Optical viewing and near infrared tracking system for a portable missile launcher

Abstract: An optical system (10) for portable missile launcher (23) includes an objective lens (12) for receiving the entire visual and near infrared light energy emanating from a target (14) and immediate background of the target. The light energy from the objective lens then passes through a Porro prism assembly (16) which acts to direct visual light along a first path to an eyepiece (20) and near infrared along a second path at approximately a right angle to the first path where it is received by a tracker assembly (22) and monitored. The Porro prism (16) is constructed in accordance with this invention to include a multi-layer interference dichroic coating (18) disposed between the two prisms (34, 36) which make up the prism assembly. This coating is of such a nature as to readily permit light energy in the visual range to pass directly through the coating, but at the same time to reflect the near infrared components along a second path bringing them to focus on the tracker assembly (22).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to a missile launcher, and, more particularly, to an optical and infrared TRACKING system according to the preamble of claim 1 for use in such a missile launcher to split the optical information received from a target into visible light portions and near infrared components. 2. Description of Related Art Known portable missile launchers at the present time include both direct viewing capability and near infrared traffic equipment (specifically, direct viewing is utilized during daylight when the target is relatively exposed and not camouflaged). The near infrared tracking system is used to follow a beacon mounted in the rear of the missile with signals generated thereby utilized to redirect the missile, if needed, along such a path as to effect engagement with the target. Known dual systems of this kind utilize separate visual and near infrared objective lenses with separate optical axes which require periodic in-field adjustments to assure and maintain boresight. These are both inconvenient and time consuming in necessitating the user of the equipment to make adjustments in both systems for realignment purposes to achieve boresight control. Further relevant prior art is known from the following documents. US-A-3 974 383 relates to a missile tracking and guidance system comprising an objective lens for receiving both visible and infrared energy directed on a first prism. This prism separates the visible energy from the infrared energy by reflecting the visible energy while transmitting the infrared energy. The visible energy is directed to an eye-piece for a viewer. The prism comprises two prisms having facing surfaces. US-A-4 038 547 relates to a tracking and sighting instrument including a system for receiving visible light and infrared light from a target having a beamsplitter for separating the visible and IR light by reflecting the visible light, an achromatic lens in the path of the IR light, a further achromatic lens in the path of the visible light and an eye-piece lens. US-A-4 441 668 relates to an infrared optical aiming and guiding device which comprises a night aiming device having a partially transmitting mirror for filtering the missile beams of a narrow spectral band out of a beam bundle and reflecting the beam to a guiding device, and further having an infrared/visible light image converter emitting visible light rays reflected into an ocular lens of the aiming device for viewing by an operator. Further, a second separate path for daylight conditions is provided. GB-A-1 405 121 relates to a missile guidance system including an objective lens and a beam splitter for transmitting visual light and for reflecting infrared light emitted by a small source within the system, and an eye-piece for viewing the visual light. US-A-3 992 629 relates to a telescope cluster having a broad, intermediate and narrow field of view infrared telescopes, broad and narrow field of view visual telescopes and a narrow field of view laser telescope, together with an apparatus for boresighting said telescopes, all of the narrow field of view telescopes having a common objective lens with means for separating the optical pathways. These means for separating the optical pathways are a first dichroic mirror reflecting received infrared radiation and radiation at the frequency of the boresight laser with an aperture extending through said first dichroic mirror, and a second dichroic mirror embedded in a multifaced prism transmitting visual frequency radiation and reflecting the radiation at the frequency of the boresight laser. It is the object of the invention to provide a compact and smaller optical system as defined in the preamble of claim 1. This object is solved in such a system by means of the characterizing features of claim 1. The optical apparatus for the missile launcher includes an objective lens for receiving the entire visual and near infrared light energy emanating from the target, immediate background of the target and missile. The light energy from the objective lens then passes through a Porro prism assembly which acts to direct visual light energy along a first path to an eyepiece and near infrared energy along a second path at approximately a right angle to the first path where it is received by a tracker assembly and monitored. The Porro prism is specially constructed in accordance with this invention to include a multi-layer interference dichroic coating disposed between the two prisms which make up the prism assembly. This coating is of such a nature as to readily permit light energy in the visual range to pass directly through the coating, but at the same time to reflect the near infrared components along the second path bringing them to focus on the tracker assembly. DESCRIPTION OF THE DRAWING In the accompanying drawings: FIG. 1 is an elevational schematic of the system of the present invention; FIG. 2 is a side elevational view of a portable launcher with which the present invention is especially advantageously employed; and FIG. 3 is an enlarged schematic showing the various layers of a multi-layer dichroic beamsplitter for use in this invention. DESCRIPTION OF A PREFERRED EMBODIMENT Turning now to the drawing and particularly FIG. 1, the optical system of the present invention is enumerated generally as 10 which in its broadest comprehension is seen to include an objective lens 12 for receiving light energy from an object 14,namely, both visible energy as well as energy in the near infrared. Light from the objective 12 passes through a Porro prism 16 which is a device having two right-angle prisms assembled together at right angles to one another for inverting and reverting images. For further details of the Porro prism, reference is made to page 13-26 of Military Standardization Handbook , OPTICAL DESIGN MIL-HDBK-141, October 5, 1962. As will be more particularly described, the prism contains a multi-layer coating 18 which permits electromagnetic energy in the visible range to pass therethrough for direct receipt and examination by an eyepiece 20. In addition, the coating 18 redirects impinging near infrared energy along a different path for receipt by near infrared tracking processing equipment 22. FIG. 2 shows, in a side elevational view, portable apparatus 23 for accomplishing missile launcher target sighting both by a visual method and by infrared sensing. On a common base 24, which may include a carrying handle 26, the electromagnetic energy from a target object is collected by the object lens 12 at the forward end and monitored by the eyepiece 20 at the rear. The near infrared processing equipment 22 is mounted conveniently adjacent to the visible optical system. Auxiliary missile launch controls 28 are mounted immediately adjacent the eyepiece 20 and a hand switch 30 is provided to control a reticle light, for example. With reference now to FIG. 3 there is shown in enlarged section the assembly of a plurality of thin film layers to form the multi-layer coating 18 for use in the Porro prism to act as a dichroic beamsplitter. More particularly, the coating 18 is composed of a plurality of very thin films 32 deposited on, say, the angular face of one of the prism elements 34. Finally, the second prism element 36 is then abutted against the coating 18 as shown in FIG. 3. This coating 18, which may also be referred to as an "interference coating", serves to spectrally control the light entering the Porro prism passing the near infrared light components through the second prism element 36 and allowing the visible light components to proceed through prism element 34 on toward the eyepiece. The coating 18 can be a dichroic dielectric coating constructed of an MgF 2 layer followed by a three-layer stack repeated six times where the three-layer stack consists of two MgF 2 layers sandwiched about a ZnS layer. The initial MgF 2 layer preferably has a thickness of 0.78 of a wavelength, the three-layer stack MgF 2 layers are each 0.5 wavelength thick, and each ZnS layer has a thickness of 1.0 wavelength. For present purposes, the wavelength will preferably be about 0.75 µm (microns). Through the practice of the present invention both near infrared light energy and visual light energy emanating from a target object are received through a single objective lens. Visual examination is made through an eyepiece, and, by the action of the beamsplitter in the Porro prism, the infrared components are shunted off the optical viewing axis for sensing and processing. In this way, the difficulties encountered in use of the present known systems requiring separate adjustments and manipulations for near infrared tracking and for visual examination are avoided.

