Black pattern correction for charge transfer sensor

• US 6,144,408 A
• Filed: 12/22/1997
• Issued: 11/07/2000
• Est. Priority Date: 02/24/1995
• Status: Expired due to Term

• Current Assignee: Apple Inc.
• Original Assignee: Eastman Kodak Company
• Inventors: MacLean, Steven D.
• Primary Examiner(s): Flynn, Nathan
• Assistant Examiner(s): SRIVASTAVA, VIVEK
• Application Number:
  US08/995,955
• Time in Patent Office: 1,051 Days
• Field of Search: 348/243, 348/245, 348/251, 348/271
• US Class Current: 348/241
• CPC Class Codes: H04N 23/125 Colour sequential image capture, e.g. using a colour wheel
• CPC Class Codes: H04N 25/63 applied to dark current

Abstract

An electronic imaging device employs black pattern correction for dark current in a charge transfer image sensor. The sensor is composed of image pixels having a characteristic black pattern of dark current in which the amplitude of the dark current for each pixel is dependent upon exposure time. A reference dark frame exposure is captured from the image sensor in the absence of light and dark frame pixel values are obtained. An exposure section regulates the exposure time of image light upon the image sensor and provides a corresponding plurality of image frame exposures; the image sensor thus generates a corresponding plurality of image frames each comprised of image frame pixel values. A processor then generates a correction factor from the dark frame pixel values and applies the correction factor to the image frame pixel values for the plurality of image frames to obtain corrected image frame pixel values that are modified for the black pattern. As a result, performance efficiency is increased by using a single reference dark frame exposure in the correction of many image frame exposures.

Claims

1. A electronic imaging device employing black pattern correction for dark current in an image sensor, said dark current imposing a fixed-pattern noise on image signals obtained by exposing the image sensor to image light from a subject, said imaging device comprising:

a charge transfer image sensor composed of image pixels having a characteristic black pattern of dark current corresponding to the array of pixels in the sensor, the amplitude of the dark current for each pixel being dependent upon exposure time;

means for capturing a reference dark frame exposure from the image sensor in the absence of light and generating dark frame pixel values therefrom;

an exposure section for regulating the exposure time of the image light upon the image sensor and providing a corresponding plurality of image frame exposures in the presence of the image light, which is non-uniform across the sensor, the image sensor generating a corresponding plurality of image frames each comprised of image frame pixel values representative of the non-uniform image light; and

a processor section for generating a unique black pattern dark current correction factor for each image frame based on processing both the dark frame pixel values for the reference dark frame exposure and the image frame pixel values for each image frame exposure, said processor section applying each black pattern correction factor to the image frame pixel values for each of the plurality of image frames to obtain corrected image frame pixel values that are modified for the black pattern due to dark current in the image sensor, wherein a single reference dark frame exposure is used in the correction of many image frame exposures.

2. An electronic imaging device as claimed in claim 1 wherein the correction factor is used to generate a pattern-corrected version of the image frame pixel values.

3. An electronic imaging device as claimed in claim 2 wherein the correction factor is used to scale the dark frame pixel values and the processor section subtracts the scaled version of the dark frame pixel values from the image pixel values to obtain the pattern-corrected version of the image frame pixel values.

4. An electronic imaging device as claimed in claim 1 wherein the correction factor is used to calculate a single scale factor that is used to correct all the image frame pixel values for an image frame exposure.

5. An electronic imaging device as claimed in claim 1 wherein the correction factor is based on a Laplacian operation.

6. An electronic imaging device employing black pattern correction for dark current in an image sensor, said dark current imposing a fixed-pattern noise on image signals obtained by exposing the image sensor to image light from a subject, said imaging device comprising:

a charge transfer image sensor composed of image pixels having a characteristic black pattern of dark current corresponding to the array of pixels in the sensor, the amplitude of the dark current for each pixel being dependent upon exposure time;

means for capturing a reference dark frame exposure from the image sensor in the absence of light and generating dark frame pixel values therefrom;

a memory section for storing the dark frame pixel values corresponding to the dark current amplitude for each pixel from the reference dark frame;

an exposure section for regulating the exposure time of the image light upon the image sensor and providing a corresponding plurality of image frame exposures in the presence of the image light, which is non-uniform across the sensor, the image sensor generating a corresponding plurality of image frames each comprised of image frame pixel values representative of the non-uniform image light; and

a processor section for generating a unique black pattern dark current scaling factor for each image frame from an arithmetic operation on both the dark frame pixel values stored in the memory section and the image frame pixel values generated by the image sensor for each image frame, the operation generating scaled dark frame image values that are subtracted from the image frame pixel values for each image frame to obtain corrected image frame pixel values that are modified for the black pattern due to dark current in the image sensor, wherein a single reference dark frame exposure is used in the correction of many image frame exposures.

