Abstract:
A window mounting system for optical sensors including a removable connection for window replacement, an alignment system to assure precision registration of the window or dome with the sensor internal optics, and thermal insulation to limit heat transfer from the window to the optics housing and sensor. The removable connection maintains precise optical alignment of curved domes with significant optical power, yet also is effective with simple flat windows.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to optical sensors and, more particularly, to mounting of a window over an optical sensor. 
   2. Brief Description of Prior Developments 
   Most methods for mounting a dome window to an optics housing include large, flat contact surfaces on either the outside diameter or on the base flat or equivalent which readily transfer thermal energy. Layer(s) of insulating material can be added to provide a thermal barrier, but these typically add greatly to the tolerances for spacing, centering, and wedge of sensitive optical or electrical components. Retention grooves in the outside surface of a dome window create high stress points, and they attract dirt and moisture which can also contribute to early failure. Large contact areas on the outside of a dome are high thermal transfer rate areas, and this is undesirable in most applications. 
   SUMMARY OF THE INVENTION 
   In accordance with one aspect of the present invention, an optical sensor cover system is provided including an optics housing adapted to house an optical sensor therein; and a window connected to the optics housing by a removable connection. The removable connection comprises a thermal transfer path between the window and the optics housing which comprises a heat transfer limiting system to limit heat transfer from the window to the optics housing. 
   In accordance with another aspect of the present invention, an optical sensor cover system is provided comprising an optics housing adapted to house an optical sensor therein; and a dome shaped window connected to the optics housing by a removable connection. The dome window contacts the optics housing only along a concave interior surface at a protruding surface of the optics housing to align the dome window relative to the optics housing and limit conductive heat transfer from the dome window to the optics housing. 
   In accordance with one method of the present invention, a method of mounting a dome window to an optics housing is provided comprising steps of positioning the dome window on the optics housing with only a portion of an interior concave surface of the dome window contacting the optics housing at an outer tip of the optics housing; and applying force to the dome window by an O-ring seal to align the dome window at a predetermined position on the optics housing. An area of contact between the outer tip of the optics housing and the dome window is small to limit conductive heat transfer from the dome window to the optics housing. The seal can be removed to remove the dome window from the optics housing. 
   In accordance with another method of the present invention, a method of mounting a window to an optics housing is provided comprising steps of bonding the window to a window frame; removably connecting the window frame to the optics housing at a removable connection, the removable connection comprising alignment members extending between the window frame and the optics housing to align the window relative to an open area of the optics housing; and providing heat transfer insulation between the window and the optics housing. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing aspects and other features of the present invention are explained in the following description, taken in connection with the accompanying drawings, wherein: 
       FIG. 1  is a partial cross sectional view of an optical sensor cover system incorporating features of the present invention; 
       FIG. 2  is a partial enlarged cross sectional view of an area of the cover system shown in  FIG. 1 ; 
       FIG. 3  is a diagrammatic cross sectional view of an alternate embodiment of the cover system shown in  FIG. 1 ; 
       FIG. 4  is a diagrammatic cross sectional view of another alternate embodiment of the cover system shown in  FIG. 1 ; 
       FIG. 5  is a schematic diagram of another alternate embodiment of the cover system shown in  FIG. 3 ; 
       FIG. 6  is a schematic diagram of another alternate embodiment of the cover system shown in  FIG. 3 ; 
       FIG. 7  is a partial diagrammatic view of one form of connecting a heater to a window; 
       FIG. 8  is a partial diagrammatic view of one form of connecting a heater to a dome shaped window; 
       FIG. 9  is a partial diagrammatic view of another form of connecting a heater to a window; 
       FIG. 10  is a partial diagrammatic view of another form of connecting a heater to a dome shaped window; 
       FIG. 11  is a partial diagrammatic view of another embodiment for heating a flat window; 
       FIG. 12  is a partial diagrammatic view of another embodiment for heating a dome shaped window; 
       FIG. 13  is a partial cross sectional view of one type of window for use with the present invention; and 
       FIG. 14  is a partial cross sectional view of another type of window for use with the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to  FIG. 1 , there is shown a partial cross sectional view of an optical sensor system incorporating features of the present invention. Although the present invention will be described with reference to the exemplary embodiments shown in the drawings, it should be understood that the present invention can be embodied in many alternate forms of embodiments. In addition, any suitable size, shape or type of elements or materials could be used. 
