Cryogenically cooled adjustable apertures for infra-red cameras

• US 20060255275A1
• Filed: 11/14/2005
• Published: 11/16/2006
• Est. Priority Date: 05/28/2003
• Status: Active Grant

• Current Assignee: OPTO-Knowledge Systems, Inc.
• Original Assignee: OPTO-Knowledge Systems, Inc.
• Inventors: Gat, Nahum, Garman, John
• Granted Patent: US 7,427,758 B2
• Time in Patent Office: Days
• US Class Current: 250/338.100
• CPC Class Codes: G01J 5/06 Arrangements for eliminating effects of disturbing radiation; Arrangements for compensating changes in sensitivity for adjusting of solid angle of collected radiation G01J5/07; means for wavelength selection G01J5/0801
• CPC Class Codes: G01J 5/061 by controlling the temperature of the apparatus or parts thereof, e.g. using cooling means or thermostats
• CPC Class Codes: G01J 5/08 Optical arrangements
• CPC Class Codes: G01J 5/0831 Masks; Aperture plates; Spatial light modulators
• CPC Class Codes: G01J 5/084 Adjustable or slidable
• CPC Class Codes: G01J 5/0865 having means for replacing an element of the arrangement by another of the same type, e.g. an optical filter
• CPC Class Codes: H04N 23/75 by influencing optical camera components
• CPC Class Codes: H04N 5/33 Transforming infrared radiation cameras or camera modules for generating image signals from infrared radiation H04N23/20; circuitry of SSIS for transforming infrared radiation into image signals H04N25/20

Abstract

A thermal infrared camera may be used under a wide variety of target-scene radiation conditions, with interchangeable or zoom lenses requiring matching or different size cold stops. A variable aperture assembly of a thermal infrared camera integrates a rigid open truss-like framework that'"'"'s capped by an aperture ring and bottomed by a driving ring, and a radiation shield, located inside the framework, that contains an aperture ring at an upper side. A plurality of blades that collectively define an aperture positioned between the upper aperture rings. Opposite blade ends are coupled to respective ones of the two aperture rings, permitting pivotal movement in one ring and radial movement in the other ring, when the rings are rotated relative to one another, to change the size of the formed aperture. Both refractive and reflective infrared telescopes may be retrofitted with variable aperture devices to enhance infrared imaging performance.

Claims

1. A variable aperture assembly for an infrared camera comprising:

a radiation shield;

a rigid structural framework, said rigid structural framework having an upper end and a lower end, a central axis and large open areas in the sides;

said radiation shield being positioned inside said rigid structural framework coaxial to the central axis thereof and including a central infrared radiation admitting passage, an input end and an output end for propagating infrared radiation admitted within said radiation shield from said input end out through said output end;

said input end facing said upper end of said rigid structural framework and including a first aperture ring;

said first aperture ring including a central circular opening positioned coaxial with said central axis for passing infrared radiation and a circumferential portion surrounding said central circular opening;

said upper end of said rigid structural framework including a second aperture ring;

said second aperture ring including a central circular opening positioned coaxial with said central axis for passing infrared radiation and a circumferential portion surrounding said central circular opening, and having an end that opens into said central circular opening;

said lower end of said rigid structural framework including a driving ring, said driving ring including a central circular opening positioned coaxial with said central axis, said driving ring having a peripherally extending rim for supporting said rigid structural framework;

one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring including a plurality of pivot holes and the other one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring including a plurality of slots, said slots having an open end that opens into said central circular opening;

each of said pivot holes and said slots being evenly distributed about said central axis;

a plurality of aperture blades arranged to define an aperture for passage of infrared radiation, said defined aperture being coaxial with said central circular openings in said first and second aperture rings;

each of said aperture blades including a front blade surface, a rear blade surface, a first pivot pin attached to, located proximate one end of said blade and projecting from and normal to one of said front and rear blade surfaces, and a second pivot pin attached to, located proximate an opposite end of said blade and projecting from and normal to the other one of said front and rear blade surfaces, and comprising a flat curved geometry;

said first pivot pin of respective ones of said plurality of aperture blades being received within respective ones of said pivot holes; and

said second pivot pin of respective ones of said plurality of aperture blades being received within respective ones of said slots, whereby rotation of said second aperture ring in one direction relative to said first aperture ring increases the size of said defined aperture in said blades and rotation in an opposite direction decreases the size of said defined aperture;

wherein rotation of said driving ring while maintaining said second end of said rigid structure framework fixed in position rotates said rigid structural framework and thereby rotates said second aperture ring relative to said first aperture ring to change the size of said defined aperture.