7. An electronic imaging device as claimed in claim 6 wherein the operation performed by the processor section calculates a single scale factor that is used to correct all the image frame pixel values for an image frame exposure.

8. An electronic imaging device as claimed in claim 6 wherein the operation performed by the processor section is based on a Laplacian operation.

9. A method of black pattern correction for an image sensor in order to remove the effect of dark current pattern noise from an array of image pixels, said dark current pattern noise imposed on image signals obtained by exposing the image sensor to image light from a subject, said method comprising the steps of:

exposing the sensor in the absence of light for a reference exposure time in order to capture a dark frame exposure and generate dark frame image pixels;

exposing the sensor in the presence of the image light for a variable exposure time in order to capture a plurality of image frame exposures in the presence of the image light, which is non-uniform across the sensor, and to generate a plurality of frames of image frame pixel values therefrom representative of the non-uniform image light;

generating a unique black pattern dark current correction factor for each image frame based on processing both the dark frame pixel values from the dark frame exposure and the image frame pixel values for each image frame exposure; and

applying the correction factor to the image frame pixel values from the plurality of frames to obtain corrected image frame pixel values that are modified for the black pattern due to dark current in the image sensor.

10. A method of black pattern correction for an image sensor in order to remove the effect of dark current pattern noise from an array of image pixels, said dark current pattern noise imposed on image signals obtained by exposing the image sensor to image light from a subject, said method comprising the steps of:

exposing the sensor in the absence of light for a reference exposure time in order to capture a dark frame exposure;

saving dark frame pixel values from the dark frame exposure;

exposing the sensor in the presence of the image light for a selected exposure time in order to capture an image frame exposure in the presence of the image light, which is non-uniform across the sensor, and generate image frame pixel values therefrom representative of the non-uniform image light;

generating a unique black pattern dark current scale factor for each image frame exposure by an arithmetic operation on both the dark frame pixel values and the image frame pixel values;

modifying the dark frame pixel values with the scale factor to generate scaled dark frame pixel values unique for each image frame exposure; and

applying the scaled dark frame pixel values to the image frame pixel values to remove the dark current pattern noise.

11. An electronic imaging device employing black pattern correction for dark current in an image sensor, said dark current imposing a fixed-pattern noise on image signals obtained by exposing the image sensor to image light from a subject, said imaging device comprising:

a charge transfer image sensor composed of image pixels having a characteristic black pattern of dark current corresponding to the array of pixels in the sensor, the amplitude of the dark current for each pixel being dependent upon exposure time;

means for capturing a reference dark frame exposure from the image sensor in the absence of light and generating dark frame pixel values therefrom;

an exposure section for regulating the exposure time of the image light upon the image sensor and providing an image frame exposure in the presence of the image light, which is non-uniform across the sensor, the image sensor generating an image frame comprised of image frame pixel values representative of the non-uniform image light; and

a processor section for generating a unique black pattern dark current correction factor for the image frame based on processing both the dark frame pixel values for the reference dark frame exposure and the image frame pixel values for the image frame exposure, said processor section applying the black pattern correction factor to the image frame pixel values for the image frame to obtain corrected image frame pixel values that are modified for the black pattern due to dark current in the image sensor.

12. An electronic imaging device as claimed in claim 11 wherein the correction factor is used to generate a pattern-corrected version of the image frame pixel values.

13. An electronic imaging device as claimed in claim 12 wherein the correction factor is used to scale the dark frame pixel values and the processor section subtracts the scaled version of the dark frame pixel values from the image pixel values to obtain the pattern-corrected version of the image frame pixel values.

14. An electronic imaging device as claimed in claim 11 wherein the correction factor is used to calculate a single scale factor that is used to correct all the image frame pixel values for the image frame exposure.

15. An electronic imaging device as claimed in claim 11 wherein the correction factor is based on a Laplacian operation.