Abstract:
An illumination device includes a light source that emits an electromagnetic radiation input beam, a dichroic first mirror that reflects a first output beam and transmits a second output beam, a second mirror positioned to reflect the second output beam, and an absorber for absorbing radiation emitted by the light source. An adjustable support for the dichroic and second mirror is also provided and is adjustable between a first position where the first beam is transmitted and a second position where the second beam is transmitted, with the other beam being directed to the absorber.

Description:
BACKGROUND 
     The present invention relates to searchlights and more particularly to covert infrared filters for selectively filtering the light source of a searchlight. 
     Vehicles, such as aircraft and more particularly helicopters, generally have searchlights mounted thereon for providing illumination during take-off, landing, or during search operations. Searchlights may also be useful for identifying aircraft or providing primary or supplemental lighting during operation of the aircraft in adverse conditions, including night operations, rain, and other particle storms. Alternatively, searchlights may be hand held or used in a smaller configuration, such as a flashlight, headlamp, or night vision imaging device. 
     With the increased use of night vision imaging systems for covert operations, a need has arisen for landing lights, searchlights, and portable light sources that are capable of filtering out visible light and illuminating an area solely with infrared light. While separate infrared and visible spectrum lights may accomplish this objective, there has been recognized a need in the art for a light source which may be converted between infrared and visible illumination. 
     One method of early convertible night-vision compatible lighting systems utilized a filter over the searchlight cover that allows only infrared light to pass through the filter. This type of filter, however, may be undesirable because of the difficulty in removing or altering the filter, requiring manual access to the searchlight. This limitation restricted the usefulness of the product by limiting flights to either visible-light or infrared-light. 
     A further improvement was the use of a lamp within a lighting system that has both a visible and an infrared filaments, allowing the operator to switch between the two. Further controls allowed the lamp head to be extended, refracted, and rotated through the use of electrical relays and a selector switch. One example of this improvement is described in U.S. Pat. No. 5,695,272 to Snyder et al. 
     This improvement allowed significant advantages over the prior art, including the ability to switch between the infrared and visible light spectrum from within the cockpit, thereby eliminating the need to manually remove and replace the searchlight cover to switch modes. However, these lamps do not emit the same intensity as a dedicated infrared or visible light system as the bulb surface is divided between infrared and visible light filtering covers. 
     U.S. Pat. No. 6,962,423 to Hamilton et al. describes another multi-mode visible and infrared lighthead for use as a landing light or searchlight. This patent describes two separate diodes, one for emitting infrared and the other for emitting visible light, spaced apart in a searchlight with each diode having its own reflector and lens cover. However, this arrangement similarly limits the amount of light that may be transmitted from the searchlight by dedicating a portion of the light-producing elements to only the infrared or visible spectrum. 
     Another dual mode searchlight is described in U.S. Pat. No. 7,518,133 to Giffen et al. This integrated searchlight lighthead includes separate infrared and visible light illumination sources each positioned within a reflector. The reflectors are merged and separated by an insulating material and air gap, providing cooling of the illumination sources. The merged reflector assembly provides an improved light distribution over previous light sources. However, the merged reflectors are inferior to a single reflector and the combination of separate lighting elements reduces the intensity of the light that may be produced. 
     Therefore, there has been recognized a need in the art for an improved searchlight capable of selectively transmitting infrared or visible light. There is further a need in the art for an improved searchlight which can be easily switched between infrared and visible illumination without the need for modification of the searchlight housing or cover. Finally, there is a need in the art for an improved searchlight which improves light distribution and efficiency. 
     SUMMARY 
     One embodiment of the apparatus is a searchlight for illuminating a distant site. The searchlight may include a light source emitting electromagnetic radiation, a mirror assembly, and a window through which light is projected. The mirror assembly includes a first mirror that splits light from the light source into a first beam that is reflected by the mirror and a second beam that passes through the mirror. A second mirror reflects the second beam. The mirror assembly is adjustable to project one of the first or second beams through the output window. 
     According to a further embodiment, the first and second mirrors are perpendicular to one another and angled at 45° relative to the light source so that the output beams are projected in opposite directions. The searchlight may also include a motor for rotating the mirror about an axis to selectively project the first or second beam through the window. 
