Abstract:
A fluorescence imaging system including a light source, an optical system, camera and an excitation light filter, the optical system produces a non-uniform fluence excitation illumination beam for illuminating an object and promoting fluorescence emissions, the optical system is positioned between the light source and the object, the optical system modifies the non-uniform fluence illumination beam into a uniform fluence illumination beam and changes the divergence of the uniform fluence illumination beam, the camera has an array of pixels, the camera detects the fluorescence emissions and performs pixel intensity measurements for each of the pixels, the excitation light filter is positioned between the object and the camera and filters out the excitation illumination beam, such that the excitation illumination beam does not reach the camera.

Description:
[0001]    This application claims benefit of Serial No. 234766, filed 21 Sep. 2014 in Israel and which application is incorporated herein by reference. To the extent appropriate, a claim of priority is made to the above disclosed application. 
       FIELD OF THE DISCLOSED TECHNIQUE 
       [0002]    The disclosed technique relates to laser illumination devices, in general, and to methods and systems for producing uniform illumination for fluorescence imaging by employing a laser diode and a vibrating diffuser, in particular. 
       BACKGROUND OF THE DISCLOSED TECHNIQUE 
       [0003]    Fluorescence imaging is employed for imaging tissues. The fluorescence imager detects fluorescence light emitted from a fluorescence agent that is excited by appropriate illumination. For example, a fluorescent microscope is an imaging system for imaging internal blood flow, visible through the skin, by promoting fluorescence radiation from a fluorescent dye added to the flowing blood. The microscope can be employed, for example, during surgery for visualizing the blood flow, and for evaluating tissue perfusion and vessel patency. 
         [0004]    The excitation illumination should be strong (i.e., of high intensity) because the fluorescence signal is often fairly weak, especially with employing IndoCyanine Green (ICG) as the fluorescence agent. One approach to create such an excitation light source is to use a laser diode. A laser diode allows relatively high power to be concentrated in a narrow wavelength region. This is advantageous for exciting fluorescent agents without interfering with the fluorescence image (i.e., as the excitation radiation can be easily filtered out in the camera). 
         [0005]    It is noted that the fluence (i.e., output distribution) of a laser diode is Gaussian, with the center of the beam having significantly higher energy than the edges of the beam. Put another way, the fluence of the laser diode beam is non-uniform. This presents a significant problem for fluorescence imaging since fluorescence intensity is generally proportional to excitation light. Any light source that is not uniform will create artificial differences in fluorescence intensity due to non-uniformity of the excitation light. These artificial differences may be misinterpreted as physiological effects by the user of the system viewing the image. Such a misinterpretation may have very significant clinical consequences. Additionally, if image pixel intensity measurements (of any kind) are used, a non-uniform source will produce incorrect measurement results. 
         [0006]    When designing laser-based illumination systems, one design requirement is to provide systems that are skin-safe and eye-safe. That is, the output of the illumination system should be within the safety limits for light incident on human skin and on human eye. 
         [0007]    A diffuser that vibrates along the plane its plane is known in the art. Reference is now made to Datasheet: LSR-3000 Series for Laser Speckle Reducer LSR-3000 Series, published on March 10, 2013 at: http://www.optotune.com/images/products/Opotune%20LSR-000%20Series.pdf. LSR Speckle reducer is basically a diffuser that is moved (i.e., vibrated). Speckle noise from a laser-based system is reduced by dynamically diffusing the laser beam. The diffuser is bonded to a thin elastic membrane, which includes four independent electro-active polymer electrodes that induce a circular oscillation of the diffuser in X and Y directions. The oscillation frequency is set to the measured resonant frequency of the LSR speckle reducer during production. However, both voltage and frequency of the electro-active polymer can be controlled. 
         [0008]    Semiconductor diode lasers are electrically pumped semiconductor lasers in which the active medium is formed by a p-n junction of a semiconductor diode. Semiconductor diode lasers include several configurations, such as edge-emitters laser diodes and Vertical-Cavity Surface-Emitting Diode Lasers (VCSEL). Edge-emitter laser diodes are made up of bars diced from the wafers on which the diode layers are grown. The high index of refraction contrast between air and the semiconductor material at the side facets of the diced bars act as mirrors. Thus, the light oscillates parallel to the layers and escapes sideways. 
