Abstract:
An active infra-red surveillance illuminator uses a statistically mono-directional micro-diffractive material overlaid on a bank of light emitting diodes (LEDs) to refract light from the LEDs onto a target image. This delivers energy distribution profiles out to a distance to match the aspect ratios of current wide-angle target and wide-angle camera surveillance systems. The distribution of refracted light is elliptical. This distribution can vary by using different diffractive material in conjunction with various numbers, brightness and angles of the LEDs in an LED bank. The effective range of the illuminators is greatly extended with this type of illumination. By limiting infrared (IR) radiation down to a 10 degree vertical window it renders the IR illumination much more effective for surveillance imaging by providing much more effective power on wide, ground-level scenes, and particularly enables multi-lane license plate capture.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of infra-red illumination for low-light video surveillance, and to the application of refractor technology, in particular micro-diffractive engineered material. It also relates to wide-angle video surveillance for such as multi-lane traffic license plate reading. 
     BACKGROUND 
     Due to the inverse square law of illumination, the intensity of light falling on a target area decreases in proportion with the square of the distance. When using a camera and lens arrangement to view a typical surveillance scene a certain proportion of the image contains the foreground and a certain amount the background. The amount of light required or optimal to illuminate the foreground of a target area is usually much lower than that required to illuminate the background of a target area. Additionally target areas in surveillance are typically more extended horizontally than vertically, because most target areas are based on a horizontal ground area, across which peoples or vehicles travel. 
     Most illumination systems produce a circularly diverging beam which when used with a camera, which requires the installer to point the peak of the beam at the farthest target point. For a fixed target distance there is an optimum beam profile in the vertical orientation. When viewing at the same distance with a wider and wider view and matching the circular illumination the illuminator moves further away from the optimum in the vertical orientation and wastes more light. Moreover, as the peak of the light source is pointed above the line of the target, a large proportion of the light is above the target area and is not utilized. 
     There are many systems which use infrared illumination for low light photography or video photography. There are also many systems which use LEDs for photographic or video illumination. Some of these systems utilize refractive or reflective elements to diffuse or focus illumination. 
     Illuminators using LEDs with refractive or reflective elements to enhance illumination are varied and include a number of different types of refractors which channel light from the LEDs so as to alter the distribution of illumination on the target and/or to make illumination more efficient by conserving light. 
     There are also a number of devices in which lights are fitted with micro-prisms or similar constructions to refract light onto a target. Some of these devices are used in projectors or similar systems or in media effects systems for backlighting. 
     Several necessities prior to this invention have combined to limit the technology of low light illumination for wide-angle nighttime surveillance video photography, particularly for license plate capture and reading, when several vehicles&#39; headlights may be pointed toward a surveillance camera. There is a need to pulse the surveillance illumination to conserve energy. There is a need to synchronize a surveillance camera to the pulsed illumination returning to the camera after having fallen on a moving target. There is a need to illuminate a wide area in the case of vehicles traveling across lanes or where multiple lanes of vehicles are targeted. There is a need for wide-angle effective illumination matching wide-angle high pixel density cameras in order to capture tiny fast-moving license plates out of a large wide-angle scene such as a multi-lane freeway. 
     SUMMARY OF THE INVENTION 
     The present invention provides a surveillance illuminator system in which a micro-diffractive material comprising is placed in front of a light-emitting manifold such as a bank of infra-red LEDs to alter the shape of illumination on a target area for wide-angle surveillance under low light conditions. The illumination field produced has an elliptical Gaussian shape. The illuminator is designed to be used in connection with a wide-angle surveillance camera having an aspect ratio sensitivity matching the light pattern produced by the illuminator. This novel combination represents a breakthrough for the quality of background illumination for surveillance and nighttime video and photography as it results in wide illumination patterns without moving away from the optimum vertical profile. 
     This invention results in the ability to produce an asymmetrical illumination pattern enabling increased sensitivity for wide area photographic or video coverage, such as wide-angle views of multi-lane traffic; optimization of the vertical spread of illumination under the cosecant squared distribution for more efficient background illumination; in the reduction in the number of LEDs which would normally be used for a new angle configuration; and in reduced light pollution including non-visible light pollution. 
     This invention is useful in an intelligently secured transportation system, in which surveillance of a multi-lane roadway is performed with a mega-pixel camera having a wide-angle aspect, in conjunction with a mega-wide monitor. The surveillance illuminator system of the present invention provides less light where it is unneeded in the top or bottom of the field of view and more light in an extended central area. The surveillance illuminator system therefore conserves power. It enables the available power and heat dissipation characteristics of the illuminator system&#39;s light source to effectively illuminate the wide-angle horizontal problem area. Less light overall is thereby needed to achieve ultra-high quality image capture of small target sub-sections of the scene because the light is more efficiently focused on the target sub-sections by the micro-diffractive material. 
