GB1589462A - Optical arrangement for infrared scanning device - Google Patents

Description

(54) OPTICAL ARRANGEMENT FOR INFRARED SCANNING DEVICE (71) We, HUGHES AIRCRAFT COM PANY, a company organized and existing under the laws of the State of Delaware, United States of America of Centinela and Teale Street, Culver City, State of California, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following state ment The invention relates to an optical arrangement having particular utility for use in association with an infrared scanning device and adapted to provide a clear and stable image in the visible spectrum to a viewing operator.

It is well known that all existing objects having a temperature level above absolute zero emit radiation in the invisible infrared spectrum. The utilization of this emitted radiation to locate and define particular objects in a field of view has developed into a well known art in recent years. A particular known application involves the use of conventional detecting materials to sense the infrared radiation. Electrical pulses produced thereby energize light sources and transfer the sensed image into a visible pattern allowing a human operator to identify the sensed object even when all visible light sources are absent such as at night. Night viewing arrangements such as the type described have been embodied and attempts have been made to use such devices in many ways, for example, in target identification in military operations.

In one such prior art embodiment a viewing device attempted to utilize conventional infrared detecting devices arranged in a multielement array. A conventional infrared optical system focused on a determined field of view. The detector array, being aligned with the axis of the optical system, was oscillated to provide a scan of the field of view. The infrared-sensitive detectors comprising the array were selectively energized by the infrared radiation received. The prior art device now being described utilized electrical signals generated by each detector ofthe array to energize a battery of small lamps remote from the array, the lamps being in an operator-viewing optical system. Each lamp of the lamp array was successively turned on and off in response to the electrical impulse of the related detector and a viewing mirror was oscillated over the plane of the lamp array synchronously with the oscillation of the detector array. Thus, an attempt was made to present, at the oscillating mirror, a visible image of the object in the infrared telescope field of view responsive to the infrared radiation received therefrom.

While in principle the device described was well conceived, it was found that in actual practice the images created did not provide the desired resolution. That is, did not accurately reflect the object being observed in the field of view. This may have been due to the fact that energized light bulbs were merely switched off and on in response to detector triggering and not modulated in light intensity. Additional difficulties involved the use of complicated mechanical devices in an attempt to provide concurrent synchronous motion between the oscillating detector array and the oscillating presentation mirror. Further, the oscillations generated were sinusoidal in character with the result that as the oscillating devices left and approached the end points of each oscillating cycle, the angular velocity thereof progressively diminished until halt and progressively increased upon direction reversal.

This vairation in angular velocity, and the coupled oscillation of both the array and mirror, did not provide the desired image at the viewing eyepiece.

Accordingly, it is a primary object of the invention to provide an optical system, in combination with an appropriate infrared viewing and detection arrangement, having a simple, direct, visible viewing display technique.

It is a further object of the invention to provide an optical system of the type described having low distortion and high resolution to provide high quality image reproduction.

It is a further object of the invention to provide a combined and embodied night viewing device that is light weight and easily portable, thereby increasing its utility and efficient use.

It is a further object of the invention to provide a novel scanning arrangement coupled with relatively fixed detector and light source arrays to provide the quality resolution and image creation desired.

These and other objects and advantages of the invention will be more fully understood in the course of the following description and from an examination of the related drawings, wherein: Figure 1 is a diagrammatic functional illustration of an arrangement embodying the invention; Fig. 2 is a front elevational view of a scanning element utilized in the invention Fig. 3 is a partially fragmentary view taken along line 3-3 of Fig. 2; Fig. 4 is a side-elevational view of the structure shown in Fig. 2 with the electromagnetic coils omitted; and Fig. 5 is a graphical display of scanning velocity relative to time during arrangement operation.

Describing the invention in detail and directing attention to Fig. 1, a conventional infrared telescope arrangement is indicated generally at 10. The scope is arranged to view an appropriate field indicated by the area 12.