         [0009]    In a VCSEL, the active layer is sandwiched between two highly reflective mirrors (also referred to as distributed Bragg reflectors) composed of several layers of alternating high and low refractive index. The light oscillates perpendicular to the layers and escapes through the top (or bottom) of the device. A VCSEL array is an X-Y array of thousands of laser sources packed into a rectangular illuminator (e.g., 2.8 millimeter×2.8 millimeter). Each individual illuminator in the array is fairly low power (e.g., a few milliwatts). However, taken together the thousands of illuminators make up a powerful illuminator array. VCSEL array products are known in the art, for example, a “6 W CW 808 nm VCSEL Array” by Princeton Optronics (http://www.princetonoptronics.com/products/pdfs/PCW-CS6-6-W0808%20revB-0514.pdf). It is noted that each ray (produced by a single illuminator) in the array has a non-uniform fluence (e.g., Gaussian shaped fluence). Therefore, while the fluence of a beam formed by the multitude of rays is more uniform then that of any of the rays, it still resembles a pin cushion, and cannot be considered as smoothly uniform. 
         [0010]    Reference is now made to U.S. Pat. No. 8,016,449 issued to Liu et al., and entitled “Surface Light Emitting Apparatus Emitting Laser Light”. This publication relates to a surface light emitting apparatus, which can be employed, for example, as a backlight for a screen. The apparatus includes a laser light source, and an optical element. The surface of the optical element has optical power, and it converts the intensity distribution of the laser beam emitted by the laser light source into a uniform intensity distribution. 
         [0011]    Reference is now made to International Patent Application Publication No. WO2011/059383 to Ivarsson et al., and entitled “Optical Sensor System Based on Attenuated Total Reflection and Method of Sensing”. This publication relates to an optical sensor system employing surface plasmon resonance (SPR). The system includes a laser light source, an SPR detector and a distribution device. The distribution device is located between the laser light source and the SPR detector. The laser light source can emit IR radiation. The distribution device distributes the laser beam emitted by the laser light source and converts it into a uniform intensity distribution beam. The system determines the dip in the detected spectrum intensity profile (i.e., the location of the low point in the intensity profile). 
         [0012]    Reference is now made to an article by Reinhard Voelkel et al., and entitled “Laser Beam Homogenizing: Limitations and Constraints”, published at SPIE — 7102 — 19, Optical Design Conf., Laser Beam Homogenizing, Glasgow 2008. This publication relates to laser homogenizing systems. For example, this publication describes a laser homogenizing system employing an array of lenses for converting the intensity distribution of a laser beam into a uniform intensity distribution. 
       SUMMARY OF THE DISCLOSED TECHNIQUE 
       [0013]    It is an object of the disclosed technique to provide a novel method and system for fluorescence imaging, which overcomes the disadvantages of the prior art. In accordance with the disclosed technique, there is thus provided a fluorescence imaging system. The system includes a light source, an optical system, a camera, and an excitation light filter. The light source is configured to produce a non uniform fluence excitation illumination beam having a non uniform beam fluence. The excitation illumination beam illuminates an object for promoting fluorescence emissions. The optical system is optically coupled with the light source and is positioned between the light source and the object. The optical system is configured to modify the non uniform fluence illumination beam into a uniform fluence illumination beam having a uniform beam fluence. The optical system is further configured to change the divergence of the uniform fluence illumination beam. The camera has an array of pixels, and is configured for detecting the fluorescence emissions and for performing pixel intensity measurements for each of the pixels. The excitation light filter is positioned between the object and the camera. The excitation light filter is configured for filtering out the excitation illumination beam, such that the excitation illumination beam does not reach the camera. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The disclosed technique will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
           [0015]      FIG. 1  is a schematic illustration of a fluorescence imaging system, constructed and operative in accordance with an embodiment of the disclosed technique; 
           [0016]      FIG. 2  is a schematic illustration of a fluorescence image detection system, constructed and operative in accordance with another embodiment of the disclosed technique; 
           [0017]      FIG. 3  is a schematic illustration of a uniform illumination system, constructed and operative in accordance with a further embodiment of the disclosed technique; 
           [0018]      FIG. 4  is a schematic illustration of a uniform illumination system, constructed and operative in accordance with yet another embodiment of the disclosed technique; 
           [0019]      FIGS. 5A ,  5 B and  5 C are schematic illustrations of the FOV of an illumination system and a detection system of a fluorescence imaging system, operative in accordance with yet a further embodiment of the disclosed technique; and 
           [0020]      FIG. 6  is an illustration of an output distribution across one axis of an illumination beam, operative in accordance with yet another embodiment of the disclosed technique. 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0021]    The disclosed technique overcomes the disadvantages of the prior art by providing a uniform illumination system including a light source and an optical system. The light source produces a non-uniform fluence light beam (having non-uniform beam fluence). The optical system modifies the non-uniform light beam into a uniform fluence light beam having uniform beam fluence (i.e., modifies the light beam to be a top hat light beam, also referred to as a flat top beam). 
