Abstract:
This invention in phase homogenization of coherent light from illuminators used for surveillance imaging systems provides a coherent light emitter aligned with a light source ray path along an optical axis with at least two separated microdiffractive diffusers, the microdiffractive diffusers comprising holographically generated surfaces of microdiffractive structure that statistically tend to diffract light at selectable angles or distribution patterns, the microdiffractive diffusers disposed upon a substrate and separated from each other by a distance determined by wavelength phase and irradiance characteristics of the coherent light emitter and by refractive characteristics of at least two microdiffractive diffusers, followed by a homogenized exit ray path in order to produce a diffused pattern of illumination optimized for long distance surveillance imaging.

Description:
FIELD OF INVENTION 
       [0001]    This invention relates to a novel device in the general field of illumination, and more specifically to phase homogenization of coherent light from illuminators used for surveillance imaging systems. 
       BACKGROUND OF THE INVENTION 
       [0002]    Surveillance imaging requires sufficient and evenly distributed on-scene illumination for adequate and timely target recognition. As distance to target increases, so must the range of effective illumination sources. Heavy, fragile, power-hungry, long-range filament bulb illuminators have been surpassed by more effective and power efficient arrays of light emitting diodes (LEDs) and now laser diodes (LDs). 
         [0003]    While LEDs are commonly used in surveillance illuminators, and often employ some method of optical diffuser to evenly distribute light on target, an optimal solution is not as simple when using laser diode (LD) illuminators. The current method of diffusion at the macroscopic level employs an array of small refractive lenses or lens-like patterns imprinted on thin transparent plastic sheets or films. Since LEDs emit a broad range of wavelengths combined from a multiplicity of single sources into overlapping beams, their randomized light can readily be diffused by common refractive diffusers to produce a homogenized distribution. 
         [0004]    The irradiance emitted by a laser diode (LD) is a single coherent source emitting at a much narrower wavelength band. Coherent light is susceptible to causing interference patterns when diffused or refracted which creates a speckle structure on the target which makes it more difficult to produce high quality images of illuminated objects with a surveillance video camera. Speckle noise is a field intensity pattern produced by the mutual interference of coherent beams that are subject to temporal and spatial fluctuations. To eliminate speckle noise, and to take advantage of the range boosting capabilities of laser diode based illuminators, a special means of homogenizing LD output should be employed. 
         [0005]    Existing methods for minimizing speckle noise include use of higher energy lasers whose non-linear effects can reduce source coherence, or inserting optical delay lines in beam paths with higher delay time than the laser coherence time. Another method is time averaging of many randomized speckle-patterns of an object image, through use of rapidly movable optical elements introducing non-stationary phase modulation of a laser beam. 
         [0006]    Short range solutions for removing LD speckle have been employed in photocopiers, digital projectors and point-of-sale barcode readers by the use of single diffusers in front of a refracting lens, rotating diffusers, and diffusers tailored for each color (projection systems). But surveillance illuminators must be capable of projecting diffused light across a wide range of distances (1 to 1000 meters and beyond) in order to create a recognizable image for camera. For this reason a speckle homogenizer employed with LD illuminators must be highly efficient at long distances. 
         [0007]    A LD speckle homogenizer employing microdiffractive diffusion materials for long distance surveillance illuminators is needed for improved imaging outcomes. The following disclosure will describe such a solution. 
       BRIEF SUMMARY OF THE INVENTION 
       [0008]    The illumination system of the present invention provides more efficient speckle homogenization of coherent light to be used in long distance surveillance illuminators, comprising a phase homogenizer with at least two microdiffractive diffusers aligned on an optical axis, which together thoroughly mix coherent light so that speckle noise has little effect on imaging of LD illuminated targets. True microdiffractive diffusers are efficient transmitters of light, but also are capable of refracting at highly selective beam angles, as well as creating asymmetric distributions. Unlike short range devices which project LD light through macroscopically diffused optics to produce images, the phase homogenizer provides microdiffractiveally diffused illumination for long distance surveillance imaging. The microdiffractive diffusers are also effective at homogenizing light from an array of coherent light emitters such as an LD array. 
