JPH06245140A - Exposure controller and image pickup device using the same - Google Patents

Abstract

PURPOSE:To suppress flicker by providing a photoelectric conversion element, a comparator means converting a signal from the element into a binarized signal, and a timing control means taking a predetermined period from a point of time when the binarized signal is changed to the controller. CONSTITUTION:A detection signal Sp outputted from a photoelectric conversion element 5 is received by a comparator circuit 6 via an amplifier circuit. The circuit 6 generates a binarized signal SHL being logical H when an amplitude of a detection signal Sp' is larger than a reference voltage and a binarized signal SHL being logical L when the amplitude of the detection signal Sp' is smaller than the reference voltage and gives the signal to a timing control circuit 4. The circuit 4 applies an exposure start control signal SaBN used to command start of exposure synchronously with inversion of the signal SHL changed from logical L into logical H, an exposure end control signal SEN commanding end of exposure at a point of time when a predetermined exposure period tau0 elapses from the point of that time, and a scanning read control signal Sc used to make scanning read from the point of time of the end of exposure to a CCD solid-state image pickup device 1. Thus, even when an irregular illuminance change is in existence, an object with an optimum luminous intensity is always picked up.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure control apparatus and an image pickup apparatus using the same, which makes it possible to take an image clearly even if the brightness of the image pickup object is not constant.

[0002]

2. Description of the Related Art As is well known, in an optical device having an image pickup function such as a video camera, a facsimile or the like, suppressing flicker is one of the most important technical problems for clearly picking up an image pickup object. Is. That is, as a typical case, the light emission illuminance of the external lighting device for illuminating the imaging target changes periodically according to the frequency change of the commercial power supply, and the imaging target whose light intensity changes accordingly is imaged. When an image is reproduced based on the video signal obtained by the image pickup, flicker occurs in the reproduced image, and a clear reproduced image cannot be obtained.

Conventionally, the following technical measures have been taken to suppress such flicker. Incidentally, CC
A conventional technique in an image pickup apparatus to which a solid-state image pickup device such as D is applied will be described.

As a first conventional example, it is assumed in advance that an image pickup object is illuminated by an external lighting device that operates with a commercial power source, and the exposure period of the solid-state image pickup device is set to a fixed cycle synchronized with the frequency of the commercial power source. It is a thing. Therefore, since the commercial power supply of 50 Hz or 60 Hz is applied in Japan, the fixed-period exposure period is predetermined with a timing synchronized with these frequencies and capable of suppressing the occurrence of flicker.

As a second conventional example, instead of fixing the exposure period in advance, the change in the actual light intensity of the object to be imaged is measured one by one, and the measurement result is statistically processed to obtain the optimum exposure. There is one that determines the period. Specifically, when using an external illumination device that operates with a commercial power supply of 50 Hz or 60 Hz, a predetermined exposure period T _D
The solid-state imaging device is exposed with, and the video signal (charge signal) accumulated in the pixel is sampled, for example, 20 times within the exposure period T _D , and the dispersion value of the luminance level of each sampled video signal is statistically calculated. To do.
The exposure period T _D corresponds to a commercial power source of 50 Hz and is 10 mS in advance, and the commercial power source of 60 Hz is 8.3 in advance.
Two types of time of 3 mS are set, and the above 20 times of sampling and each dispersion value are calculated for each exposure period T _D. Then, it is determined whether the flicker cycle T _f is 50 Hz or 60 Hz or another flicker cycle based on these variance values.

Next, the flicker period T _f and the exposure period T _D thus obtained and the time T of the least common multiple of these
_{For LCM} and the number of samplings n ₁ (= 20), a coefficient n ₂ that satisfies the relational expression of the following expression (1) is calculated.