         [0022]    In accordance with an embodiment of the disclosed technique, the illumination system further includes a vibrating diffuser. The optical system includes an optical fiber transmitting the light beam produced by the light source. The optical system reimages the output of the optical fiber at an intermediate image plane. The vibrating diffuser is located on the intermediate image plane, and is vibrating along two non-parallel axes. The vibrations of the diffuser compensate for any irregularities on the diffuser surface, thereby smoothing spots on the diffused light beam. 
         [0023]    In accordance with another embodiment of the disclosed technique, the light source is a VCSEL array producing an array of light rays forming together a non-uniform light beam. The array light source is positioned off the focal plane of the optical system, such that a slight defocus is introduced to the light beam, thereby smoothing the light beam and making it a uniform fluence beam. 
         [0024]    Reference is now made to  FIG. 1 , which is a schematic illustration of a fluorescence imaging system, generally referenced  100 , constructed and operative in accordance with an embodiment of the disclosed technique. Fluorescence imaging system  100  includes an image detection system  102  and an illumination system  104 . Fluorescence imaging system  100  acquires images of an object  106  located at a distance D 2  from the imaging system (e.g., 20-40 Centimeters). Detection system  102  is located at a distance of D 1  from illumination system  104 . 
         [0025]    Detection system  102  detects fluorescent emissions emitted from object  106 , and produces accordingly a fluorescent image of object  106 . Detection system  102  can further detect a visible image of object  106  by detecting visible light reflected from object  106 . The structure and components of detection system  102  are further elaborated herein below with reference to  FIG. 2 . 
         [0026]    Illumination system  104  illuminates object  106 . In particular, illumination system  104  provides at least an excitation illumination beam that excites a fluorescent agent in object  106 , which emits fluorescent light in response to the excitation light. The excitation illumination beam produced by illumination system  104  should cover the Field of View (FOV) of detection system  102 . In other words, an area of object  106  which would not be illuminated with excitation light, would not emit fluorescent light even if it includes a fluorescent agent. Thereby, that area would not be captured in the fluorescent image, which might lead to clinical misinterpretations. 
         [0027]    Additionally, the fluence of the illumination beam should be spatially uniform. That is, the excitation light intensity at each location within the FOV of the illumination beam should be substantially similar. The structure and components of illumination system  104  are further elaborated herein below with reference to  FIG. 3 . 
         [0028]    It is noted that illumination system  104  and detection system  102  are not parallel, but are slightly angled toward each other. Thus, the overlap between the FOV of detection system  102  and the FOV of illumination system  104  is increased. In particular, the distance (D 1 ) and the angle, between detection system  102  and illumination system  104  are determined according to the desired object distance (D 2 ), such that the FOV of illumination system  104  would cover that of detection system  102 . 
         [0029]    Reference is now made to  FIG. 2 , which is a schematic illustration of a fluorescence image detection system, generally referenced  200 , constructed and operative in accordance with another embodiment of the disclosed technique. Detection system  200  includes an excitation light filter  202 , a dichroic mirror  204 , a mirror  206 , a short pass filter  208 , a long pass filter  210 , a visible light camera  212  and an IR light camera  214 . 
         [0030]    Excitation light filter  202  is located on the object side of dichroic mirror  204  and is optically coupled therewith. Dichroic mirror  204  is further optically coupled with a visible light camera  212  via short pass filter  208 , and with IR camera  214  via mirror  206  and long pass filter  210 . 
         [0031]    Excitation light filter  202  blocks (or at least attenuates) reflected excitation light, and admits fluorescence light  218  and reflected visible light  216  into detection system  200 . It is noted that reflected visible light  216  is either reflected ambient light (i.e., preexisting light in the surrounding of the microscope), or is light provided from a dedicated light source other than the excitation light source. Dichroic mirror  204  reflects IR light  218  (i.e., the fluorescence emissions light) while enabling visible light  216  (i.e., the reflected visible light) to pass therethrough. Visible camera  212  detects a visible image of the object (e.g., object  106  of  FIG. 1 ), while IR camera  214  detects the fluorescence image of the object. 
