A conventional solid-state image sensing element such as a CCD (Charge Coupled Device) suffers a so-called defective pixel in which the sensitivity is lower than that of a nondefective pixel or a very large dark current flows. Such defective pixel appears as isolated point noise in an image sensed by the solid-state image sensing element, degrading the image quality. In order to eliminate an adverse effect on the image quality, an image sensing apparatus using the solid-state image sensing element corrects a defective pixel.
As conventional defective pixel correction methods, the following two methods have generally been known.
According to the first correction method, the position of a defective pixel is detected in shipping an image sensing apparatus from the factory. The information is stored in a nonvolatile memory such as an EEPROM (Electrically Erasable and Programmable Read Only Memory) incorporated in the image sensing apparatus. The defective pixel is corrected by replacing it with a pixel interpolated by peripheral pixels on the basis of the information during image sensing operation.
According to the second method, a pixel having an abnormal output is detected as a defective pixel on the basis of the signal level difference between adjacent pixels during image sensing operation. The defective pixel is replaced with a pixel interpolated by peripheral pixels. As a concrete example of the second method, Japanese Patent Laid-Open No. 11-239298 discloses an electronic camera capable of properly correcting white scratches which increase/decrease in accordance with the exposure time.
The first correction method achieves a high detection precision because stable image sensing conditions can be set in detecting a defective pixel. However, this method requires a nonvolatile memory for storing pieces of position information corresponding to the number of assumed defective pixels. This results in high cost and high power consumption.
The second correction method does not require any nonvolatile memory for storing position information of a defective pixel. However, if a defective pixel is detected while an image such as a natural image (image with a complicated brightness difference or color saturation difference) is sensed, the defective pixel detection precision decreases, and the image quality degrades due to correction based on erroneous detection.