2. The variable aperture assembly as defined in claim 1 wherein said rigid structural framework further comprises a poor heat transmissive material.

3. The variable aperture assembly as defined in claim 2, wherein said poor heat transmissive material comprises a metal.

4. The variable aperture assembly as defined in claim 3, wherein said metal comprises titanium.

5. The variable aperture assembly as defined in claim 2, wherein said poor heat transmissive material comprises a non-metallic composite.

6. The variable aperture assembly as defined in claim 1, wherein said one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring comprises:

said circumferential portion of said first aperture ring; and

wherein said other one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring comprises;

said circumferential portion of said second aperture ring.

7. The variable aperture assembly as defined in claim 1, wherein said first blade surface of each of said plurality of blades is oriented facing said second aperture ring.

8. The variable aperture assembly as defined in claim 1, further comprising:

electric motor means for rotating said driving ring.

9. The variable aperture assembly as defined in claim 8, wherein said electric motor means comprises a plurality of electric motors.

10. The variable aperture assembly as defined in claim 8, wherein said electric motor means comprises a plurality of piezoelectric actuators, said plurality of piezoelectric actuators being equally spaced about the periphery of said driving ring and adapted to be energized simultaneously to produce a rotational force on said driving ring without producing a net radial force thereon transverse said central axis of said driving ring.

11. The variable aperture assembly as defined in claim 10, further comprising a ceramic ring, said ceramic ring being attached to the exterior of said driving ring for rotational movement concurrent therewith and said ceramic ring including a peripheral rim.

12. The variable aperture assembly as defined in claim 1, wherein said driving ring is axially spaced from each of said first and second aperture rings.

13. The variable aperture assembly as defined in claim 1, wherein said slots comprise radially directed straight slots.

14. An infrared imaging camera comprising:

a housing;

an optical lens for coupling infra-red images of objects incident on said lens from the exterior to the interior of said housing, said optical lens including a central axis and having an f-number;

a vacuum chamber located inside said housing, said vacuum chamber being maintained in-vacuum and including a vacuum window located at a front end of said vacuum chamber for admitting infra-red images that enter said housing through said optical lens to the interior of said vacuum chamber, said vacuum window being positioned coaxial of said central axis and in spaced relation to said optical lens;

an imaging type infra-red photocell having a plurality of radiation sensitive pixels for converting incident infra-red images to electrical signals, said infra-red photocell being located inside said vacuum chamber, spaced from said vacuum window, positioned coaxial of said central axis and in heat insulated relationship with the walls of said vacuum chamber;

an integrated variable aperture and radiation shield assembly located in said vacuum chamber for permitting passage of infra-red images admitted into said vacuum chamber via said vacuum window to said imaging type infra-red photocell and defining an aperture of said f-number of said optical lens, said variable aperture device including a front side facing said optical lens and a rear side facing said infrared photocell and further comprising;

a radiation shield;

a rigid structural framework, said rigid structural framework having an upper end and a lower end, a central axis and large open areas in the sides and comprising a poor heat transmissive material;

said radiation shield being positioned inside said rigid structural framework coaxial to the central axis thereof and including a central infrared radiation admitting passage, an input end and an output end for propagating light energy admitted through said input end through said output end thereof;

said input end facing said upper end of said rigid structural framework and including a first aperture ring;

said first aperture ring including a central circular opening positioned coaxial with said central axis for passing infrared radiation and a circumferential portion surrounding said central circular opening;

said upper end of said rigid structural framework including a second aperture ring;

said second aperture ring including a central circular opening positioned coaxial with said central axis for passing infrared radiation and a circumferential portion surrounding said central circular opening;

said lower end of said rigid structural framework including a driving ring, said driving ring including a central circular opening positioned coaxial with said central axis, said driving ring having a peripherally extending rim for supporting said rigid structural framework;

one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring including a plurality of pivot holes and the other one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring including a plurality of slots, said slots having an open end that opens into said central circular opening;

each of said pivot holes and said slots being evenly distributed about said central axis;

a plurality of aperture blades arranged to define an aperture for passage of infrared radiation, said defined aperture being coaxial with said central circular openings in said first and second aperture rings;

each of said aperture blades including a front blade surface, a rear blade surface, a first pivot pin attached to, located proximate one end of said blade and projecting from and normal to one of said front and rear blade surfaces, and a second pivot pin attached to, located proximate an opposite end of said blade and projecting from and normal to the other one of said front and rear blade surfaces, and comprising a flat curved geometry;

said first pivot pin of respective ones of said plurality of aperture blades being received within respective ones of said pivot holes; and

said second pivot pin of respective ones of said plurality of aperture blades being received within respective ones of said slots, whereby rotation of said second aperture ring in one direction relative to said first aperture ring increases the size of said defined aperture in said blades and rotation in an opposite direction decreases the size of said defined aperture;

wherein rotation of said driving ring while maintaining said second end of said rigid structure framework fixed in position rotates said rigid structural framework and thereby rotates said second aperture ring relative to said first aperture ring to change the size of said defined aperture.

