Abstract:
An optical coherent processor or correlator for processing an input image produced by an imaging device illuminated by a coherent light source utilizes an optical key for preventing unauthorized use of the processor. The principle underlying processing apparatus and method according to the invention uses an optical lock in combination with an encoded software key superimposed on a filter image. The optical lock, which is preferably in hardware form, consists of a first optical mask implementing a locking mask function that is preferably complex (phase and/or amplitude), which first optical mask is included in the optical path of the processor. The mask is fixed during assembly of the processor or correlator and a unique pattern is encoded thereon. The software key consists of a pattern defined by a key mask function that is displayed preferably using a spatial light modulator as part of a second optical mask included on the processor optical path. The key mask function is designed to compensate for the wave-front distortion generated by the locking mask function of the first optical mask. Therefore, an optical processor or correlator provided with its unique hardware lock cannot generates useful correlation, unless a corresponding unique software key is used.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of optical processing, and more particularly to optical coherent processors. 
     DESCRIPTION OF PRIOR ART 
     Optical processors or correlators have been used for years in many different applications among which are target tracking, quality control and pattern recognition. In a typical optical correlator, a coherent source such as a laser generates a light beam that is collimated to illuminate an input imaging object or device as part of the correlator for generating an input image to be processed. The correlator comprises a first lens used to perform a first Fourier transform of the input image, which transform appears in the Fourier or filter plane. As well known by one skilled in the art, when applied to optical processing, the Fourier transform is a complex (real and imaginary parts) function resulting to an optical pattern lying in the spatial frequency domain. The correlator further comprises a second imaging device positioned within the Fourier plane to display a selected filter. At the filter plane, the Fourier transform of the input image is multiplied by the transmission function displayed on the filter device, to produce a combined image. Typically, the characteristics of the filter can be adapted either to perform pattern recognition, wherein the filter characteristics are based on the Fourier transform of a reference object to be recognized, or to perform filtering or other processing operations based on a predetermined mathematical function. The correlator further comprises a second lens for performing the inverse-Fourier transform of the combined image, resulting to a correlated, convoluted or filtered image, depending of the particular processing or filter function used. Known optical correlators or processors commonly use a spatial light modulator as the second imaging device, which modulator is conveniently computer-controlled using a specific software implementing a plurality of processing or filter functions that can be selected by one or more users. Especially in the case where the use of an optical correlator or processor should be limited to a reduced number of persons within an organization, it is desirable to provide particular means for limiting system access to authorized persons only. Furthermore, each individual user might require that information specific to his work, e.g. operation parameters, specific processing functions, as stored in the computer memory of the system could not be accessed by unauthorized users. 
     Phase masks have been used for long time mainly in the domain of kinoforms as described by L. B. Lesem et al. in “ The kinoform, a new wavefront reconstruction device ”, IBM J. Res. Develop, vol. 13, p. 150, 1969, and by A. Tanone et al. in “ Phase modulation depth for a real-time kinoform using a liquid crystal television ”, Optical Engineering, vol. 32, no. 3, p. 517, 1993. In the design of kinoforms, phase masks have been used to generate pattern diffraction so that, when illuminated by coherent light, the encoded pattern is observed in the far-field of propagation. 
