PATENT ABSTRACT
It is now possible to correct flicker without the risk of correcting flicker by error if a moving subject exists in the scene by using values detected in the past in order to prevent degradation of image quality from taking place. The integral value obtained by integrating an input video signal that includes a flicker component over not less than a horizontal period and the difference value between integral values of adjacent fields or frames is normalized. Then, the amplitude component and the phase component of the flicker component are estimated. A flicker coefficient for canceling the amplitude component and the phase component of the flicker component estimated is generated on the basis of the probability of being under the lighting of a fluorescent lamp. The flicker coefficient and the input video signal are computationally determined to obtain a video signal with a reduced flicker component.

PATENT DESCRIPTION
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2005-261095 filed in the Japanese Patent Office on Sep. 8, 2005, the entire contents of which being incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a flicker-reduction method and a flicker-reduction circuit to be used for an image pickup apparatus such as a video camera or a digital still camera including an XY address scanning type imaging element (imager, image sensor), which may typically be a CMOS (complementary metal oxide semiconductor) imaging element, under the lighting of a fluorescent lamp and also to an image pickup apparatus adapted to use such a method and such a circuit. 
     2. Description of the Related Art 
     When an image of a subject is picked up by means of a video camera in direct light from a fluorescent lamp energized by a commercial AC power supply, temporal fluctuations, or so-called flicker, occur in the lightness of the video signal output as a result of the image pickup operation due to the difference between the frequency (twice as high as a commercial AC power supply) of luminance change of the fluorescent lamp (change in the quantity of light) and the vertical synchronizing frequency of the camera. 
     For example, when an image of a subject is picked up by a CCD camera of the NTSC system (with the vertical synchronizing frequency of 60 Hz) under the lighting of a non-inverter type fluorescent lamp in a geographical area where the frequency of the commercially supplied AC is 50 Hz, the exposure value of each pixel changes every field as shown in  FIG. 1  of the accompanying drawings because of that a field period is 1/60 seconds while the period of luminance change of the fluorescent lamp is 1/100 seconds and hence the timing of exposure of each field is shifted relative to the luminance change of the fluorescent lamp. 
     The timing of exposure relative to the luminance change of the fluorescent lamp returns to the original one in every three fields and therefore the change of lightness is cyclic and repetitive with a period of three fields. In other words, the luminance ratio of each field (how flickering appears) changes with the exposure period but the period of flicker does not change. 
     However, the change of lightness in every three frames is repeated with progressive type cameras such as digital cameras when the vertical synchronizing frequency is 30 Hz. 
     In the case of an XY address scanning type imaging element, which may typically be a CMOS imaging element, the timing of exposure of each pixel is sequentially shifted from that of the preceding pixel in the horizontal direction by a clock (pixel clock) period and hence the timings of exposure of all the pixels differ from each other so that flicker arises in each frame and recognized as a pattern of black strips in the image. In other words, there arises a seriously degraded image. 
     Techniques have been proposed to reduce flicker under the lighting of a fluorescent lamp that arises in video signals coming from an XY address scanning type imaging element by extracting still parts where no moving subject exists, detecting flicker from the extracted areas and correcting the flicker (see, for example, Patent Document 1: Jpn. Pat. Appln. Laid-Open Publication No. 2001-119708). 
     SUMMARY OF THE INVENTION 
     However, when a moving subject comes into the scene to occupy the entire area of the picked up image, it is no longer possible to detect flickering because there is no still part in the image. Additionally, since the above-described arrangement uses a plurality of areas produced by dividing an image, it is accompanied by a disadvantage of increasing the circuit dimensions. 
     Therefore, it is desirable to prevent degradation of image quality from taking place by correcting flicker without using a plurality of areas in an image and correcting by using values detected in the past without the risk of correcting flicker by error if a moving subject exists in the scene. 
     Other purposes and specific advantages of the present invention will become apparent from the description given below by way of embodiments. 
     According to an embodiment of the present invention, there is provided a flicker-reduction method for reducing a flicker component of a fluorescent lamp contained in a video signal obtained by shooting a subject by means of an XY address scanning type imaging element under a lighting of a fluorescent lamp, the method including: an integration step of integrating the input video signal obtained by shooting the subject over a time period greater than a horizontal period; a normalization step of normalizing an integral value obtained in the integration step or the difference value between integral values of adjacent fields or frames: an extraction step of extracting the spectrum of the integral value or the difference value normalized in the normalization step; an estimation step of estimating the flicker component from the spectrum extracted in the extraction step; a subtraction step of acquiring the current integral value obtained by integrating the input video signal over a time period greater than a horizontal period and the integral value of a field preceding the current field by several fields and determining the difference of the integral values; a computation step of computing the probability of being under the lighting of a fluorescent lamp from the difference of the integral values determined in the subtraction step; a flicker-reduction signal generation step of generating a flicker-reduction signal for canceling the flicker component estimated in the estimation step according to the probability of being under the lighting of a fluorescent lamp as computed from the difference of the integral values; and an arithmetic operation step of subjecting the flicker-reduction signal generated in the flicker-reduction signal generation step and the input video signal to an arithmetic operation. 
