Vehicles, such as aircraft, may have lights mounted thereon for providing illumination during take-off, landing, or during search operations. Lights also may 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 advent of night vision imaging systems for covert military operations, a need arose for landing lights, searchlights, and portable light sources that were compatible with night vision equipment by producing infrared light. While this need may be met by providing separate sets of infrared and visible spectrum lights, single light sources that may be converted between infrared and visible illumination have been developed.
One method of early lighting systems that were convertible between visible and infrared light utilized an infrared filter that was selectively placed over a visible light searchlight. Using such filters is cumbersome because of the difficulty in attaching and removing the filter. This limitation restricted the usefulness and applicability of lights equipped in this manner by limiting flights either to visible light or infrared light.
Lighting systems have been developed that include a lamp that has both visible and infrared filaments that allow an operator to switch between the two. Additional controls permit the lamp head to be extended, retracted, and rotated by electrical relays and a selector switch. An example of this design is described in U.S. Pat. No. 5,695,272 to Snyder et al.
Such designs provide significant advantages, including the ability to switch between infrared and visible light spectrums from within the cockpit, thereby eliminating the need to remove and replace an infrared filter manually to switch illumination modes. However, such lamps do not emit light at the same intensity as a dedicated infrared or visible light system because 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 light head for use as a landing light or searchlight. That patent describes an illumination source having 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 filter cover. However, that arrangement also limits the amount of light that may be transmitted from the searchlight by dedicating a portion of the light-producing elements either to the infrared or to the visible spectrums.
Another dual mode searchlight is described in U.S. Pat. No. 7,518,133 to Giffen et al. That patent discloses an integrated searchlight light head having 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 that provide cooling of the illumination sources. The merged reflector assembly provides 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.
There is a need for an improved method and apparatus for filtering light emitted by an illumination device, such as a searchlight, capable of selectively switching the illumination device between infrared and visible light. There is also a need for an improved filtering apparatus that can be switched easily between infrared and visible illumination modes without modifying the searchlight housing or reflector. Further, there is a need for a multi-mode filter with improved light distribution and efficiency.