Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
1997-09-18
2003-05-06
Ho, Tuan (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S317000, C348S314000, C348S362000
Reexamination Certificate
active
06559889
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a solid-state imaging apparatus, a driving method therefor, and a camera using a solid-state imaging apparatus as an imaging device. In particular, the invention relates to a solid-state imaging apparatus that uses a CCD (charge coupled device) solid-state imaging device having the overflow drain (OFD) structure (hereinafter referred to as “CCD imaging device”), a driving method therefor, and a camera that is equipped with a light interrupting means such as a mechanical shutter.
As for readout schemes of the CCD imaging device,
FIG. 1
shows what is called the field readout scheme which realizes interlace scanning by causing each pixel to accumulate signal charge for {fraction (1/60)} second (corresponding to one field), and mixing, in the vertical CCDs, signal charges read out from the respective pixels such that signal charges of two pixels adjacent to each other in the vertical direction are mixed together, with the manner of combining two pixels in the vertical direction for mixing changed for odd fields and even fields.
FIG. 2
shows what is called the frame readout scheme in which signal charges of odd lines and signal charges of even lines are read out alternately on a field-by-field basis with an accumulation time of {fraction (1/30)} second.
FIG. 3
shows what is called the all pixels readout scheme in which signal charges of the respective pixels are read out independently at the same time point without being mixed in the vertical CCDs.
Among the above readout scheme, the field readout scheme is suitable for use in a video camera or the like. This is because the signal charge accumulation time in each pixel of the field readout scheme is {fraction (1/60)} second, i.e., a half of {fraction (1/30)} second of the frame readout scheme, and hence the field readout scheme enables satisfactory imaging of moving pictures. On the other hand, the field readout scheme has a drawback of low vertical resolution, which results from the mixing of signal charges of two pixels that are arranged vertically. For this reason, the frame readout scheme with the use of a mechanical shutter or the all pixels readout scheme is used in an electronic still camera which also requires high vertical resolution.
In the frame readout scheme, signals of two fields are needed to obtain one image (one-frame image). On the other hand, to obtain one image by a single exposure as is the case of an electronic still camera, it is necessary that an image remain the same in two fields. To this end, it is necessary to effect light interruption after completion of an exposure by using a light interrupting means such as a mechanical shutter.
However, when light interruption is effected after completion of an exposure in a CCD imaging device having the overflow drain structure, part of the signal charges are emitted to the overflow drain in the following manner. That is, as time elapses, due to thermal fluctuation, signal charges having energies higher than the Fermi level of the accumulated signal charges go over the overflow barrier until the Fermi level becomes close to a given value.
Therefore, the saturation signal charge amount Qs decreases over time during a period from closing of the mechanical shutter to readout of signal charges. In the second field, the period from closing of the mechanical shutter to readout of signal charges is longer than in the first field by a time corresponding to one field, and hence the saturation signal charge amount Qs further decreases as much. Thus, a level difference occurs between the saturation signal charge amounts Qs of the first and second fields. The decrease in saturation signal charge amount Qs is unfavorable because it will deteriorate such characteristics as the S/N ratio and the dynamic range.
SUMMARY OF THE INVENTION
The present invention has been made in view of the above problems in the art, and an object of the invention is therefore to provide a solid-state imaging apparatus, a driving method therefor, and a camera which prevent such characteristics as the S/N ratio and the dynamic range from being deteriorated due to decrease in saturation signal charge amount that occurs in the period from completion of an exposure to readout of signal charges.
A solid-state imaging apparatus according to the invention comprises a solid-state imaging device having a plurality of sensor sections that have an overflow drain structure and are arranged in matrix form, and a charge transfer section for transferring signal charges read out from the sensor sections; and a driving system for performing a drive so as to lower an overflow drain bias of the solid-state imaging device at least during a signal charge readout period in a given operation mode.
In a solid-state imaging apparatus comprising a solid-state imaging device having a plurality of sensor sections that have an overflow drain structure and are arranged in matrix form, and a charge transfer section for transferring signal charges read out from the sensor sections, a driving method according to the invention lowers an overflow drain bias of the solid-state imaging device at least during a signal charge readout period in a given operation mode.
A camera according to the invention comprises a solid state-imaging apparatus comprising a solid-state imaging device having a plurality of sensor sections that have an overflow drain structure and are arranged in matrix form, and a charge transfer section for transferring signal charges read out from the sensor sections, and a driving system for performing a drive so as to lower an overflow drain bias of the solid-state imaging device at least during a signal charge readout period in a given operation mode; an optical system for guiding incident light to an imaging area of the solid-state imaging device; and light interrupting means for determining an exposure period by interrupting the incident light to prevent it from reaching the imaging area of the solid-state imaging device.
With the above constitution, part of signal charges in the sensor sections are released to the overflow drain over time during light interruption. To avoid this phenomenon, the overflow drain bias is lowered in anticipation of the above reduction of signal charges at least during signal charge readout period. With this measure, the saturation signal charge amount (in terms of the value before it decreases over time during the light interruption) can be increased in anticipation of the amount of its reduction. As a result, a desired saturation signal charge amount can be secured.
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Hirota Isao
Tanaka Hiroaki
Ho Tuan
Sonnenschein Nath & Rosenthal
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