     According to a further embodiment, the dichroic mirror is a cold mirror that reflects visible light and allows infrared light to pass through where it is reflected by the second mirror. 
     Alternatively, the first and second mirrors may be parallel to one another such that in a first arrangement both visible and infrared light are reflected through the window, but in a second arrangement only infrared light is reflected through the window. 
     Also described is a novel mirror assembly for a searchlight that includes a light input and an output window. The mirror assembly includes a first mirror that reflects a portion of the input light and a second mirror that reflects at least a portion of the remaining light. The mirror assembly is adjustable so as to project either the first or second portion of the light through the window. 
     According to a further embodiment, the searchlight may include a heat sink so that when one light source is projected through the window the other is projected into the heat sink. 
     According to a further embodiment, the first and second mirrors may be either parallel or perpendicular to one another. 
     Also described is a novel method for projecting light to a target location. A searchlight with a light source, mirror assembly, and output window is provided. A source beam of light is projected from the light source to the mirror assembly. The beam is split into a first beam and a second beam by means of a dichroic mirror and the first beam is reflected from the dichroic mirror to the target location through the output window. 
     According to a further embodiment, motors for rotating the mirror about a yaw and pitch axis are provided to direct the light reflected from the dichroic mirror to the target location. The motor for rotating the mirror assembly about the pitch axis may also be utilized to switch between the first beam and second beam, for example between a visible and infrared light. 
     The features, functions, and advantages that have been discussed can be achieved independently in various embodiments of the present invention or may be combined in yet other embodiments further details of which can be seen with reference to the following description and drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows a perspective view of a searchlight. 
         FIG. 1B  shows a side plan view of the searchlight. 
         FIG. 2  shows a side cutaway view of the searchlight of  FIG. 1 . 
         FIG. 3  shows a perspective view of a mirror assembly. 
         FIG. 4A  shows a detail view of the mirror assembly in a first arrangement. 
         FIG. 4B  shows a detail view of the mirror assembly in a second arrangement. 
         FIG. 5A  shows a detail view of the mirror assembly according to an alternative embodiment in a first arrangement. 
         FIG. 5B  shows a detail view of the mirror assembly according to an alternative embodiment in a second arrangement. 
         FIG. 6A  shows a detail view of a light source according to a first arrangement. 
         FIG. 6B  shows a detail view of the light source according to a second arrangement. 
         FIG. 6C  shows a detail view of the light source according to a third arrangement. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a perspective view of a searchlight  100  according to one embodiment. The searchlight  100  may include a base section  102  and a projection section  104 . The base section  102  and projection section  104  may both be rotated about a yaw axis  106  that may be substantially centered on the searchlight  100 . The projection section  104  may include a window  108  through which an output beam  110  may be projected, as will be explained in further detail below. 
       FIG. 2  shows a cutaway front view of the searchlight  100  demonstrating the various components of the searchlight  100 . As shown in this figure, light may be generated by a light source  112  and directed onto a parabolic mirror  114  that reflects light from the light source  112  emitting electromagnetic radiation into the mirror assembly  116 . Mirror  114  can also be embodied as a lens and a turning mirror According to the embodiment illustrated, the mirror assembly  116  includes at least one mirror that reflects light through the window  108  to illuminate the desired area. 
     Also shown in  FIG. 2  is a yaw motor  120  for rotating the projection section  104  about the yaw axis  106 , a filter  122  and fan  124  which filter and provide ambient air for cooling the various optical components, a mirror motor  126  for pivoting the mirror assembly  116  about a pitch axis  128  and a heat absorber  130 . The heat absorber  130  absorbs reflected light, converts it into heat, and dissipates the heat energy into either the ambient air or the filtered airstream provided by the fan  124 . 
     The mirror assembly  116  shown in a front view in  FIG. 2  and in perspective in  FIG. 3  may include a substantially cylindrical frame  132 , visible light mirror  134  and infrared mirror  136 . On opposite ends of the frame  132  are a bearing assembly  138  and a mount  140  for receiving the mirror motor  116 . The visible light mirror  134  is preferably a dichroic mirror. The mirrors  134 ,  136  are shown at approximately 45° relative to the pitch axis  128  and at right angles to one another. The 45° orientation is preferred to maximize the effectiveness of the dichroic mirrors and reflect incident light at a right angle. The mirrors  134 ,  136  are also arranged perpendicular to one another so that light is reflected from the first mirror  134  in the opposite direction of light reflected from the second mirror  136 . While this arrangement of the mirrors  134 ,  136  prevents mixing of the reflected light, it is only one orientation and other arrangements are anticipated. For example, the angle may be selected so that both beams are not allowed to exit the window at the same time. 
