Patent Abstract:
A noise reduction apparatus of a digital camera which uses an imaging device for imaging a photographic subject, carries out a specified number of dark exposure operations, to obtain the same specified number of dark output values for each picture element of the imaging device, in which the photographic subject is imaged, under the condition where the imaging device is shaded, after a normal exposure operation in which the photographic subject is imaged. The apparatus calculates a representative value for each dark exposure operation, based on the dark output values. The apparatus calculates a ratio based on the representative values. The apparatus calculates noise components caused by the dark current in each picture element of the imaging device in the normal exposure operation on the basis of the ratio. The apparatus reduces the noise components from respective output values of each picture element, produced in the normal exposure operation.

Full Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a noise reduction apparatus for a digital camera, and in particular to noise reduction in long time exposure operations such as night-view imaging and astrophotography imaging.  
         [0003]     2. Description of the Related Art  
         [0004]     When a long time exposure operation is carried out in a digital camera which has an imaging device such as a CCD, there is a problem that a dark output by a certain picture element of the imaging device, becomes bigger than that of other picture elements due to the influence of the dispersion of dark current in each picture element on the imaging device, so that the dark output by a certain picture element turns out as a bright point in a dark area on the image.  
         [0005]     Even for an optical black output, which is obtained when a partly shaded CCD is used, the dark current component is not able to be cut and removed, so that an image having generally increased brightness and damaged image quality is obtained.  
         [0006]     Japanese unexamined patent publication (KOKAI) No. 2000-209506 discloses a noise reduction apparatus that images normally (normal exposure operation), stores the time length of an exposure operation, immediately exposes again while shading the CCD (the dark exposure operation) for the same length of time as the time length of the normal exposure operation, and reduces noise components by taking away a dark output obtained by the dark exposure operation from a normal output obtained by the normal exposure operation, for every picture element. In other words canceling the dark output of each picture element in the normal exposure operation by using the dark output of each picture element in the dark exposure operation, is the usual method of reducing the fixed pattern noise occurring due to dispersion of dark current in each picture element.  
         [0007]     However, the above-discussed conventional noise reduction method does not consider the rise in temperature of the CCD associated with the time length of the dark exposure operation which is carried out immediately after the normal exposure operation. Dark current occurs due to heat, and the temperature of a CCD goes up according to the time length of the exposure operation.  
         [0008]     Accordingly, an error results between the dark current component Im, obtained from the normal exposure operation, and the dark current component Id, obtained from the dark exposure operation, for every picture element. That is to say, the temperature of the CCD further goes up in the dark exposure operation, because the CCD has been used continuously from the normal exposure operation, so that the dark current in the dark exposure operation is more than the dark current in the normal exposure operation, corresponding to the rise in temperature of the CCD (Im&lt;Id). The images obtained by taking away the dark current component Id, obtained from the dark exposure operation, from the normal output obtained by the normal exposure operation, create a condition where the dark output is pulled too much, accordingly the color balance of the image collapses and the brightness decreases (under exposure).  
       SUMMARY OF THE INVENTION  
       [0009]     Therefore, an object of the present invention is to provide a noise reduction apparatus for a digital camera which uses an imaging device such as a CCD, that can reduce noise in a long time exposure image, caused by the dispersion of dark current in each picture element of the CCD. The noise-reduction apparatus must consider the temperature rise of the CCD associated with continuous use.  
         [0010]     According to the present invention, a noise reduction apparatus for a digital camera which has an imaging device for imaging a photographic subject, comprises a dark exposure processor, a representative value calculating processor, a ratio calculating processor, a noise component calculating processor, and a noise reducing processor.  
         [0011]     The dark exposure processor carries out a specified number of dark exposure operations to obtain the same specified number of dark output values for each picture element of the imaging device, in which the photographic subject is imaged, under the condition where the imaging device is shaded, after a normal exposure operation in which the photographic subject is imaged.  
         [0012]     The representative value calculating processor calculates a representative value for each dark exposure operation, based on the dark output values.  