     According to an embodiment of the present invention, there is also provided a flicker-reduction circuit for reducing a flicker component of a fluorescent lamp contained in a video signal obtained by shooting a subject by means of an XY address scanning type imaging element under a lighting of a fluorescent lamp, the circuit including: an integration means for integrating the input video signal obtained by shooting the subject over a time period greater than a horizontal period; a normalization means for normalizing an integral value obtained by the integration means or the difference value between integral values of adjacent fields or frames; an extraction means for extracting the spectrum of the integral value or the difference value normalized by the normalization means; an estimation means for estimating the flicker component from the spectrum extracted by the extraction means; a subtraction means for acquiring the current integral value obtained by integrating the input video signal over a time period greater than a horizontal period and the integral value of a field preceding the current field by several fields and determining the difference of the integral values; a computation means for computing the probability of being under the lighting of a fluorescent lamp from the difference of the integral values determined by the subtraction means; a flicker-reduction signal generation means for generating a flicker-reduction signal for canceling the flicker component estimated by the estimation means according to the probability of being under the lighting of a fluorescent lamp as computed from the difference of the integral values; and an arithmetic operation means for subjecting the flicker-reduction signal generated by the flicker-reduction signal generation means and the input video signal to an arithmetic operation. 
     According to an embodiment of the present invention, there is also provided an image pickup apparatus having a flicker-reduction circuit for reducing a flicker component of a fluorescent lamp contained in a video signal obtained by shooting a subject by means of an XY address scanning type imaging element under a lighting of a fluorescent lamp, the circuit including: an integration means for integrating the input video signal obtained by shooting the subject over a time period greater than a horizontal period; a normalization means for normalizing an integral value obtained by the integration means or the difference value between integral values of adjacent fields or frames; an extraction means for extracting the spectrum of the integral value or the difference value normalized by the normalization means; an estimation means for estimating the flicker component from the spectrum extracted by the extraction means; a subtraction means for acquiring the current integral value obtained by integrating the input video signal over a time period greater than a horizontal period and the integral value of a field preceding the current field by several fields and determining the difference of the integral values; a computation means for computing the probability of being under the lighting of a fluorescent lamp from the difference of the integral values determined by the subtraction means; a flicker-reduction signal generation means for generating a flicker-reduction signal for canceling the flicker component estimated by the estimation means according to the probability of being under the lighting of a fluorescent lamp as computed from the difference of the integral values; and an arithmetic operation means for subjecting the flicker-reduction signal generated by the flicker-reduction signal generation means and the input video signal to an arithmetic operation. 
     Thus, according to the present invention, it is possible to detect and correct the flicker of a fluorescent lamp specific to XY address scanning type imaging elements, which may typically be CMOS elements, regardless of the subject being shot, the video signal level and the type of the fluorescent lamp and reduce erroneous corrections due to a moving subject without using a light receiving element and a plurality of areas. Therefore, it is possible to correct flicker by using values detected in the past without erroneous corrections and prevent the image quality from being degraded by flicker even when the moving subject exists. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of image pickup apparatus according to an embodiment of the present invention; 
         FIG. 2  is a schematic block diagram of the flicker reducing section arranged in the digital signal processing section of the embodiment of image pickup apparatus of  FIG. 1 ; 
         FIG. 3  is a schematic illustration of the flicker of an image where the shot subject is a uniform pattern; 
         FIG. 4  is a schematic block diagram of the parameter control section arranged in the system controller of the image pickup apparatus of  FIG. 1 ; 
         FIG. 5  is a graph of the function of the gain computing section of the parameter control section of  FIG. 4 ; 
         FIG. 6  is a flowchart of the sequence of control operation of the parameter control section of  FIG. 4 ; 
         FIG. 7  is a schematic block diagram of another image pickup apparatus according to the embodiment of the present invention; 
         FIG. 8  is a schematic block diagram of the flicker reducing section arranged in the digital signal processing section of the embodiment of image pickup apparatus of  FIG. 7 ; 
         FIG. 9  is a schematic block diagram of another parameter control section arranged in the system controller of the image pickup apparatus of  FIGS. 1 and 7 ; 
         FIG. 10  is a graph of the function of the coefficient computing section of the parameter control section of  FIG. 9 ; 
         FIG. 11  is a schematic block diagram of the LPF arranged in the parameter control section of  FIG. 9 ; 
