JP4471055B2 - Imaging apparatus and method thereof - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an imaging apparatus and method, and is suitable for application to, for example, a video camera.
[0002]
[Prior art]
2. Description of the Related Art Conventionally, a video camera enters incident light from a subject into a solid-state imaging device (CCD: Charge Coupled Device), and photoelectrically converts the incident light incident on the CCD into an electrical signal, thereby converting an image of the subject. It is made to take an image.
[0003]
The CCD has a photo sensor unit corresponding to each pixel, and incident light incident from a subject is received by each photo sensor unit. Each photo sensor unit sweeps away the accumulated charges at the timing when the electronic shutter pulse supplied from the timing generator is turned on, obtains charges by photoelectrically converting the received incident light, and accumulates the charges.
[0004]
The electronic shutter pulse is turned on at a timing synchronized with the horizontal period, and each photo sensor unit sequentially stores the accumulated charge at each timing when the electronic shutter pulse is sequentially turned on at a constant time interval. The charge accumulation starts at the timing when the electronic shutter pulse is finally turned on.
[0005]
In this state, each photo sensor unit ends the accumulation of charges at the timing when the readout pulse supplied from the timing generator is turned on, and transfers the accumulated charges to the subsequent vertical transfer register.
[0006]
Thus, the time for accumulating photoelectrically converted charges, that is, the exposure time, is the time from the timing when the electronic shutter pulse is last turned on to the timing when the readout pulse is turned on.
[0007]
Therefore, this video camera can adjust the exposure time roughly at a constant time interval at which the electronic shutter pulses are sequentially turned on by adjusting the timing at which the electronic shutter pulse is turned on last, and the constant time interval. The exposure time can be finely adjusted by adjusting the timing at which the readout pulse is turned on in the range of.
[0008]
[Problems to be solved by the invention]
By the way, in the video camera having such a configuration, the generation timing of the readout pulse is generated by dividing the master clock having the same frequency as that of the horizontal drive signal. There arises a problem that a deviation occurs between the time intervals when the shutter pulse is turned on, and therefore it is inevitable that the exposure control becomes discontinuous every horizontal period in the entire vertical period.
[0009]
The present invention has been made in consideration of the above points, and an object of the present invention is to propose an imaging apparatus and method capable of performing exposure control with high accuracy and high continuity.
[0010]
[Means for Solving the Problems]
In order to solve such a problem, in the present invention, the timing at which charge accumulation is started at each pixel arranged on the imaging surface of the image sensor is changed at the first time interval, and the charge accumulated from each pixel is changed. In the imaging device that changes the exposure time by changing the readout timing for reading out the image at a second time interval shorter than the first time interval within the range of the first time interval, Read pulse generation means for generating a read pulse for reading out charges at a second time interval and read pulse generation adjusting means for adjusting the generation timing of the read pulse every time a predetermined number of the read pulses are generated are provided. did.
[0011]
As a result, the exposure time can be smoothly changed over the entire vertical period, and therefore exposure control with high accuracy and high continuity can be performed with a simple configuration.
[0012]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, an embodiment of the present invention will be described in detail with reference to the drawings.
[0013]
In FIG. 1, reference numeral 1 denotes the overall configuration of a video camera, and incident light L1 incident from the subject side is incident on a solid-state imaging device (CCD: Charge Coupled Device) 2. The CCD 2 photoelectrically converts the received incident light L1 into an electric signal, and sends the image signal S1 obtained as a result to the amplifier 3.
[0014]
Specifically, the CCD 2 has a photo sensor unit corresponding to each pixel, and receives incident light L1 incident from a subject by each photo sensor unit. Each photo sensor unit has a V from the timing generator 4. _sub The electronic shutter pulse SHT is supplied via the setting circuit 5, and each photosensor unit sweeps away the accumulated charges at the timing when the electronic shutter pulse SHT is turned on, and photoelectrically converts the received incident light L1. To obtain electric charge and accumulate the electric charge.
[0015]
The electronic shutter pulse SHT is turned on at a timing synchronized with the horizontal period, and each photosensor unit stores the accumulated charge at each timing when the electronic shutter pulse SHT is sequentially turned on at a constant time interval. Are sequentially swept away, and charge accumulation is started at the timing when the electronic shutter pulse SHT is finally turned on.
