JP3141997B2 - Exposure control method - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure control method, and more particularly, to an exposure control method in which an image sensor functions as a shutter in an imaging device such as an electronic still camera.

[Conventional technology]

2. Description of the Related Art In an imaging apparatus such as an electronic still camera, an exposure control method in which an imaging element such as a CCD functions as a shutter is generally used. This method uses an interline transfer type, a frame transfer type, or a frame interline transfer type image sensor.
In an image sensor having a vertical analog shift register as a charge transfer unit, if a readout pulse is given to a photoelectric conversion unit in which pixels are arranged in a matrix at the start of exposure, unnecessary charges accumulated in the photoelectric conversion unit But,
At this point, it is transferred to the vertical shift register,
The transfer pulse applied to the vertical shift register shifts from a normal transfer speed frequency to a high-speed transfer frequency, and unnecessary charges are discharged at a high speed. By this high-speed transfer, the next signal charge is accumulated as effective charge in the photoelectric conversion unit within the time during which the unnecessary charge is being discharged. Then, when the next read pulse is given at the end of the exposure, the effective charges are simultaneously transferred to the vertical shift register, and then sequentially transferred from the vertical shift register to the horizontal shift register by a normal-speed transfer pulse. An image output corresponding to the exposure time is output from the horizontal shift register. Here, the exposure time, that is, the shutter time can be controlled by controlling the timing at which the first read pulse and the next read pulse are applied.

The advantage of this method compared to the method using a mechanical shutter is that accurate exposure control can be performed because there is no mechanical delay. At the start and end of exposure, a time difference occurs at both ends of the imaging screen in the case of a focal plane shutter, and at a position near the center and the outer periphery of the imaging screen in the case of a rotary lens shutter. At the same time, since the exposure starts and ends, the entire imaging screen has a uniform exposure amount.

[Problems to be solved by the invention]

By the way, in the case of an interline type or frame transfer type image sensor, it takes about several hundreds μs to 1 ms for all unnecessary charges to be discharged from the vertical shift register. Therefore, if the exposure time is shorter than the time required for discharging the unnecessary charges, that is, if the shutter time is fast, effective signal charges are transferred to the vertical shift register before the unnecessary charges are discharged. Therefore, effective signal charges and unnecessary charges are mixed, and there is a problem that an image is stained by noise or the like.

On the other hand, in the case of an image sensor having a vertical overflow drain, unnecessary charges are not transferred to the vertical shift register, so that signal charges and unnecessary charges do not mix in the vertical shift register. The potential fluctuation of each part in the image sensor due to emission into the
Since it lasts for 0 microseconds, giving a transfer pulse during this period has an effect such as uneven brightness and has a problem that the video signal is stained by noise or the like.

The present invention has been made in view of such a problem, and an object of the present invention is to provide an exposure control method capable of obtaining a clear image without mixing signal charges and unnecessary charges.

[Means and actions for solving the problem]

In order to achieve the above object, an exposure control method according to the present invention includes a vertical overflow drain for discharging unnecessary charges and a charge transfer unit for transferring signal charges. An exposure control method in which the image pickup device functions as a shutter by simultaneously transferring signal charges accumulated in the photoelectric conversion unit of the device over time to the charge transfer unit at the end of exposure, wherein the vertical overflow In a first time interval until a time point substantially coincident with the end time point of the potential change period depending on unnecessary charge discharge to the drain, charge transfer to the charge transfer unit is prohibited. It is characterized in that charge transfer is allowed in a subsequent second time section.

〔Example〕

FIG. 2 is a block diagram showing a basic system of an electronic still camera to which the exposure control method of the present invention is applied.

A photometric IC 100 including a photometric photoelectric conversion element, a gate array 200, a photometric microcomputer 300, and a system control microcomputer 400 are connected to each other by a bus and other signal lines so as to exchange signals. The gate array 200 is connected to an in-lens circuit 500 for controlling the aperture and a CCD drive circuit 600 including a CCD image sensor.