     More recently, phase masks have been applied to optical image encryption and decryption of information encoded on an object or to authenticate the object in itself. In a typical phase encryption/decryption application, a phase key is incorporated in an input external object presented to a correlator which comprises a fixed key. The use of phase masks for various security purposes is abundantly referred to in the literature. In U.S. Pat. No. 5,485,312 issued on Jan. 16, 1996 to Horner et al., there is disclosed an optical pattern recognition system and method for verifying the authenticity of an object, which employ a joint transform coherent optical processor. An unreadable and hence counterfeit-proof encrypted phase mask is coupled to the object and the optical processor compares the phase mask with a reference phase mask having the same phase code thereon. The processor produces a correlation spot having an intensity that exceeds a given level if the phase mask is genuine. In “ Optical pattern recognition for validation and security identification ”, Optical Engineering, vol. 33, no. 6, 1994, p. 1752, and in “Fully phase encoded key and biometrics for security versification” Optical Engineering, vol. 36, no. 3, p. 935, 1997, B. Javidi et al. teach encryption and decryption techniques for authenticating an object with a phase mask in a spatial plane, external to a correlator, without discussing alignment and/or rotation problems that are likely to occur with such techniques. In “ Optical image encryption based on input plane and Fourier plane random encoding ”, Optics Letters, vol. 20, no. 7, p. 767, P. Refregier and al. teach the use of a two-phase mask for carrying out image encryption and decryption, without consideration of alignment and speckle noise problems that are likely to be observed. In “ Incoherent optical correlators and phase encoding of identification codes for access control of authentication ” Optical Engineering, Vol. 36, no. 9, p. 2409 1997, J. Brashner et al. propose the use of incoherent processors for encryption and decryption, for the purpose of authenticating separate objects. In “ Distributed kinoforms in optical security applications ” Optical Engineering, vol. 35, no. 9, p. 2453, 1996, P. Stepien and al. teach decryption and encryption of information techniques that are based on computer generated holograms. In “ Optical implementation of image encryption using random phase encoding ”, Optical Engineering, vol. 35, no. 9, p. 2459. 1996, G. Neto et al. propose a correlator architecture for encryption and decryption, where speckle noise problems are taken into consideration. In “ Random phase encoding for optical security ”, Optical Engineering, vol. 35, no. 9, p. 2464, 1996, R. K. Wang teaches encryption and decryption techniques also using an optical correlator, without considering alignment and/or rotation problems that are inherent to such techniques. In “ Practical image encryption scheme by real-valued data, Optical Engineering ”, vol. 35, no. 9, p. 2473, 1996, H.-G. Yang et al. describe encryption and decryption schemes that are based on amplitude reference mask and object. In “ Experimental demonstration of the random phase encoding technique for image encryption ”, Optical Engineering, vol. 35, no. 9, p. 2506, 1996, Javidi et al. report experimental results of encryption and decryption performed with techniques using an optical correlator, wherein bending, noise, and scratches problems were observed with these techniques. 
     SUMMARY OF THE INVENTION 
     It is therefore a main object of the present invention to provide image processing apparatus and method with a locking feature, for limiting access thereof to authorized persons only. 
     It is a further object of the present invention to provide image processing apparatus and method with locking feature ensuring that processing information specific to a user could not be accessed by unauthorized users. 
     It is a still further object of the present invention to provide a lock device for controlling the use of an optical image processor. 
     It is another object of the present invention to provide image processing apparatus and method as well as lock device and method for controlling the use of an optical image processor, which make use of an optical mask implementing a locking mask function and without including any movable part, thereby obviating problems of alignment in position/rotation, bending, scratches, space bandwidth, or speckle inherent to the use of an external object as taught in the prior art. 
     The present invention can be generally defined as an optical key for preventing unauthorized use of an optical coherent processor and more precisely of an optical correlator. The principle underlying this invention uses an optical lock in combination with an encoded software key superimposed on a filter image. The optical lock, preferably in hardware form, consists of a first optical mask implementing a locking mask function that is preferably complex (phase and/or amplitude) which first optical mask is included in the optical path of the correlator. The mask is fixed during assembly of the correlator and a unique pattern is encoded thereon. The software key consists of a pattern defined by a key mask function that is displayed preferably using a spatial light modulator as part of a second optical mask included on the correlator optical path. The key mask function is designed to compensate for the wave-front distortion generated by the locking mask function of the first optical mask. Therefore, a correlator provided with its unique hardware lock cannot generate useful correlation, unless a corresponding unique software key is used. 
     According to the above mentioned objects, from a broad aspect of the present invention, there is provided an apparatus for processing an input image produced by an imaging device illuminated by a coherent light source. The apparatus comprises first Fourier transform means for performing the Fourier transform of the input image to generate a corresponding transformed input image in the spatial frequency domain within an area defined by a Fourier transform filter plane, and first optical mask means being disposed within said area, said first optical mask implementing a locking mask function. The apparatus further comprises data processor means for generating filter mask function control data, second optical mask means disposed within said area, said second optical mask means implementing a filter mask function according to the filter mask function control data to generate with the locking mask function a combined image in the spatial frequency domain, and second Fourier transform means for performing the inverse Fourier transform of the combined image to generate a processed image only if the filter mask function control data include key control data corresponding to a key mask function complementary to the locking mask function for substantially cancel the locking effect thereof. 
