Abstract:
The present invention relates to an active imaging device for imaging a scene, comprising a scene illuminator that illuminates said scene with radiation at multiple illumination frequencies or an illumination frequency range covering multiple illumination frequencies, a radiation detector that detects radiation received from said scene in response to said illumination and that generates detection data from said detected radiation, a feature identifier that analyses said detection data and identifies different features in said scene, a frequency selector that separately selects for the identified features one or more selected illumination frequencies resulting in the minimum speckle noise in an image of the respective feature constructed from the detection data, which have been generated from radiation received in response to the illumination of the scene with radiation at said one or more selected illumination frequencies, and an image constructor that constructs a final image from the detection data, wherein the image portions of the identified features are constructed from the detection data, which have been generated from radiation received in response to the illumination of the scene with radiation at said one or more selected illumination frequencies, and wherein said image portions are combined into the final image.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority of European patent application 10 161 881.7 filed on May 4, 2010. 
       FIELD OF THE INVENTION 
       [0002]    The present invention relates to an active imaging device and a corresponding method for imaging a scene. The present invention relates further to an image processing device and method for use in an active imaging device and method. Still further, the present invention relates to a computer readable non-transitory medium storing a computer program. The present invention particularly directed to the reduction of speckle noise. 
       BACKGROUND OF THE INVENTION 
       [0003]    Speckle noise (also called “speckle” in the following) is a granular noise that inherently exists in and degrades the quality of images obtained by active imaging devices, such as active radars, and synthetic aperture radars (SARs). Speckle noise in conventional radar results from random fluctuations in the return signal from an object that is no bigger than a single image-processing element. It increases the mean grey level of a local area. 
         [0004]    Various techniques have been proposed to reduce the effect of speckle for illumination systems such as laser projectors, but these techniques concentrate on removing the phase coherence of the transmitted signal, which help to decrease the speckle on the final image. Such techniques however cannot be used for an active imaging device, since it is important that the transmitted signal in an active imaging device maintains its phase coherence. 
         [0005]    The use of multiple frequency techniques to reduce the effects of speckle in an active imaging device has been proposed in I. Jaeger et al, “Comparison of speckle reduction diversity tools for active millimetre-wave imaging”, Journal of the Optical Society of America, Vol. 25, No. 7, July 2008. This paper describes (in section 3) the use of more than one frequency to reduce the effects of speckle. In particular, it explains that when an object is illuminated with two frequencies, the amount of speckle noise can be reduced as the difference between these frequencies is increased. It also further indicates that to obtain a desired speckle reduction for a given object, the difference frequency between these two frequencies may have to be adaptively changed, in accordance with the optical properties of the object, which undesirably may need a large bandwidth. 
         [0006]    The use of the properties of different image feature/structures for selecting how to best filter the final image to reduce the effects of speckle in an active imaging device is described in Zengguo et al, “Research and Improving on Speckle MMSE Filter Based on Adaptive Windowing and Structure Detection”, IEEE International Conference on Vehicular Electronics and Safety 2005, pp. 251-256. This paper describes a technique to reduce speckle for an image in which different image features are first classified into heterogeneous or homogenous areas. Features in the heterogeneous areas are then further identified and for each type of feature (line, edge, point) different types of filtering are used to reduce the speckle noise. 
       BRIEF SUMMARY OF THE INVENTION 
       [0007]    It is an object of the present invention to provide an active imaging device and method, by which speckle noise occurring during active imaging can be effectively reduced with small efforts. It is a further object of the present invention to provide a corresponding image processing device and method as well as a corresponding computer program for implementing said image processing method and a computer readable medium. 
         [0008]    According to an aspect of the present invention there is provided an active imaging device for imaging a scene, comprising: 
         [0009]    a scene illuminator that illuminates said scene with radiation at multiple illumination frequencies or an illumination frequency range covering multiple illumination frequencies, 
         [0010]    a radiation detector that detects radiation received from said scene in response to said illumination and that generates detection data from said detected radiation, 
         [0011]    a feature identifier that analyses said detection data and identifies different features in said scene, 
         [0012]    a frequency selector that separately selects for the identified features one or more selected illumination frequencies resulting in the minimum speckle noise in an image of the respective feature constructed from the detection data, which have been generated from radiation received in response to the illumination of the scene with radiation at said one or more selected illumination frequencies, and 
         [0013]    an image constructor that constructs a final image from the detection data, wherein the image portions of the identified features are constructed from the detection data, which have been generated from radiation received in response to the illumination of the scene with radiation at said one or more selected illumination frequencies, and wherein said image portions are combined into the final image. 
