Removal of dark current spikes from image sensor output signals

• US 3,904,818 A
• Filed: 02/28/1974
• Issued: 09/09/1975
• Est. Priority Date: 02/28/1974
• Status: Expired due to Term

• Current Assignee: Michael George Kovac
• Original Assignee: Michael George Kovac
• Inventors: Kovac, Michael George
• Primary Examiner(s): Richardson, Robert L.
• Application Number:
  US05/446,893
• Time in Patent Office: 558 Days
• Field of Search: 178/7.1,7.2,DIG.26 250/211J,211R,578 315/169R
• US Class Current: 348/243
• CPC Class Codes: H04N 25/63 applied to dark current

Abstract

There is produced for each location of an image sensor, such as an X, Y addressed photodiode array or a self-scanned charge transfer array, a signal indicative of the dark current amplitude at that location. During operation of the sensor, when a location producing excessive dark current is read out, rather than using the signal stored at that location, there is instead substituted a signal level which is the average of that stored in adjacent locations of the sensor.

Claims

1. A circuit for processing the signals produced by an image sensor in response to photoexcitation of the image receiving locations of said sensor comprising, in combination:

means for producing for each location of the image sensor, a control signal indicative of the amplitude of the dark current at that location; and

means responsive to the control signal indicative of dark current at a location and the signals, hereinafter termed '"'"''"'"''"'"''"'"'information'"'"''"'"''"'"''"'"' signals, produced at adjacent locations when exposed to photo-excitation, for substituting for the information signal produced at a location having a dark current component which exceeds a given threshold level a second signal of an amplitude related to that of the information signals present in said adjacent locations.

2. A circuit as set forth in claim 1 in which said means for producing a control signal indicative of dark current comprises means for operating the sensor in the dark for a given interval of time to produce at each location a dark current signal.

3. A circuit as set forth in claim 2 wherein said means for producing a control signal indicative of dark current comprises a shutter in the path of the radiation creating said photo-excitation and means for periodically closing said shutter.

4. A circuit as set forth in claim 3 wherein said shutter comprises an electro-optic shutter.

5. A circuit as set forth in claim 1 wherein the means for producing a control signal indicative of the amplitude of the dark current comprises means for comparing the amplitude of the information signal produced at each location with the amplitude of the information signals produced at the locations on each side thereof, and when the difference between them, in both cases, is greater in a given sense than a given amount, producing a signal to so indicate.

6. A circuit as set forth in claim 1 wherein said image sensor comprises an array which includes locations arranged in columns and rows and wherein the means for producing a control signal indicative of dark current comprises means for permitting the sensor to accumulate signals in the dark for a given interval of time;

means for removing these dark current signals from the sensor a row at a time; and

means for comparing the removed signals, one at a time, with a threshold level for ascertaining which of the removed signals exceed said threshold level.

7. A circuit as set forth in claim 6, wherein said means for substituting comprises circuit means receptive of two signals, proportional to an information signal taken from a location adjacent to and on one side of a particular location exhibiting dark current of greater than a given value and the second proportional to aN information signal taken from a location adjacent to and on the other side of said particular location, for producing an output signal proportional to the average value of said two information signals.

8. A circuit as set forth in claim 1 wherein said image sensor comprises a photodiode array, and wherein said means for producing a control signal indicative of dark current comprises means for charging a row of said photodiodes, during one portion of a line time, to a back bias voltage level such as to enhance the production of dark current relative to production of photoexcited current and, means for reading from said row of photodiodes the dark currents stored therein, after a given dark current integration time still within said line time and during which said array is exposed to said photoexcitation.

9. A circuit for processing the signals produced by an image sensing array in response to photo-excitation of said array comprising, in combination:

means for exposing said array to said image to produce a charge pattern corresponding to said image;

first and second storage means;

means for transferring at least a portion of the charge pattern stored in said array to said first storage means;

means for obtaining a charge pattern of the dark currents produced in the same region of said array from which said charge pattern is transferred;

means for transferring said charge pattern of dark currents from said same region of said array to said second storage means;

means for sequentially reading the contents of said second storage means for producing a control signal indicative of dark current amplitude;

an output terminal; and

means for sequentially reading the contents of said first storage means concurrently with the readout of said second storage means and responsive to said control signal for applying the signals read from said first storage means to said output terminal in response to a value of said control signal indicative of dark current of lower than a given level and for applying a second signal having a value close to that of adjacent signals read from said first storage means to said output terminal, in response to a second value of said control signal indicative of dark current of greater than a given level.

10. A circuit as set forth in claim 9 wherein said means for obtaining a charge pattern of dark currents comprises means for maintaining at least said region of the array in the dark for a given interval of time.

11. A circuit as set forth in claim 9 wherein said means for obtaining a charge pattern of dark currents comprises a plurality of switches, one at each location in said region, means for concurrently closing the switches in said region and applying through each closed switch a voltage to charge the image sensing means at each location to a level at which the dark current production is enhanced many times more than the photoexcitation current and then opening said switches, and means for then permitting the dark current to integrate for a given interval of time.