         [0013]     The ratio calculating processor calculates a ratio of each representative value.  
         [0014]     The noise component calculating processor calculates noise components caused by the dark current in each picture element of the imaging device in the normal exposure operation, on the basis of the ratio.  
         [0015]     The noise reducing processor reduces the noise components from respective output values of each picture element in the normal exposure operation. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The objects and advantages of the present invention will be better understood from the following description, with reference to the accompanying drawings in which:  
         [0017]      FIG. 1  is a block diagram of the digital camera of this embodiment;  
         [0018]      FIG. 2  is a flowchart showing the normal exposure operation and the dark exposure operation;  
         [0019]      FIG. 3  is a timing chart showing the normal exposure operation and the first and second dark exposure operations;  
         [0020]      FIG. 4  is a timing chart showing the normal exposure operation and the dark exposure operation; and  
         [0021]      FIG. 5  is a graph showing the change of the dark current over time. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0022]     The present invention is described below with reference to the embodiments shown in the drawings.  FIG. 1  shows a block diagram of a digital camera of this embodiment.  
         [0023]     The digital camera  1  which is a single-lens reflex camera, having mount pins  12  and  13 , a CPU  31 , and an iris driving circuit  32 . An interchangeable lens  11  is connected with electric circuits of the digital camera  1 , through the mount pins  12  and  13 . A lens barrel of the interchangeable lens  11  has a front lens  14 , a rear lens  15 , an iris  16 , and a lens control circuit  17 . The iris  16  is set between the front lens  14  and the rear lens  15 . Focusing is carried out by moving the front and rear lenses  14  and  15  along an optical axis LX and is controlled by the lens control circuit  17 . The lens control circuit  17  is controlled by control signals which are transmitted from the CPU  31  through the mount pin  12 . The iris  16  is controlled by control signals which are transmitted from the iris driving circuit  32  through the mount pin  13 , in such a way that a degree of opening of the iris  16  is adjusted. The iris driving circuit  32  is controlled by the CPU  31 .  
         [0024]     The digital camera  1  has a quick return mirror  21  in line with the optical axis LX of the front and rear lenses  14  and  15 . The quick return mirror  21  can be changed between an inclined down position which is depicted, and a level up position which is above the inclined position.  
         [0025]     The digital camera  1  has a focusing glass  22  above the quick return mirror  21  in the level condition, a pentagonal prism  23  above the focusing glass  22 , and an ocular lens  24  of a view finder at the rear of the pentagonal prism  23  (the opposite side of the interchangeable lens  11 ).  
         [0026]     The digital camera  1  has a shutter  25  at the rear of the quick return mirror  21 , an infrared cut off filter  26  and an optical low-pass filter  27  at the rear of the shutter  25 . A CCD (an imaging device)  33  is located at the rear of the optical low-pass filter  27 . Accordingly, the quick return mirror  21 , the shutter  25 , the infrared cut off filter  26 , the optical low-pass filter  27 , and the CCD  33  are in line with the optical axis LX of the front and rear lenses  14  and  15 .  
         [0027]     The digital camera  1  has a mirror driving circuit  34  and a shutter driving circuit  35 . The rotation of the quick return mirror  21  is driven by the mirror driving circuit  34 . The opening and closing action of the shutter  25  is driven by the shutter driving circuit  35 . Further, the mirror driving circuit  34  and the shutter driving circuit  35  are controlled by the CPU  31 .  
         [0028]     The quick return mirror  21  is usually in the inclined position, so that the light which enters the changeable lens  11  is guided to the pentagonal prism  23  by the quick return mirror  21 . At this time, the shutter  25  is closed, so that the light path toward the CCD  33  from the changeable lens  11  is blocked by the shutter  25 .  
         [0029]     During the imaging process, the quick return mirror  21  is rotated upward by the mirror control circuit  34 , so that the quick return mirror  21  is leveled. The shutter  25  is opened by the shutter driving circuit  35  corresponding to the rotation of the quick return mirror  21 , so that the light which passes through the changeable lens  11  is guided to the light-receiving surface of the CCD  33 . An image obtained through the front and rear lenses  14  and  15 , is formed on the light-receiving surface of the CCD  33 , so that imaging signals corresponding to the image are formed by the CCD  33 .  
