Patent Grant
7157706
1. Field of the Invention
The invention relates to radiant energy, photocell arrays for capturing such energy, apparatus containing such photocells, including thermal infrared cameras, and more particularly to the optical or pre-photocell system, and to light valves, apertures, diaphragms, irises, and to the temperature control thereof. The method and apparatus of the invention shows particular utility in fire fighting, military, law enforcement, search and rescue, and agricultural applications or in any other application utilizing infrared imaging or detecting.
2. Background
The background of this invention involves thermal infrared cameras (throughout, the use of “infrared camera” is meant to be inclusive of “thermal infrared camera”), variable diaphragms and swappable fixed apertures. Infrared cameras are well known in the art. The typical infrared camera is comprised of at least several of the below described basic parts, with the most sophisticated infrared cameras comprised of most or all of these prior art parts. The variable diaphragm and swappable fixed aperture are also well known in the art, as are logic control systems.
Thermal infrared radiation is the emission of photons by all objects that are at a temperature above absolute zero. Thermal infrared radiation decreases very rapidly as the temperature of an object decreases.
Infrared Cameras:
Central to all infrared cameras is an infrared sensitive photocell. The photocell is highly sensitive to thermal infrared radiation (hereinafter referred to as “radiation”). The photocell is exposed to radiation emanating from the object or scene being imaged. However, the camera enclosure also emits radiation that can reach the photocell. This undesired radiation negatively affects the operation of the camera and therefore the photocell can be enclosed within a cold structure (referred to as a “radiation shield”). The design of the radiation shield is dictated simply: if an observer were to look out from the photocell, anything the observer could see would emit radiation that would arrive at the photocell. In order for the radiation shield to block the undesired radiation, it must be the only internal camera structure that the photocell can “see.” The “cold stop,” which is simply a name for a cooled aperture, provides the only path for external radiation to reach the photocell, through the focusing optics. The radiation shield should therefore also not emit an excess of radiation itself. The cold stop size is a compromise between the effectiveness of blocking the unwanted radiation (requiring a small aperture) and excessive vignetting (requiring a large aperture).
In order to keep the photocell and other components of the infrared camera cool, there is often an active cooler integrated into the camera. Typically, the cooling system must maintain a fixed temperature in order to control the unwanted radiation seen on the photocell, although ideally the radiation shield is cold enough to produce a negligible amount of radiation onto the photocell. This fixed temperature has a known effect on the photocell that can be removed through image post processing. The photocell is also cooled to improve its radiation sensitivity and reduce the internally generated current, as the higher the temperature of the photocell, the lower its usable dynamic range. A thermoelectric cooler, such as a Peltier cooler, typically cools such infrared cameras. However, in more sensitive systems where the signal to noise ratio is far more important, the system must be cooled to as low a temperature as reasonably possible, to minimize any unwanted radiation loading. In these systems, there are several options for achieving the necessary cooling, including integrating the cameras into dewars for liquid nitrogen or liquid helium, Stirling cryogenerators, Gifford-McMahon mechanical coolers, and other such devices.
To reduce thermal load on the cooling system, infrared camera designers often place all of the cooled elements into a vacuum vessel. Within the vacuum vessel, the radiation shield and the photocell are maintained at a low, sometimes cryogenic, temperature, based on the photocell requirements and the desired performance. The vacuum vessel, (if one is present) often constitutes a camera housing, which also often contains, or provides, a mounting apparatus for the infrared focusing lens (throughout, “lens” is inclusive of all light collecting devices including refractive or reflective systems).
Thermal infrared cameras must be able to accommodate both hot and cold target objects and scenes, while distinguishing target from background radiation. Although the thermal control methods described above can allow a camera to be used in a wide variety of thermal scenes, drastic changes in radiation quantities require different camera settings. If the scene is too cool for ideal use with the camera, the camera operator can take a longer exposure. This method may adversely affect the frame rate and may lead to resolution problems if the camera or target is moving. Another solution typically used in the art is to change the electronic gain of the signal from the photocell, although a higher gain also increases the noise in the electronic signal. Conversely, in hot scenes, reduced exposure time, reduced signal gain, or a combination of the two can allow an infrared camera to capture the scene.
