Radiant energy – Photocells; circuits and apparatus – Photocell controlled circuit
Reexamination Certificate
2001-02-14
2003-12-02
Allen, Stephone (Department: 2878)
Radiant energy
Photocells; circuits and apparatus
Photocell controlled circuit
C313S524000
Reexamination Certificate
active
06657178
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to devices and methods to image or detect useful images at low light levels utilizing passive pixel sensors in an electron bombarded mode using a photocathode for detection or imaging at low light levels.
BACKGROUND OF THE INVENTION
The copending parent application is directed to the use of active pixel sensors in creating images, particularly of low light level subjects. Active pixel sensor devices comprise a structure or system in which there is gain associated with each pixel in the production of viewable images. Although the use of active pixel sensors enables the production of images from very low light sources or the production of image frames at speeds extending present day capabilities of imaging at low light levels, the use of passive pixel sensors improves upon the sensitivity of certain active pixel sensor systems and thus can produce improved performance in certain low light level conditions. In imaging in which electrons strike the front surface of the pixel, those striking the surface of an active pixel sensor must pass through more transistors to be recognized as compared to the number of transistors encountered in a passive system. This is meaningless if the losses that occur are not important. However, in those systems where each electron is important to the final result and bombardment occurs at the front surface, then a passive system is likely to show less loss as compared to an active one. On the other hand if the amplification of the incoming bombarding electrons is more important to the results than the losses that may be incurred, then an active pixel sensor is to be preferred.
Additionally the use of passive pixel sensors simplifies the making or manufacture of the resulting system. These advantages will become more apparent as this invention is fully discussed hereinafter. For a complete understanding and discussion of the use of active pixel sensor systems, there is incorporated herein by reference the disclosure appearing in Ser. No. 09/356,800, the parent of this application.
Cameras that operate at low light levels have a number of significant applications in diverse areas. These include, among others, photographic, night vision, surveillance, and scientific uses. Modern night vision systems, for example, are rapidly transforming presently used direct view systems to camera based arrangements. These are driven by the continued advances in video display and processing. Video based systems allow remote display and viewing, recording, and image processing including fusion with other imagery such as from a forward looking infra-red sensor. Surveillance applications are also becoming predominately video based where camera size, performance, and low light level sensitivity are often critical. Scientific applications require cameras with good photon sensitivity over a large spectral range and high frame rates. These applications, and others, are driving the need for improved low light level sensors with the capability of a direct video output.
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. Cameras in the art have utilized Charge Coupled Devices (CCD) as the image sensor. Image sensors which incorporate an amplifier into each pixel for increased sensitivity are known as active pixel sensors (sometimes referred to herein as APS). Image sensors without an amplifier incorporated in each pixel are known as passive pixel sensors (sometimes referred to herein as PPS). Both APS and PPS imagers belong to the general family of image sensing devices known as CMOS imagers. 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. Passive pixel sensors are disclosed, for example in U.S. Pat. No. 3,465,293 to Weckler; U.S. Pat. No. 4,631,417 to Brilman; and U.S. Pat. No. 5,345,266 to Denyer. Extensive background on passive and active pixel sensor devices is contained in the paper by Fossum, “CMOS Image Sensors: Electronic Camera-On-A-Chip”, IEEE Transactions on Electron Devices, Vol. 44, No. 10, pp. 1689-1698, (1997) and the references therein.
In general, it is desirable to provide cameras which generate high quality images over a wide range of light levels including extremely low light levels such as those encountered under starlight and lower illumination levels. In addition, the camera should have a small physical size and low electrical power requirements, thereby making portable, head-mounted, and other battery-operated applications practical. CMOS image sensor cameras (both APS and PPS) meet the small size and low power requirements, but have poor low light level sensitivity with performance limited to conditions with 0.1 lux (twilight) or higher light levels. Generally APS image sensors have greater sensitivity than PPS image sensors due to the inclusion of amplification in each pixel but amplification, as discussed above requires more transistors per pixel which in turn can result in more photon losses for optical imagers and electron losses for electron sensitive CMOS imagers, which can destroy utility for some applications.
Night vision cameras which operate under extremely low light levels are known in the art. The standard low light level cameras in use today are based on a Generation-III (GaAs photocathode) or Generation-II (multi-alkali photocathode) image intensifier fiber optically coupled to a CCD to form an Image Intensified CCD or ICCD camera. The scene to be imaged is focused by the input lens onto the photocathode faceplate assembly. The impinging light energy liberates photoelectrons from the photocathode to form an electron image. The electron image may, for example, be proximity focused onto the input of the microchannel plate (MCP) electron multiplier, which intensifies the electron image by secondary multiplication while maintaining the geometric integrity of the image. The intensified electron image may also be proximity focused onto a phosphor screen, which converts the electron image back to a visible image, which typically is viewed through a fiber optic output window. A fiber optic taper or transfer lens then transfers this amplified visual image to a standard CCD sensor, which converts the light image into electrons which form a video signal. In these existing prior art ICCD cameras, there are five interfaces at which the image is sampled, and each interface degrades the resolution and adds noise to the signal of the ICCD camera. This image degradation which has heretofore not been avoidable, is a significant disadvantage in systems requiring high quality output. The ICCD sensor tends also to be large and heavy due to the fused fiber optic components. A surveillance system having a Generation-III MCP image intensifier tube is described, for example, in U.S. Pat. No. 5,373,320 issued Dec. 13, 1994 to Johnson et al. A camera attachment described in this patent converts a standard daylight video camera into a day
ight video camera.
In addition to image degradation resulting from multiple optical interfaces in the ICCD camera a further disadvantage is that the MCP is a relatively noisy amplifier. This added noise in the gain process further degrades the low light level image quality. The noise characteristics of the MCP can be characterized by the excess noise factor, Kf. Kf is defined as the ratio of the Signal-to-Noise power ratio at the input of the MCP divided by the Signal-to-Noise power ratio at the output of the MCP after amplification. Thus Kf is a measure of the degradation o
Allen Stephone
Cole Stanley Z.
Intevac, Inc.
LandOfFree
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