Method and apparatus for reduction of residual signal in digital images captured by image sensors

1. A method for reducing residual signal generated during a first image exposure and released during the image exposure and readout of a second image, the method comprising the steps of:

(a) flushing an image sensor using a predefined fixed flush time;

(b) after flushing with the fixed flush time, capturing the first image on an the image sensor having a known exposure that includes exposure time and a peak light intensity;

(c) after the first image is captured, reading out the first image;

(d) estimating a flush time for the second image based on the known exposure in capturing the first image;

(e) after the first image is read out, flushing the image sensor using the estimated flush time for the second image; and

(f) capturing the second image, wherein the estimated flush time for the second image substantially reduces the residual signal in the second image.

2. The method as in claim 1 , wherein step (d) includes providing either lookup tables or fit-curves in estimating the flush time.

3. The method as in claim 2 , wherein step (d) includes providing the lockup tables or fit-curves each representing the relationship between the residual signal and exposure.

4. The method as in claim 3 , wherein step (d) includes calculating the flush time based on a predetermined threshold of a tolerance of the residual signal.

5. The method as in claim 4 , wherein the lookup tables each includes values of the residual signal versus the flush time, and the fit-curves each includes parameters used to interpolate a relationship between the residual signal versus the flush time.

6. The method as in claim 1 , further comprising the step of (g) reading out the second image.

7. The method as in claim 6 , further comprising repeating for each subsequent image to be captured the steps of:

(h) estimating a flush time for a subsequent image based on a known exposure in capturing a previous image;

(i) after the previous image is read out, flushing the image sensor using the estimated flush time for the subsequent image;

(j) capturing the subsequent image; and

(k) reading out the subsequent image.

8. A digital camera for reducing residual signal generated during a first image exposure and released during the image exposure and readout of a second image, the digital camera comprising:

(a) an image sensor that captures the first image having a known exposure that includes exposure time and a peak light intensity; and

(b) a microcontrol processor that estimates a flush time for the second image based on the known exposure in capturing the first image and determines whether to use the estimated flush time or a maximum allowable flush time; wherein the image sensor captures the second image after the image sensor is flushed using either the estimated flush time or the maximum allowable flush time for substantially reducing the residual signal in the second image.

9. The digital camera as in claim 8 further comprising onboard memory that includes either lookup tables or fit-curves used in estimating the flush time.

10. The digital camera as in claim 9 , wherein the lookup tables or fit-curves comprises a plurality of curves each representing the relationship between the residual signal and exposure.

11. The digital camera as in claim 10 , wherein the microcontrol processor calculates the flush time based on a predetermined threshold of a tolerance of the residual signal.

12. The digital camera as in claim 11 , wherein the lookup tables each includes values of the residual signal versus the flush time, and the fit-curves each includes parameters used to interpolate a relationship between the residual signal versus the flush time.

13. A method for reducing residual signal generated during a first image exposure and released during the image exposure and readout of a second image, the method comprising the steps of:

(a) flushing an image sensor using a predefined fixed flush time;

(b) after flushing the image sensor with the fixed flush time, capturing the first image using a known exposure that includes exposure time and a peak light intensity;

(c) after the first image is captured, reading out the first image;

(d) estimating a residual signal level using the known exposure in capturing the first image, and based on the estimated residual signal level, estimating a flush time for the second image;

(e) determining whether to use the estimated flush time or a maximum allowable flush time;

(f) after the first image is read out, flushing the image sensor using the flush time determined in step (e); and

(g) capturing the second image.

14. The method of claim 13 , further comprising the step of (h) reading out the second image.

15. The method as in claim 14 , further comprising repeating for each subsequent image to be captured by the image sensor the steps of:

(i) estimating a residual signal level using a known exposure in capturing a previous image, and based on the estimated residual signal level, estimating a flush time for a subsequent image;

(j) determining whether to use the estimated flush time or the maximum allowable flush time;

(k) after the previous image is read out, flushing the image sensor using the flush time determined in step (j);

(l) capturing the subsequent image; and

(m) reading out the subsequent image.

16. The method as in claim 13 , wherein step (d) includes providing either lookup tables or fit-curves in estimating the flush time.

17. The method as in claim 16 , wherein step (d) includes providing the lookup tables or fit-curves each representing the relationship between residual signal levels and exposure times.

18. The method as in claim 17 , wherein step (d) includes estimating the flush time for the second image based on a predetermined threshold of a tolerance of the residual signal level.

19. The method as in claim 18 , wherein the lookup tables each includes values of the residual signal level versus the flush time, and the fit-curves each includes parameters used to interpolate a relationship between the residual signal level versus the flush time.