IL154894A - Autonomous focusing of staring systems - Google Patents

Description

'κηχν ODiK ip'n1? JiDiyn AUTONOMOUS FOCUSING OF STARING SYSTEMS AUTONOMOUS FOCUSING OF STARING SYSTEMS FIELD OF THE INVENTION The present invention relates generally to the maintenance of staring systems. More specifically the present invention provides a method for focusing of staring optical systems in the field.

BACKGROUND OF THE INVENTION Collimation and focusing of staring optical system is performed in the production line as a part of the calibration procedure. The term collimation in general, means arranging the optical components (such as lens, mirrors and prisms) to align the rays coming out from the objective lens. To perform focusing means to make changes in components of an optical system's location, to bring a parallel bundle of rays entering the objective lens to converge to a point image at the sensor's plane. In the course of the operational life of an optical device, calibration may be lost, causing degradation of image quality and misinterpretations, both qualitative and quantitative. A staring system operating under varying environmental conditions and mechanical stresses in the field is prone to loose calibration. For example, an air - borne staring system can undergo changes of several degrees centigrade while being transferred from a land level, to a high atmospheric cruise level. For space - borne system the temperature variability may also be severe depending on the exposure of the system to sunlight. The effect of the temperature is essentially of two kinds: a. temperature changes can bring about mechanical stresses, because of extension and contraction of components, and b. influence of temperature changes on physical properties of materials which generate changes in the performance of specific components or assemblies. In optical components, changing temperatures produce a correlated change in the refraction index, the consequences of which is changing of the focus, causing defocusing a previously focused system.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic isometric view of an embodiment of the present invention; Fig. 2A is a schematic median sectional view of a staring device, in which an embodiment of the invention is implemented, with an external mirror applied; Fig. 2B is a schematic sectional view of a staring device in which an embodiment of the invention is implemented, with the external mirror retracted; Fig. 3A is a schematic sectional view of a staring device in which an embodiment of the invention is implemented, wherein the illumination source for the imaging array is external; Fig. 3B is a schematic representation of the device as in Fig. 3A configured for staring.

DETAILED DESCRIPTION OF THE PRESENT INVENTION The present invention provides an autonomous adjusting system that can be employed during the operational period of the device, in order to preserve image quality of a staring device, which is prone to loose calibration and is inaccessible to calibration gear.

A general schematic description of a staring optical assembly in which the present invention is implemented, is shown in Fig. 1 to which reference is now made. Staring device 20 collects light rays 22 and focuses them by a focusing lens assembly 24 on the focal plane, where an imaging array 26 is located. Other features of a device in which the present invention is implemented are described in Fig. 2A - B, to which reference is now made. In Fig. 2A, a median longitudinal cross sectional view in a staring device 40 shows the calibration configuration. Objective lens 42 is disposed at the front end of the staring device. Focusing lens assembly 44 is capable of focusing the incoming light on the imaging array 46.

Another structural aspect of the device of the invention is an illumination means 60 for illuminating the imaging array. The illumination means is typically a light bulb or a light emitting diode (LED) secured in a housing 62 that produces rays of light 64 which illuminate the surface 66 of the imaging array 46. Light is scattered as indicated by the direction of arrow 68, to be reflected by mirror 70. This mirror reflects back an image of the imaging array onto the array's own surface. Viewing the scene, mirror 70 is removed from the optical path. This can be seen in Fig. 2B, in which mirror 70 rotates around axis Focusing the staring device in the field In a staring configuration, objective lens 42 collects the radiation arriving from the scene, including target 82. The light is then focused by focusing lens assembly 44 onto imaging array 46. If the image is defocused, the staring device requires calibration (re-focusing). Returning to Fig. 2A to which reference is now again made, the external mirror 70 obscures objective lens 42, light source 60 is now turned on, illuminating surface 66 of imaging array 46. Some light is reflected and scattered in the direction of arrow 68, to be reflected from mirror 70 back to the imaging array 46. The image obtained, is the image of the illuminated face of the imaging array 46. Now the focusing lens assembly 44 is moved away or towards the imaging array, until the sharpest image is obtained. As an alternative to the displacement of the focusing lens, the imaging array may be displaced away from or towards the focusing lens. Various distinct features on surface of the imaging array - typically linear metal stripes - can be of assistance in the attainment of the focus.

Other embodiments A system of the present uses a reflecting means to reflect the image of the imaging array or a part of it on itself to perform autonomous focusing. It can be implemented in one of several ways. In Figs. 3A - B to which reference is now made, another embodiment in which the light source is set external to the objective lens is described schematically. In Fig. 3A a staring device 90 is described, configured for autonomous focusing. Objective lens 92 is flooded by light from light sources 94, some of which is reflected by mirror 96 to the direction of mirror 98. Light is reflected towards focusing lens 100, and further towards imaging array 102. The light is then reflected from the imaging array towards the focusing lens 100, through mirror 98 and objective mirror 96 to external mirror 104. From this mirror it is reflected back to imaging array 102. To adjust the focus, the focusing lens 100 is moved up and down, away and towards the imaging array 102, respectively, until the sharpest image is obtained. To facilitate forward staring, a staring configuration is created by rotating the objective lens upwards around axis 106, together with the associated mirror 96. This situation is described schematically in Fig. 3B. Objective lens 92 is shown oriented forwards facilitating forward staring.

Spectral considerations The method of the present invention can be employed to imaging applications at various parts of the spectrum of the electromagnetic energy. Accordingly, staring devices in the X -ray, visible, near infra - red (IR) and thermal IR are suitable for using a system of the invention. In Thermal IR applications, especially with cooled detectors, a special source for illuminating the imaging array is required. In such a case, an electric heating element can supply the required illumination.

Applications of the invention.

Although the method of the invention can be ubiquitously applied to staring systems requiring calibration of focus, the uniqueness of the present invention is manifested in staring devices operating under field conditions. In such conditions under which no calibration means and gear are available this invention can provide the solution to the need for focusing. Specific systems to which the present invention can provide the focal calibration means are air -borne and space - borne staring devices.

Claims (1)

1. A system for autonomously focusing of a staring device system, comprising: • an imaging array onto which a focusing lens assembly focuses rays collected by an objective lens; • at least one lighting element for illuminating said imaging array; and • a means for retro - reflecting light scattered and reflected from said imaging array. A system for autonomously focusing of a staring device as in claim 1 and wherein said at least one lighting element is a LED. A system for autonomously focusing of a staring device as in claim 1 and wherein said at least one lighting element is a light bulb. A system for autonomously focusing of a staring device as in claim 1 and wherein said staring device is a thermal IR staring device and wherein said at least one lighting element is a heating element. 154894/2 A method for autonomously focusing of a staring assembly device, comprising the steps of: • applying electromagnetic radiation to an imaging array of said staring device, • diverting at least a part of said electromagnetic radiation scattered and reflected from said imaging array through said optical assembly and back to said imaging array; • observing said self image of said imaging array, and • adjusting the distance between said imaging array and a focusing lens assembly of said complete optical assembly to obtain the sharpest image. For the Applicant