The gate array 200 performs a predetermined signal processing operation based on transmission and reception of signals with the photometric IC 100 and the photometric microcomputer 300, and transmits and receives signals to and from the photometric block 210 and the photometric microcomputer 300. Circuit 500 and CCD drive circuit 600
A circuit is formed including each block (circuit) of the exposure control block 220 that transmits and receives control signals to and from the exposure control block 220. In the present system, the lens has a diaphragm and its driving means, but the above-described lens circuit 500 can also be configured to include a microcomputer. Photometric microcomputer 30
To 0, a switch circuit 310 for externally performing exposure correction and other function settings by an operator's operation is connected. Similarly, the system control microcomputer 400 includes a switch circuit 410 for selecting a photographing mode such as aperture priority and shutter priority and for triggering photographing.
Is connected.

The outline of the operation of the electronic still camera system is as follows.

A description will be given of a case where the present system performs a pre-metering operation in accordance with a command from the system control microcomputer 400. First, a photoelectric conversion output corresponding to the amount of incident light is obtained by the photometric IC 100, which is performed in a so-called open photometric state in which the aperture is fully opened. The counter corresponding to the photoelectric conversion output is performed by a counter in the photometric block 210 in the gate array 200, and the photometric microcomputer 300 calculates the value of the amount of incident light based on the count value of the counter. The shutter speed and aperture value corresponding to the photometric value are also calculated by the photometric microcomputer 300. These values are transferred to the system control microcomputer 400. The system control microcomputer 400 generates a signal for performing a predetermined display operation related to a shutter speed or an aperture value corresponding to the amount of incident light at that time based on the transferred data. In the photometric microcomputer 300, the count output of the counter of the gate array 200 is monitored, and when there is a possibility that the counter overflows, the counting condition is repeatedly changed and reset.

Next, the exposure control operation will be described. Switch circuit 410
Is pressed, the system control microcomputer 400 gives a command to the photometric microcomputer 300 to switch the system operation from the pre-photometric operation to the exposure control operation. The exposure control operation of the present system is performed by a so-called direct photometry method based on the photoelectric conversion output from the same photometry IC 100 as in the above-described pre-photometry. When the exposure control operation starts, the operation of accumulating the photoelectric conversion signal in the photometric IC 100 is started, and at the same time, the CCD drive circuit 600 is instructed to perform the signal integration operation. The level of the integrated value of the photoelectric conversion signal in the photometric IC 100 (more precisely, the voltage of the capacitor corresponding to the time integral of the current corresponding to the photoelectric conversion) is set for the photometric IC 100 by the photometric microcomputer 300 When the set level is reached, the IC 100 provides an integration end pulse to the photometry block 210 of the gate array 200. The photometry block 210 counts and counts the integration end pulse generation circuit, and immediately after each pulse,
Reset IC100. The integrated count value in the photometry block 210 is controlled by the microcomputer 40 for system control.
When a value (preset value) measured via the photometric microcomputer 300 from 0 is reached, the photometric block 210
Supplies a control signal to the CCD drive circuit 600 via the exposure control block 220. The CCD drive circuit 600 regulates the exposure time based on this control signal, that is, closes the element shutter,
An operation for terminating the exposure is performed, such as stopping the signal charge accumulation operation of the CCD image sensor. Here, the level of the integral value set for the photometric IC 100 from the photometric microcomputer 300 and the integrated count value set for the photometric block 210 of the gate array 200 from the microcomputer 300 are necessary for exposure correction and the like. It may be changed to take an appropriate value accordingly.

The preset value can be set to one or more arbitrary values. For example, if the preset value is set to 2, the exposure ends when the integration end pulse is counted twice. In a shooting mode using a strobe, a strobe light emitting circuit (not shown) is synchronized with an integration end pulse.
Can be started and stopped.

Next, a more detailed configuration and functions of the electronic still camera system will be described with reference to FIG. First
In the figure, parts corresponding to the respective parts in FIG. 2 are denoted by the same reference numerals.

For convenience of explanation, in the explanation based on FIG. 1, an extremely general CC of the form of transferring unnecessary charges to a vertical shift register is used.
D Describes the method in the case of applying the image sensor, and the method-specific features of the present invention in which the image sensor having a vertical overflow drain for discharging unnecessary charges and a charge transfer unit for transferring signal charges is applied. Will be described later.