         [0014]    According to a further aspect of the present invention there is provided an image processing device for use in an active imaging device as defined above and as proposed according to an aspect of the present invention, said image processing device comprising corresponding radiation detector, frequency selector and image constructor. 
         [0015]    According to a still further aspect a computer readable non-transitory medium having instructions stored thereon which, when carried out on a computer, cause the computer to perform the steps of the method according to the present invention is provided. 
         [0016]    Preferred embodiments of the invention are defined in the dependent claims. It shall be understood that all the claimed devices and methods, the claimed computer program and the claimed computer readable medium have similar and/or identical preferred embodiments as the claimed active imaging device and as defined in the dependent claims. 
         [0017]    The inventors of the present invention have found that the known speckle reduction noise technique using multiple frequencies shows disadvantages. In particular, just summing (or averaging) the received images from the different illumination frequencies, without any intelligent selection, can cause the speckle, for certain image features, to be worse than the sum of some of the received images from the individual frequencies. This is because the resulting speckle for a given feature is a function of the illumination wavelength (or frequency) and the roughness for each feature. 
         [0018]    Further, if the multiple frequency techniques described in the above cited paper of I. Jaeger is used, the frequency spacing between illuminated frequencies would have to be dynamically changed, depending upon the properties of the material. It is particularly proposed this paper that optical properties of the object could be measured to do this. Such a scheme would have the following drawbacks:
   a) It requires that the object is optically (or otherwise) visible, so that, the physical properties can be measured.   b) Feedback would have to be provided to the transmitter (or illumination menas) for each feature, which includes information about the physical properties of each feature.   c) The transmitter would have to be capable of dynamically changing its transmitter frequencies in accordance with the feedback information.   d) If it is required that each feature of the image has minimum speckle, steps a)-c) would have to be done in series for each feature. This would be very time consuming.   
 
         [0023]    Hence, it is proposed according to an aspect of the present invention to illuminate the scene (including an object to be imaged, e.g. a person) by a number of different frequencies (which may or may not be sent at the same time), which are fixed by the capabilities of the transmitter (or illumination means) and that the whole scene (or area to be scanned) is illuminated by these frequencies. At the receiver (or radiation detection means and its connected elements) different features in the image are then identified, and for some or all identified features (and, preferably, also the background, which is also treated like a feature), the illumination frequencies, which yield the lowest level of speckle noise, are selected. These selected frequencies are then combined to yield the final image feature. The final image comprises these final image features, which may have resulted from the combination of different illumination frequencies. 
         [0024]    For a transmitter with a given set of illuminated frequencies, the best selected combination of these illuminated frequencies at the receiver (to reduce speckle) is thus found and combined for each image feature. The selection of combinations for each feature can be done in parallel at the receiver and therefore the object can be scanned very quickly. 
         [0025]    Further, according to the present invention the transmitter does not have to dynamically change its set of transmitter frequencies and it does not require any feedback information from the receiver, as is required according to the multiple frequency technique described in the above cited paper of I. Jaeger. Still further, unlike this technique according to the present invention no extra information of the object (or item) is required. 
         [0026]    The present invention is also different to the concept described in the above cited paper of Zengguo, since generally no filtering techniques are used according to the present invention. Different image features are identified, but the speckle reduction is achieved by adaptively selecting and combining the received frequencies, which have the lowest amount of speckle for each feature. 