         [0032]    Reference is now made to  FIG. 3 , which is a schematic illustration of a uniform illumination system, generally referenced  300 , constructed and operative in accordance with a further embodiment of the disclosed technique. Uniform illumination system  300  includes a point light source  302 , an optical fiber  303 , a first aspheric lens  304 , a field lens  306 , a pre-diffuser window  308 , a vibrating diffuser  310 , a post-diffuser window  312 , a second aspheric lens  314 , a third aspheric lens  316 , and an output window  318 . 
         [0033]    Point light source  302  can be, for example, a laser diode light source, or another laser light source. Generally speaking, point light source  302  can be any point light source providing excitation illumination adapted to excite the fluorescence agent, or otherwise to promote fluorescent emissions to be detected. It is noted that the output distribution (i.e., fluence) of point light source  302  is non-uniform. For example, for an edge-emitter laser diode light source, the fluence is Gaussian shaped with the center of the beam having significantly higher energy than the edges of the beam. 
         [0034]    For acquiring fluorescence images, the FOV of an illumination beam  324  (i.e., the beam diameter at the imaged area) should cover at least the FOV of the fluorescence image detector (e.g., detection system  102  of  FIG. 1 , or detection system  200  of  FIG. 2 ). Furthermore, the fluence of illumination beam  324  should be distributed substantially evenly (i.e. uniformly) across the FOV. Otherwise, for example in case of Gaussian fluence, the fluorescent emissions at the center of the imaged area would be much stronger than those at the edges of the imaged area, thereby producing an erroneous image that may lead to clinical misinterpretation. 
         [0035]    Optical fiber  303  is an optical fiber for transmitting the light produced by light source  302  toward the lens  304 . For example, fiber  303  can be a 400 micrometer diameter fiber having a numerical aperture of 0.22. Alternatively fiber  303  can be of other dimensions to adapt to the dimensions of light source  302  and those of lens  304 . Fiber  303  receives illumination beam  324  at the output of light source  302 . The output port of fiber  303  (not referenced) is positioned next to lens  304 . 
         [0036]    Each of first aspheric lens  304 , field lens  306 , second aspheric lens  314  and third aspheric lens  316  is a converging lens having positive optical power. Lenses  304  and  306  reimage the output of the fiber  303  onto an intermediate image plane on which the surface of diffuser  310  is positioned. Fiber  303  and lenses  304  and  306  flatten the output of light source  302 . In other words, fiber  303  and lenses  304  and  306  turn the non-uniform fluence of beam  324  at the output of light source  302  into substantially uniform fluence when beam  324  arrives at diffuser  310 . 
         [0037]    Lenses  314  and  316  expand the output of diffuser  310  to an angle required for the FOV of the fluorescence system. In other words, lenses  314  and  316  increase the divergence of beam  324 . It is noted that as lenses  314  and  316  are converging lenses, the lenses converge the illumination beam toward the focal point of thereof, and the beam divergence is increase only after passing the focal point. Therefore, the illumination system is placed well away from the focal point of lens  316 . Windows  308  and  312  protect the diffuser surface, and window  318  protects the output of the illumination system  300 . 
         [0038]    Vibrating diffuser  310  is a planar diffuser that is vibrated in two perpendicular axes along its plane (i.e., vibrated along the X axis and the Y axis of  FIG. 3 ). Diffuser  310  might have surface irregularities that might produce respective irregularities in the fluence of beam  324 . Any irregularities in the output beam will translate to artificial variations in the intensity of areas of the fluorescence image and may lead to clinical misinterpretations. The vibrations of diffuser  310  smooth the diffused beam at the output of diffuser  310 , thereby eliminating fluence irregularities caused by possible surface irregularities of diffuser  310 . In particular, the frequency of vibrations of vibrating diffuser  310  exceeds the frame rate of the camera, thereby compensating for structural irregularities of the diffuser surface and smoothing the illumination beam. In other words, the diffuser face is imaged as part of the illumination optics functionality and any microstructures on the face will show up in the output beam profile. Diffuser vibrations in two non-parallel axes eliminate the effect of microstructures on the diffuser. 
         [0039]    Diffuser  310  also improves the safety of the illumination system, by diffusing the narrowly concentrated light emitted from point light source  302 . When designing laser-based illumination systems, one design requirement is to provide systems that are skin-safe and eye-safe. That is, the output of the illumination system should be within the safety limits for light incident on human skin and on human eye. A standard laser diode is generally considered a point source. For a point source illuminator, skin safety and eye safety limits are much lower than for a diffused source. This is because a point source can be focused (by the lens of the eye for example) onto a very small area and can therefore cause thermal damage to the area. As such, a diffusion mechanism is highly desirable with laser diodes to convert the illuminator from a point source to a safer diffused source. 