N ₁ × T _f = n ₂ × T _D = T _LCM (1) Next, the video signal accumulated in the pixel is exposed to the exposure period T _D.
Within the period of time T _LCM , sampling is performed n ₂ times, and the average value of the luminance level of each sampled video signal is obtained.
Further, by repeating this process, for example, 20 times in total, 20 average values X _{1 to} X ₂₀ are calculated, and further, the average value AT _D = (X ₁ + X ₂₀ of these 20 average values X _{1 to} X _20.
Calculate ₂ + ... + X ₁₉ + X ₂₀ ) / 20.

Next, the least common multiple time T _LCM is set as the exposure period, and the video signals accumulated in the pixels are sampled n ₂ times within the time T _LCM , and the brightness level of each sampled video signal is the average value AT. _The final exposure period T _D 'is determined so as to be equal to _D.

When such a statistical process is performed, when the light intensity of the image pickup object is low (that is, when the illuminance of the external lighting device is low), the determined exposure period T _D 'is long, and conversely. When the light intensity of the imaging target is high (that is, when the illuminance of the external lighting device is high), the determined exposure period T _D 'is short, and therefore the exposure period T _D ' depending on the actual light intensity of the imaging target. Is variably adjusted, and flicker is suppressed.

[0010]

However, such a conventional technique has the following problems.

(Problem of the first conventional example) In this conventional technique, the cause of flicker is assumed in advance, for example, 5
When using an external lighting device that operates with a commercial power source of 0 Hz or 60 Hz, a fixed exposure period is set corresponding to the frequency of the commercial power source. Therefore, there is a problem that flicker cannot be suppressed when an external lighting device in which the illuminance changes periodically at another frequency is used.

Further, according to this conventional technique, the exposure period is set to an integral multiple of 1 or more of the flicker period, and the exposure period shorter than the flicker period cannot be set. That is, it is impossible to realize high-speed exposure in a shorter time than the flicker cycle. As a result, it is not possible to variably control the sequential scanning readout, for example, the sequential scanning readout of the video signal (charge signal) generated in each pixel of the image sensor is performed at high speed at some times and at low speed at other times.

When applied to an image pickup device for performing color image pickup, since the exposure period for all pixels of red (R), blue (B) and green (G) is fixed uniformly. It is not possible to perform white balance adjustment one by one while independently controlling the exposure period of each pixel of red (R), blue (B) and green (G).

(Problem of Second Prior Art) In this prior art, since flicker is suppressed by variably adjusting the exposure period, it is more versatile than the first prior art. I can say. However, there is a limit to this, and as described above, it is only possible to suppress the flicker by adjusting the exposure period within an extremely narrow range that can accommodate commercial power sources of 50 Hz and 60 Hz.

In particular, since the statistical processing algorithm is complicated, it takes a long time to determine the optimum exposure period T _D '. For example, when the time T _{LCM, which} is the least common multiple of the exposure period and the flicker cycle, has a relatively small value, there is no problem, but when the control range of the exposure period is completely arbitrary, the time T _LCM is Since it may be extremely large, the calculation amount of statistical processing becomes enormous, and there arises a problem that it cannot be applied to a technique such as repeated high-speed exposure.

Further, the means for changing the exposure period according to the change in the light intensity of the image pickup object is effective in terms of suppressing flicker, but as is well known, a dark current is generated in the solid-state image pickup device. Therefore, the influence of dark current on the pixel signal (charge signal) generated in the pixel is
It depends on the length of the exposure period. Therefore, variably controlling the exposure period in this manner causes a problem of deterioration of image quality.

[0017]

In order to achieve such an object, the present invention sets a photoelectric conversion element for detecting a change in brightness of an image pickup object and an amplitude of a detection signal of the photoelectric conversion element in advance. And a comparison means for converting to a first logic level when the threshold level is higher than the threshold level, and to a second logic level when the threshold level is lower than the threshold level, and a binary signal. The timing control means has an exposure period that is a predetermined period from the time when the binary signal of the means changes.

