Patent Publication Number: US-5249501-A

Title: Visualization device for near-IR laser designator

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to visualization devices for use in weapon&#39;s systems and, more particularly, to a visualization device for a near-IR laser designator. 
     Near-infrared (IR) lasers, which operate in the 0.8 um.-1.5 um. range, are used in some weapons systems as target designators and rangers. That is, the laser projects a beam of light at a target to illuminate the target or range upon it. In turn, this allows a gunner to train his weapon on the spot of light produced by the laser beam striking the target. If the laser is boresighted with the weapon, it can be used by the gunner as an aiming mark. Although the gunner can be fitted with goggles or other devices sensitive to the wavelength of the laser, these encumber the gunner and are therefore unsatisfactory. What is required is a solution which 1) is easy for an operator to use, 2) provides the operator freedom to move his head without incurring parallax errors, 3) works well in a high vibration environment, 4) is easily visible to the operator, and 5) does not require high power levels to transmit energy all the way to a target and back. This invention solves the above problems by using a small percentage of backscatter of the laser energy as it passes through an object. The backscattered energy impinges upon a medium which emits visible light in response to being irradiated with the laser energy. By placing the medium at a focal point of the object, a virtual image is created. This image is at, or near, infinity along the projected laser beam line. Because the image is at infinity, there is no parallax. Further, because of the interaction of the laser beam with the object, any movement (vibration) of the object relative to the laser or the medium does not produce any shift in the virtual image. Similarly, any movement in the medium does not result in any shift of the image. 
     SUMMARY OF THE INVENTION 
     Among the several objects of the present invention may be noted the provision of a visualization device for use as a target designator in a weapons system, the device projecting a spot of light into a scene including the target; the provision of such device which employs a laser operating in the near infrared portion of the light spectrum and which converts this longer wavelength light to provide an image in the visible portion of the spectrum; the provision of such a device which employs a window/reflector and an infrared sensor for producing a visible spot of light which a gunner can use to designate a target; the provision of such device which produces the light so it can be seen by the gunner with his unaided vision or using special vision devices such as night goggles; the provision of such a device which eliminates parallax; the provision of such a device which is inherently immune from vibrations so the gunner has a stable platform at all times and in all operational environments; the provision of such a device in which the reflector is of a material which reflects only a small portion of the incident laser beam in order for the device to provide the light spot used by the gunner; the provision of such a device which utilizes off-the-shelf components; and, the provision of such a device which is a low cost optical system that is easy to use and with which the gunner is readily trained. 
     In accordance with the invention, generally stated, a tracking system is for use in a weapons system, or any aiming system using longer infrared wavelengths. The tracking system is used to assist a gunner in designating and tracking a target which the gunner can fire upon using a weapon under his control. A laser projects a laser beam at the target which illuminates a portion of the target struck by the beam. The laser is boresighted with the weapon and moves with it as the gunner tracks the target. A window is interposed between the laser and the target. The window passes a portion of the incident laser beam on it for the laser beam to strike the target. The window also reflects back a portion of the laser beam. The amount of the laser beam passing through the window is substantially greater than the portion reflected. Further, the reflected portion of the laser beam is reflected at an angle with respect to the incident beam. A sensing element upon which the reflected portion of the beam impinges produces a visible spot of light observable by the gunner. The gunner maintains the laser beam on the target by moving his weapon as the target moves. Movement of the target corresponds to movement of the spot of light on the sensing element. Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic representation of the tracking system of the present invention; and, 
     FIG. 2 is an illustration of the tracking system in use with a weapon. 
     Corresponding reference characters indicate corresponding parts throughout the drawings. 
    