         FIG. 12  is a flowchart of the sequence of control operation of the parameter control section of  FIG. 9 ; 
         FIG. 13  is a schematic block diagram of an alternative parameter control section that can be arranged in the system controller of the image pickup apparatus of  FIGS. 1 and 7 ; 
         FIG. 14  is a flowchart of the sequence of control operation of the parameter control section of  FIG. 13 ; 
         FIG. 15  is a schematic block diagram of another alternative parameter control section that can be arranged in the system controller of the image pickup apparatus of FIGS 1  and.  7 ; 
         FIG. 16  is a graph of the function of the switching control section arranged in the parameter control section of  FIG. 15 ; 
         FIG. 17  is a schematic block diagram of the delay circuit arranged in the parameter control section of  FIG. 15 ; and 
         FIG. 18  is a flowchart of the sequence of control operation of the parameter control section of  FIG. 15 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, preferred embodiments according to the present invention will be described in greater detail by referring to the accompanying drawings. However, the present invention is by no means limited to the embodiments described below, which may be modified and altered in various different ways without departing from the spirit and scope of the present invention. 
     For example, the present invention is applicable to an image pickup apparatus having a configuration shown in  FIG. 1 . 
     Referring to  FIG. 1 , the image pickup apparatus  10  is a video camera realized by using an XY address scanning type imaging element, which is a CMOS imaging element  12 . The image pickup apparatus  10  includes an imaging optical system  11 , a CMOS imaging element  12 , an analog signal processing section  13 , a system controller  14 , a lens driving driver  15 , a timing generator  16  and a digital signal processing section  17 . 
     With this image pickup apparatus  10 , light from a subject enters the CMOS imaging element  12  by way of an imaging optical system  11  and is subjected to photoelectric conversion in the CMOS imaging element  12  so that analog video signals are obtained from the CMOS imaging element  12 . 
     The CMOS imaging element  12  is formed by arranging a plurality of pixels having photodiodes (photo gates), transfer gates (shutter transistors), switching transistors (address transistors), amplifier transistors and reset transistors (reset gates) two-dimensionally on a CMOS substrate along with a vertical scanning circuit, a horizontal scanning circuit and a video signal output circuit. 
     The CMOS imaging element  12  may be of the primary color system or of the complementary color system and the analog video signals obtained from the CMOS imaging element  12  may be primary color signals of RGB or color signals of the complementary color system. 
     The analog video signals from the CMOS imaging element  12  are subjected to a sample hold process for each color by analog signal processing section  13  that is realized as an IC (integrated circuit) and controlled for the gain by AGC (automatic gain control) before being converted into digital signals by A/D conversion. 
     The digital video signals from the analog signal processing section  13  are processed by digital signal processing section  17  that is realized as an IC and the flicker component of each signal is reduced by flicker reducing section  18  in the digital signal processing section  17  before they are ultimately converted into luminance signals Y and color difference signals R-Y, B-Y and output from the digital signal processing section  17 . 
     The system controller  14  is typically realized as a microcomputer so as to control the components of the camera. 
     More specifically, a lens drive control signal is supplied from the system controller  14  to lens driving driver  15  that is realized as IC and the lenses of the imaging optical system  11  are driven by the lens driving driver  15 . 
     Similarly, a timing control signal is supplied from the system controller  14  to the timing generator  16  and various timing signals are supplied from the timing generator  16  to the CMOS imaging element  12  to drive the CMOS imaging element  12 . 
     Additionally, the detection signal of each signal component is taken into the system controller  14  from the digital signal processing section  17  so that color signals of different colors are controlled for gain by the analog signal processing section  13  according to the AGC signal from the system controller  14  as described above and the signal processing operation of the digital signal processing section  17  is also controlled by the system controller  14 . 
     As shown in  FIG. 2 , the flicker reducing section  18  arranged in the above-described digital signal processing section  17  includes a normalization process block  20  and an arithmetic operation block  30 , to which digital video signals are supplied from the above-described analog signal processing section  13 , as well as a DFT block  40  connected the normalization process block  20  and a flicker generation block  41  connected to the arithmetic operation block  30 . 
     The normalization process block  20  by turn includes an integration block  21 , to which input video signals In′(x, y), or digital video signals, are supplied from the above-described analog signal processing section  13 , an integral value retaining block  22  connected to the integration block  21 , an average value computing block  23 , a difference computing block  24 , and a normalization block  25 . 
     As shown in  FIG. 3  that illustrates the flicker of an image when the subject is a uniform pattern, the flicker component is generally proportional to the signal intensity of the subject. 
     Thus, if the input video signal (a luminance signal or an RGB primary color signal before being subjected to a flicker reducing process) of a subject at an arbitrarily selected pixel (x, y) in an arbitrarily selected field n is In′ (x, y), In′ (x, y) is the sum of the signal component that does not contain the flicker component and the flicker component that is proportional to the signal component as expressed by the formula (1) shown below.
 