         [0040]    A vibrator (not shown) vibrates diffuser  310 . As mentioned above, vibrating diffuser  310  is vibrated in two non-parallel axes (e.g., perpendicular axes X and Y). The frequency and the amplitude of vibrations in each of the axes can be controlled. 
         [0041]    In accordance with an alternative embodiment of the disclosed technique, diffuser  310  can be composed of several diffusers. For example, a first diffuser vibrating along the X axis and positioned at a first intermediate image plane of the output of the light source, and a second diffuser vibrating along the Y axis and positioned at a second intermediate image plane of the output of the light source. 
         [0042]    In accordance with yet another alternative embodiment of the disclosed technique, the diffuser can be non-planar (e.g., dome shaped or aspheric diffuser). Thereby, the shape of the diffuser can be employed to disperse the illumination beam evenly across its FOV. 
         [0043]    As can be seen in  FIG. 3 , Illumination system  300  includes a light source (point light source  302 ) and an optical system (i.e., fiber  303  and lenses  304 ,  306 ,  316  and  318 ). Specifically, the optical system of uniform illumination system  300  is formed of two optical subsystems, a first subsystem  320  includes fiber  303 , first aspheric lens  304  and field lens  306 . Fiber  303  receives the non-uniform illumination beam produced by light source  302  and relays it toward lens  304 . First optical subsystem  320  is a double telecentric system re-imaging an intermediate image of the output of fiber  303  on the plane of vibrating diffuser  310 . That is, diffuser  310  is positioned on a plane of an intermediate image of the output of fiber  303  for diffusing the illumination light beam. At the output of subsystem  320  (i.e., after diffuser  310 ) the illumination beam is substantially a top hat beam having substantially uniform fluence. In other words, first optical subsystem  320  is configured to transform the fluence of beam  324  from a non-uniform fluence at the output of light source  302  to a uniform fluence at the input to diffuser  310 . That is, first optical subsystem  320  is composed of optical elements that are constructed and located as to flatten the fluence of beam  324 . 
         [0044]    Second optical subsystem  322  includes second and third aspheric lenses  314  and  316 . Subsystem  322  reimages a second image plane of the output of light source onto a working plane (e.g., an open surgery area to be fluorescently imaged). The illumination beam at the second image plane remains a top hat beam (as is the beam at the output of diffuser  310 ). Second optical subsystem  322  increases the divergence of light beam  324  to match the required field of view of the system. 
         [0045]    It is noted that the optical system described herein above with reference to  FIG. 3  is an example optical system and can be replaced by other optical systems having different components. The optical system is designed to modify the illumination beam fluence to become substantially uniform, thereby producing a uniform top hat beam. Additionally, the optical system increases the beam diameter of beam  324  at the imaged area. The optical system of the uniform illumination system reimages the output of the fiber at an intermediate image plane, at which the vibrating diffuser would be located. The diffuser diffuses the light beam. 
         [0046]    Reference is now made to  FIG. 4  which is a schematic illustration of an illumination system, generally referenced  400 , constructed and operative in accordance with yet another embodiment of the disclosed technique. Uniform illumination system  400  includes a Vertical-Cavity Surface-Emitting Laser (VCSEL) array light source  402 , a first aspheric lens  404 , a second aspheric lens  406 , and an output window  408 . 
         [0047]    VCSEL array light source  402  provides excitation illumination adapted to excite the fluorescence agent, or otherwise to promote fluorescent emissions to be detected. VCSEL array light source  402  produces uniform output (i.e., producing a top hat beam having uniform fluence). Specifically, while each of the thousands of illuminators in the VCSEL array of light source  402  has a Gaussian beam profile, the illuminators placed in close proximity to each other and added together produce a uniform output. Additionally, the output of VCSEL array light source  402  is much larger (e.g.: 2.8 milimeter×2.8 milimeter) than that of a point source making it easier for the system to be skin-safe and eye-safe. 