         [0030]     The digital camera  1  has a DSP (Digital Signal Processor)  40 , an AF (Auto Focus) sensor  52 , a photometric sensor  53 , an operation switch  54 , and a setting indicating apparatus  55 . The lens control circuit  17 , the iris driving circuit  32 , the DSP  40 , the AF sensor  52 , the photometric sensor  53 , the operation switch  54 , and the setting indicating apparatus  55  are connected to the CPU  31 , and are controlled by the CPU  31 , so that the CPU  31  controls the action of the changeable lens  11  which is mounted on the digital camera  1 , and also the digital camera  1 , generally.  
         [0031]     In this embodiment, a dark exposure operation is carried out in addition to the normal exposure operation when the digital camera  1  images a photographic subject for a long time exposure operation which is defined as an exposure operation that has an exposure time over a specified time (a first and second standard time). In this embodiment, a normal exposure operation is defined as an operation where the digital camera images the photographic subject normally, and a dark exposure operation is defined as an operation where the digital camera obtains an image while shading the CCD  33 .  
         [0032]     The first standard time is 60 seconds, and the second standard time is 1 second.  
         [0033]     The dark exposure operation is carried out one time or two times depending on the time length of the normal exposure operation (the normal exposure time Tv), and the first and second standard times.  
         [0034]     The DSP  40  is a control circuit for controlling the exposure operation of the CCD  33  and for processing the image data obtained by the exposure operation. Therefore, the DSP  40  runs the image processes for noise reduction from the data obtained from the normal exposure operation. The noise reduction processes are carried out on the basis of the image data etc. obtained during the normal exposure operation and during the dark exposure operation, both of which are controlled by the CPU  31 .  
         [0035]     The digital camera  1  has a memory  41  which has sufficient capacity to store the digital image data corresponding to images of the photographic subject in the normal exposure and the first and second dark exposure operations, and is connected to the DSP  40 . The memory  41  stores a normal exposure time Tv, and first and second standard times which are used for comparison with the length of the normal exposure time Tv.  
         [0036]     The digital camera  1  has a PPG (Programmable Pulse Generator or Pulse Pattern Generator) circuit  37 , a CCD driving circuit  38 , and an A/D (Analogue/Digital) converter  39 . The PPG circuit  37  is connected with the DSP  40 , so that the PPG circuit  37  generates various pulse signals according to the DSP  40 . The CCD driving circuit  38  is driven on the basis of these various pulse signals, so that the action of the CCD  33  is controlled by the CCD driving circuit  38 . That is, the imaging signals which are read out from the CCD  33 , are converted to digital signals by the A/D converter  39 , and are subjected to specified image processes by the DSP  40 .  
         [0037]     An AFE (Analogue Front End)  36  is composed of the PPG circuit  37 , the CCD driving circuit  38 , and the A/D converter  39 . The AFE  36  outputs a vertical synchronous signal Vd to the CCD  33  with a first or a second cycle, every stated period. The start of the exposure operation, the termination of the exposure operation, and the reading of the data obtained in the exposure operation are carried out according to the vertical synchronous signal Vd and the drive pulse for transferring electric charge.  
         [0038]     The first cycle is shorter than the second cycle. The first cycle is 5 ms. The second cycle is 168.3 ms which is equal to the time needed for the exposure data for one field to be read out.  
         [0039]     The vertical synchronous signal Vd is output with the first cycle except during the normal exposure operation, the dark exposure operations, and the reading of the exposure data, because the response speed corresponding to the setting of the AFE  36  etc. should be high, and because the image which is indicated should be changed with proper timing for the through image.  
         [0040]     The vertical synchronous signal Vd is output with the second cycle during the normal exposure operation, the dark exposure operations, and the reading of the exposure data.  
