ERROR CORRECTION ARRANGEMENT FOR IMAGERS

Abstract: Two or more CCD imagers (12, 14), which have random defects are optically coupled to form a single image. The CCD's are in registry. Because of the random nature of the location of the defects, the defective photosensors of one imager (12) are aligned with good photosensors of another (14). Memories (34, 36) keep track of which locations of which CCD are defective. The imagers (12, 14) are operated synchronously and the signals from good photosensor locations are summed for improved signal-to-noise ratio. When a location is addressed at which one imager (12) has a defective photosensor, the respective memory (34) decouples it from the good photosensor of the other imager (14). The signal level is restored for that location by increasing the gain of a preamp (46) by the correct amount. If the imagers (12, 14) are made from the same mask, they may have almost identically corresponding defective photosensors. Then one imager (12) is mounted upside-down relative to the other (14) and scanned backwards so the defects do not coincide.

Description: ERROR CORRECTION ARRANGEMENT FOR IMAGERS The present invention relates to imagers having discrete photosensors. More particularly this invention relates to an imaging method and apparatus using such imagers where there may be defective photosensors . Present CCD (charge coupled device) imagers suffer from low yields due to imperfections and defects in the integrated- circuit chips from which they are fabricated. Various schemes have been used to correct for the signal disturbances that result from such defects. For example U.S. Patent No. 3,904,818 shows a system that in effect does dropout compensation. The system detects when a photosensor provides excessive dark current, and if so, substitutes a signal that is the average of signals provided by photosensors surrounding the defective one. However, an average of signals from surrounding photosensors may not be a close enough approximation to be satisfactory. Other photosensors in the area may also be affected by the defect and averaging may not produce an- acceptable correction.- Other schemes use a ROM (read only memory) programmed with correction factors for the signal from each photosensor. However, if a particular photosensor is totally defective, the signal therefrom cannot be corrected. It is therefore an object of the present invention to correct for the effects of defective photosensors of a discrete photosensor imager. The present invention is defined by an imaging method and apparatus comprising imaging a scene onto a plurality of imagers having a plurality of discrete photo¬ sensors, the photosensors being subject to defects; obtaining signals from the imagers, the signals being subject to portions which are not representative of the scene due to the defects; and combining the signals to reduce the visible effects of such portions. In the attached drawings : FIGURE 1 is a block diagram of a first embodiment of the invention; and FIGURE 2 is a block diagram of a second embodiment of the invention. Referring to the drawings, FIGURE 1 shows a block diagram of the invention wherein light 60 from a scene being viewed passes through a lens 10 into a prism arrangement 16. Arrangement 16 comprises two contiguous triangular cross-section prisms with a semi-reflecting half-silvered mirror 18 disposed therebetween. Imagers 12 and 14 comprise discrete photosensors and mechanisms for scanning the- photosensors to produce a signal representative of the light incident thereon, as in CCDs or photodiode arrays, for example. In accordance with one aspect of the invention, they need not be defect free and therefore are inexpensive. The imagers 12 and 14 are bonded in registration to two perpendicular sides of the arrangement 18 and at 45 degree angles with respect to the mirror 18. Thus half the light 60 is applied to each of the imagers 12 and 14. The lens 10 is designed and spaced from the arrangement 18 so that light froπ. the scene will be in focus in the plane of the photosensors (not shown) of the imagers 12 and 14. An arrangement for bonding photosensor array to a prism with the imagers in registration is described in U.S. Patent Application Serial No. 180,897 filed August 25, 1980 in the name of S. . Bendell. A clock generator 38 synchronously causes scanning to take place within the imagers 12 and 14 so that both imagers are simultaneously providing a signal from corresponding photosensors. Most of the tine there will be good signals from both imagers 12 and 14, and therefore it is desired to add the signals from the two imagers together for improved signal-to-noise ratio. To achieve this, the signals from the imagers 12 and 14 are applied to switches 22 and 24 respectively. The switches, the positions of which are independently controlled by the signals on conductors 30 and 32 respectively, are normally in position A as shown. The output signals from the o:..p. switches 22 and 24 are applied to an adder 40 including resistors 42 and 44. The output of the adder 40 is coupled 5 to ' an inverting amplifier 46. Resistors 48 and 50 normally both provide negative feedback for the amplifier 46 due to a switch 52 which is in position A as shown. The position of the switch 52 is controlled by the output of an OR gate 54. A video output signal is derived from the output of the amplifier 46. Memories 34 and 36 are programmed with the ^ locations of defective photosensors of the imagers 12 and 14 respectively. For example-, the memorys could be a ROM, which is preprogrammed with the locations of defective photosensors of the particular imager. (See U.S. Patent 4,237,488.) These memories are driven synchronously with the imagers by the clock generator 38. When the memory 34 indicates that a defective photosensor of the imager 12 is being read out, it provides a signal on the conductor 30 that switches the switch 22 to position B (an open terminal) so that no signal from the imager 12 is provided to the adder 40. Simultaneously therewith, the signal on the conductor 30 is passed by the OR gate 54 to the switch 52 to switch it to position B (an open terminal) . Thus only the resistor 48 provides the negative feedback, which feedback is thereby reduced, increasing the gain. By proper selection of the resistors 48 and 50, the gain will increase by a factor of two to maintain peak video signal levels. Similarly, when the memory 36 indicates that a defective photosensor is being read out from the imager 14, a signal is applied to the conductor 32 to the switch 24 to switch it to position B (an open terminal) , thereby preventing the signal from being applied to the adder 40. Further, the signal on the conductor 32 is applied to the OR gate 54 to achieve the gain increase as explained above. Thus at least one imager will always supply a signal to the adder 40, provided that the defects do not coincide, which is normally rare. Thus, in the embodiment of FIGURE 1 when neither photosensor is defective, the signals are D 0'*P\ additively combined. If one of the photosensors is defective, only the signal from the other photosensor is - used, and the gain of the system is doubled so that the peak signal level is maintained constant. In the above discussion it has been assumed that the defective photosensors of the imagers 12 and 14 do not coincide. However, repetitive defects on a die arising 0 from systematic errors, such as mask defects, could produce CCD imagers with systematic defects (defects all in the same location) . In accordance with another aspect of the invention, one of each pair of the imagers 12 and 14 with systematic defects is mounted upside-down relative to the _~ other. This imager is then scanned backwards to eliminate the systematic nature of the defects, making them appear random so as not to occur at the same location. If the hal -silvered mirror 18 divides the light 60 by other than one half, one imager receives more light 20 than the other and the signal from one will be larger than from the other. This may be corrected by changing the values of the summing resistors 42 and 44 to correspond to the differing outputs. A second embodiment of the invention is shown in 25 FIGURE 2, wherein corresponding reference numbers are applied to corresponding elements. In this embodiment, the output signals from the imagers 12 and 14 are applied to an adder 62 and summed together. At least one photo¬ sensor of a pair of corresponding photosensors will provide 30 an output signal, assuming that the defects do not coincide. At each location at which a photosensor of an imager is defective, the signal from the defective imager is averaged with the signal from the corresponding good photosensor of the second imager, thereby reducing the magnitude of the error. Again one of the imagers can be mounted upside down, and scanned backwards, if there are systematic defects. Other embodiments of the invention will be apparent to those skilled in the art. For example, the * imagers may be mounted by means other than bonding to the faces of prisms, and any number of imagers may be used, rather than the two shown. For color operation, an arrangement such as that shown may be used to respond to one or more of the primary colors. Further, in a color camera, two or more (preferably three) imagers can be used, each imager being responsive to a different spectral distribution. Therefore if there is a defect in one imager, a signal from a defect-free imager can be combined with the signal from the defective photo¬ sensor. In particular, the imagers can be responsive to complementary colors, such as magenta, cyan, and yellow, which makes each imager responsive to two primary colors, thereby making the signal combining process more effective.