    
     DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring to the drawings, apparatus 10 of the present invention comprises a targeting system for use with a weapon W. As shown in FIG. 2, the weapon comprises a machine gun MG. However, it will be understood that the machine gun is for illustrative purposes only, and that the invention can be practiced with other weapons, or weapon systems, as well. The machine gun, or other weapon, is operated by a gunner G who designates a target (not shown) by, in the case of the machine gun, pointing it at the target. He then tracks the target by moving the muzzle of the weapon with the target as it moves. In battlefield situations, where the target may be obscured by dust, smoke, etc., it is desirable to be able to range or designate the target with longer wavelength laser beams. Further, it is also desirable that the targeting and tracking be done so that if, for example, the machine gun were mounted on a movable platform such as a personnel carrier, tank, or helicopter, the targeting and tracking would not be effected by vibrations, shock, etc. 
     To accomplish this, the targeting system 10 of the present invention first includes means 12 for projecting a beam B of long wavelength coherent light at the target. Means 12 includes a laser beam generator 14 for projecting a laser beam at the target. The laser beam can be in the near infrared portion of the light spectrum; i.e. it generates a light beam having a wavelength of 0.8 um.-1.5 um. It will be understood that lasers operating in other portions of the light spectrum can also be used. For safety reasons, for example, the laser may have a wavelength greater than 1.5 um. The principles discussed herein, apply however to whatever wavelength laser is used. The laser beam illuminates the portion of the target which it strikes. As shown in FIG. 2, the laser beam generator is mounted on the machine gun. Consequently, it is movable by the gunner as he moves the weapon in tracking the target. Preferably, laser beam generator 14 is boresighted with the machine gun. 
     Next, a window means 16 is interposed between the laser beam projector and the target. Means 16 passes a portion of the impinging laser beam through to the target, and reflects back the remainder of the beam. The amount of the beam passing through the window means is substantially greater than that reflected. Means 16 includes a spherically shaped window 18 which is, for example, approximately 40 cm.-50 cm. in diameter. The window is concave in form and is either fabricated of, or its inner and outer surfaces, 20i and 20o respectively are coated with, a material which reflects approximately 1% of the laser light impinging on it. The geometry of window 18 results in a focal length of approximately 50-100 cm, for example. Again, the principles discussed herein are not constrained by focal length. 
     System 10 next includes sensing means 22 upon which the reflected portion of the laser beam impinges. Means 22 includes an infrared sensor card 26. Card 26 is a commercially available electron trap, upconverting device which is energized either by exposure to daylight or to fluorescent light. The card is responsive out to 1.6 um. For other, longer wavelengths, similar upconverters are available. In addition, these commercially available cards have signal rise and decay times which are on the order of 10 nsec. As a result, the system can respond rapidly to beam motion on the card resulting from any vibration. This is also advantageous because it prevents depletion of the stored energy at any one location on the card. 
     The card is positioned off-axis with respect to the axis of the laser beam projector. As seen in FIG. 1, reflected beam Br is reflected at an angle θ with respect to the incident beam Bi. Angle θ is preferably between 10° and 15°. When the reflected laser beam strikes card 26 it produces a spot 28 of light which is observable by gunner G. The spot 28 of light is in the visible portion of the spectrum. The spot of light is reflected onto surface 20i of the window so to be observable by the gunner. By maintaining the spot of light on the image of the target, he can track the target. And, he can do this with his unaided eye, or, if appropriate, with night goggles. 
     The effect of system 10 is to produce a virtual image of the target object using a laser operating at a wavelength the eye cannot see. The curved, reflective mirror 18, and sensor card 26, together with the laser beam produce a psuedo image of the target in the visible band. While the real object is effectively at, or near, infinity, from an optics point of view, the gunner is nevertheless able to track it using by its virtual image. This has a number of advantages, a primary one of which is elimination of parallax. 
     Of particular importance, system 10 is inherently immune to vibrations associated with the weapon&#39;s platform. This is because, as ray tracing demonstrates, the image of the light spot 28 is congruent with the laser beam. This is because the double pass of the beam with respect to the concave window/reflector, first from the laser to card 26, and then from the spot on the card to the gunner. It will be understood that, as shown in FIG. 2, gunner G uses the system by observing the target through window 18. If he looks around the window, the spot disappears from his view. 
     What has been described is a low cost, simple, easy to use system which is employed with a wide variety of weapons to enable the user of the weapon to readily designate and track a target. This despite smoke, dust, and any platform vibrations which the system may experience. 
     In view of the foregoing, it will be seen that the several objects of the invention are achieved and other advantageous results are obtained. 
     As various changes could be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.