         [0009]    The invention can be summarized as an illumination system comprising a coherent light emitter aligned with a light source ray path along an optical axis with at least two separated microdiffractive diffusers, the microdiffractive diffusers comprising holographically generated surfaces of microdiffractive structure that statistically tend to diffract light at selectable angles or distribution patterns, the microdiffractive diffusers disposed upon a substrate and separated from each other by a distance determined by wavelength phase and irradiance characteristics of the coherent light emitter and by refractive characteristics of at least two microdiffractive diffusers, followed by a homogenized exit ray path in order to produce a diffused pattern of illumination optimized for long distance surveillance imaging. 
         [0010]    Microdiffractive diffusers are optical elements comprising tiny lens-like surface structures measured at the nanometer scale which individually refract light in a randomized manner due to their non-periodic structures such that they collectively channel light into much smoother distributions than can be achieved with conventional lenses. The microdiffractive diffusers cause light to diverge, which smoothes and homogenizes the coherent light sources, providing uniform light without speckle or hotspots. The degree of smoothing achieved by the microdiffractive diffusers depends on the angle. For applications in which a greater degree of homogenization is required, large angle microdiffractive diffusers are utilized. 
         [0011]    Microdiffractive diffusers can work with all types of light, but are particularly effective when a collimated light source. Color diffraction is eliminated and incoming light is channeled towards well defined areas. Microdiffractive diffusion angles are measured at FWHM and microdiffractive diffusers have an effective angular output (EAO) defined by the equation: 
         [0000]      EAO=SQRT[(light source angle)̂2+(microdiffractive diffuser angle)̂2].
 
         [0012]    When combined in the arrangement herein described, microdiffractive diffusers play an important role in the phase homogenizer of coherent light illuminators used for surveillance imaging systems. 
         [0013]    In a preferred embodiment, the phase homogenizer comprises at least two microdiffractive diffusers aligned on an optical axis and a lens aligned on the optical axis and situated between the two diffusers. 
         [0014]    The microdiffractive diffusers should be separated at a distance along the ray path such that coherent light emitted along the ray path is mixed and speckle noise effect is reduced on imaging of distant targets illuminated by the coherent light. 
         [0015]    The coherent light source can be a laser diode or an array of laser diodes. 
         [0016]    Using more more than two separated microdiffractive diffusers can also be more effective in reducing speckle effect on the target, and in increasing the effective size of the light emitter from a perspective of an illuminated observer so that the diffused pattern of illumination is of lesser areal intensity for retinal safety of illuminated target individuals. 
         [0017]    The salutary effects are achieved if a first microdiffractive diffuser is separated from a second microdiffractive diffuser by a distance in a range from approximately 2.5 centimeters to 10 centimeters. The most legible resolutions are achieved in the 7 to 9 centimeter subrange. Beyond that range, there is additional benefit, but it diminishes and approaches full illegibility at about 16 inches. Any separation beyond the optimal range of 7 to 9 centimeters has the disadvantage of having a longer physical embodiment of the surveillance equipment. 
         [0018]    In addition to the advantages outlined above, the use of microdiffractive diffuser with LDs can increase the effective size of the light source as seen from an illuminated observer. By decreasing laser beam areal intensity, microdiffractive diffusers prevent damage to the human retina if directly observed. This feature can prevent safety hazards to human sight when LD illuminators are used in public spaces or on unintended targets. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0019]      FIG. 1  is a side view showing the basic elements of Surveillance Illumination System including a Phase Homogenizer. 
           [0020]      FIG. 2  is a side view of a Random Source (LED) illuminating a Microdiffractive Diffuser and projecting a Homogenized distribution. 
           [0021]      FIG. 3  is a side view of a Coherent Source (LD) illuminating a Microdiffractive Diffuser and projecting a Speckled distribution. 