 In ′( x, y )=[1 +Γn ( y )]× In ( x, y )  (1)
 
     In the formula (1), In (x, y) is the signal component and Γn (y)×In (x, y) is the flicker component, where Γn (y) is the flicker coefficient. Since a horizontal period is very short relative to the light emitting period of the fluorescent lamp, it is possible to assume that the flicker coefficient is constant for the same line in the same field. Therefore, the flicker coefficient is expressed by Γn (y). 
     To generalize Γn (y), it will be developed into a Fourier series as indicated by formula (2) shown below. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
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     Then, it is possible to express a flicker coefficient in a form of comprehensively including different light emission characteristics and afterglow characteristics that vary as a function of the type of fluorescent lamp. 
     Referring to the formula (2), λo is the wavelength of the in-image flicker shown in  FIG. 3 . If the number of lines read out per field is M, it corresponds to L (=M/λo) lines. In the formula (2), ωo is the standardized angular frequency that is normalized by λo. 
     In the formula (2), γm is the amplitude of the flicker component of each degree (m=1, 2, 3, . . . ) and Φmn indicates the initial phase of the flicker component of each degree that is determined as a function of the light emission period of the fluorescent lamp and the exposure timing. Note, however, that Φmn is determined by the vertical synchronizing frequency and the frequency of the fluorescent lamp and hence the difference of Φmn between a field and an immediately preceding field, or ΔΦm, n is expressed by formula (3) shown below.
 
ΔΦ m,n   =−m×λ   0 ×2π  (3)
 
     In the integration block  21  of the flicker reducing section  18 , the input video signal In′(x, y) is integrated over a line in the horizontal direction of the image plane to determine the integral value Fn (y) as expressed by formula (4) shown below in order, which reduces the influence of the image pattern when detecting flicker. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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     In the formula (4), αn (y) is the integral value over a line of the signal component In(x, y) that is expressed by formula (5) shown below. 
     
       
         
           
             
               
                 
                   
                     
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     The computationally determined integral value Fn (y) is stored and retained in integral value retaining block  22  for the purpose of flicker detection in the subsequent fields. The integral value retaining block  22  is so designed as to be able to retain the integral values of at least K fields. Note that K is the number of fields necessary for canceling the flicker component that is obtained from the vertical synchronizing frequency fv and the frequency of the fluorescent lamp by means of formula (6) shown below. In the formula (6), GCD is a function for determining the greatest common divisor. 
     
       
         
           
             
               
                 
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     If the subject is a uniform one, the integral value αn (y) of the signal component In (x, y) is a constant value and hence it is easy to extract the flicker component αn (y)×Γn (y) from the integral value Fn (y) of the input video signal In′ (x, y). 
     However, a subject generally contains m×ωo component in αn (y) and hence it is not possible to isolate the luminance component and the color component of the flicker component from the luminance component and the color component of the signal component of the subject itself. In short, it is not possible to purely extract only the flicker component. Additionally, since the flicker component of the second term is very small relative to the signal component of the first term in the formula (4), the flicker component is substantially buried in the signal component. 
     Additionally, the flicker reducing section  18  uses the integral value of continuous K fields in order to remove the influence of αn (y) from the integral value Fn (y). 
     More specifically, in this example, when computationally determining the integral value Fn (y) of a line, the integral value Fn−(K−1) (y) of the same line in the (K−1) preceding field and the integral value Fn — 1 (y) of the same line in the immediately preceding field are read out from an integral value retaining block  22  and the average vale AVE [Fn (y)] of K integral values Fn (y), . . . , Fn−(K−1) (y) is computationally determined. 
     If the subject can be regarded substantially same in the period of the K consecutive fields, it is possible to regard the value of αn (y) same for the K consecutive fields. If the movement of the subject is sufficiently small in the K fields, the above assumption does not practically give rise to any problem. Additionally, when computing the average value of the integral values of K consecutive fields, the signals where the phase of the flicker component is sequentially shifted by −λ0×m×2π are added as seen from the relationship of the formula (3) so that consequently the flicker components are cancelled. Therefore, the average value AVE [Fn (y)] is expressed by formula (7) shown below. 
     