         [0041]     A noise reduction apparatus relating to the present invention is composed of the CPU  31 , the AFE  36 , and the DSP  40 .  
         [0042]     The digital camera  1  has a monitor interface  42 , a card interface  43 , and a PC interface  44 . The monitor interface  42 , the card interface  43 , and the PC interface  44  are connected to the DSP  40  and are controlled by the DSP  40 .  
         [0043]     The digital camera  1  has an LCD (Liquid Crystal Display) driving circuit  45 , a backlight  46 , an LCD device  47 , a card connector  48 , a PC connector  49 , a video output driving circuit  50 , and a video output terminal  51 .  
         [0044]     The monitor interface  42  is connected with the backlight  46  and the LCD device  47  through the LCD driving circuit  45 , and is connected with the video output terminal  51  through the video output circuit  50 . The LCD driving circuit  45  is controlled on the basis of the image data read out from the memory  41 , so that the image corresponding to the image data is indicated on the LCD device  47 . The image data is converted to the specified format by the video output driving circuit  50 , so that the converted image data is output to external output devices which are not depicted, through the video output terminal  51 .  
         [0045]     The card interface  43  is connected with the card connector  48 , and the PC interface  44  is connected with the PC connector  49 . The card connector  48  can be fixed to the IC memory card which can store image data etc. and is not depicted. The PC connector  49  can be connected to a personal computer which is not depicted.  
         [0046]     The AF sensor  52  and the photometric sensor  53  are connected with the CPU  31 . The AF sensor  52  measures the focus adjustment condition of the front and rear lenses  14  and  15 . The photometric sensor  53  carries out the photometry to automatically decide the degree of opening of the iris  16  during the normal exposure operation and the electric charge accumulation time (the time length of the normal exposure operation).  
         [0047]     The operation switch  54  and the setting indicating apparatus  55  are connected with the CPU  31 . The operation switch  54  has a photometric switch and a release switch etc. The digital camera  1  has a release button which is not depicted. The photometric switch is turned to the on state when the release button is half way depressed. When the photometric switch is in the on state, the photometry is carried out by the photometric sensor  53 . The shutter release switch is tuned to the on state when the release button is fully depressed. And, the shutter  25  is opened and closed, so that the CCD  33  is exposed, and the CCD  33  generates imaging signals corresponding to the image of the photographic subject. The setting indicating apparatus  55  has an LCD device which indicates the various settings of the digital camera  1 .  
         [0048]     Next, the flow of the normal exposure operation and the dark exposure operations is explained (see FIGS.  2  to  4 ).  FIG. 3  shows a timing chart for the case where the normal exposure time Tv is longer than the first standard time.  FIG. 4  shows a timing chart for the case where the normal exposure time Tv is longer than the second standard time and is shorter than or equal to the first standard time.  
         [0049]     The horizontal axes of  FIGS. 3 and 4  represent time.  FIGS. 3 and 4  show the timings where the vertical synchronous signal Vd is output and where pixel data accumulated by the CCD  33  during a normal exposure operation and a dark exposure operation, is read in, corresponding to the pulse input (the release sequence) at the start and termination of the long time exposure operation.  
         [0050]     The flowchart in  FIG. 2  shows the action of the DSP  40  controlled by the CPU  31  after the power switch (not depicted) is turned on.  
         [0051]     The flow starts in step S 11 . It is judged whether the release switch is in the on state by the operator, in step S 12 . When the release switch is in the on state (T 11  in  FIG. 3 , and T 21  in  FIG. 4 ), the normal exposure operation is started (T 12  in  FIG. 3 , and T 22  in  FIG. 4 ), in step S 13 .  
         [0052]     The time length of the normal exposure operation (the normal exposure time Tv) which is the period from T 12  to T 13  in  FIG. 3  is manually set by the operator or is automatically set by the photometry before the release switch is turned to the on state. When the operator manually sets the normal exposure time Tv with the bulb exposure, the length of time during the on state of the release switch is controlled by the operator.  