           [0022]      FIG. 4  is a side view of a Coherent Source (LD) illuminating a Phase Homogenizer (Lensed) and projecting a homogenized distribution. 
           [0023]      FIG. 5  is a side view of a Coherent Source (LD) illuminating a Phase Homogenizer (Lensless) and projecting a partially homogenized distribution. 
           [0024]      FIG. 6  shows the effects of different embodiments of the invention in differing quality of resolution of licence plates being surveilled. 
       
    
    
     DETAILED DESCRIPTION 
       [0025]      FIG. 1  gives an overview of the basic elements of a surveillance illuminator employing a phase homogenizer  10 . Light from a coherent source  14 , a laser diode array in this example, enters the phase homogenizer  10  by means of the source ray path  46  along the optical axis  42 , and exits along the homogenized ray path  48 , which then illuminates the target  28  with a homogenized distribution  30 . 
         [0026]      FIG. 2  illustrates how light from a random source  12  such as a light emitting diode array, projects a homogenized distribution  30  on the target  28 , by means of a microdiffractive diffuser  16  on a substrate  18 . Illumination through source points  20  (A, B, C) is diffused along a diffusion ray path  22 , to their respective diffusion points  24 , (A 1 , A 2 , B 1 , B 2 , C 1 , C 2 ) on the distribution plane  26 . 
         [0027]      FIG. 3  illustrates how light from a coherent source  14  such as a laser diode, projects a speckled distribution  32  on the target  28 , by means of a microdiffractive diffuser  16  on a substrate  18 . Illumination through source points  20  (D, E, F) is diffused along a diffusion ray path  22 , to their respective diffusion points  24 , (D 1 , D 2 , E 1 , E 2 , F 1 , F 2 ) on the distribution plane  26 . 
         [0028]      FIG. 4  illustrates how light from a coherent source  14  can be made to project a homogenized distribution  30  on the target  28  by means of a microdiffractive diffuser  16  positioned at front and rear focal plane ( 34  &amp;  38 ) of a refractive lens  36 . The light diffused ( 22  &amp;  24 ) from the source points  20  (D, E, F) at the front focal plane  34  is refracted by the lens  36  along the refracted ray path  40  to a second diffuser  16  at the rear focal plane  38 . The output of the phase homogenizer  10  (lensed) follows the homogenized ray path  48  and projects a homogenized distribution  30  onto the target  28 . 
         [0029]      FIG. 5  illustrates the lensless phase homogenizer  44 , an alternate embodiment of the preferred phase homogenizer  10 . Two microdiffractive diffusers  16  are placed at an appropriate distance apart to create the diffusion ray paths  22  illustrated. The resulting distribution on the target  28  is homogenized  30  at its center and speckled  32  at its periphery. 
         [0030]    LED arrays employ diffusers on top of beam-shaping optics and can be regarded as equivalent to an array of secondary light sources excited by a primary LED. An LED emits the combined radiation of many single-mode sources, each with very short coherence time. The resulting target  28  irradiance is an arithmetic sum of irradiances created by different parts of a microdiffractive diffuser  16 . LED sources  12  generate randomized wavelengths that are seldom in phase, whereas an LD has a much longer coherence time, and so it can be put out of phase by diffraction or diffusion. 
         [0031]    The above is illustrated in  FIG. 2 , where the randomized source (LED)  12 , is shown emitting non-coherent light through the microdiffractive diffuser  16 , which spreads the random non-coherent light across the distribution plane  26 , partly mixing it further. The resultant illumination on the target  28  presents a homogenized distribution  30 . 
         [0032]    The target  28  irradiance of a coherent laser diode (LD) source  14  is defined by the vector sum of light waves sent to a target  28  by different parts of an LD or of a microdiffractive diffuser  16  inserted in its beam-path. A vector sum of light waves depends not only on irradiances created by different parts of a light source, like an arithmetic sum, but also on the phases of radiation coming from different parts of the source. Therefore, phase homogenization for a LD source  14  is a much more complicated problem than just intensity homogenization for an LED source  12 . 