   The system generally comprises a window  10 , a sensor housing  11 , an optics housing  12 , and a removable retention ring  18 . Optics housing  12  includes lens element  13  and  14 , and other components not illustrated. Optics housing  12  mounts within a central opening in sensor housing  11 , and is held in alignment by face  15  and inside diameter  16 , or by other means not critical to this invention. Elastomeric “O-ring”  17  seals the interior of the optics assembly against dust and moisture, by pressing against sensor housing  11  and optics housing  12 . These details of mounting the optic are not critical to this invention, so other designs can be used without conflicting with the principles of this invention. 
   Window element  10  is attached to the sensor housing  11  by a removable connection that comprises retention ring  18 , seal member  19 , and fastener  20 . Multiple fasteners  20  are located at several places about the diameter of retention ring  18 , and hold window  10  firmly against optics housing  12  and seal member  19 . Fasteners  20  engage the sensor housing  11 , within which optics housing  12  is mounted. The seal member  19  is preferably a flexible or elastomeric O-ring. However, in an alternate embodiment, any suitable type of seal member could be provided. Retention ring  18  is sized and shaped to receive the O-ring seal  19  therein. As flange  18  presses the O-ring seal  19  against both window  10  and sensor housing  11 , a seal is created against dust and moisture intrusion. 
   Referring to  FIG. 2 , the window retention and general position is assured by fastening to sensor housing  11 , as previously described. Window element  10  has optical surfaces defining the window exterior  21  and interior  22 , and customary window edge  23 , which are conventional optics details. Window element  10  also has additional precision cylindrical diameter  24  that mates with corresponding precision diameter in optics housing  12 . The window also has a precision flat surface  25  that similarly mates with a corresponding precision height reference in optics housing  12 . These two precision surfaces assure accurate alignment of the window element  10  with optics housing  12 , even if the optics housing  12  is slightly misaligned with sensor housing  11 . The elastomeric nature of seal member  19  accommodates such small misalignments, and enables the window to be mounted to the overall sensor housing  12 . As the seal  19  is pressed down against the window  14 , the seal  19  applies a force to the window  10  to center the window towards a center established by precision surfaces  24  and  25  of the optics housing  12 . 
   Window  10  is in contact with optics housing  12  at only a small area defined by precision surfaces  24  and  25 . Flat edge  23  makes broad contact with heating element  26  and insulating element  27 . Insulating element  27  and small contact areas  24  and  25  block heat flow into sensor housing  11  and optics housing  12 . Dome heating for defrosting, thus, is efficient in cold conditions, and unwanted aerodynamic heating is blocked from the sensor in hot conditions. 
   In addition, this mounting to housing  12  by retaining flange  18  and fasteners  20  allows replacement of window  10  without disassembly of optics housing  12  from sensor housing  11 . When the window is mounted to the sensor housing  11 , it is precisely aligned to optics  12 . User replacement of window  10 , thus, can be done without sensor disassembly or risk of window misalignment. 
   Commercial application for the present invention could comprise any application requiring or preferring very low thermal transfer rate from one component to another component while maintaining accurate positioning and/or alignment. The dome mounting method can be made a part of a wide angle, infrared airborne optical system. In some applications, such as described in U.S. provisional patent application entitled “Submicron Tolerance Lens Design Method” Application No. 60/340,162 filed in 2001, which is hereby incorporated by reference in its entirety, the dome must be positioned with precision. In addition to accomplishing this precision, the dome mounting method of the present invention also minimizes conductive heat load to the housing. Suitable means could also be provided to address radiated heat transfer, such as use of a special coating on the inside surface to transmit desired wavelengths while reflecting other wavelengths that merely cause radiative heating of the optical surface. 
   With the present invention, the dome window can be replaceable on an aircraft without risk of the optics being dislodged. With an O-ring in the triangular cavity, complete sealing is accomplished between the dome, the optics housing and the sensor housing. Loosening mounting screws for the mounting assembly could allow movement of the optics assembly with the dome. That would release tension on the O-ring such that the O-ring could now be removed. That would allow the dome to be removed and replaced, and a new sealing O-ring installed. 