Apertures and Cold Stops:
A cold stop is simply a temperature-controlled aperture. In its most basic form, the cold stop is a fixed aperture, similar to the aperture found in some disposable light cameras. Variable diaphragms (hereinafter used interchangeably with an “iris”) and swappable fixed apertures for light cameras have been described in patent art for many years (see e.g., U.S. Pat. No. 24,356 to Miller and Wirsching in 1859, U.S. Pat. No. 582,219 to Mosher in 1897). The variable diaphragm works by allowing more or less of the radiation (visible light, in the case of visible light cameras) that reaches the focusing lenses to pass through to the photocell or film. The focusing lens receives radiation and focuses it based on the distance from the radiation source to the lens and the prescription of the lens. The prescription includes the focal length and the f-number. In conventional light cameras, the aperture is typically built into the compound lens assembly. That aperture then lets pass a certain desired portion of the radiation intercepted by the lens.
With a very large aperture, nearly all of the light arriving at the focusing lens passes through the aperture. By reducing the size of the aperture, the mechanism of the aperture itself blocks a portion of the light from entering. In typical light cameras, the aperture is located at the point where the cone of light from the object is wide and thus diminishes the light intensity without affecting the image quality. Lenses may have specific aperture requirements, which determine the optimum position and size of the aperture. This is typically a function of the f-number (hereinafter interchangeably also referred to as “f/#”), the focal length of the lens, and the construction. However, in infrared cameras, the aperture cannot be located in the lens since the lens is uncooled. The aperture is typically located in the converging path of the light; that is, between the lens and the focal plane, so the aperture first reduces the image intensity and then, with reducing aperture size, begins to vignette, or cut off, the outer edges of the image. The aperture thus defines an effective f/# for the system.
As a result, when interchangeable lenses of a different f/# are used with an infrared camera, the system f/# may not match the lens f/#. There is heretofore no solution to this problem in the prior art. A variable diaphragm or aperture, however, can correct this situation and match the system f/# to the specific lens in use. By lens, we refer to all light collecting devices including refractive or reflective systems.
U.S. Pat. No. 6,133,569 to Shoda and Ishizuya discloses a thermal infrared camera incorporating all of the above-mentioned features. The Â□569 patent further describes the promising idea of using variable diaphragms in thermal feedback infrared cameras, that is, in cameras with thermal sensors controlling cooling elements. Specifically, Shoda and Ishizuya suggest the use of an optically variable diaphragm optionally thermally coupled to the infrared radiation shield. However, due to the limitations discussed below in regards to cooling the variable diaphragm, the Â□569 patent has not made possible the use of such a variable diaphragm.
The use of continuously variable diaphragms or swappable fixed apertures in thermal infrared cameras has to date not been viable because of fundamental packaging and thermal control problems. As described above, the aperture must be cooled. While an effectively cooled variable diaphragm is difficult to design, the problem becomes considerably more difficult if the aperture must be kept at cryogenic temperatures and located inside a vacuum chamber. Within a vacuum chamber, the aperture and the associated drive mechanisms cannot outgas. Depending on the depth of vacuum, this may require a completely dry iris and specially designed lubricants, electrical wiring, motors, and gears. Moreover, the drive mechanism cannot add heat load onto the cooling system, nor allow conductive heat load from the ambient vacuum enclosure to affect the cooling system. Equally important, the aperture must dissipate energy from the radiation that it blocks. These and other considerations for the aperture itself have made implementing a variable diaphragm impossible given the prior art.
Further, with continuously variable diaphragms or swappable fixed apertures, there must be some mechanism for changing the aperture. There must be mechanical, electromagnetic, piezoelectric, or other such control means to change the diaphragm size or swap fixed apertures. The control means must be strong enough to operate the variable diaphragm or swappable fixed aperture in a timely manner, and either thermally isolated from the photocell or able to operate at cryogenic temperatures. As mentioned above, if the aperture is in a vacuum, the control means must be small enough to contain within the vacuum chamber or must provide a means for transferring mechanical force through the wall of the vacuum chamber. Where such transfer of mechanical force occurs, there must be complex seals to ensure the vacuum is not compromised and that excessive heat is not conducted into the radiation shield.