         [0027]    According to still a further aspect of the present invention there is provided an active imaging device for imaging a scene, comprising: 
         [0028]    an illumination means for illuminating said scene with radiation at multiple illumination frequencies or an illumination frequency range covering multiple illumination frequencies, 
         [0029]    a radiation detection means for detecting radiation received from said scene in response to said illumination and for generating detection data from said detected radiation, 
         [0030]    a feature identification means for analysing said detection data and identifying different features in said scene, 
         [0031]    a frequency selection means for separately selecting for the identified features one or more selected illumination frequencies resulting in the minimum speckle noise in an image of the respective feature constructed from the detection data, which have been generated from radiation received in response to the illumination of the scene with radiation at said one or more selected illumination frequencies, and 
         [0032]    an image construction means for constructing a final image from the detection data, wherein the image portions of the identified features are constructed from the detection data, which have been generated from radiation received in response to the illumination of the scene with radiation at said one or more selected illumination frequencies, and wherein said image portions are combined into the final image. 
         [0033]    According to a further aspect of the present invention there is provided an image processing device for use in an active imaging device as defined above and as proposed according to an aspect of the present invention, said image processing device comprising corresponding radiation detection means, frequency selection means and image construction means. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0034]    These and other aspects of the present invention will be apparent from and explained in more detail below with reference to the embodiments described hereinafter. In the following drawings 
           [0035]      FIG. 1  shows a first embodiment of an active imaging device, 
           [0036]      FIG. 2  shows a first embodiment of an active imaging device, 
           [0037]      FIG. 3  shows a diagram illustrating a rough surface of an object, 
           [0038]      FIG. 4  shows a diagram illustrating the generation of speckle noise on a rough surface, 
           [0039]      FIG. 5  shows a first embodiment of an active imaging device according to the present invention, 
           [0040]      FIG. 6  shows a flow chart of a first embodiment of an active imaging method according to the present invention, 
           [0041]      FIG. 7  shows a diagram illustrating feature selection according to the present invention, 
           [0042]      FIG. 8  shows a second embodiment of an active imaging device according to the present invention, and 
           [0043]      FIG. 9  shows a flow chart of a second embodiment of an active imaging method according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    Active imaging systems are becoming increasingly more popular at ultrasonic, microwave, millimetre and terahertz frequencies for a number of applications including medical and security applications. 
         [0045]    The arrangement of transmitter and receiver in an active imaging system may take on many different forms, but in the most general sense active systems use a transmitter (generally an “illumination means”) to illuminate the object (or item, in general the “scene”) to be studied by transmitting at the frequency (or frequencies) of interest and analyse the received signal due to reflections and/or transmission characteristics of the object. The present invention is primarily directed to active imaging using the reflected signal, although the idea may also be used in active imaging using the transmitted signal. 
         [0046]    The general layout of simple arrangements of an active imaging device is schematically depicted in  FIGS. 1A and 1B .  FIG. 1A  shows a bi-static arrangement  10  in which the transmitter  12  (labelled as TX), including a transmit antenna  14 , and the receiver  16  (labelled as RX), including a receive antenna  18 , are not collocated.  FIG. 1B  shows a mono-static arrangement  20 , in which both the transmitter and the receiver  22 , including one or more (separate or shared) antenna(s)  24 , are collocated. In these simple general arrangements  10 ,  20  beam antennas  14 ,  18 ,  24  for both transmitter and receiver are shown, which may be implemented in many different ways. 
         [0047]    The object  30  (generally, the scene, which usually includes the object, e.g. a person or item) is illuminated by radiation  32 , and the reflected radiation  34  is detected, from which detected radiation a radiation signal is generated for further processing. In these simple arrangements  10 ,  20  it is generally assumed that the transmitted wave of the radiation  32  is hitting a flat surface  36  of the object and that no significant amount of transmitted energy penetrates into the object  30  (or item). The active imaging device could be using any type of transmission scheme (i.e. pulse based, multi-carrier, frequency modulated continuous wave (FMCW), etc.) and could be using any frequency. 
         [0048]    This case of not penetrating the object or item&#39;s surface would typically be similar to an aircraft fuselage being detected by air traffic control radar or an active imaging security body scanner where the transmitted wave does not penetrate the human skin. 