   The dome location is controlled and toleranced by single surfaces  24 ,  25  which are part of the lens cell assembly. Due to the small contact area of the dome and the optics housing, the heat transfer rate across this interface will be very low and, thus, beneficial to image quality during dome friction heating. 
   This dome mounting method minimizes the contact area with media that will readily transfer or absorb heat from the dome to optical housing or components. Large contact areas mate with insulating O-ring seals and with a heater assembly which is insulated from other optics. Since a dome normally requires an accurate centering tolerance, nearly mating pilot diameters on the dome and on the optics housing are designed for minimum contact and heat transfer. By designing the support and control interface area to be on the inside dome surface near the outside perimeter, the outside surface of the dome is clear of any trapped dirt in grooves or freezing moisture in surface retention cavities. The pressure generated by the outside surface O-ring to seal and retain the dome also causes the dome to be accurately positioned and tolerance controlled by the inner mounting interface surfaces. This arrangement generates very low distortion stresses in the dome surface, and thus the dome is likely to survive fracture impacts that would otherwise destroy a moderately to highly stressed dome. Most of the heat generated by aerodynamic friction is contained within the dome, but because of the very low mount distortion stresses, there is little or no optical distortion of the dome. 
   Using the O-ring to generate a balanced, controlled, thrust vectored force while sealing multiple surfaces from the environment is unique. Using a heater for de-icing of the dome, mounting the heater directly to a large flat dome surface, but mounting it such that no heat is transferred to the optics or housing is also unique. By putting the precision, tolerance controlled, mounting interface on the inside surface of the dome opposite the O-ring pressure, and by minimizing the optics housing contact area, we have created a thermal transfer path that is very restrictive. High speed airflow over the dome or the de-iceing heater can generate very hot dome surfaces, but there is a very small path for thermal transfer into the optics or optics housings. Almost all of the conductive thermal transfer must take place at the only optics housing surface that is in full, intimate contact with the dome. That surface is normal to the optics axis and only wide enough to keep the dome stress concentrations very low. The adjacent radial pilot diameters are sized such that those surfaces will only touch, without excess pressure, in one small spot if all of the control tolerances go in one direction. 
   The transfer of heat from a dome or window to optics or the optics mounting housing can be a difficult problem, especially on optical systems mounted in high-speed aircraft. The high-speed airflow over an optics dome generates a large amount of heat, part of which is transferred to some optical elements. The transient heat absorbed may cause localized distortion of a lens or lenses and optical housings, out of focus, etc. 
   Another problem often encountered is that the allowable de-centering and spacing tolerance of a dome with respect to the next optical element is larger than allowable per the optical prescription. Since most applications require that the dome be removable from the outside of the aircraft, the tolerance stack-up has a number of contributors. The contributors generally are; (1) dome outside pilot diameter and surface to the dome mount in the aircraft with (2) additional radial clearance to allow for the materials differing Coefficient of Thermal Expansion (CTE) over the operating temperature range, (3) aircraft dome mount-to-optics assembly mount dimensions, (4) optics assembly mount to the lens mount housing, (5) lens mount to the potting diameter and surface, (6) lens potting positional tolerance, and finally, (7) the lens outside diameter to the true lens optical centerline and mounting surface. 
   This dome mounting method minimizes the thermal transfer contact area between the dome and the optics housing, while drastically reducing the number of tolerance contributors. The dome mounting surface to lens mounting surface tolerance is controlled by one dimension on a common housing. Likewise, the centering of the dome (inside surface) pilot diameter to the lens pilot diameter tolerance is controlled by one dimension on a common housing. 
   As portrayed in the figures, by moving the dome interface to an inside radius surface, a common housing can be used to mount both the dome  14  and first lens  18 . Thus, one dimension controls centering and one dimension controls spacing. By selecting a material for the housing that minimizes the CTE difference with the dome, very little additional clearance is needed to allow for material size changes during wide temperature excursions. Given the proper dimensions, the dome does not touch any other thermally conductive surface, except the added defrost heater element shown in the figure. While the heater makes excellent thermal contact with the dome, the heater is thermally insulated from all other optical elements and housings. 