     The invention provides a surveillance illuminator system in which a micro-diffractive material that is “mono-directional” in that is statistically more directional for light in a first plane of output than in another output plane, such as in a horizontal output plane than in a vertical output plane, is mounted in front of a light-emitting manifold comprising a plurality of light sources, such as a planar array of light-emitting diodes (LEDs) array emitting light having a wavelength of approximately 850 nanometers, in the infrared range from 700 to 1000 nanometers. In an especially efficient embodiment, the system has a surveillance camera having an aspect ratio sensitivity substantially matching a light pattern projected by the mono-directional micro-diffractive material mounted in front of the light-emitting manifold. 
     This invention may employ a double layer of the micro-diffractive material which combines horizontal and vertical sets of material to produce different asymmetrical combinations from a smaller subset of lens shapes and in some combinations may extend the maximum angle of diffraction. 
     This invention can also use different micro-diffractive material above the mid-line of the illuminator than below may allow an asymmetrical vertical beam profile which enables more efficient use of the light resulting in increased illumination distances. 
     The invention thus provides significantly increased usable distance from prior illuminators, for example 110 meters compared to 54 meters for a light source comprising LED&#39;s having a 60 degree angle of light dispersion before micro-diffraction. The foreground/background ratio of illumination is also much more even with the present invention, making for a significantly better image because the camera needn&#39;t try to adjust the exposure to the foreground; it is able to make better use of the light on the target scene. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is a top down view of a section of the illuminator showing light rays from the light emitting manifold traveling through the mono-directional micro-diffractive material and being diffracted horizontally. 
         FIG. 1B  is a side view of the section of the illuminator showing light rays from the light emitting manifold traveling through the mono-directional micro-diffractive material with negligible vertical diffraction. 
         FIG. 2A  is a horizontal cross-sectional view of the mono-directional micro-diffractive material. 
         FIG. 2B  is a vertical cross-sectional view of the mono-directional micro-diffractive material. 
         FIG. 3  shows the structure of the micro-diffractive material under high magnification 
         FIG. 4  is a perspective view of the rectangular LED array showing the heat sink and the constant current power source. 
         FIG. 5  is a perspective view of the mono-directional micro-diffractive material overlaid on the rectangular LED array. 
         FIG. 6  is a view of the target image illuminated by a conventional infra-red illuminator. It illustrates how the power density for a circular beam pattern reduces as it gets wider to take in the extent of an asymmetrical target. 
         FIG. 7  is a view of the target image illuminated by the mono-directional micro-diffractive material rectangular LED array illuminator. It shows that using an elliptical or asymmetrical beam pattern there is practically no wasted light. 
         FIG. 8  shows another embodiment of this invention using different material above the mid line of the illuminator than below to allow an asymmetrical vertical beam profile. 
         FIG. 9A  plots the distribution function of light emitted by the illuminator in terms of quantity of light and horizontal divergence angle. The full width-at half maximum (FWHM) of the function is shown. 
         FIG. 9B  plots the distribution of light emitted by the illuminator in terms of quantity of light and vertical divergence angle. The full width-at half maximum (FWHM) of the function is shown. 
         FIG. 10A  shows a vehicle and its license plate illuminated by a conventional illuminator with a circular beam. 
         FIG. 10B  shows two vehicles and their license plate illuminated with an illumination beam of similar area using the micro-diffractive light emitting manifold illuminator. 
         FIG. 11A  shows a four lane roadway scene illuminated by the micro-diffractive light emitting manifold illuminator and the same scene captured by a wide-angle CCTV camera and displayed on a wide-angle monitor. 
         FIG. 11B  shows a four lane roadway scene illuminated by the micro-diffractive light emitting manifold illuminator and captured two lanes at a time by four CCTV video cameras. The integrated image is displayed on a wide-angle monitor. 
         FIG. 12A  is a top down view of a section of the illuminator showing light rays from the light emitting manifold traveling through a spherical lens and narrowing before passing through the mono-directional micro-diffractive material and being diffracted horizontally. 
