Television – Camera – system and detail – Low light level
Reexamination Certificate
1999-07-20
2001-10-23
Christensen, Andrew B. (Department: 2612)
Television
Camera, system and detail
Low light level
C250S2140VT
Reexamination Certificate
active
06307586
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to electron bombarded active pixel sensor cameras and, more particularly, to electron bombarded active pixel sensor cameras which operate over a wide dynamic range and which incorporate gain control.
BACKGROUND OF THE INVENTION
Night vision cameras which operate under extremely low light levels are known in the art. Such cameras frequently utilize a microchannel plate or a highly sensitive photocathode for sensing of low light levels. A focused electron bombarded hybrid photomultiplier tube comprising a photocathode, focusing electrodes and a collection anode disposed in a detector body is disclosed in U.S. Pat. No. 5,374,826 issued Dec. 20, 1994 to LaRue et al and U.S. Pat. No. 5,475,227 issued Dec. 12, 1995 to LaRue. The disclosed hybrid photomultiplier tubes are highly sensitive but do not sense images. A surveillance system having a microchannel image intensifier tube is disclosed in U.S. Pat. No. 5,373,320 issued Dec. 13, 1994 to Johnson et al. A camera attachment converts a standard daylight video camera into a day
ight vision video camera.
Image sensing devices which incorporate an array of image sensing pixels are commonly used in electronic cameras. Each pixel produces an output signal in response to incident light. The signals are read out, typically one row at a time, to form an image. Image sensors which incorporate an amplifier into each pixel for increased sensitivity are known as active pixel sensors. Active pixel sensors are disclosed, for example in U.S. Pat. No. 5,789,774 issued Aug. 4, 1998 to Merrill; U.S. Pat. No. 5,631,704 issued May 20, 1997 to Dickinson et al; U.S. Pat. No. 5,521,639 issued May 28, 1996 to Tomura et al; U.S. Pat. No. 5,721,425 issued Feb. 24, 1998 to Merrill; U.S. Pat. No. 5,625,210 issued Apr. 29, 1997 to Lee et al; U.S. Pat. No. 5,614,744 issued Mar. 25, 1997 to Merrill; and U.S. Pat. No. 5,739,562 issued Apr. 14, 1998 to Ackland et al.
In general, it is desirable to provide cameras which generate high quality images and which operate over an extremely large dynamic range, including both daytime and nighttime conditions. In addition, the camera should have a small physical size and low electrical power requirements, thereby making head-mounted and battery-operated applications practical.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, a camera is provided. The camera comprises a photocathode operable in a night mode wherein electrons are generated in response to incident light, an active pixel sensor comprising an array of pixels for sensing electrons in the night mode, a power supply for energizing the photocathode in the night mode in response to a control signal, and a power supply control circuit for providing the control signal to the power supply in response to a sensed incident light level. The power supply provides to the photocathode an average energy level that increases as the sensed incident light level decreases.
Preferably, the power supply comprises a gated power supply, and the control signal comprises a gating signal having a duty cycle that increases as the sensed incident light level decreases. The duty cycle is defined as the fraction of the time, within any given field time of the active pixel sensor, that the gating signal requests the high voltage supply to be in its energized state. The active pixel sensor has an associated field time for sensing an image and for outputting signals representative of the image. The gating signal preferably has a period that corresponds to the field time. The gating signal may be a bi-level signal in which one level corresponds to the state where the high voltage power supply is energized and the other level controls the power supply to its quiescent state. The field time of the active pixel sensor includes readout times when pixels are being read out and non-readout times when pixels are not being read out. The control circuit preferably includes means for switching states of the gating signal during the non-readout times when pixels are not being read out.
In a first embodiment, the control circuit generates the power supply control signal in response to a video output level of the active pixel sensor. In a second embodiment, the image sensor further comprises an electron shielded light detector that is shielded from the electrons generated by the photocathode. The light detector generates a light detector signal in response to incident light, and the control circuit generates the power supply control signal in response to the light detector signal. In a third embodiment, the control circuit generates the power supply control signal based on both the light detector signal and the video output level of the active pixel sensor.
According to another aspect of the invention, the photocathode may be operable in a day mode in which a fraction of the incident light is transmitted through the photocathode and wherein the pixels sense light in the day mode. The power supply control circuit may further comprise means for switching the power supply off in the day mode when the sensed incident light level is greater than a predetermined value and for supplying the control signal to the power supply in the night mode when the sensed light level is less than the predetermined value.
According to yet another aspect of the invention, the camera further comprises a lens positioned between the object being imaged and the photocathode, and a lens position actuator for moving the lens., in response to a position control signal, between a night position wherein an image of the object is focused on the photocathode, and a day position wherein an image of the object is focused on the active pixel sensor. A position control circuit provides the position control signal to the lens position actuator in response to the sensed incident light level. The image sensor may further comprise an electron shielded light detector that is shielded from the electrons generated by the photocathode. The electron shielded light detector generates a light detector signal which is used by the position control circuit to control movement of the lens between the night position and the day position.
In a preferred embodiment, the active pixel sensor, the power supply control circuit and the position control circuit are fabricated on a single substrate.
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Christensen Andrew B.
Cole Stanley Z.
Intevac, Inc.
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