         [0033]    As illustrated in  FIG. 3 , a coherent source (LD)  14  emits waves which are in phase with each other, then pass through the microdiffractive diffuser  16 , and arrive at the same diffusion points  24  on the distribution plane  26  as does a randomized source  12 . But unlike light from a non-coherent random source  12 , light from a coherent source  14  constructively interferes (phases add) or destructively interferes (phases cancel) at its diffusion points  24 . Depending on whether the wavelength phases of two intersecting diffusion ray paths  22  may be added together or cancel each other out, any given diffusion point  24  may present an island of double brightness or complete darkness, respectively. The speckled distribution  32  illustrated in  FIG. 3  is the result of these interference patterns of diffused coherent light  14 . 
         [0034]    The following is a more detailed explanation of the nature of the speckle effect created by coherent emitters. As described by equation (1) below, a single-mode laser emits the smallest possible Entendu (product of the beam waist size 2R 0  by its divergence  2 θ 0  sine): 
         [0000]        E   SM   =R   0  sin(θ 0 )=λ/π  (1)
 
         [0000]    where λ is the wavelength. The laws of physics forbid radiation with an Entendu smaller than that defined by equation (1). Monochromatic radiation consists of a multiplicity of single modes (speckles) so that a “snapshot” of coherent illumination looks like an aggregate of these grain-like speckles. (see  FIG. 3 ) Generally, this effect is not apparent by observing light bulbs or LEDs because their multi-chromatic radiation changes every few picoseconds, so we see a homogenized illumination. However, laser radiation is coherent and a laser keeps its modal structure over time. If we look at a multi-mode LD emitting zone through a microscope, we see a series of single-mode speckles that appear similar to the straight line of “grains” observed along a p-n junction. 
         [0035]    For the purposes of this disclosure, the term homogenization will be defined as an optical energy distribution free of speckle noise over a specifically delimited area of illumination. 
         [0036]    A brief note on the properties of the microdiffractive diffuser used in this invention is necessary. Incorporated into the surface of a plastic-like substrate is a diffuser comprised of a holographically generated surface of microdiffractive structure that statistically tends to diffract light at a selectable angle or distribution pattern. Microdiffractive diffraction has a rough equivalence to macro-scale diffusion, but performs with greater optical efficiency and control over its light shaping properties. In addition, microdiffractive diffusers do not require precise alignment of discrete point sources to transmit maximum illuminance, as when an array of laser diodes projects a light manifold through a microdiffractive diffuser  16 . 
         [0037]    The preferred embodiment of the phase homogenizer will now be described in further detail.  FIG. 4  illustrates the preferred embodiment of the phase homogenizer  10  which employs microdiffractive diffusers  16  positioned at the front focal plane  34  and rear focal plane  38  of a refractive lens  36 . The diffusion ray paths  22  are the same as those illustrated in  FIGS. 2 &amp; 3 , but have been shortened in this drawing to make room for the full embodiment. Note that source points  20  (D, E, F) are representative, and that any points of illumination from the LD source  14  could be used to demonstrate how the phase homogenizer  10  removes speckle noise. 
         [0038]    From the D source point  20  at the front focal plane  34 , the diffuser  16  splits the light into a cone bounded by two diffusion ray paths  22  which enter the refractive lens  36  at the D 1  &amp; D 2  diffusion points  24 . Notice how the lens  36  bends the LD light along the refracted ray paths  40  to illuminate the entire inside surface of the second diffuser  16 . Diffracted light from source points  20  E &amp; F are also refracted so as to illuminate the inside surface of the second diffuser  16  positioned at the rear focal plane  38  of the lens  36 . By this means each point of source light  14  is spread by the first diffuser  16 , then collimated by the lens  36  onto the second diffuser  16 , which then spreads it towards the target  28  via the homogenized ray path  48 . The phase homogenizer  10  uses each point (D) of coherent light  14  to create a beam (D 1  to D 2 ) that shines through the second diffuser  16 . Since there are theoretically an infinite number of source points  20 , there are an infinite number of generated beams that wash through the second diffuser  16 , and by this means coherent light  14  is randomized and diffusive speckle noise is removed. 