         FIG. 12B  is a side view of the section of the illuminator showing light rays from the light emitting manifold traveling through a spherical lens and narrowing before passing through the mono-directional micro-diffractive material with negligible vertical diffraction. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1A , the light emitting manifold  11  seen in a top down view, which contains a number of near point-source lights such as  12  and  13 , sends rays of light such as  17  through a mono-directional micro-diffractive material  14  which causes the rays of light to diverge horizontally as at point  18 , so that the outgoing light rays  15  and  16  travel out from the illuminator at greater angles. 
     Referring to  FIG. 1B , the light emitting manifold  1  is seen in a side view, with point-source light  12  shown at the top, and point-source lights  2  &amp;  3  which are below the visible plane of  FIG. 1A . Light from the manifold  1  passes through the micro-diffractive material  4 , but the mono-directional nature of the micro-diffractive material enables the light to pass through without significant vertical divergence as a point  8 . The outgoing rays of light such as  5  and  6  continue close to their initial trajectories. 
     Referring to  FIG. 2A , a beam of light from an LED  20  characterized by light rays  25  &amp;  26  passes through a micro-diffractive lens  21  on a magnified horizontal cross-section of micro-diffractive sheet  22 . The rays  25  &amp;  26  are diffracted to greater incidence angles at points  23  and  24  on the micro-diffractive lens  21 . The new trajectory of the rays is shown by  27  &amp;  28  respectively as contrasted with the original path represented by dotted lines. 
     Referring to  FIG. 2B , a beam of light from an LED  30  characterized by light rays  34  &amp;  35  passes through a magnified vertical cross-section of the micro-diffractive sheet  32 . The new trajectory of rays  34  &amp;  35  is shown by  36  &amp;  37  respectively to be very nearly identical to the original trajectory. This is due to the extreme flatness of the lens structure of the micro-diffractive sheet in the vertical dimension. At this level of magnification, no curvature can be seen to define a lens and no inflection is visible at points  31  &amp;  33 . 
     Referring to  FIG. 3 , a microscopic segment of the micro-diffractive material is shown, at very high magnification with nanoscopic refractive structures as at  122 ,  123 ,  124 , &amp;  125  that appear like waves. The length and relative flatness of the wave crests allows for less diffraction in the vertical plane than in the horizontal plane. The aligned nanoscopic structures cause the micro-diffractive sheet statistically to tend to refract light at a different angle in one plane such as a horizontal plane than in another plane perpendicular to the first, such as a vertical plane. 
     Referring to  FIG. 4 , the rectangular LED array  41 , is mounted on the illumination housing  43 , within a frame  42 . The housing  43  is equipped with a heat sink  45 , and a constant current power source  46 . It can be mounted with a bracket  44 . The front window  47  for the light emitting manifold has optical filter properties that pass substantially all infrared light energy while blocking light at shorter wavelengths; 
     Referring to  FIG. 5 , a sheet composed of micro-diffractive material  50  is shown overlaying a small section of the LED array  51 . LEDs such as  52  &amp;  54  appear slightly blurred beneath the sheet of micro-diffractive material  50 . The LED housing  53  contains the LED array  51 . 
     Referring to  FIG. 6 , the power density for a circular beam pattern  71  reduces as it gets wider to take in the extent of the asymmetrical targets  72  (house) and  73  (roof). The total power is divided over the area of the larger circle  70 . Much of the power is then wasted above and below the target. 
     Referring to  FIG. 7 , in using an elliptical or asymmetrical beam pattern  81  there is practically no wasted light in the area where a circular beam  80  extends above and below the elliptical beam  81  while the beam illuminates wide targets such as roof  82  and house  83 . This is very efficient use of the light available and thus can be used to increase the obtainable imaging distance or reduce the number of multiple illuminators that may be required for a particular application or reduce the size of illuminator required and decrease the electrical power required for a particular application. 
     Referring to  FIG. 8 , using different material above the mid line  91  of the illuminator than below allows an asymmetrical vertical beam profile  93 . More efficient use of the light can be achieved resulting in increased illumination distances. A conventional illuminator would have a beam profile more like  92  with a much greater extent along the vertical axis  90 . 
     Referring to  FIG. 9A , the distribution of light emitted by the illuminator is plotted as a bell-shaped curve. On the vertical axis the quantity of light is shown against the horizontal divergence angle of the light from the illuminator on the horizontal axis. The full width-at half maximum (FWHM) of the function is shown between −67.5 and +67.5 indicating a significant horizontal divergence of 130 degrees of the light from the illuminator, the top of the curve being zero angle toward the center of the field of illumination. 
     Referring to  FIG. 9B , the distribution of light emitted by the illuminator is plotted as a bell-shaped curve. On the vertical axis the quantity of light is shown against the vertical divergence angle of the light from the illuminator on the horizontal axis. The full width-at half maximum (FWHM) of the function is shown between 6 and 12 degrees, indicating a very low vertical divergence of the light from the illuminator. 