Aperture control means located in a vacuum chamber have several constraints that make their implementation significantly less feasible. First, the materials used in conjunction with the control means cannot outgas, as vaporized materials not only destroy the vacuum that provides the thermal isolation for the cold components, but also condense on the photocells. For this reason, bearings, linings, coatings, winding insulation, and any cements or glues must be eliminated or replaced with a fluorinated polymer or polytetrafluoroethylene based insulation or otherwise coated or manufactured with special non-outgassing materials.
Moreover, the motor control means must also be able to cool itself effectively without the typical convection of heat into air. This means that all heat generated in the motor must be dissipated through conduction to the motor mounting apparatus. The control means must therefore be thermally isolated from the aperture it controls. The motor must incorporate heat-reducing technology, including bipolar drives, low current standby systems, and other such options. Furthermore, the diaphragm control means must not produce electromagnetic interference (EMI) that can distort the electronic signal produced by the photocell. Mechanical or other temperature control means must often also be associated with the motor.
Finally, for control means located in a vacuum, there is an additional potential problem created by high voltage to exposed conductors in the motor apparatus. In extremely low-pressure vacuums, the remaining air molecules subject to high voltage can ionize and current will flow as if the vacuum chamber were an electron tube, creating strong corona effects. These effects are particularly problematic near highly sensitive photocells, so careful insulation is needed on any exposed electric contacts.
An additional packaging problem exists where a variable diaphragm system must fit within the same confines as an existing fixed aperture camera. In these retrofit cases, the entire aperture control means must fit within very small confines that were not designed to accommodate such hardware.
Accordingly, there is a need in the art for a continuously variable diaphragm or swappable fixed aperture along with a detailed method of implementing such a means that overcomes or avoids the above problems and limitations.
The invention comprises a continuously variable diaphragm or swappable fixed aperture for infrared cameras that can be cooled to cryogenic temperatures while minimizing other associated thermal problems. The invention further comprises diaphragm control means suitable, if necessary, either for mounting within a vacuum vessel or for mounting outside the vessel and transferring aperture control means into the vessel, in a thermally isolated manner to avoid excess radiation load on the photocell.
The invention additionally encompasses a method of arranging the diaphragm and diaphragm control means to make possible the object of using a single thermal infrared camera under a wide variety of target-scene radiation conditions that may be rapidly changing, with interchangeable camera optics requiring different size cold stops, and under other such dynamic situations. The invention and inventive method also make possible the object of upgrading and retrofitting fixed aperture infrared cameras with variable diaphragm hardware.
A further object of the invention is to allow the use of interchangeable optics, including interchangeable compound lenses, with a single infrared camera, by providing a means to aperture match the camera to the lenses.
The scope of application of the inventive method and apparatus is broad, as a number of alternative thermal isolation and diaphragm control means may suggest themselves to those skilled in the art as suitable for a wide variety of thermal infrared camera applications. These applications include military thermal signature identification (including aircraft, vehicle, missile identification), military and other field of view changes (switching camera use from wide area search to narrow field of view as a target is acquired and tracked, used in target tracking and fire control systems), police surveillance (detecting the presence of people, objects, etc.), general security and surveillance applications (detecting and identifying intrusions), search and rescue (finding people or vehicles), firefighting (finding victims in smoke-filled rooms), and general zooming in or out with infrared cameras, to name a few.
The accompanying views of the drawings are incorporated in, and constitute a part of, this specification and illustrate one or more exemplary non-limiting embodiments of the invention, which, together with the description, serves to explain the principles of the invention. In the drawings:
a and 4b show two schematic views of main components located around the photocell in a liquid nitrogen dewar based infrared camera of the type in
a and 5b show a schematic layout views of an embodiment of the inventive apparatus, arranged as in
a, 6b, 6c, 6d, 6e, 6f, and 6g show schematic views of several embodiments of the inventive aperture drive mechanisms, highlighting implementations of the aperture with an exemplary worm gear driven swappable fixed aperture drive (6a), a gear cog driven swappable fixed aperture (6b), a simple two-aperture worm gear driven swappable fixed aperture drive (6c), an exemplary piezoelectric driven swappable fixed aperture (6d), an exemplary piezoelectric driven variable diaphragm (6e), and two exemplary embodiments of electromagnetic aperture control means (6f and 6g); and
The following detailed description illustrates the invention by way of example, not by way of limitation of the principles of the invention. This description will clearly enable one skilled in the art to make and use the invention, and describes several embodiments, adaptations, variations, alternatives, and uses of the invention, including what are presently believed to be the best modes of carrying out the invention.