         [0053]     Immediately after the normal exposure operation is terminated (T 13  in  FIG. 3 , and T 23  in  FIG. 4 ), the operation mode of the AFE  36  is changed from a first mode for the exposure operation, to a second mode for the reading of the data obtained in the exposure operation, from the vertical synchronous signal output point (T 14  in  FIG. 3 , and T 24  in  FIG. 4 ).  
         [0054]     The first mode for the exposure operation of the AFE  36  is the operation mode where electric charge signals, which occur in the imaging device, due to light striking the light-receiving surface of the imaging device and forming the electric charge, are accumulated.  
         [0055]     The second mode for reading of the data obtained in the exposure operation of the AFE  36  is the operation mode where accumulated electric charge is transferred from a receiving-unit in the imaging device to a transferring-unit in the imaging device; the transferring-unit is driven by the transferring-drive pulse signals (not depicted); and the electric charge from the imaging device is gradually read.  
         [0056]     In  FIG. 3 , the normal exposure time is strictly from point T 12  to point T 14 , however during the short time from point T 13  to point T 14 , the exposure operation under the condition of shading of the CCD  33 , is carried out, so that the normal exposure time is actually from point T 12  to point T 13 . Similarly in  FIG. 4 , the normal exposure time is strictly from point T 22  to point T 24 , however during the short time from point T 23  to point T 24 , the exposure operation under the condition of shading of the CCD  33 , is carried out, so that the normal exposure time is actually from point T 22  to point T 23 .  
         [0057]     After the operation mode of the AFE  36  is changed to the second mode for reading of the data obtained in the exposure operation (T 15  in  FIG. 3 , and T 25  in  FIG. 4 ), the exposure data of the normal exposure operation is read for every field, or the reading of the electric charge that was accumulated in each picture element of the CCD  33  for every field, is carried out.  
         [0058]     In the first field, the exposure data of the normal exposure operation is read in for 168.3 ms from point T 15  in  FIG. 3  or point T 25  in  FIG. 4 . In the second field, the exposure data of the normal exposure operation is read in for 168.3 ms from a point which is 168.3 ms passed point T 15  in  FIG. 3  or point T 25  in  FIG. 4 .  
         [0059]     In the normal exposure operation, accumulation of the electric charge is carried out under the condition that the CCD  33  receives light from the photographic subject, so that the accumulated electric charge is composed of the accumulated electric charge corresponding to the light from the photographic subject, and the accumulated electric charge corresponding to the dark current of the CCD  33 . In other words, the accumulated electric charge is the sum of the accumulated electric charge corresponding to the light from the photographic subject, and the accumulated electric charge corresponding to the dark current of the CCD  33 .  
         [0060]     When the reading in the accumulated electric charges in the first and second fields is terminated (T 16  in  FIG. 3 , and T 26  in  FIG. 4 ), the length of the normal exposure time Tv is stored in the digital camera  1  in step S 14 , so that the dark exposure operation is carried out.  
         [0061]     The number of dark exposure operations, and whether or not the dark exposure operation is carried out, are judged according to the length of the normal exposure time Tv. The judgment is carried out according to the first and second standard times which are stored in the memory  41  in advance, and the length of the normal exposure time Tv.  
         [0062]     In step S 15 , it is judged whether or not the length of the normal exposure time Tv is longer than the first standard time (60 sec). When the length of the normal exposure time Tv is longer than 60 sec, the dark exposure operation is carried out two times (step S 16 -S 19  in  FIG. 3 ).  
         [0063]     When the length of the normal exposure time Tv is shorter than or equal to 60 sec, it is judged whether or not the length of the normal exposure time Tv is longer than the second standard time (1 sec). When the length of the normal exposure time Tv is longer than 1 sec, the dark exposure operation is carried out one time (step S 21 -S 22  in  FIG. 4 ).  
         [0064]     This is because there is little temperature rise of the CCD  33  associated with the time length of the dark exposure operation, even when using the noise reduction method in the prior art, where the dark output value due to the dark current at the time of the normal exposure operation is calculated from the dark output value due to the dark current in one dark exposure operation, in the case where the normal exposure time is comparatively short, for example from 1 sec to 60 sec.  
