D ESCRI PTI ON

FI ELD OF I NVENTION

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 TH E INVENTION

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) .

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 .

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 .

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 .

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 1 000 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.

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 .

BRI EF SUMMARY OF TH E INVENTION

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.

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 .

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 .

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 :

EAO = SQRT[ (light source angle) ^A 2 + (microdiffractive diffuser angle) ^A 2 ] ,

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.

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.

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.

The coherent light source can be a laser diode or an array of laser diodes.

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.

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.

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 Figure 1 is a side view showing the basic elements of Surveillance I llumination System including a Phase Homogenizer.

Figure 2 is a side view of a Random Source (LED) illuminating a Microdiffractive Diffuser and projecting a Homogenized distribution .

Figure 3 is a side view of a Coherent Source (LD) illuminating a Microdiffractive Diffuser and projecting a Speckled distribution .

Figure 4 is a side view of a Coherent Source (LD) illuminating a Phase Homogenizer (Lensed) and projecting a homogenized distribution .

Figure 5 is a side view of a Coherent Source (LD) illuminating a Phase Homogenizer (Lensless) and projecting a partially homogenized distribution .

Figure 6 shows the effects of different embodiments of the invention in differing quality of resolution of licence plates being surveilled ..

DETAI LED DESCRI PTION

Figure 1 gives an overview of the basic elements of a surveillance illuminator employing a phase homogenizer 1 0. Light from a coherent source 14, a laser diode array in this example, enters the phase homogenizer 1 0 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.

Figure 2 illustrates how light from a random source 1 2 such as a light emitting diode array, projects a homogenized distribution 30 on the target 28, by means of a microdiffractive diffuser 1 6 on a substrate 1 8. I llumination through source points 20 (A, B, C) is diffused along a diffusion ray path 22, to their respective diffusion points 24, (A1 , A2, B1 , B2, C1 , C2) on the distribution plane 26.

Figure 3 illustrates how light from a coherent source 1 4 such as a laser diode, projects a speckled distribution 32 on the target 28, by means of a microdiffractive diffuser 1 6 on a substrate 1 8. I llumination through source points 20 (D, E, F) is diffused along a diffusion ray path 22, to their respective diffusion points 24, (D1 , D2, E1 , E2, F1 , F2) on the distribution plane 26.

Figure 4 illustrates how light from a coherent source 1 4 can be made to project a homogenized distribution 30 on the target 28 by means of a microdiffractive diffuser 1 6 positioned at front and rear focal plane (34 & 38) of a refractive lens 36. The light diffused (22 & 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 1 6 at the rear focal plane 38. The output of the phase homogenizer 1 0 (lensed) follows the homogenized ray path 48 and projects a homogenized distribution 30 onto the target 28.

Figure 5 illustrates the lensless phase homogenizer 44, an alternate embodiment of the preferred phase homogenizer 1 0. Two microdiffractive diffusers 1 6 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.

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 1 6. LED sources 1 2 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 .

The above is illustrated in Fig . 2, where the randomized source (LED) 1 2, is shown emitting non-coherent light through the microdiffractive diffuser 1 6, 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.

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 1 6 inserted in its beam-path . A vector sum of light waves depends not only on irradiances created by different parts of a l ight 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 1 4 is a much more compl icated problem than just intensity homogenization for an LED source 1 2.

As il lustrated in Fig . 3, a coherent source (LD) 1 4 emits waves which are in phase with each other, then pass through the microdiffractive diffuser 1 6, and arrive at the same diffusion points 24 on the distribution plane 26 as does a randomized source 1 2. But unl ike l ight from a non-coherent random source 1 2 , light from a coherent source 1 4 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 il l ustrated in Fig . 3 is the result of these interference patterns of diffused coherent l ight 1 4.

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 smal lest possible Entendu (product of the beam waist size 2R ₀ by its divergence 2θ ₀ sine) :

E _SM = R ₀ sin (9 ₀ ) = λ/π ( 1 ) where λ is the wavelength . The laws of physics forbid radiation with an Entendu smal ler than that defined by equation ( 1 ) . Monochromatic radiation consists of a multipl icity of single modes (speckles) so that a "snapshot" of coherent ill umination looks l ike an aggregate of these grain-like speckles, (see Fig . 3) General ly, this effect is not apparent by observing light bul bs or LEDs because their multi- chromatic radiation changes every few picoseconds, so we see a homogenized il l umination . 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 l ine of "grains" observed along a p-n junction .

For the purposes of this disclosure, the term homogenization wil l be defined as an optical energy distribution free of speckle noise over a specifical ly del imited area of il l umination .

A brief note on the properties of the microdiffractive diffuser used in this invention is necessary. Incorporated into the surface of a plastic-l ike substrate is a diffuser comprised of a holographically generated surface of microdiffractive structure that statistical ly tends to diffract light at a selectable angle or distribution pattern .

M icrodiffractive diffraction has a rough equivalence to macro-scale diffusion, but performs with greater optical efficiency and control over its l ight shaping properties. I n addition, microdiffractive diffusers do not require precise al ignment of discrete point sources to transmit maximum il l uminance, as when an array of laser diodes projects a light manifold through a microdiffractive diffuserl 6.

The preferred embodiment of the phase homogenizer will now be described in further detail . Figure 4 ill ustrates the preferred embodiment of the phase

homogenizer 1 0 which employs microdiffractive diffusers 1 6 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 ill ustrated in Figs. 2 & 3, but have been shortened in this drawing to make room for the ful l embodiment. Note that source points 20 (D, E, F) are representative, and that any points of il l umination from the LD source 1 4 could be used to demonstrate how the phase homogenizer 1 0 removes speckle noise .