[0016]
Next, each photo sensor unit ends the accumulation of charges at a timing based on the read pulses SG1 and SG2 supplied from the timing generator 4, and transfers the accumulated charges to the subsequent vertical transfer register.
[0017]
The charges transferred to the vertical transfer register are transferred from the vertical transfer register to the horizontal transfer register at a timing based on the vertical transfer pulses V1 to V4 supplied from the timing generator.
[0018]
Subsequently, the charges transferred to the horizontal transfer register are transferred from the horizontal transfer register to the output unit at a timing based on the horizontal transfer pulses H1 and H2 supplied from the timing generator 4, and then reset supplied from the timing generator 4. The image signal S1 is output from the output unit at a timing based on the pulse RG.
[0019]
The amplifier 3 reduces the noise component of the image signal S1 by a correlated double sampling (CDS) and then amplifies it to a desired level, and the resulting image signal S2 is converted from analog to digital (A / D). To the device 6.
[0020]
The A / D converter 6 converts the image signal S2 from analog to digital, and sends the image data S3 obtained as a result to a camera DSP (Digital Signal Processor) circuit 7. The camera DSP circuit 7 performs predetermined image processing on the image data S3 to convert it to, for example, a television signal S4 of the NTSC (National Television System) system, and outputs this to the outside.
[0021]
The timing generator 4 uses the source oscillation signal S5 supplied from the oscillator 8 as the master clock MCKD, generates the master clock MCK by dividing the source oscillation signal S5 by 2, and uses the master clocks MCKD and MCK as a camera. It is sent to the DSP circuit 7. Thus, the timing generator 4 and the camera DSP circuit 7 are operated by the same master clocks MCKD and MCK.
[0022]
As described above, the camera DSP circuit 7 operates in accordance with the master clocks MCKD and MCK, thereby generating the horizontal synchronization signal TGHD and the vertical synchronization signal TGVD and sending them to the timing generator 4. The camera DSP circuit 7 determines the optimum exposure time of the CCD 2 based on the luminance information obtained from the image data S3, and sends the determined exposure time to the timing generator 4 as the exposure time setting value T.
[0023]
The timing generator 4 is based on the horizontal synchronization signal TGHD, the vertical synchronization signal TGVD and the exposure time setting value T supplied from the camera DSP circuit 7, and the electronic shutter pulse SHT, the readout pulses SG1 and SG2, the vertical transfer pulses V1 to V4, The flat transfer pulses H1 and H2 and the reset pulse RG are generated.
[0024]
Then, the timing generator 4 converts the electronic shutter pulse SHT to V _SUB Send to setting circuit 5 _SUB In the setting circuit 5, the electronic shutter pulse SHT is converted into a voltage so that the reset potential V of the CCD 2 is converted. _SUB Is sent to the CCD 2.
[0025]
In addition, the timing generator 4 superimposes the read pulse SG1 on the vertical transfer pulse V1, superimposes the read pulse SG2 on the vertical transfer pulse V3, vertical transfer pulses V1 to V4 on which the read pulses SG1 and SG2 are superimposed, and horizontal transfer. Pulses H1 and H2 and a reset pulse RG are sent to the CCD 2.
[0026]
Thus, the timing generator 4 adjusts the exposure time of the CCD 2 by sending the electronic shutter pulse SHT, the readout pulses SG1 and SG2, the vertical transfer pulses V1 to V4, the horizontal transfer pulses H1 and H2, and the reset pulse RG to the CCD 2. However, the CCD 2 is driven.
[0027]
Here, the exposure time (time for accumulating the photoelectrically converted charge) is the time from the timing when the electronic shutter pulse SHT is last turned on to the timing when the readout pulses SG1 and SG2 are turned on.
[0028]
The timing generator 4 adjusts the timing at which the electronic shutter pulse SHT is finally turned on based on the exposure time setting value T supplied from the camera DSP circuit 7, thereby sequentially turning on the electronic shutter pulses SHT. The exposure time can be roughly adjusted at a constant time interval, and the exposure time can be finely adjusted by adjusting the timing at which the readout pulses SG1 and SG2 are turned on within the range of the constant time interval. .