         [0065]     When the length of the normal exposure time Tv is shorter than or equal to 1 sec, the dark exposure operation is not carried out. This is because a dark current has little influence on the dark output value for a normal exposure time, which is comparatively short, for example below 1 sec.  
         [0066]     The case where the dark exposure operation is carried out twice is explained below. In step S 16 , the first dark exposure operation is started (T 16  in  FIG. 3 ). The first dark exposure operation is terminated (T 17  in  FIG. 3 ), after only half of the normal exposure time Tv. That is to say, half the distance from point T 12  to point T 13  which is the normal exposure time Tv, is equal to the distance and therefore the time from point T 16  to point T 17 .  
         [0067]     After the first dark exposure operation, the exposure data of the first dark exposure operation is read for every field, or the reading of the electric charge that has accumulated in each picture element of the CCD  33  for every field is carried out (T 17  in  FIG. 3 ).  
         [0068]     In the first dark exposure operation, the imaging and thereby the accumulating of the electric charge is carried out under the condition that the CCD  33  does not receive light from the photographic subject, so that the accumulated electric charge is composed of the accumulated electric charge corresponding to the dark current of the CCD  33 .  
         [0069]     When the reading of the accumulated electric charge for the first and second fields is terminated (T 18  in  FIG. 3 ), the second dark exposure operation is carried out.  
         [0070]     Next, in step S 17 , the second dark exposure operation is started (T 18  in  FIG. 3 ). The second dark exposure operation is terminated (T 19  in  FIG. 3 ), after half of the normal exposure time Tv. That is to say, half the distance from point T 12  to point T 13  which is the normal exposure time, is equal to the distance and therefore the time from point T 18  to point T 19 , similar to the first dark exposure operation.  
         [0071]     After the second dark exposure operation, the exposure data of the second dark exposure operation is read for every field, or the reading of the electric charge that has accumulated in each picture element of the CCD  33  for every field is carried out (T 19  in  FIG. 3 ).  
         [0072]     In the second dark exposure operation, the imaging and thereby the accumulating of the electric charge is carried out under the condition that the CCD  33  does not receive light from the photographic subject, so that the accumulated electric charge is composed of the accumulated electric charge corresponding to the dark current of the CCD  33 , as in the first dark exposure operation.  
         [0073]     When the reading of the accumulated electric charges in the first and second fields is terminated (T 20  in FIG.  3 ), a mean value, which corresponds to the integration value Sd of the dark current, of the data that is output from each picture element due to the dark current in the normal exposure operation is calculated on the basis of mean values, which respectively correspond to the integration values S2 and S3 of the dark current, of the data that is output from each picture element due to the dark current in the first and second dark exposure operations, in step S 18 .  
         [0074]     In step S 19 , an output value which is deducted from the influence of the dark current of each picture element in the normal exposure operation, in other words which is reduced noise components in the normal exposure operation, is calculated on the basis of the calculated mean value. In step S 23 , the flow is terminated. The method in steps S 18  and S 19  is described later.  
         [0075]     Next, the case where the dark exposure operation is carried out once is explained. After step S 20 , the dark exposure operation is started (T 26  in  FIG. 4 ), in step S 21 . The dark exposure operation is terminated (T 27  in  FIG. 4 ), after the normal exposure time Tv. That is to say, the length of time corresponding to the distance between point T 22  and point T 23 , which is the normal exposure time Tv, is equal to the distance and therefore the time between point T 26  and point T 27 .  
         [0076]     After the dark exposure operation, the exposure data of the dark exposure operation is read for every field, or the reading of the electric charge that has accumulated in each picture element of the CCD  33  for every field, is carried out (T 27  in  FIG. 4 ).  
         [0077]     In the dark exposure operation, the imaging and thereby the accumulating of the electric charge is carried out under the condition that the CCD  33  does not receive light from the photographic subject, so that the accumulated electric charge is composed of the accumulated electric charge corresponding to the dark current of the CCD  33 .  