A scanning mirror 14 is provided rearwardly of and optically aligned with the telescope 10. The mirror, as will hereinafter be described, is arranged to oscillate about a vertical axis 16. At any given instant the mirror 14 may be considered stationary and it provides, therefore, an instantaneous linear field of view in the area 12 as is shown by line 18. By oscillating the mirror 14 through a determined angle, the linear field of view scans the area 12 as, for example, between lincs 20 and 22.

The mirror 16 has one surface thereof optically aligned with a generally planar surfaced detector array 24. In the usual mode, the detector array 24 comprises a plurality of infrared-sensitive elements 26, 26, disposed in plane over the surface of array 24. A low temperature refrigerator source 28 may be used to cool the detectors 26 to a temperature level whereat optimum sensitivity to impinging infrared radiation exists. As the mirror 14 is oscillated through a field of view, its instantaneous linear field of view is reflected to the planar surface of the array 26 and the area between lines 20 and 22 is effectively scanned over the surface of the array 24. The infrared reflected radiation is thus sensed by the respective detectors 26, 26.

A generally planar source of visible light is indicated generally at 30. The visible source may comprise a plurality of individual light bulbs 32, 32, arrnaged in the surface plane of the source 30. Each detector 26 is electrically connected via amplifiers, diagrammatically indicated at 34, to one of the light bulbs or sources 32. Thus, as each detector 26 is electrically activated by impinging infrared radiation, the associated bulb 32 is energized and provides a source of visible light. The intensity of the light emitted by each bulb 32 varies in accordance with the amount of infrared radiation received by the associated detector 26.

A first reflecting mirror 36 is optically aligned with the light source 30 and reflects the entire planar face thereof to the rear aspect of oscillating mirror 14. The rear face 37 of oscillating mirror 14 is also a reflective surface and during its oscillation results in a linear scan over the reflected face of optical source 30. Thus, the mirror 14 provides a common scanning element in both the infrared optical system and in the visible optical system and automatic and concurrent scan synchronization is thus obtained.

The image scanned by the rear surface of mirror 14 is reflected through a relay lens 38 to a folding mirror 40, the latter being optically aligned with a filter 42 and a viewing eyepiece 44.

Attention is now directed to Figs. 2-4 which illustrate an operative embodiment of the oscillating mirror 14. Here the mirror 14 is mounted in a frame 50. The frame 50 is mounted from an independent lug 52, the latter carrying an armature 54 at its lower end. The armature extends on opposed sides of lug 52. Centrally of the lug 52, a flexure pivot 56 is provided which supports the mirror frame 50 for oscillating movement.

A pair of conventional electromagnets indicated generally at 60, 60, are provided, said magnets comprising cores 62, 62, arranged in slight spaced relation to the opposed ends of armature 54. Conventional windings 64, 64, are utilized to create a magnetic field in the cores 62. A movable flexure spring 66 is connected to the pivot 56 and movable with it and the frame 50. The spring 66 carries oppositely directed contact 67. The fixed contacts 68 are provided in operative association with contact 67 and alternately energize or short the electromagnets 60 as the moving contact 67 sequentially closes with one of the fixed contacts as a result of oscillation of the frame 50 and flexure spring 66. It will be noted that the coils 64 are omitted from Fig. 4.

In one mode of operation of the described structure, one of the electromagnets 60 is energized by virtue of contact between one face of contact 67 and one of the fixed contacts 68. In this circumstance the mirror 14 and frame 50 may be considered at the angular end point of travel in one direction.

The energizing of the electromagnet 60 magnetically attracts the armature 54 causing the armature and related mirror 14 and frame 50 to accelerate angularly in the opposite direction. This acceleration is rapidly induced for a very small segment of the total angular oscillatory rotation of the frame 50, at which time, the contact 67 breaks the circuit with the previously engaged contact 68, deenergizing the electromagnet 60. The armature and related mirror 14 and frame 50 thereafter continue to move in the accelerated direction on the relatively frictionless pivot 56 and at a relatively uniform velocity without further acceleration. As the armature 54, frame 50, flexure spring 66 and contact 67 approach the end point of angular travel in the noted direction, the armature engages the deenergized core 62 and concurrently the contact 67 engages the other fixed contact 68.