Further, an image pickup element such as a solid-state image pickup device for picking up an image pickup object, a photoelectric conversion element for detecting a change in brightness of the image pickup object, and an amplitude of a detection signal of the photoelectric conversion element are preset. When it is larger than the threshold level, it is converted to a first logic level, and when it is smaller than the threshold level, it is converted into a binary signal by converting it into a second logic level. A configuration is provided that includes timing control means for setting a predetermined period from the time when the binarized signal changes as the exposure period of the image pickup device.

[0019]

According to the exposure control apparatus and the image pickup apparatus having such a configuration, the comparing means detects the change point of the brightness of the image pickup object, the exposure is started in synchronization with the change point, and the exposure start time point is reached. Therefore, the exposure is performed in a constant exposure period, and therefore, the exposure is performed in synchronization with the actual brightness change (flicker cycle) of the imaging target, so that the flicker can be suppressed.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to FIGS. First, the configuration of an image pickup apparatus equipped with an exposure control apparatus according to the present invention will be described with reference to FIG. In this embodiment, as an image pickup element, a pixel group composed of a plurality of photodiodes and a charge transfer path provided along this pixel group via a transfer gate are provided, and charges accumulated in the pixel group by exposure. A CCD that outputs a video signal by scanning and reading a signal through a charge transfer path
The solid-state imaging device 1 is applied.

In front of the CCD solid-state image pickup device 1, an optical system 3 for forming an image of the image pickup target 2 on a pixel group is arranged. The timing control circuit 4 includes an exposure start control signal S _BN for designating and controlling the exposure start time of the CCD solid-state imaging device 1, an exposure end control signal S _EN for designating and controlling the exposure end time, and charges accumulated in the pixel group by imaging. A video signal (pixel signal) corresponding to each pixel by transferring a signal to a charge transfer path via a transfer gate and performing a charge transfer operation on the charge transfer path.
Scan read control signal S _C for scanning and reading Pi in a dot-sequential manner
And so on. As will be described in detail later, the timing control circuit 4 includes a main control circuit 4a such as a microprocessor for centrally controlling a series of image pickup operations, and control signals S _BN and S _BN .
It is provided with a timing generator 4b which forms _EN and a scan read control signal S _C.

The photoelectric conversion element 5 such as a photodiode is
It is provided adjacent to the optical system 3 in order to detect the brightness of the measurement target 2. The detection signal S _P output by the photoelectric conversion element 5 by its photoelectric conversion function is amplified by the amplification circuit 5, and the amplified detection signal S _P ′ output from the amplification circuit 5 is non-inverted by the comparison circuit 6. It is supplied to the input contacts, any of the inverting input contact of the comparator circuit 6 the reference voltage V _R
Is connected to a variable voltage circuit 7 that can be set. That is, the comparison circuit 6, the detection signal S _P 'logic when the amplitude is greater than the reference voltage V _R is "H", the detection signal S _P' logic when the amplitude of is less than the reference voltage V _R is "L" A binary signal S _HL is generated, and this binary signal S _HL is supplied to the timing control circuit 4.

The timing control circuit 4 outputs an exposure start control signal S _BN for instructing the start of exposure in synchronization with the inversion of the binary signal S _HL from logic "L" to "H", and from that time point. The exposure end control signal S _EN for instructing the end of exposure at the time when a predetermined exposure period τ ₀ has elapsed and the scan read control signal S _C for performing the scan read from the time when the exposure is completed
The process of supplying the solid-state imaging device 1 is repeated.

Next, the operation of the embodiment having such a configuration will be described with reference to FIG. As a typical example for clarifying the operation, as shown in FIG. 1, the measurement target 2 is illuminated by an external lighting device A such as a fluorescent lamp operated by a commercial power supply of 50 Hz or 60 Hz, and Under some lighting, some moving object B may be between the external lighting device A and the measuring object 2
The light intensity of the measurement target 2 fluctuates due to factors such as the illumination being blocked by moving in the direction or the like, or the illuminance of the measurement target 2 being increased by lighting of another external light (not shown). The explanation will be made assuming the case.