     From another broad aspect of the invention, there is provided a lockable coherent optical processing apparatus comprising a laser source for generating substantially coherent light, an input imaging device receiving the coherent light to produce an input image, first Fourier transform means for performing the Fourier transform of the input image to generate a corresponding transformed input image in the spatial frequency domain within an area defined by a Fourier transform filter plane, and first optical mask means being disposed within said area, said first optical mask implementing a locking mask function. The apparatus further comprises data processor means for generating filter mask function control data, second optical mask means disposed within said area, said second optical mask means implementing a filter mask function according to the filter mask function control data to generate with the locking mask function a combined image in the spatial frequency domain, and second Fourier transform means for performing the inverse Fourier transform of the combined image to generate a processed image only if the filter mask function control data include key control data corresponding to a key mask function complementary to the locking mask function for substantially cancel the locking effect thereof. 
     From still another broad aspect of the invention, there is provided an optical correlator for analyzing an input image produced by an imaging device illuminated by a coherent light source. The optical correlator comprises first Fourier transform means for performing the Fourier transform of the input image to generate a corresponding transformed input image in the spatial frequency domain within an area defined by a Fourier transform filter plane and first optical mask means being disposed within said area, said first optical mask implementing a locking mask function. The correlator further comprises data processor means for generating filter mask function control data, second optical mask means disposed within said area, said second optical mask means implementing the filter mask function according to the filter mask function control data to generate with the locking mask function a combined image in the spatial frequency domain, and second Fourier transform means for performing the inverse Fourier transform of the combined image to generate a correlation indicating image only if the filter mask function control data include key control data corresponding to a key mask function complementary to the locking mask function for substantially cancel the locking effect thereof. 
     From a further broad aspect of the invention, there is provided a lock device for controlling the use of an optical image processor including a laser source for generating and directing substantially coherent light onto an input imaging device producing an input image, first Fourier transform means for performing the Fourier transform of the input image to generate a corresponding transformed input image in the spatial frequency domain within an area defined by a Fourier transform filter plane, second Fourier transform means for performing the inverse Fourier transform of the transformed input image to be combined in the spatial domain with a filter mask image to generate a processed image. The lock device comprises first optical mask means being disposed within said area, said first optical mask implementing a locking mask function, data processor means for generating filter mask function control data and second optical mask means disposed within said area, said second optical mask means implementing a filter mask function according to the filter mask function control data to generate with the locking mask function the filter mask image, wherein the processed image is generated by the image processor only if the filter mask function control data include key control data corresponding to a key mask function complementary to the locking mask function for substantially cancel the locking effect thereof. 
     From a still further broad aspect of the invention, there is provided a method of processing an input image produced by an imaging device illuminated by a coherent light source. The method comprises the steps of a) performing the Fourier transform of the input image to generate a corresponding transformed input image in the spatial frequency domain within an area defined by a Fourier transform filter plane; b) combining the transformed input image with a filter mask image and a locking mask image respectively defined by a filter mask function and a locking mask function to generate a combined image in the spatial frequency domain; and c) performing the inverse Fourier transform of the combined image to generate a processed image only if the filter mask function include a key mask function complementary to the locking mask function for substantially cancel the locking effect thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     Preferred embodiments of the processing apparatus, method and locking device according to the present invention will be now described in view of the accompanying drawing in which 
     FIG. 1 is as schematic view of an optical processor linked to a computer-based controller represented by a block diagram. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference is now made to FIG. 1 illustrating an optical processing apparatus generally designated at  10 , which comprises an optical module  12  including a coherent light source  13  such as a laser or laser-diode, which could be of a He—Ne type or any other suitable type, for generating a beam  14  of coherent light that is directed toward a collimator formed by an objective  16  followed by a collimating lens  18  for directing a collimated beam  20  of coherent light toward an input imaging device  22 , which generates an input image when illuminated by the incident collimated coherent light. For example, in a pattern recognition application, the input imaging device can be a spatial light modulator such as a liquid crystal display or other suitable display device generating a pattern for which validation or identification has to be made using one or more known reference patterns. With a spatial light modulator, the pattern can be dynamically displayed allowing it to be conveniently changed when analyzing several patterns. The pattern