cameras.

10. A system according to claim 1, wherein said at least two lights is infrared light or ultraviolet light.

11. A system according to claim 1, further comprising: a unit for analyzing images captured by said one or more cameras (13) to determine the positions of said at least two reflective markers (12).

12. A method of capturing images, comprising steps of:

- generating, by an illumination means (11), at least two lights having different characteristic;

- reflecting, by each of at least two reflective markers (12), only one of said at least two lights respectively; - capturing, by a camera (13), lights reflected by said at least two reflective markers

(12).

13. A method according to claim 12, wherein, in said generating step, generating said at least two lights in sequence with a given frequency; and in said capturing step, capturing, by said camera (13), lights with a capturing frequency corresponding to said given frequency.

14. A method according to claim 12, wherein, in said generating step, generating said at least two lights simultaneously; and said capturing step further comprises a step of:

- placing filters (14) in front of the lens of said camera (13) in sequence, wherein each of said filters (14) allowing only one of said at least two lights to pass through respectively.

15. A method according to claim 12, wherein the method further comprises a step of:

- analyzing images captured by said camera (13) to determine the positions of said at least two reflective markers (12).