[0029]
Here, FIG. 2 shows a timing chart when the exposure time is roughly adjusted by the electronic shutter pulse SHT. 2A shows the vertical synchronization signal TGVD, FIG. 2B shows the horizontal synchronization signal TGHD, FIG. 2C shows the readout pulses SG1 and SG2, and FIG. 2D shows the electronic shutter. Pulse SHT is shown.
[0030]
One horizontal period, which is one period of the horizontal synchronization signal TGHD, is 63.5 [μsec] (FIG. 2B), and the electronic shutter pulse SHT is a pulse that is turned on in synchronization with the horizontal period. (FIG. 2D), the adjustment of the exposure time by the electronic shutter pulse SHT is performed at a time interval of 63.5 [μsec].
[0031]
That is, the electric charge accumulated in each photo sensor unit is swept away at every timing when the electronic shutter pulse SHT is turned on at a time interval of 63.5 [μsec], and the timing when the electronic shutter pulse SHT is turned on last. The exposure time starts.
[0032]
Next, FIG. 3 shows a timing chart when the exposure time is finely adjusted by the read pulses SG1 and SG2. 3A shows the horizontal synchronization signal TGHD, FIG. 3B shows the electronic shutter pulse SHT, and FIGS. 3C to 3D show the readout pulses SG1 and SG2. Here, the intermediate timing between the falling timings of the readout pulses SG1 and SG2 is the readout timing for reading out charges from the photosensor unit.
[0033]
The fine adjustment of the exposure time by the read pulses SG1 and SG2 is performed by changing the read timing based on the read pulses SG1 and SG2 stepwise within a range of one period (that is, one horizontal period) of the horizontal synchronization signal TGHD. In this case, the read timing is set to change the range of one horizontal cycle in 128 steps, for example. Note that the number of stages when the read timing is changed stepwise is called the number of control stages.
[0034]
Incidentally, the exposure time set value T defines a read timing set value Hz in which the charge read timing is expressed by the number of control stages. FIG. 3C shows the read timing when the timing set value Hz is 0. 3D shows the read timing when the timing set value Hz is 20, and FIG. 3E shows the read timing when the timing set value Hz is 127.
[0035]
This read timing is generated by dividing the master clock MCK, and the number of divisions is determined by the number of master clocks corresponding to one horizontal period and the number of read timing control stages. Incidentally, the number of master clocks is determined according to the CCD 2 to be used, and the number of control stages for the read timing is an integer power of 2.
[0036]
Here, adjustment of readout timing when the CCD 2 having about 380,000 effective pixels is applied to the video camera 1 will be described. In the CCD 2, when one horizontal cycle corresponds to 910 master clocks and the number of control stages is 128 (27), it is optimal to divide by 7.
[0037]
FIG. 4 shows the configuration of the read pulse generation circuit 20 inside the timing generator 4, which is composed of a 7-frequency divider block 21 and a read pulse generation block 22.
[0038]
The 7-divided block 21 divides the master clock MCK (FIG. 5A) by 7 to generate a pulse P1 every 7 master clocks (FIG. 5B), which is read pulse generation block. 22 to send.
[0039]
The readout pulse generation block 22 selects the pulse P1 corresponding to the readout timing setting value Hz by counting the pulses P1 up to the readout timing setting value Hz supplied from the camera DSP circuit 7, and uses this as the reference readout pulse. It is set as PSG (FIGS. 5C to 5E). Here, FIG. 5C shows the reference read pulse P when the read timing set value Hz is zero. _SG FIG. 5D shows the reference readout pulse P when the readout timing set value Hz is 3. _SG FIG. 5E shows the reference read pulse P when the read timing set value Hz is 15. _SG Indicates.
[0040]
The timing generator 4 generates a reference read pulse P generated by the read pulse generation circuit 20. _SG Based on the above, read pulses SG1 and SG2 are generated, and the generated read pulses SG1 and SG2 are supplied to the CCD 2.
[0041]
By the way, it is desirable to change the read timing based on the read pulses SG1 and SG2 for every 7.1 (≈910 / 128) master clock. However, in the timing generator 4 having the read pulse generation circuit 20, the read timing is changed every 7 master clocks, so that there is a deviation between the actual time interval and the ideal time interval of the read timing. Occurs, and the deviation becomes large at the final stage in the entire horizontal period.