         [0048]    Each of first aspheric lens  404  and second aspheric lens  406  is a converging lens having positive optical power. Lenses  404  and  406  expand the output of light source  402  (i.e., increase the divergence of the light beam) to an angle required for illuminating the FOV of the fluorescence system. Window  408  protects the output of illumination system  400 . It is noted that the optical design of system  400 , including lenses  404  and  406 , and window  408 , is similar to subsystem  322  of  FIG. 3 . Specifically, in accordance with the embodiment of the disclosed technique presented in  FIG. 4 , subsystem  320  of  FIG. 3 , is replaced by VCSEL array light source  402 , while subsystem  322  (directed at expanding the excitation illumination beam) remains similar. 
         [0049]    VCSEL array light source  402  is placed at some distance in front of an entry focal plane  410  of the optical system (lenses  404  and  406 ) of illumination system  400 , to affect a desired degree of uniformity. In other words, by placing light source  402  off the focus plane of lens  404  a slight defocus (blur) is introduced to the array output. The amount of defocus can be adjusted as needed by changing the distance between the light source  402  and the lens  404  while viewing the system illumination pattern. 
         [0050]    As mentioned in the background section VCSEL array light source  402  actually produces an array of rays, each having a Gaussian output distribution. By introducing some defocus (by placing the light source off the focus plane of the optical system), the multitude of rays are merged for forming together a smoothed uniform beam. Essentially, the de-focused output of VCSEL array  402  is similar to the output of vibrating diffuser  310  of  FIG. 3 . That is, the illumination system produces a uniform fluence light beam having uniform flat top fluence, and being skin-safe and eye-safe (within safety levels defined by applicable standards). 
         [0051]    As can be seen in  FIG. 4 , illumination system includes a light source (i.e., VCSEL array light source  402 ) and an optical system (i.e., lenses  404  and  406 ). The light source produces a non-uniform light beam. The optical system modifies the non-uniform light beam into a uniform light beam having uniform flat top fluence. 
         [0052]    Reference is now made to  FIGS. 5A ,  5 B and  5 C which are schematic illustrations of the FOV of an illumination system and a detection system of a fluorescence imaging system, operative in accordance with yet a further embodiment of the disclosed technique. With reference to  FIG. 5A , FOV  502  of an illumination system and FOV  504  of a detection system are depicted for an object distance of 20 centimeters (i.e., the object is located 20 centimeters from the illumination and detection systems). With reference to  FIG. 5B , FOV  502  of the illumination system and FOV  504  of the detection system are depicted for an object distance of 30 cm. With reference to  FIG. 5C , FOV  502  of the illumination system and FOV  504  of the detection system are depicted for an object distance of 40 centimeters. 
         [0053]    As can be seen in  FIGS. 5A-5C , FOV  502  of the illumination system is larger than FOV  504  of the detection system, for fully illuminating the imaged object. At a relatively short object distance (e.g., 20 centimeters as in  FIG. 5A ), the FOVs of the illumination system and the detection system are at an offset to each other. This occurs because the systems are located adjacently to one another and not coaxially. At a relative longer distance (40 cm as depicted in  FIG. 5C ), the distance between the systems is small with respect to the object distance and the FOVs appear substantially concentric. 
         [0054]    Reference is now made to  FIG. 6 , which is an illustration of an output distribution, generally referenced  600 , across one axis of an illumination beam, operative in accordance with yet another embodiment of the disclosed technique. As can be seen, output distribution  600  is substantially of a top hat shape. That is, the fluence of the illumination beam is dispersed evenly across its FOV. As mentioned above, for example with reference to  FIG. 3 , the optical system that reimages the output of the light source at an intermediate image plane (at which the diffuser is located) is configured to transform the light beam outputted by the light source into a uniform fluence light beam as depicted in  FIG. 6 . 
         [0055]    It is noted that the imaging system described herein above with reference to  FIG. 2 , serves only an example of a fluorescence imaging system, which employs the illumination system of the disclosed technique. Generally, the illumination system of the disclosed technique can be employed for any fluorescence imaging system having a non-uniform fluorescence light source. Moreover, in the examples set forth herein above, the uniform illumination system is employed for producing uniform excitation illumination beam for a fluorescence imaging system. It is noted however, that the uniform illumination system can be employed for other situations in which the light source output is non-uniform and its fluence should be made uniform and smooth. For example, microscope or endoscope imaging systems may also benefit from a uniform high power illumination source as described in this invention. In fact, any system that is intended to perform measurements of output intensity of image pixels will benefit from this type of illumination source. 
         [0056]    It will be appreciated by persons skilled in the art that the disclosed technique is not limited to what has been particularly shown and described hereinabove. Rather the scope of the disclosed technique is defined only by the claims, which follow.