         [0078]     When the reading of the accumulated electric charges in the first and second fields is terminated (T 28  in  FIG. 4 ), an output value which is deducted from the influence of the dark current of each picture element in the normal exposure operation, in other words which is reduced noise components in the normal exposure operation, is calculated, in step S 22 . In step S 23 , the flow is terminated.  
         [0079]     The dark output value obtained by using the dark current in the dark exposure operation is regarded as equal with the dark output value obtained by using the dark current in the normal exposure operation, in the calculating method of step S 22 .  
         [0080]     When the dark exposure operation is not carried out, the flow is terminated in step S 23 , after step S 20 .  
         [0081]     The calculation for reducing the dark output value of the dark current for the normal exposure data in steps S 18  and S 19  in the case where the dark exposure operations are carried out twice, is explained.  
         [0082]     It can be assumed that the normal exposure operation, the first dark exposure operation, and the second dark exposure operation are continuously carried out. This is because it can be assumed that the normal exposure time (T 12 ˜T 13  in  FIG. 3 ), the first dark exposure time (T 16 ˜T 17  in  FIG. 3 ), and the second dark exposure time (T 18 ˜T 19  in  FIG. 3 ) are sufficiently longer than the time (T 17 ˜T 18  in  FIG. 3 ) etc. required to read the accumulated electric charges.  
         [0083]     Accordingly, the increase in the dark current corresponding to the rise in temperature of the CCD  33  over time in the case where the normal exposure operation, the first dark exposure operation, and the second dark exposure operation are continuously carried out, is like that shown in  FIG. 5 . The horizontal axis of  FIG. 5  represents time and the vertical axis of  FIG. 5  represents the value of the dark current.  
         [0084]     The integration value of the dark current for a certain time section corresponds to the mean value of the dark outputs from each picture element of the CCD  33  for that certain time section.  
         [0085]     In  FIG. 5 , the normal exposure time Tv is given by the time between t0 and t2, which is equal to the time between T 12  and T 13  in  FIG. 3 . Similarly, in  FIG. 5 , the first dark exposure time is given by the time between t2 and t3, which is equal to the time between T 16  and T 17  in  FIG. 3 . Similarly, in  FIG. 5 , the second dark exposure time is given by the time between t3 and t4, which is equal to the time between T 18  and T 19  in  FIG. 3 .  
         [0086]     In  FIG. 5 , the point where only half of the normal exposure time Tv has passed is defined as t1.  
         [0087]     The integration values of the dark current which respectively correspond to the mean values of the dark output values that are output from each picture element of the CCD  33  for the time between t0˜t1, t1˜t2, t2˜t3, t3˜t4, t4˜t5, and t5˜t6, are defined as S0, S1, S2, S3, S4, and S5.  
         [0088]     The electric charge corresponding to only the dark current is accumulated in the first and second dark exposure operations. Accordingly, the integration value of the dark current in the first dark exposure operation S2 is obtained on the basis of the output values which are the same as the dark output values and which are output from each picture element of the CCD  33  in the first dark exposure operation. Similarly, the integration value of the dark current in the second dark exposure operation S3 is obtained on the basis of the output values which are the same as the dark output and which are output from each picture element of the CCD  33  in the second dark exposure operation.  
         [0089]     However, because the electric charge corresponding to not only the dark current, but also the light from the photographic subject is accumulated in the normal exposure operation, the integration value of the dark current in the normal exposure operation Sd (=S0+S1) can not be directly obtained on the basis of output values which are output from each picture element of the CCD  33  in the normal exposure operation. Accordingly, the integration value Sd needs to be obtained from the integration values S2 and S3.  
         [0090]     When the increase curve of the dark current is a linear shape, as shown in  FIG. 5 , the integration values S0, S1, S2, and S3 have the following relationship (S0&lt;S1&lt;S2&lt;S3):  
                   S1   ≈       ⁢     S2   ×     (     S2   ÷   S3     )                   S0   ≈       ⁢     S1   ×     (     S1   ÷   S2     )                     (   1   )                       ⁢     =       ⁢     S2   ×     (     S2   ÷   S3     )     ×     (     S2   ÷   S3     )                 (   2   )             
 
         [0091]     The integration value Sd has the following relationship on the basis of equations (1) and (2).  