In the photometric IC 100, both ends of the photodiode 101 are connected to both input terminals of the operational amplifier 102, the inverting input terminal and the output terminal of the operational amplifier 102 are connected to one input terminal of the comparator 103, and the other end of the comparator 103 is connected to the other input terminal. An input terminal is provided with a reference level signal Er. This reference level signal Er is supplied to a D / A converter 30 provided in the photometric microcomputer 300.
This is the output of 1. Further, an integration capacitor 104 is connected between the cathode of the photodiode 101 and the ground. A connection point between the integration capacitor 104 and the photodiode 101 is connected to a positive electrode of a battery 106 that generates a reference voltage Ei via a drain source of a switching FET 105, and a negative electrode of the battery 106 is grounded.

The gate array 200 has the respective circuits of the photometry block 210 and the exposure control block 220, as described in FIG. The photometric block 210 is provided for the comparator 103 of the photometric IC 100.
A charge / discharge control circuit 211 for receiving a pulse-like reset signal RT to the gate of the FET 105 in response to the integration end pulse IE which is an output of the FET 105 and receiving a clock pulse CL from the oscillation circuit 700 in common with the microcomputer 300 for photometry. The frequency divider 212 divides the same pulse by a frequency division ratio according to the frequency division command DV from the photometric microcomputer 300 to produce a count pulse CP, and outputs a photometric / exposure control switching signal MC from the photometric microcomputer 300. In response, the counting pulse CP from the frequency divider 212
And the integration end pulse IE of the comparator 103 of the photometric IC 100
A switching circuit 213 for selectively outputting both or one of them, an 8-bit counter 214 for counting the output pulse SC of the switching circuit 213, and a latch circuit for latching the output of the 8-bit counter 214 and supplying it to the photometric microcomputer 300. 215 respectively. 8-bit counter 214
Receives the integration end pulse IE via the switching circuit 213,
A function of giving a photometric end signal ED to the exposure control block 220 and the photometric microcomputer 300 is added. The preset value from the photometric microcomputer 300 to the 8-bit counter 214 is the integration end pulse IE
The photometry end signal ED is also output when the count value of the 8-bit counter 214 becomes 0 after the subtraction.
The charge / discharge control circuit 211 and the exposure control block 220 have a photometric start command MS from the photometric microcomputer 300.
Is given.

An element shutter control signal SHT is given from the exposure control block 220 to the CCD drive circuit 600 including the CCD image pickup device.
An element shutter end prohibition signal DST is provided from the drive circuit 600 to the exposure control block 220.

Next, the operation of the present system having the configuration shown in FIG. 1 will be described. Since the pre-metering operation of the present system is not directly related to the present invention, the exposure control operation by the direct metering method will be described below with reference to time charts shown in FIGS.

First, when the system control microcomputer 400 switches the present system to the exposure control mode upon detecting that the trigger button has been pressed, the photometry microcomputer 300 receives this and receives a photometry / exposure control switching signal M.
The switching circuit 213 is switched by C. That is, the frequency divider 212
In place of the counting pulse CP, the integration end pulse IE from the comparator 103 is supplied to the 8-bit counter 214 as a counted pulse. Further, a value corresponding to whether the exposure control operation is performed in a normal state or how many exposure corrections are performed is preset from the photometric microcomputer 300 as a preset value PS in the 8-bit counter 214 and the D / A converter 301 The level of the reference level signal Er supplied to the comparator 103 is set.

Now, it is assumed that the preset value PS is set to “1” and the level of the reference level signal Er is also set appropriately. Then, when the photometry start command MS is given from the microcomputer 300 to the charge / discharge control circuit 211 and the exposure control block 220, the charge / discharge control circuit 211 sets the reset signal RT to low level and sends it to the gate of the FET 105 of the photometry IC 100. At the same time, the exposure control block 220
At the falling time t ₀ of the element shutter control signal SH at a high level.
T is sent to the CCD drive circuit 600 to start exposure to the CCD image sensor. That is, the direct photometry operation starts.