     Referring to  FIG. 10A , a target license plate  62  and reflectors  64  and  66  are illuminated by a conventional infrared beam  60 . The images appear blurred due to insufficient concentration of light. This is a problem which arises because with a conventional illuminator, much light is wasted illuminating the background. 
     Referring to  FIG. 10B , two lanes separated by dotted line  68  are illuminated by the wider beam  61  of the micro-diffractive surveillance illuminator. In the leftmost lane the target license plate  63  and reflectors  65  and  67  are all clearly illuminated by the concentrated elliptical illumination beam  61  produced by the light emitting manifold micro-diffractive illuminator. Due to the efficiency of the illumination beam a second target license plate  69  in the rightmost lane is also illuminated despite the fact that the total area and total power usage of the illumination beam in  FIG. 10B  is comparable to that in  FIG. 10A . The micro-diffractive material is formed and arranged such that it projects an elliptical Gaussian distribution of refracted light, having a major axis of diffraction in the horizontal plane and a horizontal divergence in a range greater than double the angular divergence of the array of LEDs and a vertical divergence substantially unaffected by the micro-diffractive material. In the license plate application—the mono-directional micro diffractive material will allow multiple lanes to be covered with the same or less illumination compared to existing surveillance illuminator and camera systems by enabling the camera to make much better use of the illumination on scene. 
     Referring to  FIG. 11A , the light emitting manifold micro-diffractive illuminator  201  is shown illuminating a scene  211  consisting of a four-lane highway  213 . The image of the highway is captured by a wide-angle CCTV camera  202 , which transmits the image to a wide-angle video monitor  203  where it is displayed as  215 . The arrows (such as  200 ) which point outward from the illuminator indicate outgoing light. The arrows (such as  199 ) which point inward to the camera indicate the incoming light from the illuminated scene  211 . 
     Referring to  FIG. 11B , the light emitting manifold micro-diffractive illuminator  204  is shown illuminating a scene  212  consisting of a four lane highway  214 . The image of the highway is captured collectively by four integrated specialized cameras,  205 ,  206 ,  207 ,  208 . Cameras  205  and  206  work together to capture light represented by the downward arrows  210  and from the two leftmost lanes of the highway. One of these cameras may be optimized for daytime and the other optimized for nighttime. Alternatively, one of these cameras may be optimized for license plate image capture and the other optimized to capture images of the vehicle&#39;s driver or passengers. Cameras  207  and  208  are trained on the two rightmost lanes of the illuminated highway  214 . They can be specialized in the same manner as cameras  205  and  206 . The wide-angle light projected from the system onto a multi-lane roadway can be pulsed to synchronize with video input, of license plates or passengers in the vehicles traveling toward the system, to a wide-angle mega-pixel surveillance camera. The video input can then be processed by alphanumerics pattern recognition software in order to read license plate information of vehicles traveling on the roadway. The image of the wide, multi-lane highway  216  is shown correspondingly be displayed on a wide-angle monitor  217 . 
     An array of surface mount LEDs with standard 120 degree circular illumination pattern can be combined with 6 degree spherical lens to produce narrow beams which are then directionally micro-diffracted. In  FIG. 12A , the light emitting manifold  311  is seen in a top down view, containing a plurality of near point-source lights such as  312  and  313 , sends rays of light such as  317  through spherical lenses such as  321  and  322  which causes them to narrow as at  323  and  324 . The light rays then pass through a mono-directional micro-diffractive material  315  which causes the rays of light to diverge horizontally as at point  318 , so that the outgoing light rays  315  and  316  travel out from the illuminator at greater angles. Referring to  FIG. 12B , the light emitting manifold  331  is seen in a side view, with point-source light  312  shown at the top, and point-source lights  332  &amp;  333  which are below the visible plane of  FIG. 12A . Rays of light such as  325  from the manifold  331  pass through spherical lenses such as  326  which causes them to narrow as at  327  and  328 . The light rays then pass through the micro-diffractive material  334 , but the mono-directional nature of the micro-diffractive material enables the light to pass through without significant vertical divergence as a point  338 . The outgoing rays of light such as  335  and  336  continue close to their pre-micro-diffractive material trajectories shown at  327  and  328 . 
     The within-described invention may be embodied in other specific forms and with additional options and accessories without departing from the spirit or essential characteristics thereof. The presently disclosed embodiment is therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalence of the claims are therefore intended to be embraced therein.