In this regard, the invention is illustrated in the several figures and is of sufficient complexity that the many parts, interrelationships, process steps, and sub-combinations thereof simply cannot be fully illustrated in a single patent-type drawing or table. For clarity and conciseness, several of the drawings show particular elements in schematic and omit other parts or steps that are not essential in that drawing to a description of a particular feature, aspect, or principle of the invention being disclosed.
The prior art infrared camera is typified by the U.S. Pat. No. 6,133,569 Peltier cooled thermal infrared camera, an embodiment of which is shown in
In this case, radiation cone 122 emanating from the head of the source 120 arrives at the bottom of the photocell 110, where an image of the head is made. The same focusing applies to radiation (cone 124, drawn with dotted lines) emanating from a point at the bottom of the source. This radiation 124 focuses to the optics side of the photocell 110. In this way, the entire source 120 is imaged upside down onto the photocell 110. The invention, however, is not limited to applications with optics that invert the image. If a small aperture 41 is used, the aperture blocks a portion of the radiation being focused toward the photocell and that radiation cannot pass.
The aperture size is also dictated by the need to reduce or eliminate radiation emanating from non-cooled portions of the camera, because such undesirable radiation would otherwise arrive at the photocell. This aspect of aperture size is shown in
Further complicating the aperture design, specific coatings are required for the photocell-side of the aperture as well as the inside of the radiation shield 40, since radiation from cone 126 reflecting off the radiation shield 40, should be damped or absorbed as efficiently as possible to reduce stray light load at the photocell 110. If the photocell-side of the aperture reflects radiation, stray radiation in the radiation shield 40 may be deflected onto the photocell 110.
The problem with aperture sizing as shown in
Continuing the description of the prior art, internally, the various types of prior art thermal infrared cameras are similar.
a shows in a schematic layout view an exemplary housing and radiation shield of a liquid nitrogen dewar based camera of the type described in
b is a side view planar slice of the assembled components, showing their relative locations. The photocell 110 is shown below the radiation shield 40. The radiation shield 40 mounts on the same plane as the photocell 110, both mounting onto the LN2 dewar (not shown). The optical entrance of the radiation shield 40 is the fixed aperture 42. Outside of the radiation shield 40 is located the camera housing 34 with a transmissive window 32. The area around the radiation shield 40 is under a vacuum 28.
In order to implement a variable diaphragm, several new mechanisms are necessary.
Both worm gear examples use a worm gear screw 66 attached through a coupler 70 to the control means. In the case of the top system, the vacuum chamber seal 78 is penetrated by a rotary dial adjuster 76 and a vacuum feed-through 80. In this example, the dial adjuster 76 contacts warm air outside of the housing 34. Since the coupler 70 and the rest of the worm gear system are within the vacuum chamber 30, the dial adjuster 76 and coupler 70 must be made of minimally heat-conductive materials. Moreover, in either the manual or motorized worm gear examples, the coupler 70 for the dial adjuster 76 must minimize the heat transference to the variable diaphragm 46. In addition, there must be a high performance vacuum seal for the vacuum feed-through 80, capable of maintaining high vacuum. One of the several seals necessary is shown 82. This system allows the infrared camera operator to adjust the aperture size manually and directly.
The second worm gear system 62 shown utilizes a motor drive 72 rather than a rotary dial adjuster 76. The motor drive 72 is attached to the vacuum chamber seal 78 and the coupler 70. Through the coupler 70, the motor drive 72 turns the worm gear screw 66. In this arrangement, the motor drive 72 must be vacuum-capable, which means that it must not outgas, must use special coatings, must be capable of shedding heat through the vacuum chamber 30 and camera housing 34, and must not have exposed electrodes that may cause a corona effect.
A further complication may arise when using a stepper motor 72 of the appropriate size. Such a motor 72 is not likely to have sufficient torque to operate variable diaphragm 46. In this case, the inventive system contemplates the use of a reduction gearbox. Such a gearbox would be located between the motor 72 and the worm gear screw 66, where the coupler 70 is shown in the figure.