When the element shutter control signal SHT rises, unnecessary charges that have been accumulated in the photoelectric conversion unit of the CCD image pickup device are simultaneously transferred vertically to the shift register. The accumulation is started according to.
At this time, the unnecessary charge transferred to the vertical shift register of the CCD image sensor is discharged out of the CCD image sensor by high-speed transfer. At least during the period when the unnecessary charge is discharged, the CCD driving circuit 600 , A high-level element shutter end prohibition signal DST is sent to the exposure control block 220. That is, the element shutter end prohibition period τ of the high level of the element shutter end prohibition signal DST is a period slightly exceeding the unnecessary charge discharging operation period or the synchronous period.

While the reset signal RT is at a high level, the FET 105 conducts. During this time, the integration capacitor 104 is charged to the reference voltage Ei by the current from the battery 106.
As soon as the level changes to low level, the switch by the FET 105 is cut off and the integration operation is started, and the electric charge accumulated in the integration capacitor 104 is incident on the photodiode 101 exposed under the same conditions as the exposure intensity for the CCD image sensor. It is discharged through the photodiode 101 with a current value corresponding to the amount of light. The positive voltage of the integration capacitor 104 is
At the beginning of the completion of charging, the voltage equal to the reference voltage Ei starts to fall at a rate of change corresponding to the discharge current at this time. Accordingly, the output voltage Ec of the operational amplifier 102 also decreases, and this voltage Ec
c is the reference level signal Er set for the comparator 103
It becomes equal to the level of the comparator 103 outputs an integration end pulse IE low level at the same time t _1. This comparator 103
Low-level integration end pulse IE from the charge / discharge control circuit
When input to 211, the falling time t ₁ of the integration end pulse IE
Since the charge / discharge control circuit 211 immediately outputs the high-level reset signal RT, the FET 105 is turned on again, and the integrating capacitor 104 is charged to the reference voltage Ei. The integration end pulse IE is supplied to the 8-bit counter 2 through the switching circuit 213.
The 8-bit counter 214 sends the photometry end signal ED to the exposure control block 220 in synchronization with the fall of the integration end pulse IE.

By the way, since the subject is relatively dark, the time when the photometric end signal ED is transmitted (that is, the time t ₁ when the integration end pulse IE falls) is, as shown in FIG. past the tau, already when the element shutter termination inhibiting signal DST is at a low level, at time t ₁ when the photometric signal ED has been delivered, the exposure control block
Reference numeral 220 immediately sets the element shutter control signal SHT, which has been sent to the CCD drive circuit 600 at a high level, to a low level immediately. When the element shutter control signal SHT becomes low level, in the CCD image sensor, effective signal charges accumulated in the photoelectric conversion unit are transferred to the charge transfer unit. That is, the element shutter function is stopped, and the exposure operation ends.

In addition, as shown in FIG. 4, the subject is very bright, and before the element shutter end prohibition period τ has elapsed,
If the integration end pulse IE photometric end signal ED I falling is sent, the exposure control block 220, without the time t ₁ in the element shutter control signal SHT to the low level, the elapsed element shutter termination inhibition period τ Then, when the element shutter end prohibition signal DST changes from the high level to the low level, the element shutter control signal SHT is changed to the low level to stop the element shutter function of the CCD image sensor and end the exposure operation.

As described above, in the case of the shutter time in which the end time of the exposure is earlier than the end time of the unnecessary charge discharging operation period, the exposure end time is forcibly changed to the time when the unnecessary charge discharging operation period ends completely. I'm delaying. For this reason,
If the shutter time that is appropriate for direct photometry is a high-speed time that is shorter than the unnecessary charge discharging operation period, the shutter time that is longer than the unnecessary charge discharging operation period will be uniquely set. Since the unnecessary signal charges do not mix with the unnecessary charges, the image becomes clear without being stained with noise.

Note that the time width of the element shutter end prohibition signal DST is set to such a degree that unnecessary charges can be sufficiently discharged. Normally, charge discharge is performed before the DST signal rises to a high level in order to finish the charge discharge in a short time.