Therefore, when the illumination is performed only by the specific external illumination device A in a state where the commercial power source is stable, the illuminance of the external illumination device A changes periodically according to the periodic voltage change of the commercial power source of 50 Hz or 60 Hz. Change,
The detection signal S _P output from the photoelectric conversion element 5 is as shown in FIG.
As shown in (a), the amplitude periodically changes at 50 Hz or 60 Hz. Further, for example, in the case illuminance of the commercial power supply external illumination device voltage drop is generated at A is as drops in a period of time tau _1, as shown in FIG. 6 (a), the maximum amplitude of the detection signal S _P Becomes smaller. Further, when another external lighting device is turned on in a certain period τ ₂ and the illuminance on the measurement target 2 is increased, as shown in FIG.
_When the amplitude of _P increases as a whole and the moving object B shields the external lighting device A during a certain period τ ₃ ,
As shown in FIG. 7B, the amplitude of the detection signal S _P is entirely lowered. When such a phenomenon occurs in time series, the detection signal S output via the amplifier circuit 5 is output.
_P'is as shown in FIG.

Assuming that the detection signal S _P 'as shown in FIG. 7C is input to the comparison circuit 6, the comparison circuit 6 outputs the reference voltage V _R and the detection signal S preset in the variable voltage circuit 7. The amplitudes of _P ′ are compared, and a binary signal S _HL that produces a logic “H” when S _P ′ ≧ V _{R and} a logic “L” when S _P ′ <V _R is generated. That is, the binarized signal S _HL becomes a rectangular wave whose logic is inverted in synchronization with the cycle of the detection signal S _P 'in the period excluding the periods τ ₁ , τ ₂ and τ ₃ , as shown in FIG. , In the periods τ ₁ , τ ₂ , and τ ₃ , the logic signal becomes “H” or “L” continuously according to the magnitude of the amplitude of the detection signal S _P ′.

When such a binarized signal S _HL is supplied, the timing control circuit 4 shows a part of FIG. 6 (d) enlarged in time as shown in (e) to (i) of FIG. In addition, by generating the exposure start signal S _BN of logic “H” for a short time in synchronization with the time when the binarized signal S _HL is inverted to logic “H”,
The CCD solid-state image pickup device 1 is caused to start exposure, and further, an exposure end signal S _EN which becomes a logic "H" for a short time is generated when a certain time τ ₀ elapses from the exposure start time point. Stop the exposure. Note that well-known electronic shutter technology or the like is applied to perform exposure only during this period τ ₀ . Further, as shown in FIG. 3H, the scanning read signal S _C is generated from the time when the exposure end signal S _EN is inverted from the logic “H” to “L” to drive the charge transfer path. As shown in Figure (i), CCD
From the solid-state imaging device 1, the pixel array is followed (that is,
A dot-sequential) video signal Pi is output.

Here, like the periods τ ₁ , τ ₂ , τ ₃ ,
When the brightness of the measurement object 2 changes more drastically than in the steady state, the logic of the binarized signal S _HL becomes “H” or “L” and does not change.
τ _OFF ), the exposure start signal S _BN , the exposure end signal S _EN, and the scanning read signal S _C are not generated, so that the CCD solid-state imaging device 1 does not perform the imaging operation.

As described above, according to this embodiment, even when an object to be measured is illuminated by an external illumination device such as a fluorescent lamp, the exposure is performed in synchronization with the change in the illuminance, so that flicker is completely suppressed. It In particular, since the exposure is controlled to be performed during the period when the illuminance increases, a clear image can be obtained and the exposure period can be shortened.
Further, although the exposure period τ ₀ is fixed in advance, it can be arbitrarily changed and can be set independently of the flicker cycle. For example, high-speed exposure and high-speed imaging can be performed. You can Further, when a color image pickup device is used as the image pickup device, the exposure period can be independently set for the pixels of each color, so that the white balance can be adjusted based on the exposure period.