       
         
           
             
               
                 
                   
                     
                       
                         
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                                   k 
                                   = 
                                   0 
                                 
                                 
                                   K 
                                   - 
                                   1 
                                 
                               
                               ⁢ 
                               
                                 Γ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 n_k 
                                 ⁢ 
                                 
                                   ( 
                                   y 
                                   ) 
                                 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                         
                           
                             α 
                             n 
                           
                           ⁡ 
                           
                             ( 
                             y 
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     Note that the average value of the integral values of K consecutive fields is computationally determined in the above description on an assumption that the approximation of formula (8) shown below. However, the approximation of the formula (8) does not hold true when the movement of the subject is large.
 
α n ( y )≅α n-1 ( y )≅α n-2 ( y )  (8)
 
     Then, it is sufficient that the flicker reducing section  18  provided for a situation where the movement of the subject is large retains the integral values of not less than three fields in the integral value retaining block  22  and computationally determines the average value of the integral values of not less than (K+1) fields including the integral value Fn (y) of the current field. With this arrangement, it is possible to reduce the influence of the movement of the subject due to the effect of a temporal low pass filter that operates. 
     Additionally, in the flicker reducing section  18 , the normalization block  25  normalizes the difference value Fn (y)−Fn — 1 (y) from the difference computing block  24  as it divides the difference value by the average value AVE [Fn (y)] from the average value computing block  23  to computationally determine the normalized difference value gn (y). 
     The normalized difference value gn (y) is developed into formula (10) shown below by means of the above formula (7) and formula (9) shown below and the addition/multiplication formula of trigonometric function and expressed by formula (11) below from the relationship of the above formula (3). 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           
                             Fn 
                             ⁢ 
                             
                               ( 
                               y 
                               ) 
                             
                           
                           - 
                           
                             Fn_ 
                             ⁢ 
                             1 
                             ⁢ 
                             
                               ( 
                               y 
                               ) 
                             
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             { 
                             
                               
                                 
                                   α 
                                   n 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   y 
                                   ) 
                                 
                               
                               + 
                               
                                 
                                   
                                     α 
                                     n 
                                   
                                   ⁡ 
                                   
                                     ( 
                                     y 
                                     ) 
                                   
                                 
                                 × 
                                 Γ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   n 
                                   ⁡ 
                                   
                                     ( 
                                     y 
                                     ) 
                                   
                                 
                               
                             
                             } 
                           
                           - 
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           { 
                           
                             
                               
                                 α 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       ⁢ 
                                       _ 
                                     
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               ⁢ 
                               
                                 ( 
                                 y 
                                 ) 
                               
                             
                             + 
                             
                               
                                 α 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     
                                       n 
                                       ⁢ 
                                       _ 
                                     
                                     ⁢ 
                                     
                                         
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                               
                               ⁢ 
                               
                                 ( 
                                 y 
                                 ) 
                               
                               × 
                               Γ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               n_ 
                               ⁢ 
                               1 
                               ⁢ 
                               
                                 ( 
                                 y 
                                 ) 
                               
                             
                           
                           } 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               α 
                               n 
                             
                             ⁡ 
                             
                               ( 
                               y 
                               ) 
                             
                           
                           × 
                           
                             { 
                             
                               
                                 Γ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   n 
                                   ⁡ 
                                   
                                     ( 
                                     y 
                                     ) 
                                   
                                 
                               
                               - 
                               
                                 Γ 
                                 ⁢ 
                                 
                                     
                                 
                                 ⁢ 
                                 n_ 
                                 ⁢ 
                                 1 
                                 ⁢ 
                                 
