Abstract:
A device for image gating using an array of reflective elements is provided herein. The device includes an array of reflective elements, wherein each one of the reflective elements is movable within a range of a plurality of tilt positions, wherein the array is located at an image plane of the device, wherein the array is perpendicular to an optical axis of the device. The device further includes a control unit configured to control the reflective elements such that in at least some of the tilt positions, the reflective elements reflect the radiant flux at said image plane, to one or more projection planes. A gradual rotation of the reflective elements along the plurality of tilt positions result in a gradual increase or decrease in the intensity of the image reflected from the array of reflective elements while maintaining the image integrity.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims priority from U.S. provisional application No. 61/539,487 filed on Sep. 27, 2011 and from UK application No. GB1116474.6 filed on Sep. 26, 2011 which are incorporated herein by reference in their entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to gated systems, and more particularly, to such systems that employ an array of reflective elements for implementing the gating. 
         [0004]    2. Discussion of Related Art 
         [0005]    Active gated systems are known in the art for achieving an enhanced image of a scene in high scattering or absorption media. Gated systems are used when there is a clear advantage for a reflective image rather than a thermal (emitted) image. Since the human eye is used to perceiving a reflected image and the human brain is accustomed to process reflected images, it is easier to interpret reflected images. 
         [0006]    Thermal imagers are associated to emitted image formed by the collection of the photons emitted from the observed target. There are certain features in an image that one can observe only by using the reflected image and equally there are such that can be achieved only by using the emitted image. 
         [0007]    Active imaging benefits from a unique technological feature that enables the synchronized switching between the light source and the camera. This mode of operation is referred to as synchronized gated imaging (SGI) or burst illumination (BIL). The active imaging systems mode eliminates the reflected backscatter of near range reflectors. A reflector may be an aerosol particle or any feature located within the field of view. The SGI mode of operation enables adjustments to the illumination level at each range resulting in an effective uniform illumination regardless of the range. The depth of field is a controllable feature of an active system, controlling the opening and closing of the camera and light source in a synchronized manner along the time line. 
         [0008]    If the transparent atmosphere medium is clear there is no need for gating. When observing a target with known range with no obstacles along the line of sight there will be no reflections of close objects. When there are reflections from close objects, the gating technique eliminates the backscatter target contrast degradation. 
         [0009]      FIG. 1  is a schematic block diagram illustrating the reflection due to an obstruction media according to the existing art. An exemplary gating imaging system  10  operates as follows: pulse of light (can be laser)  13  from illuminator  12  is radiated to the atmosphere. Some of the pulses backscatter from a disturbing medium  16 . In order to eliminate the impact of the backscattering, the camera shutter  14  is closed when the backscattering radiance reaches it and the camera shutter opens when the pulse  14  returns after reflection from target  17 . 
         [0010]    There are several known methods in the art to design a gated imaging system. One method is based a single pulse per frame—in one camera frame time (normally for standard video about 30-40 msec) only one pulse of laser is radiated to the target. The camera is synchronized for the return of the pulse. Usually the laser has high energy per pulse and very narrow pulse width (˜20-100 nsec). The implementation of this method compels the use of a detector so that its internal shutter has a response time in the order of micro seconds and possibly less. 
         [0011]    Another method is based on multiple pulses per frame—in one camera frame time multiple pulses of light (normally laser) are radiated to the target with time delay between one another. The camera is synchronized for the return of each pulse. The time delay between the gate “ON” duration of the camera and the radiation of the light source is depended on the distances to the observed scene. The duration of the “ON” time is also depended on the distance. The light source can be operated in high repetition rates (even up to mega hertz) with high average power and changeable pulse width (typically 100 nsec to 50 microsec for observation systems or even femto-second for very small depth of filed imaging). The implementation of this method compels the use of specific and unique types of detectors. This is because the internal shutter needs to be opened and closed in the same repetition rate of the light source (even up to mega hertz). The common sensors that are being used in a multiple gating system are ICMOS/ICCD/EBAPS (which has this capability). In these sensors the image intensifier (II) behaves as the shutter in front of the camera (The II has very fast shuttering capabilities). The spectral sensitivity is limited to the image intensifier sensitivity. This method is illustrated in  FIG. 2A  showing the timing scheme of the gating and the light source signal over time. 
         [0012]    The laser and camera are synchronized in time. The depth of field and minimum range can be achieved by changing the synchronization and time scheme. 
         [0013]    As illustrated in  FIG. 2B  and  FIG. 2C , there is a possibility to change the depth of field and minimum range from frame to frame by playing with the timing. In this way a 3D video is achieved. The 3D video can be used for better understanding of the scene and the distance of detected objects. Moreover this method will produce better imaging performance—the illumination will be uniform over the entire depth of field. For every depth slice the illumination timing and power is optimized. All the slices can be combined to generate one image. 