In infrared cameras that do not merge the camera housing 34 and vacuum chamber 30, it can be easier to maintain a cryogenic temperature at the important parts of the camera. In such a case, the motor 72 could be mounted external to the vacuum chamber 30, to reduce heat transference to the cryogenic parts. Mounted externally, the motor 72 would not need to be vacuum-safe, and could be a normal motor, simply associated to a vacuum feed-though of the type shown at 80. This embodiment simply drives the vacuum feed-through and rotary dial adjuster using an externally mounted motor.
In either arrangement, the worm gear screw 66 then contacts the driven gear 64. This connection should be as thermally isolative as possible, using less conductive materials or insulators for the parts of the mechanisms. The driven gear 64 is attached to the variable diaphragm 46 and the assembly is mounted to the optical entrance of the radiation shield 40 at aperture mounting location 44 (which is where the fixed aperture would have been). The typical variable diaphragm 46 consists of at least three basic parts.
There are iris fingers 54 and two rings 56, 58 that form the aperture mechanism. The iris fingers 54 form smaller or larger apertures as they are manipulated. Generally, the iris fingers 54 are roughly triangular and have two pivot points. The two pivot points are attached to an inner ring 56 and an outer ring 58. In the closed position, where the variable diaphragm size is at its smallest (nearly completely closed), the pivot points are close to being in line with one another, forming a line perpendicular to the edge of the outer ring 58. As the two rings 56, 58 spin relative to each other, the pivot points move apart, causing the iris fingers to pivot away from the center of the aperture, making the aperture size greater.
The above-described aperture mechanism is known in the art. However, this mechanism allows the driven gear 64 to be attached in such a way that the whole worm gear system 62 can open and close the aperture. Although the best mode of implementation will vary by the application, one possible arrangement, which is an embodiment of the current best mode of implementation, is for the driven gear 64 to be attached to the inner ring 56 (thus, here, the inner ring 56 has features identical to the driven gear 64, allowing the two to be attached physically). The outer ring 58 is then attached to the radiation shield 40 at the aperture mounting location 44. As the worm gear screw 66 turns the driven gear 64, the inner ring 56 is turned relative to the outer ring 58, which is fixed. The variable diaphragm 46 is thus controlled by the worm gear system 62.
The variable diaphragm 46 itself must meet certain requirements. Any portion of the variable diaphragm 46 facing the photocell 110 should be coated in a radiation absorbing material or color. Typiceily, the photocell-side of the radiation shield 40 is coated in black, though the reflectivity in the infrared is more important than the visible-specfrurn “color.” The iris fingers 54 must be allowed to move along one another freely, without relying on greases or other outgassing lubricant materials. For this reason, the blades should be coated with a material functionally similar to TEFLON® brand polytetrafluoroethylenes The result is that the iris fingers 54 must be low friction and low reflectivity coated. Finally, the focusing lens side of the aperture 46, including the iris fingers, should be coated in a reflective material, such as gold. This prevents aperture 46 from warming while it is blocking radiation by reflecting that radiation away. A reflective coating prevents the aperture 46 from heating and transferring that heat from its photocell-side to the photocell, causing unwanted radiation load. As a last caveat, the reflective coating of the iris fingers 54 must be specifically designed not to shed reflective material onto the photocell-side of the overlapping iris fingers.
b shows the mechanism of
a, 6b, 6c, 6d, 6e, 6f, and 6g show schematic views of several embodiments of the inventive aperture drive mechanisms, shown without other parts of the infrared camera. For the sake of simplicity, the apertures and gears are shown as single combined units, though it is to be understood that they can be separate or joined units. Furthermore, although there are seven examples described herewith, these examples are not limiting, and serve to teach the inventive apparatus and method. Additional embodiments will become obvious to those skilled in the art.
c shows a schematic view of a third possible embodiment of the inventive system, using a swappable partial aperture wheel 50 that inserts a small aperture in front of a larger fixed aperture. This system would provide a smaller aperture under those conditions that so warranted. For example, in a targeting system that uses a wide field of view when scanning for targets and swaps optics to a zoom lens with a narrow field of view once a target has been acquired, two aperture sizes may be sufficient. Although shown with a worm gear screw 66, this would also work with a gear cog of the type in
d shows a schematic view of a piezoelectric motor driven swappable aperture. This embodiment is otherwise similar to the aperture of
e shows a schematic view of a piezoelectric motor driven variable diaphragm. As in
f and 6g show schematic views of magnetic control means. Magnetic control means offer several distinct advantages, including not requiring any physical intrusion into the vacuum chamber (reducing the possibility of a vacuum leak) and fewer parts likely to outgas being placed into the vacuum chamber. The magnetic control means are thus most useful in cases where highly sensitive photocells are used and temperature control is of the utmost concern.