This contact engagement electrically energizes the other electromagnet 60, decelerating the armature 54 and frame 50 and inducing acceleration in the opposed direction. Movement of the frame 50 and flexure spring 66 in the reverse direction breaks electrical engagement between contact 67 and related fixed contact 68 deenergizing the second electromagnet 60 and allowing the armature 54, frame 50 and carried mirror 14 to move with relatively uniform velocity on the relatively frictionless pivot 56 in said reverse direction.

The cycle repeats itself as contact 67 is brought into electrical engagement with the first mentioned fixed contact 68. It will thus be apparent that the angular velocity of the mirror 14 over a major segment of its oscillating arc movement is substantially uniform and deceleration and acceleration thereof occur only in a relatively small angular area at the terminal end points of angular movement. Attention is directed to Fig. 5 which graphically illustrates that the velocity of the mirror 14 is substantially uniform over a major segment of its angular travel. The zero line indicates terminal points of movement while graph areas above and below the zero line indicate movement in opposed directions.

It will be understood that this feature contributes substantially to both the accuracy and resolution of the image produced. Thus, an image is presented to the viewing eyepiece which accurately reproduces the object viewed in the infrared telescope viewed area.

In order to present to the viewing eyepiece an object image of high quality, it is necessary that the retention ability of the eye to viewing object be given consideration. The human eye does not observe an object instantaneously but retains a viewed light source for a short interval of time. If the rate at which the light source is presented to the human eye is too low, the viewed image is not retained for a sufficient length of time and a visible flicker results. In order to present a high quality comfortably viewed image to the observer, it is suggested that the rate of oscillation of the mirror 14 be controlled to provide 15 to 30 scans or frames per second which would be in a range compatible with comfortable viewing to the human eye. This can easily be accomplished by conventionally controlling the cycling of the electromagnets which induce mirror oscillation.

In addition, it is essential that the bulbs 32 of light source 30 be modulated in light intensity in response to the varying energy of the received radiation. It is further desirable that the bulbs utilized have a fast response time. Thus, it is preferable that bulbs be capable of 100% modulation from total dark to total bright within a time limit of less than 500 microseconds. Where the device incorporates these features, the picture being viewed accurately reflects the infrared emanating object and extremely good resolution and quality reproduction is achieved.

WHAT WE CLAIM IS: 1. A device for providing a visual image representing the distribution of infra-red energy in an area, comprising an infra-red telescope arranged to view said area, an infrared reflecting mirror arranged to reflect infra-red energy from said telescope upon a multi-element linear array of infra-red detector elements each of which is electrically connected to cause an individual light source correspondingly disposed in a linear array of light sources to emit light of which the intensity is modulated in response to the varying energy of the received infra-red radiation, means for oscillating said mirror about an axis thereby to direct infra-red energy from adjacent linear portions of said area in succession upon said detector array and an eyepiece for viewing said light source array by way of an optical path including a further mirror oscillating with said infra-red reflecting mirror.

2. A device in accordance with claim 1, wherein said first-mentioned mirror and said second-mentioned mirror are unitary.

3. A device in accordance with claim 2 wherein said mirrors oscillate at a uniform velocity through a major segment of the arc of oscillation.