From the D source point 20 at the front focal plane 34, the diffuser 1 6 spl its the l ight into a cone bounded by two diffusion ray paths 22 which enter the refractive lens 36 at the D 1 & D2 diffusion points 24. Notice how the lens 36 bends the LD l ight along the refracted ray paths 40 to il l uminate the entire inside surface of the second diffuser 1 6. Diffracted l ight from source points 20 E & F are also refracted so as to ill uminate the inside surface of the second diffuser 1 6 positioned at the rear focal plane 38 of the lens 36. By this means each point of source light 1 4 is spread by the first diffuser 1 6, then col limated by the lens 36 onto the second diffuser 1 6, which then spreads it towards the target 28 via the homogenized ray path 48. The phase homogenizer 1 0 uses each point (D) of coherent l ight 1 4 to create a beam (D 1 to D2) that shines through the second diffuser 1 6. Since there are theoretical ly an infinite number of source points 20, there are an infinite number of generated beams that wash through the second diffuser 1 6 , and by this means coherent light 1 4 is randomized and diffusive speckle noise is removed . Any coherent light 1 6 from any source point 20 at the front focal plane 34 is in phase, and by passing through the phase homogenizer 1 0, is ful ly randomized, and therefore projects LD il lumination on target 28 that is ful ly homogenized as defined above .

Proven optical principles demonstrate that the optical energy distribution in a lens' rear focal plane 38 is a Fourier transform of the optical energy distribution in its front focal plane 34. U nder 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 .

Other embodiments of the phase homogenizer incl ude, but are not limited to the use of asymmetric diffusers, and lensless phase homogenization, the latter with or without an air gap between diffusers .

Asymmetric Phase Homogenization :

Referring to Fig . 4 -- by replacing the existing microdiffractive diffuser 1 6 which creates symmetrical diffusion at the rear focal plane 38, with a microdiffractive diffuser 1 6 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 l ight 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 .

The distribution created by a lens 36 and two diffusers 1 6 pressed together and placed after the lens 36 is similar to the distribution created by the same lens 36 and a single diffuser 1 6 with doubled diffusion angle, in place of these two diffusers 1 6. However, when we increase the distance between two diffusers 1 6,

homogenization increases as the distance between the diffusers 1 6 is increased . At some distance the majority of speckle disappears and the distribution becomes homogenized 30. A similar result can be achieved j ust with two diffusers 1 6 without a lens 36. Lensless Phase Homogenization is shown in Fig . 5. A lensless phase homogenizer 44, is composed of two

microdiffractive diffusers 1 6 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 1 6 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 l ight 1 4 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 l ight. Prior art sol utions 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 sol ution .

Note that when microdiffractive diffusers 1 6 are not precisely at either focal plane of a lens 36 , the method of phase homogenization is stil l possible, but the distribution is not as uniform . Even when the diffusers 1 6 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 l ight reaching the second diffuser 1 6, and the radiation from majority of points of the first diffuser 1 6 are mixed, this method stil l gives suitable phase homogenization .

When the effective size of the first diffuser is much smal ler than the area il l uminated by the light dispersed by the first diffuser, each point of the second diffuser receives radiation from each point of the first diffuser, as il l ustrated in Fig . 5. This is not valid only for the narrow strip at the edge of the second diffuser. So the l ight 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.

Referring to Figure 6, row 1 shows the improvement in reading the license plate al phanumers from non-coherent il lumination (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 mill imeter) the laser diode . Row 2 ill ustrates the improvement upon using 2 diffusers (left) and 3 diffusers (right) , each 1 0 degrees, separated by 7 centimeters, with the main license plate al phanumerics clearly visible, and even the text of the jurisdiction of the plate coming into legibil ity. 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 . 1 0 degrees and 30 degrees (middle) and 1 0 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 1 0 centimeters (right) . Row 6 shows continuing improvement at 1 2 centimeters of separation ("gap" in the drawing's labels) , but negl igible extra improvement from increasing beyond that to 1 5 centimeters (center) or 20 centimeters (right) . There is essential ly no

improvement in resol ution from additional ly increasing the gap further, as shown in Row 7, to 25 , 30, or 35 centimeters .

The use of two separated diffusers in sequence introduces a new system qual ity not achievable with a single diffuser. It al lows suppressing the FOI non-uniformity caused by light diffraction on microdiffractive diffusers 1 6 and by a laser modal structure . The method of homogenizing speckle by two separated microdiffractive diffusers 1 6 does not fal l into the categories of el iminating speckle noise described elsewhere . The il luminance distribution created by a laser depends both on intensity and phase of al l radiation parts, coming to a particular target point from different points of primary (laser itself) or secondary (diffuser surface) light sources.

Other embodiments are not ruled out or similar methods leading to the same result.

For appl ications 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 .

For narrower FOI a simplest lens should be added and two smal l angle microdiffractive diffusers should be placed near its front and rear focal planes, with the LD manifold placed close to the first microdiffractive diffuser. I n this case the FO I 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 ill uminator.

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 appl ication 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 .

Other advantages of using the novel device over other methods or devices is described herein . A phase homogenizer is primarily used for enhancing

surveil lance ill umination for improved imaging . Other uses may incl ude, but are not l imited to the use of this invention in displays and light screens, projection systems, flat panel TVs, computer monitors, pal m-held displays, barcode scanning, flashl ights, lamps, microscope, fiber-optics, and LED il l umination, laser, LD and CCFL homogeneity and beam shapers, optical sensing, bio-medical

instrumentation, architectural, office and in-house l ighting, automotive lighting, signs, posters and cel l phone displays.

The foregoing description of the preferred apparatus and method of installation should be considered as il l ustrative only, and not limiting . Other forming

techniques and other materials may be employed towards similar ends . Various changes and modifications will occur to those skil led in the art, without departing from the true scope of the invention as defined in the above disclosure, and the fol lowing general claims .