Claims: WHAT IS CLAIMED IS: 1. Apparatus for generating a video signal representative of light characterized by: a plurality of imagers having a plurality , of discrete photosensors, said photosensors being subject to defects and scanning means for obtaining signals from said photosensors, said signals being subject to portions which are not representative of said scene due to said defects; and means for selectively combining said signals to produce said video signal so as to reduce the visible effects of said portions. 2. An apparatus as claimed in Claim 1, characterized in that said scanning means scans said photosensors of one of said imagers backwards relative to said photosensors of said remaining imagers. 3. n apparatus as claimed in Claim 1, characterized in that said combining means comprises means for adding said signals. 4. An apparatus as claimed in Claim 3, characterized in that said combining means further comprises means for disconnecting said nonrepresentative portion of said signal from said adding means. 5. An apparatus as claimed in Claim 4, characterized in that said disconnecting means comprises a plurality of memories for respectively storing locations of said defective photosensors in said plurality of imagers; and a plurality of switches respectively coupled to said plurality of memories and imagers. OMPI 6. An apparatus as claimed in Claim 3, characterized in that said combining means further comprises an amplifier, and means for changing the amplification of said amplifier when said nonrepresentative portion of said signal .is disconnected from said adding means. 7. An apparatus as claimed in Claim 6, characterized in that said amplification changing means comprises means for changing the amount of negative feedback applied to said amplifier. 8. An apparatus as claimed in Claims 1 or 5, characterized in that said plurality of imagers, plurality of memories, and plurality of switches are all two in number. 9. A method for generating a video signal representative of light from a scene characterized by steps of: 1. an image scene onto a plurality of imagers having a plurality of discrete photosensors; said hotosensors being subject to defects; 2. scan said photosensors of said imagers to obtain signals from said imagers, said signals being subject to portions which are not represenative of the scene due to said defects, and 3. combine said signals to produce said video signal so as to reduce the visible effects of said portions. 10. A method as claimed in Claim 9, characterized in that said, step 2 comprises scanning the photosensors of one of said imagers backwards relative to said remaining imagers. O PI 11. A method as claimed in Claim 9, characterized in that said step 3 comprises adding said signals. 12. A method as claimed in Claim 11 characterized in that 'said step 3 further comprises preventing said nonrepresentative portion of said signal from being added. 13. A method as claimed in Claim 12, characterized in that said step 3 further comprises changing the amplitude of said video signal when preventing said nonrepresentative portion of said signal from being combined. Q. PI

Inventors: DISCHERT ROBERT ADAMS, MEISE WILLIAM HARRY
Applicants: RCA CORP
Application year: 1982
Main classification: H04N5
IPC codes: H04N5, H04N9