4. A device in accordance with claim 3 wherein said unitary mirrors are carried in a frame mounted for pivotal movement and carrying an armature and wherein a pair of electromagnets are operatively associated with opposed ends of the armature and electrical control means are provided to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. face of contact 67 and one of the fixed contacts 68. In this circumstance the mirror 14 and frame 50 may be considered at the angular end point of travel in one direction. The energizing of the electromagnet 60 magnetically attracts the armature 54 causing the armature and related mirror 14 and frame 50 to accelerate angularly in the opposite direction. This acceleration is rapidly induced for a very small segment of the total angular oscillatory rotation of the frame 50, at which time, the contact 67 breaks the circuit with the previously engaged contact 68, deenergizing the electromagnet 60. The armature and related mirror 14 and frame 50 thereafter continue to move in the accelerated direction on the relatively frictionless pivot 56 and at a relatively uniform velocity without further acceleration. As the armature 54, frame 50, flexure spring 66 and contact 67 approach the end point of angular travel in the noted direction, the armature engages the deenergized core 62 and concurrently the contact 67 engages the other fixed contact 68. This contact engagement electrically energizes the other electromagnet 60, decelerating the armature 54 and frame 50 and inducing acceleration in the opposed direction. Movement of the frame 50 and flexure spring 66 in the reverse direction breaks electrical engagement between contact 67 and related fixed contact 68 deenergizing the second electromagnet 60 and allowing the armature 54, frame 50 and carried mirror 14 to move with relatively uniform velocity on the relatively frictionless pivot 56 in said reverse direction. The cycle repeats itself as contact 67 is brought into electrical engagement with the first mentioned fixed contact 68. It will thus be apparent that the angular velocity of the mirror 14 over a major segment of its oscillating arc movement is substantially uniform and deceleration and acceleration thereof occur only in a relatively small angular area at the terminal end points of angular movement. Attention is directed to Fig. 5 which graphically illustrates that the velocity of the mirror 14 is substantially uniform over a major segment of its angular travel. The zero line indicates terminal points of movement while graph areas above and below the zero line indicate movement in opposed directions. It will be understood that this feature contributes substantially to both the accuracy and resolution of the image produced. Thus, an image is presented to the viewing eyepiece which accurately reproduces the object viewed in the infrared telescope viewed area. In order to present to the viewing eyepiece an object image of high quality, it is necessary that the retention ability of the eye to viewing object be given consideration. The human eye does not observe an object instantaneously but retains a viewed light source for a short interval of time. If the rate at which the light source is presented to the human eye is too low, the viewed image is not retained for a sufficient length of time and a visible flicker results. In order to present a high quality comfortably viewed image to the observer, it is suggested that the rate of oscillation of the mirror 14 be controlled to provide 15 to 30 scans or frames per second which would be in a range compatible with comfortable viewing to the human eye. This can easily be accomplished by conventionally controlling the cycling of the electromagnets which induce mirror oscillation. In addition, it is essential that the bulbs 32 of light source 30 be modulated in light intensity in response to the varying energy of the received radiation. It is further desirable that the bulbs utilized have a fast response time. Thus, it is preferable that bulbs be capable of 100% modulation from total dark to total bright within a time limit of less than 500 microseconds. Where the device incorporates these features, the picture being viewed accurately reflects the infrared emanating object and extremely good resolution and quality reproduction is achieved. WHAT WE CLAIM IS:

1. A device for providing a visual image representing the distribution of infra-red energy in an area, comprising an infra-red telescope arranged to view said area, an infrared reflecting mirror arranged to reflect infra-red energy from said telescope upon a multi-element linear array of infra-red detector elements each of which is electrically connected to cause an individual light source correspondingly disposed in a linear array of light sources to emit light of which the intensity is modulated in response to the varying energy of the received infra-red radiation, means for oscillating said mirror about an axis thereby to direct infra-red energy from adjacent linear portions of said area in succession upon said detector array and an eyepiece for viewing said light source array by way of an optical path including a further mirror oscillating with said infra-red reflecting mirror.

2. A device in accordance with claim 1, wherein said first-mentioned mirror and said second-mentioned mirror are unitary.

3. A device in accordance with claim 2 wherein said mirrors oscillate at a uniform velocity through a major segment of the arc of oscillation.

4. A device in accordance with claim 3 wherein said unitary mirrors are carried in a frame mounted for pivotal movement and carrying an armature and wherein a pair of electromagnets are operatively associated with opposed ends of the armature and electrical control means are provided to

alternately energize said magnets and induce the controlled oscillation of said mirrors.

5. A device for providing a visual image representing the distribution of infra-red energy in an area substantially as described with reference to the accompanying drawing.