Here, the change in the positive electrode voltage of the integration capacitor 104 is due to the charge of the capacitor 104 being discharged through the photodiode 101, and the rate of change is proportional to the discharge current value, that is, the current value. It becomes proportional to the amount of light incident on the photodiode 101.
When the time integration value of the discharge current is
Equal to the amount of discharge charge at 1. That is, the capacitance of the integrating capacitor 101 is C, and the voltage of the capacitor is the voltage of the battery 106.
When the current drops from Ei to the reference level Er, the discharge current
i _SC , assuming that the time until the voltage changes from Ei to Er is t, From the above equation (1), Although the discharge current i _SC in the above equation (2) corresponds to the amount of incident light, since the capacitance C of the integration capacitor 101 is constant, the amount of incident light can be determined by measuring the time t. If the amount of incident light fluctuates, the value of the time t in the discharge will be different. Even if the incident light amount is constant, if the value of Er is changed, the value of t will change.

Further, in the above embodiment, the element shutter end prohibition signal DST input to the exposure control block 220 is transmitted to the CCD drive circuit 600.
Although the light is transmitted from the microcomputer 300, the light may be transmitted from the photometric microcomputer 300 as shown by a broken line in FIG. In this case, the element shutter end prohibition signal
Synchronizing DST with the timing signal of the CCD drive circuit 600 is slightly difficult, but the element shutter end prohibition signal τ of the element shutter end prohibition signal DST is
Can be easily changed, so that the element shutter end prohibition signal DST corresponding to the imaging device can be generated even in each electronic still camera having a different CCD imaging device and CCD driving circuit 600.

In addition, the CCD drive circuit outputs an element shutter end prohibition signal DST.
Instead, the CCD drive circuit may be configured not to receive the exposure end transfer pulse during the element shutter end prohibition period. Next, the features specific to the method of the present invention in which an image pickup device having a vertical overflow drain for discharging unnecessary charges and a charge transfer section for transferring signal charges is applied will be discussed. As described above, in the case of an image sensor having a vertical overflow drain, unnecessary charges are not transferred to the vertical shift register.
Although the signal charge and the unnecessary charge do not mix in the vertical shift register, the transfer pulse is given during this period because the potential change of each part in the image sensor due to the discharge of the unnecessary charge into the base region lasts for several hundred μsec. And uneven brightness. Therefore, even if the element shutter is performed using an image sensor having a vertical overflow drain, there is still a restriction on the timing at which a transfer pulse to the vertical shift register can be sent. Therefore, it is of course effective to apply the present invention to an image sensor having a vertical overflow drain.

〔The invention's effect〕

As described above, according to the present invention, an image sensor having a vertical overflow drain for discharging unnecessary charges and a charge transfer unit for transferring signal charges is applied,
In the exposure control method in which the image sensor is made to function as a shutter, there is no possibility of generating noise due to potential fluctuations of various parts in the image sensor due to discharging unnecessary charges into the base region by the vertical overflow drain. And a clear image can be obtained.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a main part of the system shown in FIG. 2 in detail, FIG. 2 is a block diagram showing an outline of an electronic still camera system to which an exposure control method of the present invention is applied, FIG. 3 and FIG. 4 are time charts showing the operation of each unit in FIG. 1 when the brightness of the subject is relatively dark and when the brightness of the subject is extremely bright. 600 CCD drive circuit (image sensor) IE Integration pulse τ Element shutter end prohibition period (first time period)

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-62-95077 (JP, A) JP-A-63-105579 (JP, A) JP-A-2-131683 (JP, A) JP-A-60-1985 125081 (JP, A) JP-A-59-40779 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 5/30-5/335

Claims (1)

(57) [Claims] 1. An image pickup device having a vertical overflow drain for discharging unnecessary charges and a charge transfer portion for transferring signal charges is accumulated with time in a photoelectric conversion portion of the device from the start of exposure. In the exposure control method in which the received signal charges are simultaneously transferred to the charge transfer unit at the end of the exposure so that the image pickup device functions as a shutter, the potential fluctuation depending on the discharge of unnecessary charges to the vertical overflow drain is provided. Charge transfer to the charge transfer unit is prohibited in a first time interval until a time substantially coincident with the end time of the period, and charge transfer is allowed in a second time interval following the first time interval. An exposure control method characterized by the above.