                                   ( 
                                   y 
                                   ) 
                                 
                               
                             
                             } 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             
                               α 
                               n 
                             
                             ⁡ 
                             
                               ( 
                               y 
                               ) 
                             
                           
                           ⁢ 
                           
                             
                               ∑ 
                               
                                 m 
                                 = 
                                 1 
                               
                               ∞ 
                             
                             ⁢ 
                             
                               γ 
                               ⁢ 
                               
                                   
                               
                               ⁢ 
                               m 
                               × 
                               
                                 { 
                                 
                                   
                                     cos 
                                     ⁡ 
                                     
                                       ( 
                                       
                                         
                                           
                                             m 
                                             × 
                                             ω0 
                                             × 
                                             y 
                                           
                                           + 
                                           
                                             Φ 
                                             ⁢ 
                                             
                                                 
                                             
                                             ⁢ 
                                             m 
                                           
                                         
                                         , 
                                         n 
                                       
                                       ) 
                                     
                                   
                                   - 
                                 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                   m 
                                   × 
                                   ω0 
                                   × 
                                   y 
                                 
                                 + 
                                 
                                   Φ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   m 
                                 
                               
                               , 
                               
                                 n_ 
                                 ⁢ 
                                 1 
                               
                             
                             ) 
                           
                         
                         } 
                       
                     
                   
                 
               
               
                 
                   ( 
                   9 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           
                             g 
                             n 
                           
                           ⁡ 
                           
                             ( 
                             y 
                             ) 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             { 
                             
                               
                                 
                                   F 
                                   n 
                                 
                                 ⁡ 
                                 
                                   ( 
                                   y 
                                   ) 
                                 
                               
                               - 
                               
                                 
                                   F 
                                   
                                     
                                       n 
                                       ⁢ 
                                       _ 
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                                 ⁡ 
                                 
                                   ( 
                                   y 
                                   ) 
                                 
                               
                             
                             } 
                           
                           
                             AVE 
                             ⁡ 
                             
                               [ 
                               
                                 Fn 
                                 ⁡ 
                                 
                                   ( 
                                   y 
                                   ) 
                                 
                               
                               ] 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             ∑ 
                             
                               m 
                               = 
                               1 
                             
                             ∞ 
                           
                           ⁢ 
                           
                             
                               γ 
                               m 
                             
                             × 
                             
                               { 
                               
                                 
                                   cos 
                                   ⁡ 
                                   
                                     ( 
                                     
                                       
                                         m 
                                         × 
                                         
                                           ω 
                                           0 
                                         
                                         × 
                                         y 
                                       
                                       + 
                                       
                                         Φ 
                                         
                                           m 
                                           , 
                                           n 
                                         
                                       
                                     
                                     ) 
                                   
                                 
                                 - 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 m 
                                 × 
                                 
                                   ω 
                                   0 
                                 
                                 × 
                                 y 
                               
                               + 
                               
                                 Φ 
                                 
                                   m 
                                   , 
                                   
                                     
                                       n 
                                       ⁢ 
                                       _ 
                                     
                                     ⁢ 
                                     1 
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             ∑ 
                             
                               m 
                               = 
                               1 
                             
                             ∞ 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 - 
                                 2 
                               
                               ) 
                             
                             × 
                             
                               γ 
                               m 
                             
                             × 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     m 
                                     × 
                                     
                                       ω 
                                       0 
                                     
                                     × 
                                     y 
                                   
                                   + 
                                   
                                     
                                       
                                         Φ 
                                         
                                           m 
                                           , 
                                           n 
                                         
                                       
                                       + 
                                       
                                         Φ 
                                         
                                           m 
                                           , 
                                           
                                             
                                               n 
                                               ⁢ 
                                               _ 
                                             
                                             ⁢ 
                                             1 
                                           
                                         
                                       
                                     
                                     2 
                                   
                                 
                                 ) 
                               
                             
                             × 
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             
                               
                                 
                                     
                                 
                                 ⁢ 
                                 
                                   
                                     Φ 
                                     
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         m 
                                         , 
                                         
                                             
                                         
                                         ⁢ 
                                         n 
                                       
                                     
                                   
                                   ⁢ 
                                   
                                       
                                   
                                   - 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     Φ 
                                     
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         m 
                                         , 
                                         
                                             
                                         
                                         ⁢ 
                                         
                                           
                                             n 
                                             ⁢ 
                                             _ 
                                           
                                           ⁢ 
                                           
                                               
                                           