         [0049]    The arrangements  10 ,  20  of the imaging device used for imaging an object  30  (or item) having multiple reflecting layers  36 ,  38  are shown in  FIGS. 2A and 2B . Here, both arrangements  10 ,  20  are generally bi-static. Typical example of this multi-layer case would be in medical diagnosis when ultrasound (&lt;10 MHz) or Ultra-wideband (typically 3-10 GHz) frequencies are used. 
         [0050]    For all of these systems, where the reflected signal is analysed at the receiver, the received signal (and resulting final image) may be degraded by a multiplicative noise known as speckle noise. Speckle noise is generated by the summing of multiple reflections (which have different phase shifts) and occurs when the object has a rough reflecting surface or layer. A simple model of roughness is shown in  FIG. 3 . Such a roughness could occur at the surface  36  of the object  30  or could occur at one of the intermediate layers  38  of the object  30  as shown in  FIG. 2 . 
         [0051]    The reason, why the speckle noise occurs, is illustrated with reference to  FIG. 4 . In  FIG. 4A  it is assumed that the transmitted signal  32  is hitting the object surface  36  (or immediate layer) at 90°.  FIG. 4B  shows the reflected signal  34 . Since the surface  36  (or immediate layer) is rough, the total reflected signal  34  is the summation of all the different reflections which can be received by the receiver antenna. Since many of the individual reflected signals  34  had to travel different distances (due to the roughness), they all have different phase shifts and the total power of reflected signal can be reduced. 
         [0052]    If some reflected signals have to travel half a wavelength (λ/2) longer than other reflected signals, the difference in phase can be 180° and the reflections can totally cancel each other out. In  FIG. 4  the example maximum roughness is (λ/4) and the signals being reflected back from the lowest part in the roughness do indeed have to travel (λ/2) longer than waves which are being reflected back from the top of the surface  36 . 
         [0053]    As an object is scanned therefore (e.g. by moving the Tx and Rx beam antenna directions, or by moving the transmitter and receiver units, or by electronically sweeping the antenna beams or radiation patterns), there can be a huge variation in the received signal depending upon the roughness of the surfaces being illuminated, the roughness of the surfaces being received and the illuminated frequency. Hence, to optimise the visibility of all objects in the image, the speckle noise needs to be reduced. Many different techniques have been proposed to combat speckle including using multiple frequencies and various filtering techniques as mentioned above, all having different advantages and disadvantages. According to the present invention, a new approach is proposed by which the disadvantages of the known techniques are overcome. 
         [0054]    A first embodiment of an active imaging device  40  for imaging a scene  60 , e.g. including an object, is schematically depicted in  FIG. 5 . The device  40  comprises an illumination means  42 , e.g. a transmitter  12  including a transmit antenna  14  as shown in  FIGS. 1 and 2 , for illuminating said scene  60  with radiation  62  at multiple illumination frequencies or an illumination frequency range covering multiple illumination frequencies. A radiation detection means  44 , e.g. a receiver  16  including a receive antenna  18  as shown in  FIGS. 1 and 2 , is provided for detecting radiation  64  received from said scene  60  in response to said illumination  62  and for generating detection data from said detected radiation  64 . Said radiation data are then processed in an image processor  46 . 
         [0055]    Said processor  46  includes a feature identification means  48  for analysing said detection data and identifying different features in said scene  60 , a frequency selection means  50  for separately selecting for the identified features one or more selected illumination frequencies resulting in the minimum speckle noise in an image of the respective feature constructed from the detection data, which have been generated from radiation  64  received in response to the illumination of the scene  60  with radiation  62  at said one or more selected illumination frequencies, and an image construction means  52  for constructing a final image from the detection data, wherein the image portions of the identified features are constructed from the detection data, which have been generated from radiation  64  received in response to the illumination of the scene with radiation  62  at said one or more selected illumination frequencies, and wherein said image portions are combined into the final image. 
         [0056]    The elements  48 ,  50 ,  52  of said processor may be implemented in hardware and/or software. Their functions and effects will be explained below in more detail. 