[0042]
Further, in the timing generator 4 having the read pulse generation circuit 20, since the 128th stage is the 896 (= 7 × 128) master clock, the interval between the 896 master clock and the 910 master clock corresponding to one horizontal period is provided. Shift of the electronic shutter pulse SHT
If the adjustment of the exposure time by the combination of the adjustment of the exposure time by the readout pulses SG1 and SG2 is combined, the exposure time cannot be changed smoothly.
[0043]
Therefore, in the case of the present embodiment, the exposure time is smoothly changed over the entire vertical period by finely adjusting the time interval of the read timing based on the read pulses SG1 and SG2.
[0044]
FIG. 6 shows that when the CCD 2 having 380,000 effective pixels is applied to the video camera 1 and the change range of the read timing based on the read pulses SG1 and SG2 is shorter than one horizontal period, the read timing is slightly reduced. The configuration of the read pulse generation circuit 30 for realizing the adjustment is shown.
[0045]
The read pulse generation circuit 30 is provided inside the timing generator 4 and includes a 7-frequency division block 31, a 64-frequency division block 32, and a read pulse generation block 33.
[0046]
The 7-divided block 31 divides the master clock MCK (FIG. 7A) by 7 to generate a pulse P1 every 7 master clocks (FIG. 7C), and this is used as a read pulse generation block. To 33.
[0047]
Further, the divide-by-64 block 32 divides the master clock MCK (FIG. 7A) by 64 to generate a pulse P2 for every 64 master clocks (FIG. 7B), which is divided into 7 blocks. 31 to reset the divide-by-7 block 31. As a result, a pulse P1 having a long interval corresponding to one master clock is generated every nine pulses (FIG. 7C).
[0048]
The read pulse generation block 33 selects the pulse P1 corresponding to the read timing set value Hz by counting the pulses P1 up to the read timing set value Hz supplied from the camera DSP circuit 7, and uses this as the reference read pulse. P _SG (FIGS. 7D to 7F). Here, FIG. 5D shows the reference read pulse P when the read timing set value Hz is zero. _SG FIG. 5E shows the reference read pulse P when the read timing set value Hz is 3. _SG FIG. 5F shows the reference readout pulse P when the readout timing set value Hz is 15. _SG Indicates.
[0049]
The timing generator 4 generates a reference readout pulse P generated by the readout pulse generation circuit 30. _SG Based on the above, read pulses SG1 and SG2 are generated, and the generated read pulses SG1 and SG2 are supplied to the CCD 2.
[0050]
FIG. 8 shows the case where the change range of the read timing based on the read pulses SG1 and SG2 becomes longer than one horizontal cycle by applying the CCD 2 having 360,000 effective pixels or 630,000 pixels to the video camera 1. The configuration of the read pulse generation circuit 40 for realizing fine adjustment of the read timing is shown.
[0051]
The read pulse generation circuit 40 is provided inside the timing generator 4 and includes a 7-frequency division block 41, a 20-frequency division block 42, an OR circuit 43, and a read pulse generation block 44.
[0052]
The divide-by-7 block 41 divides the master clock MCK (FIG. 9A) by 7 to generate a pulse P1 every 7 master clocks (FIG. 7B), and sends this to the OR circuit 43. To do.
[0053]
Further, the divide-by-20 block 42 divides the master clock MCK (FIG. 9A) by 20 to generate a pulse P2 for every 20 master clocks (FIG. 7C), which is divided into 7 blocks. 41 and resets the divide-by-7 block 41, and sends the pulse P2 to the OR circuit 43.
[0054]
The OR circuit 43 takes the logical sum of the pulse P1 and the pulse P2 to generate a pulse P3 having a short interval of one master clock every three pulses (FIG. 7D), and outputs this pulse P3 for reading. To send.
[0055]
The read pulse generation block 44 selects the pulse P3 corresponding to the read timing set value Hz by counting the pulses P3 up to the read timing set value Hz supplied from the camera DSP circuit 7, and uses this as the reference read pulse. P _SG (FIGS. 9E to 9G). Here, FIG. 9E shows the reference read pulse P when the read timing set value Hz is zero. _SG FIG. 9F shows the reference readout pulse P when the readout timing set value Hz is 3. _SG FIG. 9G shows the reference readout pulse P when the readout timing set value Hz is 15. _SG Indicates.