said sides of said variable aperture device including a radiation absorbing coating for preventing reflection of light to said infrared photocell;

said radiation shield surrounding said infra-red photocell being in thermal conductive relationship with said first aperture ring and comprising a heat transmissive material;

said radiation shield including an inside surface facing said central axis that contains a parasitic radiation absorbent coating to prevent parasitic radiation from being reflected to said photocell; and

mechanical adjusting means coupled to said variable aperture device for adjusting said aperture to match said f-number of said aperture to said f-number exhibited by said optical lens to minimize entry of extraneous rays of light, and, should the infrared radiation received when said camera f-number is so matched to said optical f-stop be too intense for said photocell, for increasing said camera f-number to reduce the infrared radiation received by said photocell; and

cryogenic cooling means for cooling said infra-red photocell, said radiation shield, and said variable aperture device to a cryogenic temperature to maintain said infra-red photocell, said variable aperture device and said radiation shield, at a significantly lower temperature than said housing and said walls of said vacuum chamber.

15. The infrared camera as in claim 14, wherein said optical lens further includes an adjustable focal length to permit a user to selectively change the focal length of said optical lens between at least a short focal length to permit a wide field of view of exterior scenes and defining a first optical f-number and a long focal length to permit a close-up or narrow field of view of said scene and defining a second optical f-number

16. The infrared camera as in claim 14, wherein said variable aperture device is controlled by a logic control means, said logic control means receiving the f-number of said optical lens and causing said mechanical adjusting means to adjust said defined aperture of said variable aperture device to maintain a desired f-number.

17. The infra-red imaging camera as defined in claim 14, wherein said variable aperture device comprises:

a variable iris, said variable iris including a plurality of blades, each of said plurality of blades including a coating of a non-outgassing greaseless solid lubricant material.

18. The infra-red imaging camera as defined in claim 14, wherein said mechanical adjusting means comprises:

a first portion external of said vacuum chamber;

a second portion internal to said vacuum chamber;

and a third portion mechanically coupling said first and second portions;

said third portion comprising a minimally heat-conductive material to prevent heat from the exterior of said vacuum chamber from entering said vacuum chamber.

19. The infra-red imaging camera as defined in claim 14, wherein said mechanical adjusting means comprises at least one piezoelectric actuator.

20. The infra-red imaging camera as defined in claim 14, wherein said one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring comprises:

said circumferential portion of said first aperture ring; and

wherein said other one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring comprises;

said circumferential portion of said second aperture ring.

21. An infra-red imaging camera comprising:

a housing;

an optical lens for coupling infra-red images of objects incident on said lens from the exterior to the interior of said housing;

a vacuum chamber located inside said housing, said vacuum chamber including a vacuum window located at a front end of said vacuum chamber for admitting infra-red images that enter said housing through said optical lens to the interior of said vacuum chamber, said vacuum window being positioned in spaced relation to said optical lens;

an infra-red photocell for converting incident infra-red images to electrical signals, said infra-red photocell being located inside said vacuum chamber, spaced from said vacuum window, and in heat insulated relationship with the walls of said vacuum chamber;

a variable aperture assembly located in said vacuum chamber for permitting passage of infra-red images admitted into said vacuum chamber via said vacuum window to said infra-red photocell, said variable aperture assembly including a front side facing said optical lens and a rear side facing said infra-red photocell comprising;

a radiation shield;

a rigid structural framework, said rigid structural framework having an upper end and a lower end, a central axis and being essentially open and comprising a poor heat transmissive material;

said radiation shield being positioned inside said rigid structural framework coaxial to the central axis thereof having a central infra-red radiation admitting passage, an input end and an output end for propagating light energy admitted through said input end through said output end thereof;

said input end including a first aperture ring;

said first aperture ring including a central circular opening for passing infra-red radiation;

said upper end of said rigid structural framework including a second aperture ring, said second aperture ring including a central circular opening for passing infra-red radiation; and

said lower end of said rigid structural framework including a driving ring;

one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring including a plurality of pivot holes and the other one of said circumferential portion of said first aperture ring and said circumferential portion of said second aperture ring including a plurality of radially directed slots, said radially directed slots having an open end that opens into said central circular opening;