       BRIEF SUMMARY 
       [0014]    One aspect of the present invention provides a device for image gating using an array of reflective elements. The device includes an array of reflective elements, wherein each one of the reflective elements is movable within a range of a plurality of tilt positions, wherein the array is located at an image plane of the device, wherein the array is perpendicular to an optical axis of the device. The device further includes a control unit configured to control the reflective elements such that in at least some of the tilt positions, the reflective elements reflect the radiant flux at said image plane, to one or more planes projection planes (other than the focal plane of the optical device). A gradual rotation of the reflective elements along the plurality of tilt positions result in a gradual increase or decrease in the intensity of the image reflected from the array of reflective elements. 
         [0015]    These, additional, and/or other aspects and/or advantages of the present invention are: set forth in the detailed description which follows; possibly inferable from the detailed description; and/or learnable by practice of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    The present invention will be more readily understood from the detailed description of embodiments thereof made in conjunction with the accompanying drawings of which: 
           [0017]      FIG. 1  is a schematic block diagram illustrating gated system according to the existing art; 
           [0018]      FIGS. 2A-2C  are graph diagrams illustrating one aspect according to the existing art; 
           [0019]      FIG. 3  is a schematic diagram illustrating the structure according to some embodiments of the present invention; 
           [0020]      FIGS. 4A-4E  are schematic diagrams illustrating one aspect according to some embodiments of the present invention; 
           [0021]      FIG. 5  is a schematic diagram illustrating the structure according to some embodiments of the present invention; 
           [0022]      FIG. 6  is a schematic diagram illustrating the structure according to some embodiments of the present invention; 
           [0023]      FIG. 7  is a schematic diagram illustrating the structure according to some embodiments of the present invention; and 
           [0024]      FIGS. 8A and 8B  are graph diagrams illustrating one aspect according to some embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]    Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction and the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is applicable to other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for the purpose of description and should not be regarded as limiting. 
         [0026]      FIG. 3  is a schematic diagram illustrating a non-limiting exemplary structure of an optical device  300  according to some embodiments of the present invention. Optical device  300  includes a main lens  310  associated with an optical axis and an array of controllable reflective shutters  330  that are associated each with a first position and a second position. Array of reflective shutters  330  may be implemented in a non-limiting example as a digital micro mirror device (DMD) and is located at an image plane of the at least one main lens. Additionally, array  330  is further perpendicular to the optical axis thereof. 
         [0027]    Optical device  300  further includes a one-way optical folding element  320  located between main lens  310  and array  330  and along the optical axis main lens. In a non limiting embodiment, one-way optical folding element  320  may be implemented as a total internal reflection (TIR) prism. Specifically, folding element  320  is configured to transfer light coming from main lens  310  and further to fold a light reflected from array  330  onto a second optical axis that is perpendicular to the optical axis of main lens  310 . 
         [0028]    Optical device  300  further includes one or more field lenses  340  located along the second optical axis and configured to focus light coming from folding element  320  onto a focal plane  350 . Additionally, optical device  300  further includes a controller (not shown) operatively associated with array  330  of reflective shutters and configured to switch the reflective shutters between the first and the second position. At the first position, light coming through main lens  310  is reflected to folding element  320  and then focused by field lenses  340 , yielding an image at focal plane  350 . At the second position and during switching to and from the first position, light coming from main lens  310  is reflected off the second optical axis (this is the optical axis of the focal plane array  350  and the field lens  340 ). 
         [0029]    Advantageously, by the aforementioned positioning of array  330  at the focal plane of main lens  310 , the image produced and reflected upon focal plane  350  does not suffer from the diffraction effect of array  330 . Because array  330  is at the focal plane all the reflected lobes due to the Brag effect are focused by field lenses  340  to respective focal points at focal plane  350 . 
         [0030]    Yet another advantage of the aforementioned positioning of array  330  at the focal plane of main lens  310  is that when the mirror rotates into their first and second positions, there is no smearing of the image on focal plane  350 . Specifically, during movement, the rays that are folded onto field lenses  340  affect the formation of the image merely by changing the amount of energy of the image at focal plane  350 , in other words, the image fades in and fades out but is not smeared. 
         [0031]      FIGS. 4A-4E  are schematic diagrams illustrating one aspect according to some embodiments of the present invention. In  FIG. 4A  reflective shutters of array  330 A are in the non-image forming position and no rays reach focal plane  350  at all. As the mirror rotate to the image forming position along  FIGS. 4B ,  4 C,  4 D and  4 E more and more rays (lobes) of the light reflected from the reflective shutters reach field lenses  340 A and then focal plane  350 A. As explained above, the transient stage from the image forming position and non-image forming is characterized by a gradual change in the intensity of the image thus avoiding the undesirable side effect of image smearing. 
         [0032]    Consistent with some embodiments of the present invention, optical device  300  may further have an array of optical sensors located at the focal plane  350  of field lens  340 . The sensors may be of any wavelength and sensitivity in accordance with the optical properties of optical device  300  and the desired use thereof. 