is revealed either through coherent light transmission forming a beam  21  as in the example shown in FIG. 1, or through coherent light reflection by setting an appropriate incident light angle with respect to the applied pattern. The optical module  12  further includes an optical processor  24  such as a four-f correlator in the example shown in FIG. 1, which has a first lens  26  disposed in front of the input imaging device  22  and having its optical plane being distant from the optical plane of imaging device  22  by a focal length (f). The first lens  26  performs the Fourier transform of the input image and generates a corresponding transformed input image in the complex spatial frequency domain, within an area defined by a Fourier transform filter plane represented at  28 , which is also distant from the optical plane of first lens  26  by one focal length (f). The optical processor  24  has a second lens  30  having its optical plane laying two focal length (2f) from the optical plane of first lens  26 , for performing the inverse Fourier transform of a combined image formed within the area defined by filter plane  28 , as will be explained later in detail. The processed image resulting from the inverse Fourier transform of the combined image is captured by a conventional optical detector array  32  generating electrical output signals indicative of the light intensity distribution resulting from the optical processing, which signals can be acquired and analyzed by any suitable instrumentation. While a typical four-f correlator is employed in the example shown in FIG. 1 for sake of simplicity, it is to be understood that any other type of optical correlator or processor using a different architecture can be employed to practice the present invention. The optical module  12  further includes a first optical mask  34  disposed within the area defined by filter plane  28 , which mask  34  implements a unique locking mask function L(u,ν), which is either a phase mask function or a complex (phase and amplitude) mask function, characterized by continuous or discrete phase and amplitude variation in the spatial domain, which function is preferably expressed by the following relation: 
     
       
           L ( u ,ν)= A ( u ,ν) e   jφ(u,ν)   (1) 
       
     
     wherein A(u,ν) is an amplitude component of the locking mask function, φ(u,ν)is a phase amplitude component of the locking mask function and (u,ν) are the spatial coordinates in the Fourier transform filter plane  28 . Although the locking mask function L(u,ν) is preferably made unique for a particular optical processor, it may also be common to a limited set of optical processors. The optical mask  34  is preferably built in a fixed, permanent form with glass or plastic material, or with any transparent or semi-transparent material showing suitable optical characteristics, provided that the wave-front distortion introduced by the resulting mask as compared to the desired locking mask function is within a maximum predetermined value, which can be generally set to less than one wavelength characterizing the light source used with the optical processor. The phase variation can be introduced by a variation of the material thickness, by a change of refractive index of the material or by any other suitable optical technique. 
     Alternatively, the optical mask  34  may be of a dynamic type allowing change of locking mask function, e.g. using a conventional computer-controlled spatial light modulator supporting phase recording, as will be describer later in more detail. 
     The optical unit further includes a second optical mask  36  also disposed within the area defined by filter plane  28 , which mask  36  implements a filter mask function to generate with the locking mask function provided by first optical mask  34  the combined image in the spatial domain. The second optical mask  36  is of a dynamic type allowing complex variation of filter mask function, being preferably a computer-controlled spatial light modulator supporting phase recording, as will be explained later in more detail. The combination of first and second optical masks  34 ,  36  may be considered as a further imaging device receiving light forming the input image as displayed by imaging device  22 , to display within the area defined by filter plane  28 , the resulting combined image as modulated by filter mask and locking mask functions. Since the optical masks  34 ,  36  are preferably disposed in an aligned, adjacent relationship, alignment problems related to position/rotation, bending, scratches, space bandwidth, or speckle is therefore prevented or reduced. The filter mask function characteristics can be adapted either to perform pattern recognition, wherein the filter mask function characteristics are based on the Fourier transform of a reference object to recognized, or to perform filtering or other processing operations based on a predetermined mathematical function. The Fourier transform of the input image is multiplied by the displayed transmission function resulting from the combination of first and second optical masks  34 ,  36 , and the resulting combined image is inverse-Fourier transformed with second lens  30 . Depending on the characteristics of the filter function used, the processed output is a correlated, a convoluted or simply a filtered image, wherein the complex characteristics (amplitude and/or phase) of the locking mask function L(u,ν) may be chosen so as to improve the filtering operation. 
     Since, the complex mask contains a certain continuous or discrete phase and/or amplitude function varying spatially, the optical module  12  cannot perform processing unless, in accordance with the present invention, the filter mask function implemented in second optical mask  36  includes a key mask function K(u,ν) complementary to the locking mask function for substantially cancel the locking effect thereof. In order to be complementary to the locking mask function defined in equation (1) above, the key mask function is preferably expressed by the following relation: 
     
       
           K ( u ,ν)=1/ A ( u ν) e   jφ(u,ν)   (2) 
       
     
     It is to be understood that the key mask function K(u,ν) may be not exactly as defined in equation (2), provided it significantly compensates for the disturbing effect of the locking mask. 