                   Sd   =       ⁢     S0   +   S1                 ≈       ⁢       S2   ×     (     S2   ÷   S3     )     ×     (     S2   ÷   S3     )       +     S2   ×     (     S2   ÷   S3     )                     =       ⁢     S2   ×     {         (     S2   ÷   S3     )     ×     (     S2   ÷   S3     )       +     (     S2   ÷   S3     )       }                     (   3   )             
 
         [0092]     The integration value of the dark current Sd is calculated by multiplying the integration value S2 by a coefficient which is calculated on the basis of the ratio of the integration values S2 and S3. The equation (3) is applied to the calculation of the dark current of each picture element, so that the calculation of the dark current component in each picture element data, that is to be removed, is decided.  
         [0093]     The output value of a particular picture element in the normal exposure operation is defined as Pm, similarly the dark output value of the particular picture element in the first dark exposure operation is defined as Pd1. The output value Pm′ of the particular picture element for a normal exposure operation, where the dark output by the dark current, or the noise component, has been removed, is given by the following equation. 
 
 Pm′=Pm−Pd 1×{( S 2÷ S 3)×( S 2 ÷S 3)+( S 2 ÷S 3)}  (4) 
 
         [0094]     If equation (4) is applied to all the picture elements, it becomes possible to calculate the output value for each picture element, where the dark output component (the noise component) of the dark current, in the normal exposure operation, has been removed.  
         [0095]     Accordingly, the mean values, which respectively correspond to the integration values S2 and S3, of the dark outputs in the first and second dark exposure operations, are calculated in steps S 16  and S 17  in  FIG. 2 , so that the mean value, which corresponds to the integration value Sd, of the dark outputs in the normal exposure operation, is calculated in step S 18  in  FIG. 2  on the basis of the integration values S2 and S3. Then the output value of each picture element, where the dark output component of the dark current, is removed, is calculated in step S 19  in  FIG. 2 .  
         [0096]     Furthermore, in this embodiment, it is explained that the integration value of the dark current corresponds to the mean value of the dark output values that are output from each picture element of the CCD  33 , however the integration value of the dark current may correspond to the total value of the dark output values that are output from each picture element of the CCD  33 .  
         [0097]     The mean or total value of the dark output values, for each dark exposure operation, may be determined on the basis of some of the picture elements of all the picture elements of the CCD  33 .  
         [0098]     For simplicity of calculation, the lengths of the first and second dark exposure times are half of the length of the normal exposure time, for carrying out the calculation of the dark current component and the reduction of the dark output value, however the length of the dark exposure time is not limited to this.  
         [0099]     However, if the exposure times were different from the above exposure times, another set of calculation equations whereby the dark output value of the dark current in the normal exposure operation is calculated on the basis of two dark output values of the dark current in the first and second dark exposure operations, would be needed, and the same effect would be obtained.  
         [0100]     The number of dark exposure operations is set at two to perform the calculation quickly. However, the number of dark exposure operations may be set to three or more, so that the accuracy of the calculation for the dark output value of the dark current in the normal exposure operation, becomes high, on the basis of the ratios of a plurality of mean values.  
         [0101]     Furthermore, the length of the first standard time is not limited to 60 seconds, similarly the length of the second standard time is not limited 1 second.  
         [0102]     The digital camera in this embodiment is a single-lens reflex camera, however the digital camera is not limited to this.  
         [0103]     Although the embodiment of the present invention has been described herein with reference to the accompanying drawings, obviously many modifications and changes may be made by those skilled in this art without departing from the scope of the invention.  
         [0104]     The present disclosure relates to subject matter contained in Japanese Patent Application No. 2003-308457 (filed on Sep. 1, 2003), which is expressly incorporated herein by reference, in its entirety.

Technology Classification (CPC): 7