     One example of a dichroic mirror is a PYREX® substrate having a borosilicate crown glass coating. Mirrors that separate infrared from visible light are known in the art as hot (reflecting infrared and passing visible light) or cold (reflecting visible and passing infrared light) mirrors. Hot or cold dichroic mirrors are optimized for a certain angle of incidence, such as 45 degrees, at which a majority (&gt;90%) of the infrared or visible light is reflected while a majority of the other (&gt;80%) light is passed through the mirror. In addition to reflecting infrared or visible light, the dichroic mirrors may also be selected to provide for other wavelengths of light to pass or be reflected. For example, low-pass dichroic mirrors allow short wavelength light to pass through while high-pass dichroic mirrors allow long wavelength light to pass through. Band pass mirrors may also be used to reflect certain wavelengths while passing other wavelengths which may be longer or shorter than those reflected. The specific wavelengths selected to be reflected or passed can be controlled through careful selection of mirrors and coatings. 
       FIGS. 4A  and B show an enlarged view of the mirror assembly  116 , heat absorber  130 , and window  108  in order to illustrate the selective filtering process. As shown in  FIG. 4A  the visible light mirror  134  may be a cold mirror that allows infrared light  146  to pass through while reflecting visible light  144 . Infrared mirror  136  may be either a hot mirror or a regular mirror that reflects all light. As further shown in this figure, when the mirror assembly  116  is in a first orientation, white light  142  that impacts the visible light mirror  134  is split into visible light  144  that is reflected away form the mirror  134  and infrared light  146  that passes through the mirror. In the first orientation, the visible light  146  is reflected out through the window  108 . The output beam  110  ( FIG. 1 ) is therefore of visible light. Infrared light  146  that passes through the visible light mirror  134  impacts the infrared mirror  136  and is reflected into the heat absorber  130 . Each of the visible  134  and infrared  136  mirrors is at approximately 45° (plus or minus 2°) to reflect visible light  144  perpendicular to the incoming white light  142  and maximize the dichroic features of the mirrors. 
     A second orientation of the mirror assembly  116  is shown in  FIG. 4B . In this orientation, white light  142  that impacts the visible light mirror  134  may also be split into visible light  144  and infrared light  146 . However, the mirror assembly  116  has been adjusted so that visible light  144  is reflected from the visible light mirror  134  into the heat absorber  130  while infrared light  146  is reflected from the infrared mirror  136  through the window  108 . This provides an output beam  110  ( FIG. 1 ) of infrared light. 
     As shown in  FIGS. 5A-B , an alternative arrangement of mirrors is presented that minimizes the size of the mirror assembly  116  while still allowing for selective control of light that may be projected through the window  108 . 
     A first arrangement for reflecting visible light  146  through the window  108  is shown in  FIG. 5A . In this figure, the visible light mirror  134  may be arranged so that white light  142  is reflected from the parabolic mirror  114  through the window  108 . As with the embodiment illustrated in  FIGS. 4A-B , the visible light mirror  134  may be a cold mirror that allows infrared light  146  to pass through. The infrared mirror  136  may be positioned to be substantially parallel to the visible light mirror  134  so that infrared light  146  that passes through the visible light mirror  134  may be reflected at the same angle as the visible light  144 . However, unlike the embodiment described above, in this case the infrared light  146  would also pass through the window  108 , and therefore the output beam  110  may include both visible  144  and infrared  146  light. 
     A second arrangement of this alternative embodiment is illustrated in  FIG. 5B . In this arrangement, the infrared mirror  136  remains stationary while the visible light mirror  134  may be pivoted to reflect visible light  144  away from the window  108 . The visible light  144  may be reflected by the visible light mirror  134  to a heat sink (not shown) that collects this light and dissipates it as heat. As previously discussed, because the visible light mirror  134  may be a cold mirror, infrared light is passed through the mirror  134  to the infrared mirror  136 . Because the position of the infrared mirror  136  has not changed, any infrared light  146  that impacts the infrared mirror  136  will be reflected through the window  108  to form an output beam  110  composed solely of infrared light. 