     The optical processing apparatus  10  further comprises a control computer used as a data processor and schematically represented at  38 , the main purpose of which is to generate through a display controller  39  and line  41  filter mask function control data for the second optical mask  36 . The control computer  38  incorporates a memory database  40  for storing encrypted data representing a plurality of selectable filter mask functions produced by a dynamic key generator  42 , whereby each filter mask function is formed by a respective processing or filter function as represented at  44  and the key mask function K(u,ν) as represented at  46 , in a programmable manner. The filter mask function control data being preferably coded in the form of an executable software format, information about the key mask function K(u,ν) cannot be easily found unless the executable code itself is decoded. Since the filter mask function control data stored in memory database  40  cannot be decoded without the code, that information is protected against any user who should not have access to it. For example, if one attempts to use a database generated with another software that does not implement the chosen key mask function K(u,ν) for the optical processor, the latter could not be operated in a useful manner. Moreover, since the data characterizing the filter are saved with the key mask function data into database  40 , it would be not feasible or at least difficult to decode or analyze the saved filter data. 
     Moreover, since the first optical mask  34  according to the preferred embodiment physically implements the locking mask function L(u,ν) whose phase information cannot be revealed without special instrumentation using a complex procedure, the locking mask function L(u,ν) cannot be easily determined to derive a suitable corresponding key mask function K(u,ν). 
     In an embodiment where first optical mask  34  is of a dynamic type using a conventional computer-controlled spatial light modulator, the computer  38  is also used as a data processor for generating through display controller  39  and line  41 ′ locking mask function control data for first optical mask  34 , whereby the locking mask function L(u,ν) is implemented accordingly. Although the improved security inherent to physical implementation of optical mask  34  is not provided in such all software-based locking embodiment, since the locking mask function control data, which may correspond to a plurality of selectable locking mask functions assigned to a plurality of users, are encrypted into memory database  40 , a high security level is still provided. In all embodiments, the locking mask function L(u,ν) implemented by first optical mask  34  is chosen to be sufficiently complicated to ensure that the desired processing or correlation is not performed or is significantly reduced if the filter mask function control data sent to the second optical mask  36  either does not include key control data or includes wrong key control data. In the case where a correlator is used without a suitable software key, correlation peaks either cannot be observed, or cannot be interpreted adequately. 
     Furthermore, the characteristics of filter mask function and key mask function K(u,ν) can be chosen so as to improve dynamic range of the spatial light modulator used as filter mask  36  or to encode some specific filtering function. 
     A preferred mode of operation by a user of the optical apparatus according to the present invention will be now explained with reference to FIG. 1, in the context of a typical pattern recognition application. The user displays a scene to be analyzed on input imaging device  22 , such scene being obtained with a standard camera, computer or other suitable imaging device. Then, using a conventional data entry device (not shown) and guided by a user interface software provided on computer  38 , the user is asked to enter login name and appropriate password, which are then checked by the software before presenting a list of filter mask functions that this particular user is allowed to select. Then, control data specific to a filter mask function selected by the user, which control data include key control data corresponding to the key mask function K(u,ν) complementary to the locking mask function L(u,ν) implemented in the first optical mask  34 , is read out from memory database  40  and sent by display controller through line  41  to the spatial modulator used as second optical mask  36 , which is modulated in accordance with the selected filter mask function. Finally, laser source  13  is switched on to generate coherent light beam  14 , becoming collimated beam  20  which then reaches imaging device  22  for generating the input image. Fourier transform of the input image is then performed by first lens  26  to generate the corresponding transformed input image in the complex spatial domain near or at Fourier transform filter plane  28 . The Fourier transform of the input image is multiplied by the displayed transmission function resulting from the combination of first and second optical masks  34 ,  36 , which implement filter mask function and locking mask function respectively, and the resulting combined image is inverse-Fourier transformed with second lens  30 . The processed image resulting from the inverse Fourier transform of the combined image is captured by optical detector array  32  generating electrical output signals indicative of the light intensity distribution resulting from the optical processing. Since the filter mask function control data that modulate second optical mask  36  include key control data corresponding to the specific key mask function K(u,ν) that is complementary to the locking mask function L(u,ν) implemented in first optical mask  34 , the locking effect thereof is substantially canceled, and the optical processing apparatus is rendered entirely functional accordingly.