Further, even if the illuminance of the external illumination fluctuates extremely or an irregular illuminance change state occurs such that the light is shielded, the exposure is stopped following the irregular illuminance change state, so that only clear image pickup can be realized. Is possible.

As a result of exhibiting such an excellent effect,
When applied to an image reader, a facsimile, or the like that optically reads a document illuminated by external illumination, excellent effects can be obtained, and a special illumination device that constantly emits light with a constant illuminance is not required. It enables high-quality image reading in a lighting environment.

In this embodiment, the case of synchronizing with the rising pulse of the comparing means 6 has been described, but the falling pulse may be used as a matter of course. Further, although the case where the CCD solid-state image pickup device is applied has been described, it can also be used for a MOS type solid-state image pickup device and other known image pickup elements.

[0033]

As described above, according to the present invention,
The exposure control device includes a photoelectric conversion element that detects a change in brightness of an imaging target, and a first logic level when the amplitude of a detection signal of the photoelectric conversion element is larger than a preset threshold level. When it is smaller than the threshold value level, a comparison means for converting it into a binary signal by performing a binary conversion to a second logic level, and a predetermined period from the time when the binary signal of the comparison means changes. Since the constitution is provided with the timing control means for setting the exposure period, the following effects can be obtained.

That is, even if there is an irregular change in illuminance such that a shadow is cast on the imaging target due to some object intervening between the illumination device and the imaging target, the image of the shadow is captured. Instead, it is possible to always capture an image of an assumed object with the optimum light intensity.
Further, even when the illuminance of the illumination device changes periodically, the exposure period is set in synchronization with the cycle, so that flicker can be suppressed. Furthermore, since the exposure period can be set arbitrarily, a wide range of applications such as high-speed exposure are possible. The exposure period can be shortened by synchronizing the exposure period with the period in which the illuminance of the illumination device is high. Further, when the control signal is not output from the exposure control apparatus of the present invention, there is a side effect that it can be detected that there is no change in illuminance of the illumination apparatus. The application is also possible in which the timing can be freely controlled. When an image is captured using a color image sensor, the exposure period of each pixel of red (R), green (g), and blue (B) can be independently controlled. Therefore, the exposure period can be variably controlled. It is possible to adjust the white balance by.

[Brief description of drawings]

FIG. 1 is a configuration explanatory diagram showing a configuration of an embodiment according to the present invention.

FIG. 2 is a timing chart for explaining the operation of the embodiment.

[Explanation of symbols]

A ... External lighting device, B ... Moving object, 1 ... CCD solid-state imaging device, 2 ... Imaging target, 3 ... Optical system, 4 ... Timing control circuit, 4a ... Main control circuit, 4b ... Timing generator, 5 ... Amplification circuit, 6 ... Comparison circuit, 7 ... Variable power supply circuit.

Claims (3)

[Claims] 1. A photoelectric conversion element for detecting a change in brightness of an image pickup target, and a first logic level when the amplitude of a detection signal of the photoelectric conversion element is larger than a preset threshold level. When it is smaller than the threshold level, comparison means for converting it into a binarized signal by binarizing it to a second logic level, and a predetermined period from the time when the binarized signal of the comparison means changes An exposure control apparatus comprising: a timing control unit for setting an exposure period. 2. An image pickup device for picking up an image pickup target, a photoelectric conversion device for detecting a change in brightness of the image pickup target, and an amplitude of a detection signal of the photoelectric conversion device is larger than a preset threshold level. The comparison means converts the signal into a binarized signal by binarizing and converting it to a first logic level when it is lower than the threshold level and a second logic level when it is lower than the threshold level. An image pickup apparatus, comprising: a timing control unit that sets a predetermined period from a change time point as an exposure period of the image pickup device. 3. The image pickup apparatus according to claim 2, wherein the image pickup element is a solid-state image pickup device.