         [0057]    A flow chart of a first embodiment of the proposed imaging method is depicted in  FIG. 6 . In said embodiment it is assumed that the transmitter or illuminator  42  uses multiple frequencies to illuminate the scene  60  (or object or item) to be examined. The proposed method, however, can also be used with a wideband band illumination signal (typically generated by a short pulse), as will be explained below with reference to other embodiments. 
         [0058]    In a first step S 10  the scene is sequentially illuminated with radiation at multiple illumination frequencies f x , which could for instance be in a range from kHz to hundreds of THz. After each illumination with a particular illumination frequency f x , radiation received from said scene in response to said illumination is detected and detection data are generated from said detected radiation in step S 12 . These steps S 10  and S 12  are carried out sequentially, e.g. in a loop as shown in  FIG. 6 . Said loop includes the question in step S 14  if there is another illumination frequency f x  at which steps S 10  and S 12  shall be carried out, in which case x (e.g. starting at x=1 is increased by 1 in step S 16 , i.e. the next (generally predetermined) illumination frequency f x  is used for carrying out S 10  and S 12  again. 
         [0059]    After the scene has been illuminated at all desired illumination frequencies f x  the obtained radiation data are subjected to image processing in step S 18 . Said image processing includes the step of analysing said detection data and identifying different features in said scene in step S 20 . Thereafter, in step S 22 , for the identified features one or more selected illumination frequencies resulting in the minimum speckle noise in an image of the respective feature constructed from the detection data, which have been generated from radiation received in response to the illumination of the scene with radiation at said one or more selected illumination frequencies, are separately selected in step S 22 . Finally, in step S 24  a final image is constructed from the detection data, wherein the image portions of the identified features are constructed from the detection data, which have been generated from radiation received in response to the illumination of the scene with radiation at said one or more selected illumination frequencies, and wherein said image portions are combined into the final image. 
         [0060]    Since the level of speckle depends upon the surface or layer roughness, the illuminated frequency and the angle of illumination, different parts or features of an object will have different levels of speckle at a given illumination frequency. Smooth parts or features of an object will have very low levels of speckle across a wide set of illumination frequencies, while rough parts of an object will have very high level of speckle when the roughness distribution has a mean value close to X/ 4 , where X is the wavelength of the illumination frequency. 
         [0061]    Depending upon the exact distribution of the roughness for each feature, using a higher or lower illumination frequency will have different effects. If, for instance, a certain feature has a roughness distribution where many component depths (see  FIG. 4 ) have a value lower than the λ f /4, where λ f  is the wavelength of the present illumination frequency, then using a higher illumination frequency causes this feature to be viewed with more speckle noise. 
         [0062]    If, on the other hand, a feature has a roughness distribution where many components depths have a value higher than the λ f /4, where λ f  is the wavelength of present illumination frequency, then using a higher illuminated frequency will enable the feature to be viewed with less speckle noise. 
         [0063]    One idea of the present invention is therefore that to obtain a minimum level of speckle across a whole image, the best set of frequencies (from the total set of frequencies used for illumination) for selected (or each) feature(s) are found and these are then combined to represent that feature in the final image. This is schematically shown in  FIG. 7 , wherein  FIG. 7A  shows an example scene including 5 features and  FIG. 7B  shows the same scene, wherein for each feature the best set of frequencies has been selected from the original set of illumination frequencies f 1  to f 6 . As can be seen, different features may have different best frequencies. 
         [0064]    To achieve this, the following steps are used in the image processor  46  and the processing step S 18 , respectively. 
         [0065]    Once the scene has been scanned, which may be achieved by moving the Tx and Rx beam antenna directions or by moving the transmitter and receiver units or by any other means, e.g. beam forming antennas or mirrors, the different features in the scene are identified in feature identification unit  48  and feature identification step S 20 , respectively. This feature identification may use in an embodiment individually a single illumination frequency, i.e. the detection data obtained for each separate illumination frequency are individually used for feature identification. In another embodiment his feature identification may combine the detection data (e.g. separate detection data sets) subsequently obtained for multiple illumination frequencies (e.g. may combine separate detection data sets, each detection data set being obtained for a different illumination frequency, e.g. by averaging or summing said detection data sets into a common detection data set for further processing). An example of a feature identification algorithm is generally described in the above cited paper of Zengguo et al., but there are many other possibilities. For instance,  FIG. 5A  shows a scene in which a number of different features have been identified. 