   The design reduces heat transfer from a dome into other elements of an optical system, while preserving accurate alignment of the dome with the other optics. This minimizes thermal distortion of the optics that could degrade image focus, and it also enables incorporation of a dome heater for dome defrosting or deicing. However, thermally isolating the dome from the sensor and lens has other important advantages. 
   Reducing heat transfer reduces undesirable image patterns in infrared sensors that are sensitive to patterns in dome thermal emissions, because heat flow through the dome into the sensor housing is a major source of dome temperature gradients. Blocking heat flow into the sensor also reduces sensor temperature rise of the sensor. This can be important to performance of infrared sensors, and reliability of infrared and other types of sensors. The unusual compactness and wide-angle optical coverage of some sensors make it particularly sensitive to dome thermal gradients and thermal coupling. In addition, while the first embodiment&#39;s approach provides extremely accurate positioning of the dome, many systems can tolerate modest placement errors that arise from other alignment means, so such schemes should be included for completeness. Another acceptable method separately aligns the dome to a common sensor housing. 
   The present invention could apply generally to any equipment, not just to infrared sensors. This design concept can apply to infrared products that are subject to harsh aerodynamic heating. It meets strenuous requirements, and is a creative solution to an unusual problem. The advantages claimed for this invention represent a unique combined knowledge of thermodynamics and optical systems. 
   This concept is important to sensors performing in the severe aerodynamic heating of required high-speed dash conditions. Such conditions apply quite commonly to infrared missile warning systems for aircraft, and probably to competing ultraviolet and visible systems. Blocking temperature gradients across the sensor dome also is important to avoiding image non-uniformity, a potentially serious problem. 
   The first embodiment achieves thermal isolation with ultra-precise registration of the dome element, and discusses thermal-mechanical concerns. However, there also are significant infrared systems benefits, which apply more broadly to sensors with or without ultra-precise dome registration. Infrared sensors in which the entrance pupil is coincident with the window element have the problem that undesirable image non-uniformities are caused by non-uniformities in dome or window temperature, particularly if the optical field-of-view is very wide. And, sensors that are compact are unusually sensitive to aerodynamic heating, due to their small thermal inertia and restricted placement of components, and their dome or outer window being a large area on the outer sensor face that is exposed to the airstream. 
   Infrared sensors usually are not ultra-compact, thus avoid this problem inasmuch as their relatively smaller exposed area couples less heat into the unit and their higher thermal inertia slows temperature rise, thereby preventing excessive sensor heating during high speed dash for typically-limited short periods. In addition the interior size is large enough that thermal paths from the housing to sensitive components can be minimized. Similarly, prior infrared seekers have short flight times and couple the dome thermally to the long missile body, which has substantial mass and is insulated from interior components. In addition, infrared sensors and seekers use telephoto optics with an entrance pupil near the outer window (placed directly or by re-imaging), which makes the sensor immune to hot-window emission patterns. 
   Referring now also to  FIG. 3 , an alternative design uses either a shallow dome or flat window  60  that is bonded to a mounting frame  62 . This frame  62  is registered to the sensor body  64  through alignment pins  66 , and held in place with screws or bolts  68 , which renders it field-replaceable. Alignment is assured by the pin  66  registration, and mounting is made secure by the bolts  68 . Between this frame  62  and the sensor body  64 , a sheet of insulating material  70  isolates the dome or window  60  from the sensor housing  64 . Alternatively, as seen in  FIG. 4 , the window frame  62 ′ itself may contain an insulating structure  70 ′ that is bonded into the frame. 
   Note that these designs are shown with a flat window, but apply equally well to mounting of a curved dome. While shown for a sensor with a flat front face, these designs also apply equally well to mounting on a missile with straight sides.  FIGS. 5 and 6  illustrate these variations of the basic concept, without repeating the details of the insulated mount. The dome  14  is attached by an insulating frame  76  with the sensor  72  shown in  FIG. 5 . The dome  14  is attached by an insulating frame  78  with the missile  74  shown in  FIG. 6 . The sensor housing is parallel with the external surface of the sensor shown in  FIG. 5  or missile shown in  FIG. 6 , presenting a smooth outer surface. This external surface for the sensor  72  is the flat front face ( FIG. 5 ), and for the missile  74  is the cylindrical outer body ( FIG. 6 ). Thus, in a detailed section view, the frame for the missile mount would be turned  90  degrees relative to that shown in  FIGS. 3 and 4  for the sensor mount. 