         [0039]    Any coherent light  16  from any source point  20  at the front focal plane  34  is in phase, and by passing through the phase homogenizer  10 , is fully randomized, and therefore projects LD illumination on target  28  that is fully homogenized as defined above. 
         [0040]    Proven optical principles demonstrate that the optical energy distribution in a lens&#39; rear focal plane  38  is a Fourier transform of the optical energy distribution in its front focal plane  34 . Under such a transform the optical wave phase variation in the RFP  38  is proportional to the square of the ray coordinates both in the FFP  34  and RFP  38 . By this means, the phase distribution of coherent light in the RFP  38  is effectively homogenized. 
         [0041]    Other embodiments of the phase homogenizer include, but are not limited to the use of asymmetric diffusers, and lensless phase homogenization, the latter with or without an air gap between diffusers. 
         [0042]    Asymmetric Phase Homogenization: 
         [0043]    Referring to FIG.  4 —by replacing the existing microdiffractive diffuser  16  which creates symmetrical diffusion at the rear focal plane  38 , with a microdiffractive diffuser  16  that creates an asymmetrical diffusion pattern, a directional distribution is possible. Directional or asymmetric diffusers have a surface comprised of a microdiffractive structure that statistically tends to refract light at a different angle in a horizontal plane than in a vertical plane. Asymmetric diffusion creates a Gaussian distribution which sends more light where it needs to be, onto targets  28  that are predominantly along the horizontal plane. 
         [0044]    The distribution created by a lens  36  and two diffusers  16  pressed together and placed after the lens  36  is similar to the distribution created by the same lens  36  and a single diffuser  16  with doubled diffusion angle, in place of these two diffusers  16 . However, when we increase the distance between two diffusers  16 , homogenization increases as the distance between the diffusers  16  is increased. At some distance the majority of speckle disappears and the distribution becomes homogenized  30 . A similar result can be achieved just with two diffusers  16  without a lens  36 . 
         [0045]    Lensless Phase Homogenization is shown in  FIG. 5 . A lensless phase homogenizer  44 , is composed of two microdiffractive diffusers  16  separated by sufficient distance to project an area of homogenized distribution  30  on the target  28  centered around the optical axis  42 , but which is also surrounded by a ring of speckle distribution  32 . If the diffusers  16  are far enough apart, the overlapping areas of speckle admit a cone of uniform mixing around the optical axis  42 , so that the interference of coherent light  14  is averaged in this region. When rays are recombining on other side of the air gap, between D and D 1  for example, path lengths become mismatched and phases become randomized. The air gap randomizes the coherence, which key to homogenizing coherent light. Prior art solutions use mirrors or gratings or multiple lenses to create the equivalent to an air gap, but two diffusers can do this partially, although two diffusers surrounding a lens is the optimal solution. 
         [0046]    Note that when microdiffractive diffusers  16  are not precisely at either focal plane of a lens  36 , the method of phase homogenization is still possible, but the distribution is not as uniform. Even when the diffusers  16  are on the same side of the lens  36 , or without a lens  36  at all, but separated by a distance large enough so that the light reaching the second diffuser  16 , and the radiation from majority of points of the first diffuser  16  are mixed, this method still gives suitable phase homogenization. 
         [0047]    When the effective size of the first diffuser is much smaller than the area illuminated by the light dispersed by the first diffuser, each point of the second diffuser receives radiation from each point of the first diffuser, as illustrated in  FIG. 5 . This is not valid only for the narrow strip at the edge of the second diffuser. So the light intensity distribution existing at the first diffuser is effectively mixed at the central area of the second diffuser. The optical wave phase at the second diffuser changes uniformly. So the phase distribution of light at the second diffuser is also effectively mixed, except at the edge of the second diffuser. 