                                           ⁢ 
                                           1 
                                         
                                       
                                     
                                   
                                 
                               
                               
                                 
                                     
                                 
                                 ⁢ 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   10 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       
                         
                           
                             g 
                             n 
                           
                           ⁡ 
                           
                             ( 
                             y 
                             ) 
                           
                         
                         = 
                           
                         ⁢ 
                         
                           
                             ∑ 
                             
                               m 
                               = 
                               1 
                             
                             ∞ 
                           
                           ⁢ 
                           
                             
                               ( 
                               
                                 - 
                                 2 
                               
                               ) 
                             
                             × 
                             
                               γ 
                               m 
                             
                             × 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     m 
                                     × 
                                     
                                       ω 
                                       0 
                                     
                                     × 
                                     y 
                                   
                                   + 
                                   
                                     Φ 
                                     
                                       m 
                                       , 
                                       n 
                                     
                                   
                                   + 
                                   
                                     
                                       Δ 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         Φ 
                                         
                                           m 
                                           , 
                                           n 
                                         
                                       
                                     
                                     2 
                                   
                                 
                                 ) 
                               
                             
                             × 
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             
                               - 
                               
                                 
                                   ΔΦ 
                                   
                                     m 
                                     , 
                                     n 
                                   
                                 
                                 2 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             ∑ 
                             
                               m 
                               = 
                               1 
                             
                             ∞ 
                           
                           ⁢ 
                           
                             2 
                             × 
                             
                               γ 
                               m 
                             
                             × 
                             
                               cos 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     m 
                                     × 
                                     
                                       ω 
                                       0 
                                     
                                     × 
                                     y 
                                   
                                   + 
                                   
                                     Φ 
                                     
                                       m 
                                       , 
                                       n 
                                     
                                   
                                   + 
                                   
                                     
                                       Δ 
                                       ⁢ 
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         Φ 
                                         
                                           m 
                                           , 
                                           n 
                                         
                                       
                                     
                                     2 
                                   
                                   - 
                                   
                                     π 
                                     2 
                                   
                                 
                                 ) 
                               
                             
                             × 
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           sin 
                           ⁡ 
                           
                             ( 
                             
                               
                                 ΔΦ 
                                 
                                   m 
                                   , 
                                   n 
                                 
                               
                               2 
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             ∑ 
                             
                               m 
                               = 
                               1 
                             
                             ∞ 
                           
                           ⁢ 
                           
                             2 
                             × 
                             
                               γ 
                               m 
                             
                             × 
                             
                               sin 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     ΔΦ 
                                     
                                       m 
                                       , 
                                       n 
                                     
                                   
                                   2 
                                 
                                 ) 
                               
                             
                             × 
                           
                         
                       
                     
                   
                   
                     
                       
                           
                         ⁢ 
                         
                           cos 
                           ⁡ 
                           
                             ( 
                             
                               
                                 m 
                                 × 
                                 
                                   ω 
                                   
                                     
                                         
                                     
                                     ⁢ 
                                     0 
                                   
                                 
                                 × 
                                 y 
                               
                               + 
                               
                                 Φ 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     m 
                                     , 
                                     
                                         
                                     
                                     ⁢ 
                                     n 
                                   
                                 
                               
                               + 
                               
                                 
                                   Δ 
                                   ⁢ 
                                   
                                       
                                   
                                   ⁢ 
                                   
                                     Φ 
                                     
                                       
                                           
                                       
                                       ⁢ 
                                       
                                         m 
                                         , 
                                         
                                             
                                         
                                         ⁢ 
                                         n 
                                       
                                     
                                   
                                 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                               
                               - 
                               
                                 π 
                                 
                                   
                                       
                                   
                                   ⁢ 
                                   2 
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           
                             ∑ 
                             
                               m 
                               = 
                               1 
                             
                             ∞ 
                           
                           ⁢ 
                           
                             
                                
                               
                                 A 
                                 m 
                               
                                
                             
                             × 
                             
                               cos 
                               ⁡ 
                               
                                 ( 
                                 
                                   
                                     m 
                                     × 
                                     
                                       ω 
                                       0 
                                     
                                     × 
                                     y 
                                   
                                   + 
                                   
                                     θ 
                                     m 
                                   
                                 
                                 ) 
                               
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   11 
                   ) 
                 
               
             
           
         
       
     
     Note that |Am| and θm in the formula (11) are respectively expressed by formulas (12) and (13) shown below. 
     