each of said pivot holes and said slots being evenly distributed about said central axis;

a plurality of aperture blades arranged to define an aperture for passage of infra-red radiation, said defined aperture being coaxial with said central circular openings in said first and second aperture rings;

each of said aperture blades comprising a flat curved geometry and having a front blade surface, a rear blade surface, a first pivot pin attached to, located proximate one end of said blade and projecting from and normal to one of said front and rear blade surfaces, and a second pin attached to, located proximate an opposite end of said blade and projecting from and normal to the other one of said front and rear blade surfaces;

said first pivot pin of respective ones of said plurality of aperture blades being received within respective ones of said pivot holes; and

said second pivot pin of respective ones of said plurality of aperture blades being received within respective ones of said slots, whereby rotation of said second aperture ring in one direction relative to said first aperture ring increases the size of said defined aperture in said blades and rotation in an opposite direction decreases the size of said defined aperture;

wherein rotation of said driving ring axially while maintaining said second end of said rigid structure framework fixed in position rotates said rigid structural framework and thereby rotates said second aperture ring relative to said first aperture ring to change the size of said defined aperture and thereby vary the f-number of said defined aperture;

a radiation shield surrounding said infra-red photocell, said radiation shield being in thermal conductive relationship with said first aperture ring and comprising a heat transmissive material;

cooling means for cooling said infra-red photocell, said radiation shield, and said variable aperture device to a low temperature to maintain said infra-red photocell, said variable aperture device and said radiation shield, at a significantly lower temperature than said housing and said walls of said vacuum chamber.

22. The infra-red imaging camera as defined in claim 21, wherein said radiation shield includes an inside surface that includes a radiation absorbent coating to prevent parasitic radiation from being reflected to said infrared photocell;

and wherein said cooling means comprises cryogenic cooling means for cooling said infrared photocell, said radiation shield, and said variable aperture device to a cryogenic temperature; and

further comprising;

mechanical adjusting means coupled to said variable aperture device for adjusting the aperture to match said camera f-number to the optical f-number exhibited by said optical lens to minimize entry of extraneous rays of light.

23. An infrared imaging camera, said camera including a lens possessing an f-number and an adjustable aperture device defining an f-number for said camera that is dependent on the size of the aperture defined by said aperture device, and means for detecting the f-number of said aperture produced by said adjustable aperture device, comprising:

an indicator ring, said indicator ring for rotating in angular position in response to a change in adjustment of said adjustable aperture device to change the size of said aperture, said degree of rotation being related to the size of said aperture;

position detecting means for determining the angular position of said indicator ring; and

converter means coupled to said position detecting means for correlating the angular position of said indicator ring determined by said position detecting means to a corresponding f-number.

24. The infra red camera as defined in claim 23, further comprising:

control loop means responsive to said f-number from said converter means for attaining a desired change in adjustment of said adjustable aperture device.

25. The infrared imaging camera as defined in claim 23 wherein said position detection means comprising a non-contacting position detecting means for determining the angular position of said ring without making physical contact with said indicator ring.

26. The infrared imaging camera as defined in claim 25 wherein non-contacting position detecting means comprises a member selected from the group consisting of:

a hall effect sensor, a magnetic flux sensor, a slotted optical switch and a photoreflector.

27. In combination:

a telescope for supplying an object image, said telescope and an infrared camera for recording infrared images received by said telescope, said infrared camera comprising a fixed f-number, wherein both said telescope and said infrared camera possess separate fixed f-numbers that are different, if said telescope lens is of a focal length that is fixed, and, if said telescope lens is of a focal length that is adjustable, possesses separate f-numbers that are different at all but one of the focal lengths to which said focal length may be adjusted;

a correction device, comprising;

an input end for receiving an image from said telescope lens;

an output end for coupling an image to said infrared camera;

a first lens system, located behind said input end;

a variable aperture device, located behind said first lens system for receiving an infrared object image from said first lens system, said variable aperture device being cryogenically cooled and adjusted in aperture to possess an f-number equal to the f-number of said telescope lens;

a second lens system, located behind said aperture device for receiving an infrared image from said variable aperture device;

third and fourth lens systems;

said third and fourth lens systems for receiving said infrared image from said second lens system, re-imaging the stop located at the variable aperture cold stop to the stop located inside said infrared camera and re-imaging said infrared image formed by said second lens system to the image plane located at the detector of the infrared camera.

28. The combination defined in claim 27, wherein said correction device comprises the characteristics prescribed in table 1.