When the worm gear 66 turns the magnetic drive ring, the magnetic field created by the permanent magnets 96 cause the permanent magnets 96 affixed to the variable diaphragm 46 to move. This actuates the diaphragm or changes the fixed apertures as described above.
g, similarly, uses magnetic fields to turn the variable diaphragm or swap fixed apertures. In this figure, the variable diaphragm 46 has permanent magnets 96 affixed and is located within the dashed vacuum chamber wall 30. Outside the vacuum chamber 30 are located one or more electromagnets 100 positioned directly outside of the non-magnetic vacuum chamber wall 30. As the current increases to the electromagnets 96, the field generated changes and the permanent magnets 96 are forced to move within the field and actuate the variable diaphragm 46 or swap fixed apertures (not shown).
A further configuration possibility includes the use of a mechanical system such as a belt or chain either directly driving the aperture or driving a pulley attached to the aperture. In fact, many similar configurations may suggest themselves to those skilled in the art and such skilled persons should be enabled to utilize such configurations by applying the inventive method described herewith.
In any of these swappable aperture cases, there must be detection means for determining when the swappable apertures are in position above the underlying fixed aperture or hole in the aperture mounting location. The detection means can involve detents that stop the aperture wheel as it rotates under the manipulation of the piezoelectric motor, or contacts on the disc that send a signal to the motor control means as they pass another electrical contact fixed to some non-moving portion of the thermal infrared camera. Furthermore, optical means can be used for positioning, as well as many other possible methods of implementing such detection means, and such methods are also contemplated by the invention.
Finally,
With at least one sensor of 112 and 110 connected, the logic control system 116 can receive information on the quantity of radiation present and can apply a programmed algorithm to determine the appropriate size for the aperture 42. The logic control module 116 can then provide a signal to cause the aperture control means 60 to affect the size of the aperture 42. If the sensor used is the photocell 110 or any other sensor located within the radiation shield 40, the logic control system 116 can be feedback based, so that as the aperture size changes, the data to the logic control module 116 changes. Generic input means 118 allow the user to modify the aperture size manually.
The aperture size can also be changed via control logic that is tied to the selection of the interchangeable lens. When the user switches from search to track mode on the infrared camera and thus swaps lens elements, the variable aperture reacts accordingly and adjusts properly to match the f/#.
It is clear that the infrared camera diaphragm apparatus and method will have wide industrial applicability, not only to military, police, search and rescue applications, but also to other applications where infrared signals may be present in a wide variety of quantities, such as in agriculture. The inventive apparatus and method will further have great applicability in any circumstance where the dynamic range of the given electronics is insufficient, where various f/#s are needed, or where additional user tunability is desired.
This invention was made with U.S. Government support under SBIR Contract No. DAAB07-03-C-P004 awarded by the Department of the Defense. The U.S. Government has a royalty-free right to the use of this invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 24356 | Miller | Jun 1859 | A |
| 582219 | Mosher | May 1897 | A |
| 4583814 | Koetser | Apr 1986 | A |
| 4783593 | Noble | Nov 1988 | A |
| 5737119 | Mladjan | Apr 1998 | A |
| 5966945 | Mengel et al. | Oct 1999 | A |
| 6122919 | Patel et al. | Sep 2000 | A |
| 6133569 | Shoda et al. | Oct 2000 | A |
| 6174061 | Cooper | Jan 2001 | B1 |
| 6414708 | Carmeli et al. | Jul 2002 | B1 |
| 6707044 | Lannestedt et al. | Mar 2004 | B1 |
| 6806471 | Matsukuma et al. | Oct 2004 | B1 |
| 20030161049 | Okada et al. | Aug 2003 | A1 |
| 20040129881 | Hamrelius et al. | Jul 2004 | A1 |
| Number | Date | Country |
|---|---|---|
| 05172635 | Jul 1993 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20040238741 A1 | Dec 2004 | US |