[0056]
The timing generator 4 generates read pulses SG1 and SG2 based on the reference read pulse PSG generated by the read pulse generation circuit 40, and supplies the generated read pulses SG1 and SG2 to the CCD 2. .
[0057]
In the above configuration, when the change range of the read timing based on the read pulses SG1 and SG2 is shorter than one horizontal period, the read pulse generation circuit 30 is provided with the 64 frequency dividing block 32, and the 64 frequency dividing block 32 is provided. By resetting the divide-by-7 block 31 every 64 master clocks by the pulse P2 generated in step 1, a pulse P1 having a long interval of one master clock is generated every nine pulses.
[0058]
On the other hand, when the change range of the read timing based on the read pulses SG1 and SG2 becomes longer than one horizontal cycle, the read pulse generation circuit 40 is provided with the 20-dividing block 42 and the OR circuit 43, and the 20-dividing block By resetting the divide-by-7 block 41 every 20 master clocks by the pulse P2 generated at 42, a pulse P3 having a short interval of 1 master clock is generated every 3 pulses.
[0059]
Here, FIG. 10 shows the relationship between the read timing set value Hz specified by the camera DSP circuit 7 and the exposure time. Incidentally, N (= 2 ^N , N is an integer) represents the number of control stages. As shown in FIG. 10, when the conventional readout pulse generation circuit 20 is applied, the deviation of the exposure time from the ideal value increases near the final stage, but the readout pulse generation circuit 30 according to the present embodiment. And 40 are applied, it can be seen that the exposure time is close to the ideal value even near the final stage.
[0060]
Thus, by adjusting the change range of the readout timing to one horizontal period, that is, the time interval when the electronic shutter pulse SHT is turned on, the adjustment of the exposure time by the electronic shutter pulse SHT and the exposure time by the readout pulses SG1 and SG2 are adjusted. Even if the adjustment is combined, the exposure time can be changed smoothly, so that exposure control with high accuracy and high continuity can be performed over the entire vertical period.
[0061]
Further, since the deviation from the ideal value of the readout timing is small, even when the number of control stages is large or small, the deviation of the readout timing with respect to the ideal value can be reduced, so that it is uniform over the entire horizontal period. High-precision exposure control can be performed.
[0062]
Further, FIG. 11 shows the relationship between the read timing setting value Hz and the exposure time when various CCDs 2 are applied to the video camera 1. Since the number of master clocks varies depending on the type of CCD 2 to be applied, the deviation of the read timing from the ideal value varies depending on the CCD 2.
[0063]
Therefore, as shown in FIG. 11A, when the conventional readout pulse generation circuit 20 is applied, the exposure time varies depending on the type of the CCD 2, but as shown in FIG. When the readout pulse generation circuits 30 and 40 according to the embodiment are applied, it is possible to reduce the variation in exposure time due to the difference in the CCD 2, and thereby the same exposure control can be performed for different CCDs 2.
[0064]
According to the above configuration, the read pulse generation circuits 30 and 40 are provided with the 64 frequency dividing block 32 and the 20 frequency dividing block 42, and by finely adjusting the time interval of the readout timing of the CCD 2, the 64 frequency dividing block 32 and By only providing the divide-by-20 block 42, the exposure time can be smoothly changed over the entire vertical period, so that exposure control with high accuracy and high continuity can be performed with a simple configuration.
[0065]
In the above-described embodiment, the case where the present invention is applied to the video camera 1 has been described. However, the present invention is not limited to this, and is disposed on the imaging surface of the imaging element, such as a digital still camera. The timing for starting the charge accumulation in each pixel is changed at the first time interval, and the read timing for reading the charge accumulated from each pixel is changed to the first time interval within the first time interval. The present invention can be widely applied to various other imaging devices that change the exposure time by changing the second time interval shorter than the second time interval.
[0066]
In the above-described embodiment, the case where the present invention is applied to the CCD 2 has been described. However, the present invention is not limited to this, and the present invention is applied to various other image pickup devices such as a CMOS (Complementary Metal Oxide Semiconductor). May be applied.
[0067]
In the above-described embodiment, the case where the divide-by-7 blocks 31 and 41 are applied as the readout pulse generating means has been described. However, the present invention is not limited to this, and readout for reading out the accumulated charges from each pixel. Various other readout pulse generating means for generating pulses at the second time interval may be applied.