         [0066]    Once the different features have been identified, in an embodiment the mean and/or variance of each feature are calculated for every illumination frequency for finding the best illumination frequencies for each feature in frequency selection unit  50  and frequency selection step S 22 , respectively. To choose the minimum amount of speckle noise, it is preferred in an embodiment to select frequencies which cause the minimum amount of variance (or standard deviation) across the feature, said condition being used as a frequency selection rule. Since the active imaging device may also have a non-flat frequency response across the range of illumination frequencies, the mean can also be measured and the best frequencies for each feature is then based on choosing the features which have the highest signal to noise ratio (mean of feature/variance of feature). 
         [0067]    The final image is then constructed in the image construction unit  52  and the image construction step  24 , respectively, by combining (e.g. summing or averaging) for selected or, preferably, each identified image feature the signals (receiver signals or image signals) from the set of best frequencies identified in the frequency selection step together. The final image therefore consists of all these optimally combined image features, resulting in an overall image with reduced speckle noise as shown in  FIG. 5B . 
         [0068]    To simplify the explanation it has been assumed that the transmitter (or illuminator) is subsequently illuminating the object using multiple discrete frequencies. The proposed imaging method can, however, also be used when the transmitter is using a wideband illumination signal coving a frequency band Δf (i.e. not consisting of discrete frequencies), such as would be produced from a pulse based active imaging system, or when the transmitter is using a illumination signal simultaneously comprising a number of discrete frequencies or small frequency bands). A corresponding embodiment  40 ′ of an active imaging device is depicted in  FIG. 8 , a corresponding embodiment of an active imaging method is depicted in  FIG. 9 . In this case the illuminator  42 ′ illumination the scene  60  with the wideband illumination signal (step  510 ′), and the detector  44 ′ receives a single detection data set (step S 12 ′). 
         [0069]    In this case the processor  46 ′ would need to examine different frequency components of the received detection data obtained from the reflected wideband illumination signal. These different frequency components would typically be obtained using a filter  54  (step S 26 ), e.g. an FFT unit, but other methods are also possible. The other units of the processor  46 ′ and the other steps of the processing step S 18  are generally identical to the units and steps explained above for the first embodiment. In particular, in this case the method and device described above are also applicable be replacing the concept of different illumination frequencies with the frequency components identified at the receiver. 
         [0070]    In summary, the proposed method and device select the best combination of receiver frequencies to reduce speckle (from the set of illuminated frequencies) for selected or each image feature. The final image therefore comprises these optimally combined image features, resulting in an overall image with reduced speckle noise. 
         [0071]    Compared to a system which uses multiple frequencies but uses no intelligence to optimally combine these frequencies, the proposed idea results in an image with reduced speckle noise. 
         [0072]    Compared to a system as described in the above cited paper of Jaeger et al., in which the optical properties of the object are measured to dynamically change the spacing between the illuminated frequencies, various advantages are obtained. In particular, the proposed method requires the transmitter to dynamically change its set of transmitter frequencies. Further, no feedback information is required from the receive and it is not require that the object is optically (or otherwise) visible, so that, the physical properties can be measured. 
         [0073]    The invention can be applied in a wide range of applications and technical fields, in particular all areas where active imaging of a scene is used and where speckle noise generally is a problem. These fields particularly include the fields of medical imaging, e.g. by use of ultrasound, active radiometry or security scanning. The applicable frequencies may be selected from a wide range, particularly dependent on the kind of application. Said wide frequency range ranges generally from 20 kHz up to hundreds of THz. Preferred frequency ranges are in the millimetre wavelength range. 
         [0074]    The invention has been illustrated and described in detail in the drawings and foregoing description, but such illustration and description are to be considered illustrative or exemplary and not restrictive. The invention is not limited to the disclosed embodiments. Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. 
         [0075]    In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single element or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
         [0076]    A computer program may be stored/distributed on a suitable non-transitory medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. 
         [0077]    Any reference signs in the claims should not be construed as limiting the scope.