         [0048]    Referring to  FIG. 6 , row  1  shows the improvement in reading the license plate alphanumers from non-coherent illumination (on the left) to direct multimode laser diode (LD) light (in the middle), and the continued improvement (on the right) upon adding a diffuser just past (1 millimeter) the laser diode. Row  2  illustrates the improvement upon using 2 diffusers (left) and 3 diffusers (right), each 10 degrees, separated by 7 centimeters, with the main license plate alphanumerics clearly visible, and even the text of the jurisdiction of the plate coming into legibility. Row  3  shows the effect of a single diffuser before the lens (left), after the lens (center), and with a diffuser before the lens and a second diffuser after (right). Row  4  shows the effect is achieved even if the second diffuser is different than the first, e.g. 10 degrees and 30 degrees (middle) and 10 degrees and 60 degrees (right). Row  5  shows the improving effect when the separation of two diffusers is increased from 2 centimeters (left) to 4 centimeters (center) to 10 centimeters (right). Row  6  shows continuing improvement at 12 centimeters of separation (“gap” in the drawing&#39;s labels), but negligible extra improvement from increasing beyond that to 15 centimeters (center) or 20 centimeters (right). There is essentially no improvement in resolution from additionally increasing the gap further, as shown in Row  7 , to 25, 30, or 35 centimeters. 
         [0049]    The use of two separated diffusers in sequence introduces a new system quality not achievable with a single diffuser. It allows suppressing the FOI non-uniformity caused by light diffraction on microdiffractive diffusers  16  and by a laser modal structure. The method of homogenizing speckle by two separated microdiffractive diffusers  16  does not fall into the categories of eliminating speckle noise described elsewhere. The illuminance distribution created by a laser depends both on intensity and phase of all radiation parts, coming to a particular target point from different points of primary (laser itself) or secondary (diffuser surface) light sources. 
         [0050]    Other embodiments are not ruled out or similar methods leading to the same result. 
         [0051]    For applications requiring moderate homogenization an LD can be used with a simplest optical beam shaping system comprising just two microdiffractive diffusers separated by a a distance determined by the wavelength phase and irradiance characteristics of the LD and by the refractive characteristics of the microdiffractive diffusers, without a lens. This is possible with the use of low divergence laser beams. FOI of such a system is a square root of sum of squares of diffusive angles of both diffusers. 
         [0052]    For narrower FOI a simplest lens should be added and two small angle microdiffractive diffusers should be placed near its front and rear focal planes, with the LD manifold placed close to the first microdiffractive diffuser. In this case the FOI is defined by the diffusion angle of a second diffuser only, which can be different in vertical and horizontal planes to match the field of view of a camera used with the illuminator. 
         [0053]    Another embodiment includes the use of multiple element objectives (multiple lens combinations) in place of a single refractive lens between microdiffractive diffusers. The diffusers would be located at the front and rear focal planes of the resultant of the combined lenses. One application of the use of a multiple element objective would be to create a variable beam pattern with all the described properties of the preferred embodiment by moving the elements in the objective. 
         [0054]    Other advantages of using the novel device over other methods or devices is described herein. A phase homogenizer is primarily used for enhancing surveillance illumination for improved imaging. Other uses may include, but are not limited to the use of this invention in displays and light screens, projection systems, flat panel TVs, computer monitors, palm-held displays, barcode scanning, flashlights, lamps, microscope, fiber-optics, and LED illumination, laser, LD and CCFL homogeneity and beam shapers, optical sensing, bio-medical instrumentation, architectural, office and in-house lighting, automotive lighting, signs, posters and cell phone displays. 
         [0055]    The foregoing description of the preferred apparatus and method of installation should be considered as illustrative only, and not limiting. Other forming techniques and other materials may be employed towards similar ends. Various changes and modifications will occur to those skilled in the art, without departing from the true scope of the invention as defined in the above disclosure, and the following general claims.