Claims: An optical system in a portable missile launcher for receiving visual and near infrared light energy from a target and missile beacon, comprising: an objective lens (12); an eyepiece (20) located on a common optical axis with the objective lens (12); a prism assembly (16) located on said optical axis between the objective lens (12) and eyepiece (20), said prism assembly (16) including first and second prisms (34, 36) with facing surfaces, characterized by an interference coating (18) located between the prism facing surfaces allowing visual light energy to be directed along the optical axis to the eyepiece (20) and the near infrared light along a path off angle to the optical axis, said interference coating (18) being constructed of a plurality of dielectric layers (32) laid down on one of the prism facing surfaces in a stack, with an outer layer of the stack constructed of MgF 2 having a 0.78 wavelength thickness, and the remaining layers in the stack being formed from a plurality of three-layer elements, each element including a pair of 0.5 wavelength layers sandwiched about a 1.0 wavelength layer of ZnS. An optical system as in claim 1, in which the wavelength is about 750 nm.

Inventors: CARLSON JAMES J, NOYES GARY R, CARLSON, JAMES J., NOYES, GARY R.
Applicants: RAYTHEON CO, RAYTHEON COMPANY
Application year: 1993
Main classification: F41G7
IPC codes: G02B23, G02B27, F41G1, G02B5, F41G7, F41F3