         [0033]    Consistent with some embodiments of the present invention, folding element  320  may be a beam splitter of any type and may also be implemented, by way of example, by a total internal reflection (TIR) prism, wherein the TIR prism is applied to light coming from array  330 . 
         [0034]    Consistent with some embodiments of the present invention, the main lens may be a photographic lens or a set thereof. In some embodiments, optical device  300  serves as a shutter mechanism for a camera. In some embodiments, the camera serves as a camera in an optical gated imaging system but other shutter-related applications may also be considered. 
         [0035]      FIG. 5  is a schematic diagram illustrating the structure according to some embodiments of the present invention. As shown herein, an alternative approach could be the use of the second state of the array of reflective shutters for gating using folding optics.  FIG. 5  shows the reflections of light when the array of reflective shutters is in the 1 st  position. In this approach the field lens  530  and  562  has two entrance apertures, and one exit aperture (near the focal plane array). An inner folding mirror  540  is used between the first and second entrance field lens aperture. The inner folding of the light by the mirrors can be made only where the light is collimated, hence between the field lenses  530  and  562 . In this method the repetition rate of the entire system is doubled. The light source of light  505  is activated when the mirrors are in transition between the 1 st  and 2 nd  states. The exact synchronization between the light source on time and the arrival of the mirrors into position will determine the beginning of the depth of field. The on time in every state will determine the full depth of field. Once the depth of field is achieved—the array of reflective shutters rotates to the second position. Again, during the rotation the laser is pulsed on. This process is repeated. 
         [0036]    This method is effective mainly for the short range where the laser pulse is limited to the transition time between states. For larger ranges, one of the channels can be obstructed using (for example) a mechanical black foil obstructing the mirror  640  or the first field lens  630  and  662  as shown in  FIG. 6 . The foil can be inserted in and out using a mechanical mechanism. The insertion does not need to be quick since the ranging is changed only on a frame level time. 
         [0037]      FIG. 7  is a schematic diagram illustrating the structure according to some embodiments of the present invention; for near distance or for increase of depth of field or for higher repetition rate of the array of reflective shutters we can use two focal plane arrays as follow. In  FIG. 7  a focal plane array is placed in the on position of the array of reflective shutters and a focal plane array is placed in the off position. The dead time is when the array of reflective shutters is shifted along the plurality of the tilt positions. In this way both extreme states of the array of reflective elements can be used. The pulse of light is radiated while the array of reflective elements is “traveling” from one state to the other. In this method, double repetition rate may be achieved in a similar manner to the aforementioned embodiment of folding mirrors inside the field lens. Can be used to increase the depth of field in the same frame for different reflective elements positions. 
         [0038]      FIGS. 8A and 8B  are graph diagrams illustrating one aspect according to the existing art. Specifically, the aforementioned requirement according to which reflective shutter array need to be perpendicular to the optical axis of the main lens is illustrated. As shown in  FIG. 8A , when imaging large distance objects light reaches the optics relatively collimated. The lens  920 A (corrected photographic lens) can focus the light into a focal plane  910 A perpendicular to its optical axis. The size of the focal plane array and the focal length determines the field of view. However, if as shown in  FIG. 8B , the focal plane array  910 B is placed not perpendicular to the optical axis, the collimated light coming from different angles will not focus on the array. Possibly, some of the points may be where light will be focused, but surely not all of them will be in focus. In  FIG. 8B  the dashed rays does not focus on the focal plane array. 
         [0039]    The reflective shutters are rotated slightly (in the order of microns) on the focal plane array and by so changing the reflected light angle. As shown in  FIG. 8B  the result of the rotation of the reflective shutters mechanical plane will result in the image being out of focus. 
         [0040]    In the above description, an embodiment is an example or implementation of the invention. The various appearances of “one embodiment”, “an embodiment” or “some embodiments” do not necessarily all refer to the same embodiments. 
         [0041]    Although various features of the invention may be described in the context of a single embodiment, the features may also be provided separately or in any suitable combination. Conversely, although the invention may be described herein in the context of separate embodiments for clarity, the invention may also be implemented in a single embodiment. 
         [0042]    Furthermore, it is to be understood that the invention can be carried out or practiced in various ways and that the invention can be implemented in embodiments other than the ones outlined in the description above. 
         [0043]    The invention is not limited to those diagrams or to the corresponding descriptions. For example, flow need not move through each illustrated box or state, or in exactly the same order as illustrated and described. 
         [0044]    Meanings of technical and scientific terms used herein are to be commonly understood as by one of ordinary skill in the art to which the invention belongs, unless otherwise defined. 
         [0045]    While the invention has been described with respect to a limited number of embodiments, these should not be construed as limitations on the scope of the invention, but rather as exemplifications of some of the preferred embodiments. Other possible variations, modifications, and applications are also within the scope of the invention. Accordingly, the scope of the invention should not be limited by what has thus far been described, but by the appended claims and their legal equivalents.