Television – Camera – system and detail – Solid-state image sensor
Reexamination Certificate
2000-09-07
2004-08-17
Vu, Ngoc-Yen (Department: 2612)
Television
Camera, system and detail
Solid-state image sensor
C348S298000, C348S312000, C348S320000, C348S250000
Reexamination Certificate
active
06778215
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for driving solid-state image pickup devices and an image pickup system and more specifically to a method for driving interlace scanning type solid-state image pickup devices and an image pickup system carrying the solid-state image pickup devices to which such driving method is applied as image pickup devices and controlling an exposure time by using light control means such as a mechanical shutter.
2. Related Art Statement
Digital still cameras (electronic still cameras) are now rapidly disseminating as an image pickup system using solid-state image pickup devices as image pickup devices. The digital still camera uses a so-called total pixel reading out type solid-state image pickup device which reads out signal charges of all pixels simultaneously and transfers the signal charges of the respective pixels independently or a so-called frame reading out type solid-state image pickup device which reads out signal charges of odd and even lines alternately per field and transfers the signal charges of the respective pixels independently is used to give priority to the resolution in taking a still picture.
Signals of two fields are required to obtain one frame of picture when the frame reading out type solid-state image pickup device is used among such reading out type solid-state image pickup devices as the image pickup device. Accordingly, it is required to shade by a mechanical shutter after completing exposure to obtain one picture by one exposure like a digital still camera in order to prevent the images of the first and second fields from changing.
By the way, the charge transfer type solid-state image pickup device typified by the CCD (Charge Coupled Device) image pickup device causes smear and blooming as its unique phenomena. Here, the ‘smear’ is a phenomenon causing whitish stripe noises across the whole angle of field in the vertical direction when there is a highly luminous subject within an picked up image. The blooming is a phenomenon by which signal charges generated by the sensor sections (pixels) become excessive as an excessive quantity of light enters and overflow to the surrounding pixels, thus causing a white part spreading around the sensor sections.
Beside the smear and blooming components, a semiconductor device causes electric charge stored as time elapses regardless of light even in the state in which light is shut off, i.e., a dark signal (dark current), as a noise component. These noise components appear as stationary pattern noises in the picked up image and become a factor of deteriorating the image quality.
Then, an operation of sweeping out the electric charges within the vertical transfer section by transferring and driving the vertical transfer section at speed higher than the normal transfer speed after closing a mechanical shutter (hereinafter referred to as a sweep-out transfer) has been carried out to sweep the dark signal component sprung out within the vertical transfer section and the smear and blooming components leaked from the sensor sections in the prior art CCD image pickup device used as an image pickup device in a digital still camera controlling an exposure time by using the mechanical shutter. After that, the signal charges from the respective sensor sections are read out.
Although it is possible to obtain one image by one time of exposure without using the mechanical shutter in case of the total pixel reading out type solid-state image pickup device, there is a case when the similar sweep-out transfer is carried out also in the total pixel reading out type solid-state image pickup device to remove the dark signal component sprung out in the vertical transfer section and the smear and blooming components leaked from the sensor sections.
In case of the frame reading out type solid-state image pickup device, the sweep-out transfer is carried out by the same number of steps in the first and the second field sides, respectively, from the point of time when the mechanical shutter is closed and the exposure period ends till the time when the signal charge is read out from the sensor section as it is apparent from a timing chart in FIG.
7
. It means that the sweep-out transfer period is equal in the first and the second field sides. That is, it also means that the quantity of handled electric charge in the sweep-out transfer of the first and the second field sides is equal.
However, a quantity of unnecessary electric charge swept out at high speed is different as follows in the first and the second field sides. That is, while the quantity of unnecessary charge swept out at high speed is the dark signal component+smear component+blooming component in the first field side because the smear and blooming components are caused by the input light and are all swept out by the sweep-out transfer in the first field side, only the dark signal component is swept out in the second field side.
Therefore, although there has been totally no problem in the sweep-out transfer in the second field side when an excessive quantity of light enters during the exposure period and when the smear and blooming components increase along that, there has been a problem in the sweep-out transfer in the first field side that the quantity of unnecessary charge to be swept out exceeds the quantity of charge to be handled in the vertical transfer section, thus causing defective sweep.
For example, there is a case of taking a picture of the sun during day time which is otherwise difficult to see directly by eyes because the digital still camera allows its user to take such picture by watching an image projected on a liquid crystal monitor without watching the sun directly by eyes through an optical finder. When such excessive quantity of light like the sun light enters, a quantity of unnecessary charge to be swept out exceeds the quantity of charge to be handled by the vertical transfer section, thus causing the defective sweep which appears as a vertical stripe in the upper part of the screen as shown in FIG.
8
.
It is conceivable to take a method of increasing the quantity of handled charge by prolonging the sweep-out transfer period of the first field side to eliminate such problem. Normally, because the dark signal component<< (smear component+blooming component), it is conceivable of not carrying out the sweep-out transfer in the second field side and of allocating it to the sweep-out transfer period of the first field side when only the smear component+blooming component are to be swept out. In this case, the sweep-out transfer period of the first field side may be doubled without increasing the total sweep-out transfer period.
However, the sweep-out transfer must be carried out also in the second field side due to the following reasons. When the sweep-out transfer period of the second field side is eliminated as shown in the timing chart in
FIG. 9A
, shading in which a dark signal output waveform (
1
) in the first field side has an inclination obliquely as shown in
FIG. 9B
occurs. However, a dark signal output waveform (
2
) of the second field side becomes constant. Thus, the dark signal output waveforms (
1
) and (
2
) are unbalanced in the first and second field sides.
Here, a case of turning on a power supply right after closing the mechanical shutter will be considered to explain it as shown in the timing chart in FIG.
9
A. At first, because the sweep-out transfer for removing the smear components and others is fast, there is less storage time. Accordingly, the dark signal component generated during this sweep-out transfer period is fully smaller than the dark signal component generated in a line shift (vertical transfer of one line) period after that, so that it may be considered to be almost zero. This sweep-out transfer sweeps and removes also the dark signal component in addition to the smear and blooming components.
Then, the dark signal increases as the storage time elapses in the vertical transfer section during the next line s
Nakashima Kazutoshi
Tanaka Hiroaki
Frommer William S.
Frommer & Lawrence & Haug LLP
Sony Corporation
Tillery Rashawn N.
Vu Ngoc-Yen
LandOfFree
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