[0068]
Further, in the above-described embodiment, the case where the 64 frequency dividing block 32, the 20 frequency dividing block 42 and the OR circuit 43 are applied as the read pulse generation adjusting means has been described. However, the present invention is not limited to this, and the read pulse is not limited thereto. Various other readout pulse generation adjusting means for adjusting the generation timing of the readout pulse every time a predetermined number of occurrences occur may be applied.
[0069]
【The invention's effect】
As described above, according to the present invention, the read pulse generation adjusting means for adjusting the generation timing of the read pulse every time a predetermined number of read pulses are generated is provided, and by finely adjusting the time interval of the read timing of the image sensor, By only providing the read pulse generation adjusting means, the exposure time can be smoothly changed over the entire vertical period, and therefore, exposure control with high accuracy and high continuity can be performed with a simple configuration.
[Brief description of the drawings]
FIG. 1 is a block diagram showing an embodiment of a video camera according to the present invention.
FIG. 2 is a schematic diagram for explaining adjustment of an exposure time by an electronic shutter pulse.
FIG. 3 is a schematic diagram for explaining adjustment of an exposure time by a readout pulse.
FIG. 4 is a block diagram showing a configuration of a conventional read pulse generation circuit.
FIG. 5 is a schematic diagram showing a conventional timing chart.
FIG. 6 is a block diagram showing a configuration of a read pulse generation circuit according to the present embodiment.
FIG. 7 is a schematic diagram showing a timing chart according to the present embodiment.
FIG. 8 is a block diagram showing a configuration of a read pulse generation circuit according to the present embodiment;
FIG. 9 is a schematic diagram illustrating a timing chart according to the present embodiment.
FIG. 10 is a chart showing a relationship between a read timing setting value and an exposure time.
FIG. 11 is a chart showing a relationship between a read timing setting value and an exposure time.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Video camera, 2 ... CCD, 3 ... Amplifier, 4 ... Timing generator, 5 ... Vsub setting circuit, 6 ... A / D converter, 7 ... Camera DSP circuit, 8 ... Oscillator, 20, 30, 40... Read pulse generation circuit, 21, 31, 41... 7 division block, 22, 33, 44... Read pulse generation block, 32. Dividing block, 43 ... OR circuit.

Claims (6)

The timing at which charge accumulation is started at each pixel arranged on the imaging surface of the image sensor is changed at a first time interval, and the readout timing at which the accumulated charge is read from each pixel is changed to the first timing. In the imaging apparatus that changes the exposure time by changing the second time interval shorter than the first time interval within the time interval,
Read pulse generating means for generating a read pulse for reading the accumulated charge from each pixel at the second time interval;
An image pickup apparatus comprising: a read pulse generation adjusting unit that adjusts a generation timing of the read pulse every time a predetermined number of the read pulses are generated. The read pulse generation adjusting means is
When the change range when the read timing is changed at the second time interval is shorter than the first time interval, the read time is changed to the second time interval every time a predetermined number of read pulses are generated. The imaging apparatus according to claim 1, wherein the readout pulse is generated at a longer time interval. The read pulse generation adjusting means is
When the change range when changing the read timing at the second time interval is longer than the first time interval, the read time is changed to the second time interval every time a predetermined number of read pulses are generated. The imaging apparatus according to claim 1, wherein the readout pulse is generated at a shorter time interval. The timing at which charge accumulation is started at each pixel arranged on the imaging surface of the image sensor is changed at a first time interval, and the readout timing at which the accumulated charge is read from each pixel is changed to the first timing. In the imaging method of the imaging apparatus that changes the exposure time by changing at a second time interval shorter than the first time interval within the range of the time interval,
A first step of generating a readout pulse for reading out the accumulated charge from each of the pixels at the second time interval;
And a second step of adjusting the generation timing of the readout pulse every time a predetermined number of the readout pulses are generated. In the second step,
When the change range when the read timing is changed at the second time interval is shorter than the first time interval, the read time is changed to the second time interval every time a predetermined number of read pulses are generated. The imaging method according to claim 4, wherein the readout pulse is generated at a longer time interval. In the second step,
When the change range when changing the read timing at the second time interval is longer than the first time interval, the read time is changed to the second time interval every time a predetermined number of read pulses are generated. The imaging method according to claim 4, wherein the readout pulse is generated at a shorter time interval.

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