1. Field of the Invention
The present invention relates to a solid-state imaging apparatus, a driving method thereof, and an imaging system.
2. Description of the Related Art
In recent years, the progress of solid-state imaging apparatus has been popularizing inexpensive digital cameras having higher image qualities. In particular, the performance advances of complementary metal oxide semiconductor sensors (CMOS sensors), which have an active element in each of their pixels and can mount peripheral circuitry on each of their chips, have been remarkable, and the CMOS sensors have been replaced with a part of charge coupled devices (CCD sensors). If moving image photographing is performed under the illumination of a fluorescent light or the like when the moving image photographing is performed using a CMOS sensor, then temporal light-dark changes, the so-called fluorescent light flicker, are caused in a pixel signal of a photographing output by the difference between the frequency of the luminance changes of the fluorescent light and the vertical synchronizing frequency (imaging frequency) of the used camera. As described in Japanese Patent Application Laid-Open No. 2005-033616 (hereinafter referred to as Patent Document 1), light-dark changes are caused not only between fields but also within one field in the flicker phenomenon, and sometimes appear as a striped pattern on a screen. The Patent Document 1 proposes the following method in regard to this problem. The method estimates the flicker component by using the continuity of the flicker from an pixel signal from an imaging element, and corrects the pixel signal from the imaging element according to the estimation result to reduce the flicker component.
Moreover, Japanese Patent Application Laid-Open No. 2005-318504 (hereinafter referred to as Patent Document 2) proposes an example of providing a pixel region dedicated for flicker detection differently from the imaging for recording or an ornamental use. The technique disclosed in the Patent Document 2 provides a pair of pixel columns for detecting flicker noises and makes exposure times different from each other.
However, the technique disclosed in the Patent Document 1 includes the restriction of setting an exposure time in a drive mode for reducing flickers (flicker reducing drive mode), and a disadvantage of the impossibility of performing the optimization of the exposure time according to the illuminance of a subject. For example, when photographing of 60 fps is being performed, the maximum exposure time is a value shorter than 1/60 second slightly since one frame is 1/60 second and there is a dead time for readout to be considered. The options of the exposure times are limited to 2/100, 1/100, and 1/120 seconds by the technique mentioned above. A system for entering the flicker reducing drive mode by the technique mentioned above when a flicker component of a light source is detected is tried to be considered. In this case, after the system has entered the flicker reducing drive mode once, the existence or inexistence of the flicker cannot be detected, and the system cannot be escaped from the flicker reducing drive mode even if the flicker component of the light source is eliminated again. Consequently, the reduction of the dynamic range of the system and the deterioration of the signal-to-noise (S/N) ratio of a low illuminance image are caused by the existence of the restriction in the degree of freedom of setting accumulation times. The problem mentioned above is caused by the performance of the detection of a flicker based on the imaged image for recording or the like itself.
Moreover, since the technique proposed by the Patent Document 2 provides the pixel columns unnecessary for imaging and must perform dependent readout processing, the proposed technique increases the cost of the camera and has the adverse effect to the miniaturization of the camera owing to the increase of the chip size thereof. Moreover, since the photographing region in which flicker detection is performed is restricted to a part of the screen, such as the end thereof, it is difficult to heighten the accuracy of detecting a flicker in distinction from a pattern at the time of photographing a moving subject.