Night vision systems are used in a wide variety of military, industrial and residential applications to enable sight in a dark environment. For example, night vision systems are utilized by military aviators during nighttime flights. Security cameras use night vision systems to monitor dark areas and medical instruments use night vision systems to alleviate conditions such as retinitis pigmentosis (night blindness).
Image intensifier devices are employed in night vision systems to convert a dark environment to an environment perceivable by a viewer. More specifically, the image intensifier device within the night vision system collects tiny amounts of light in a dark environment, including the lower portion of the infrared light spectrum present in the environment, which may be imperceptible to the human eye. The device amplifies the light so that the human eye can perceive the image. The light output from the image intensifier device can either be supplied to a camera, external monitor or directly to the eyes of a viewer. The image intensifier device is commonly employed in vision goggles that are worn on a user's head for transmission of the light output directly to the viewer. Accordingly, since the goggles are worn on the head, they are desirably compact and light weight for purposes of comfort and usability.
Image intensifier devices include three basic components mounted within a housing, i.e. a photocathode (commonly called a cathode), a microchannel plate (MCP), and a phosphor screen (commonly called a screen, fiber-optic or anode). The photocathode detects a light image and converts the light image into a corresponding electron pattern. The MCP amplifies the electron pattern and the phosphor screen transforms the amplified electron pattern back to an enhanced light image.
Referring to FIG. 1, a current state of the art Generation III (GEN III) image intensifier tube 10 is shown. Examples of the use of such a GEN III image intensifier tube in the prior art are exemplified in U.S. Pat. No. 5,029,963 to Naselli, et al., entitled REPLACEMENT DEVICE FOR A DRIVER'S VIEWER and U.S. Pat. No. 5,084,780 to Phillips, entitled TELESCOPIC SIGHT FOR DAYLIGHT VIEWING. The GEN III image intensifier tube 10 shown, and in both cited references, is of the type currently manufactured by ITT Corporation, the assignee herein. In intensifier tube 10 shown in FIG. 1, infrared energy impinges upon photocathode 12. The photocathode 12 is comprised of glass faceplate 14 coated on one side with antireflection layer 16, a gallium aluminum arsenide (GaAlAs) window layer 17 and gallium arsenide (GaAs) active layer 18. Infrared energy is absorbed in GaAs active layer 18, thereby resulting in the generation of electron/hole pairs. The produced electrons are then emitted into vacuum housing 22 through a negative electron affinity (NEA) coating 20 present on GaAs active layer 18.
A microchannel plate (MCP) 24 is positioned within vacuum housing 22, adjacent NEA coating 20 of photocathode 12. Conventionally, MCP 24 is made of glass having a conductive input surface 26 and a conductive output surface 28. Once electrons exit photocathode 12, the electrons are accelerated toward input surface 26 of MCP 24 by a difference in potential between input surface 26 and photocathode 12 of approximately 300 to 900 volts. As the electrons bombard input surface 26 of MCP 24, secondary electrons are generated within MCP 24. The MCP 24 may generate several hundred electrons for each electron entering input surface 26. The MCP 24 is subjected to a difference in potential between input surface 26 and output surface 28, which is typically about 1100 volts, whereby the potential difference enables electron multiplication.
As the multiplied electrons exit MCP 24, the electrons are accelerated through vacuum housing 22 toward phosphor screen 30 by a difference in potential between phosphor screen 30 and output surface 28 of approximately 4200 volts. As is the electrons impinge upon phosphor screen 30, many photons are produced per electron. The photons create the output image for image intensifier tube 10 on the output surface of optical inverter element 31.
FIG. 2 is a schematic representation of image intensifier tube 41. The tube includes photocathode 54, microchannel plate (MCP) 53 and imaging sensor 56. Imaging sensor 56 can be any type of solid-state imaging sensor, such as a CCD device, or a CMOS imaging sensor.
Photocathode 54 can be, but is not limited to, a material such as GaAs, Bialkali, InGaAs, and the like. Photocathode 54 includes input side 54a and output side 54b. MCP 53 has a plurality of channels 52 formed between an input surface and an output surface.
An electric biasing circuit 44 provides a biasing current to image intensifier tube 41. Electric biasing circuit 44 includes a first electrical connection 42 and a second electrical connection 43. First electrical connection 42 provides a biasing voltage between photocathode 54 and MCP 53. Second electrical connection 43 applies a biasing voltage between MCP 53 and imaging sensor 56. In this configuration, photocathode 54, MCP 53, and imaging sensor 56 are maintained in a vacuum body or envelope 61 as a single unit, in close physical proximity to each other.
Still referring to FIG. 2, in operation, light 58, 59 from an image 57 enters image intensifier tube 41 through input side 54a of photocathode 54. Photocathode 54 changes the entering light into electrons 48, which are output from output side 54b of photocathode 54. Electrons 48 exiting photocathode 54 enter channels 52 of MCP 53. Secondary electrons are generated within the plurality of channels 52 of MCP 53. The MCP 53 may generate several hundred electrons in each of channels 52 for each electron entering through the input surface. Thus, the number of electrons 47 exiting channels 52 is significantly greater than the number of electrons 48 that entered channels 52. The intensified number of electrons 47 exit channels 52 and strike the electron receiving surface of imaging device 56. The imaging device transforms the electrons into a light image which may be stored in memory or viewed on display 46.