   Defrosting can be done through a separate heating element. In the embodiments shown in  FIGS. 1 and 2  the heater  26  is placed in contact with the window edge (or dome rim), atop the thermal insulator  27 , so heat flows into the dome and not into the sensor. Heating also can be done by imbedding the heater in the window mount, as shown in  FIGS. 7–8 . In this case, the mount can include a rigid insulator  52  to block heat from the mount surface that contacts the sensor. The embodiment shown in  FIG. 7  has a flat window  60  with a heater  50  in contact therewith. A thermal insulator  52  is provided to insulate the heater  50  from the frame  64 . The embodiment shown in  FIG. 8  has a dome window  10  with a heater  50  in contact therewith. A thermal insulator  52  is provided to insulate the heater  50  from the frame  51 . 
   In an alternate embodiment, the entire mount can be insulated as shown in  FIGS. 9–10 , respectively. The embodiment shown in  FIG. 9  has a flat window  60  attached to a heater  80  and frame  82 . The heater and frame are attached by a thermal insulator  70  to the attachment frame  64 . The embodiment shown in  FIG. 10  has a dome window  10  attached to a heater and frame assembly  82 . The heater and frame assembly  82  is insulated from the attachment frame  51  by the thermal insulator  70 . 
   Heating also can be done through direct electrical conduction through the window or dome as shown in  FIGS. 11–12 . In this case, the thermal insulator doubles as an electrical insulator, and heating current is applied through electrical contacts that are suitably placed on alternative sides of the window. The embodiment shown in  FIG. 11  has a flat window  60  in direct contact with an electrical contact  84 . An electrical insulator  70  is provided between the contact  84  and the frame  64 . The embodiment shown in  FIG. 12  has a dome window  10  in direct contact with the electrical contact  84 . A spacer  86  is provided to insulate the contact  84  from the optics housing. Contacts can be made by flame spraying and electroplating, giving an interface to which wires can be soldered. Or, instead of flame spraying and plating, copper foil can be bonded with conductive adhesive. 
   The first test windows built for sensors had a conductive silicon window, as shown in  FIG. 13 .  FIG. 14  shows an alternative. For the embodiment shown in  FIG. 13 , the window comprised a mono-crystalline silicon window layer  90  doped for de-icing and anti-reflection coatings  92 ,  93 ,  94 . For scratch resistance, the conductive silicon layer  90  was covered with a bonded layer of durable aluminum oxynitride (ALON)  96  by a high-temperature IR-transmissive adhesive  98 . For the embodiment shown in  FIG. 14 , the window comprised the layer  90 , entire reflection coating  94 , polycrystalline silicon layer  100  fused directly on the layer  90 , and an exterior coating  102  comprising a one quarter wavelength zirconia exterior coating. In another common embodiment, individual windows have heater wires soldered onto copper foil conductors that were physically and electrically bonded to opposite window edges. In alternate embodiments, any suitable type of window construction could be used. 
   This invention accommodates all these window heating embodiments described above by integrating an insulating material into the window to mount block heat flow. The  FIGS. 3–12  embodiments use rigid insulating material to maintain an accurate registration during assembly and field replacement. 
   In all cases, insulating the window blocks heat flow into the cooler sensor which temperature rise within the sensor. This also lowers thermal gradients across the window that could cause image patterns, since the window rises to a uniform temperature. 
   The combined window/dome features of better temperature uniformity, lower thermal conduction into the sensor, and multiple defrosting/deicing options are not incorporated in designs of any known infrared equipment. Deicing by electric conduction does not require thermal isolation, just power. Missile seekers use solid mounting of the dome to the missile body, which protects interior parts by thermal inertia. Forward looking infrared systems (FLIRs) similarly use thermal inertia of the relatively large housing. Both seekers and FLIRs place the entrance pupil at the window or dome, thus minimize image pattering from window or dome temperature gradients. Such prior sensors/seekers neither require nor use this novel design. 
   It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.