     According to this alternative arrangement, the infrared mirror  136  may simply be a standard mirror that reflects all light or it may be a hot mirror that only reflects infrared light and allows other wavelengths of light to pass through. A hot mirror may be preferred if, for example, there is risk of other light contamination while a regular mirror may be preferred to reduce expense. 
     The above embodiments are described as utilizing a cold mirror that allows infrared light to pass through while reflecting visible light. However, it is appreciated that other types of dichroic mirrors may be utilized to create a searchlight that may be switchable between other various types of light, including ultraviolent or various colors of light. 
     According to a further embodiment illustrated in  FIGS. 6A-C , the output beam  110  may narrowed or widened. This is accomplished by adjusting the optics within the searchlight  100  to cause light rays  142  to converge or diverge. 
       FIGS. 6A-C  illustrate the relationship between the light source  112  parabolic mirror  114  and mirror assembly  116 . The light source  112  may include a lamp  148 , typically approximated as a point source, and an elliptical reflector  150  that reflects light from the lamp  148  in a converging beam to a distant focal point where the light diverges as if from a second point source. The parabolic reflector  114  may be positioned to capture light from this distant focal point and reflect it towards the mirror assembly  116 . Similarly, a lens and a turning mirror at 45 degrees can perform the same operation as the off-axis parabola  114 . As shown in  FIGS. 6B-C , the parabolic mirror  114  may be adjusted away ( FIG. 6B ) or towards ( FIG. 6C ) the light source  112 . Movement of the parabolic mirror may be according to a variety means, including a movable cylinder, screw, or other apparatus for predictably adjusting the height of the mirror  114  remotely. 
     As shown in  FIG. 6A , when the parabolic mirror  114  is in a neutral position, light rays  142  reflected from the parabolic mirror  114  are substantially parallel and therefore create an output beam of constant diameter. 
     As shown in  FIG. 6B , when the parabolic mirror  114  is in a lower position (further away from the light source  112 ), light rays  142  reflected from the parabolic mirror  114  will diverge. This diverging light profile may be useful for illuminating a wider area with the searchlight. 
     Finally, as shown in  FIG. 6C , when the parabolic mirror  114  is in an upper position (closer to the light source  112 ), light rays  142  reflected from the parabolic mirror  114  will converge. This converging profile may be useful for targeting a small area with the searchlight. 
     Also described in this application is a novel method for adjusting the location, focus, and content of an output beam  110  from a searchlight  100 . 
     Fine control of the angular position of the output beam  110  is controlled by means of the yaw motor  120  and mirror motor  126  ( FIG. 2 ). The mirror motor  126  preferably is capable of rotating 360° to fully pivot the mirror assembly  116  about the pitch axis  128 . As shown in  FIG. 1B , the window  108  extends approximately 110° around a perimeter of the mirror assembly  116 . The heat absorber  130  extends for at least an equal angle opposite the window  108  although may extend to complete the perimeter about the window  108 . Light reflected from the either the visible light  134  or infrared  136  mirror ( FIG. 3 ) in the mirror assembly  116  is shown as the output beam  110 . Therefore, by adjusting the rotational position of the mirror assembly  116  by means of the mirror motor  126 , the projected position of the output beam  110  is adjusted. 
     Conversion between an infrared or visible light output beam  110  is accomplished by adjusting the mirror assembly  116  by a sufficient rotation to expose one of the mirrors  134 ,  136  ( FIG. 3 ) to the window  108  and the other to the heat sink  130 . 
     Further controlling the position of the output beam  110  is the yaw motor  120  that may rotate the searchlight  100  about the yaw axis  106 . The yaw motor may connect the searchlight  100  to another structure, such as a helicopter or other aircraft, and be selectively engaged to rotate the searchlight  100 . By adjusting the searchlight about the yaw axis  106 , another rotational dimension is added to the output beam  110 . By controlling the position of the output beam about the yaw axis  106  and pitch axis  128 , the position of the output beam  110  may be directed to any position.