       
         
           
             
               
                 
                   
                      
                     
                       A 
                       m 
                     
                      
                   
                   = 
                   
                     2 
                     × 
                     
                       γ 
                       m 
                     
                     × 
                     
                       sin 
                       ⁡ 
                       
                         ( 
                         
                           
                             ΔΦ 
                             
                               m 
                               , 
                               n 
                             
                           
                           2 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   ( 
                   12 
                   ) 
                 
               
             
             
               
                 
                   
                     θ 
                     m 
                   
                   = 
                   
                     
                       Φ 
                       
                         m 
                         , 
                         n 
                       
                     
                     + 
                     
                       
                         ΔΦ 
                         
                           m 
                           , 
                           n 
                         
                       
                       2 
                     
                     - 
                     
                       π 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   13 
                   ) 
                 
               
             
           
         
       
     
     Since the influence of the signal intensity of the subject remains on the difference value Fn (y)−Fn — 1 (y), the level of the luminance change and that of the color change due to flickering can vary depending on an area in the image. However, the level of the luminance change and that of the color change can be equalized over all the areas of the image as a result of the above-described normalization. 
     Note that |Am| and θm that are expressed respectively by the above formulas (12) and (13) are the amplitude and the initial phase of the spectrum of each degree of the normalized difference value gn (y). Thus, by means of formulas (14) and (15) shown below, it is possible to determine the amplitude γm and the initial phase Φmn of the flicker component of each degree shown in the above-described formula (2), when the normalized difference value gn (y) is Fourier transformed and the amplitude |Am| and the initial phase θm of the spectrum of each degree are detected. 
     
       
         
           
             
               
                 
                   
                     γ 
                     m 
                   
                   = 
                   
                     
                        
                       
                         A 
                         m 
                       
                        
                     
                     
                       2 
                       × 
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             
                               ΔΦ 
                               
                                 m 
                                 , 
                                 n 
                               
                             
                             2 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   14 
                   ) 
                 
               
             
             
               
                 
                   
                     Φ 
                     
                       m 
                       , 
                       n 
                     
                   
                   = 
                   
                     
                       θ 
                       m 
                     
                     - 
                     
                       
                         ΔΦ 
                         
                           m 
                           , 
                           n 
                         
                       
                       2 
                     
                     + 
                     
                       π 
                       2 
                     
                   
                 
               
               
                 
                   ( 
                   15 
                   ) 
                 
               
             
           
         
       
     
     In the instance of the flicker reducing section  18  shown in  FIG. 2 , DFT block  40  performs a discrete Fourier transform of the data that corresponds to the wavelength (for line L) of the flicker in the normalized difference value gn (y) obtained from the normalization block  25 . 
     If the DFT operation is DFT [gn (y)] and the result of the DFT of degree m is Gn (m), the DFT operation is expressed by formula (16) shown below. 
     
       
         
           
             
               
                 
                   
                     DFT 
                     ⁡ 
                     
                       [ 
                       
                         gn 
                         ⁡ 
                         
                           ( 
                           y 
                           ) 
                         
                       
                       ] 
                     
                   
                   = 
                   
                     
                       Gn 
                       ⁡ 
                       
                         ( 
                         m 
                         ) 
                       
                     
                     = 
                     
                       
                         ∑ 
                         
                           i 
                           = 
                           0 
                         
                         
                           L 
                           - 
                           1 
                         
                       
                       ⁢ 
                       
                         
                           gn 
                           ⁡ 
                           
                             ( 
                             i 
                             ) 
                           
                         
                         × 
                         
                           W 
                           
                             m 
                             × 
                             i 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   16 
                   ) 
                 
               
             
           
         
       
     
     W in the formula (16) is expressed by formula (17) shown below. 
     
       
         
           
             
               
                 
                   W 
                   = 
                   
                     exp 
                     ⁡ 
                     
                       [ 
                       
                         
                           - 
                           j 
                         
                         × 
                         
                           
                             2 
                             ⁢ 
                             π 
                           
                           L 
                         
                       
                       ] 
                     
                   
                 
               
               
                 
                   ( 
                   17 
                   ) 
                 
               
             
           
         
       
     
     By the definition of DFT, the relationship between the above-described formulas (12) and (13) and the formula (16) is expressed by formulas (18) and (19) shown below. 
     
       
         
           
             
               
                 
                   
                      
                     Am 
                      
                   
                   = 
                   
                     2 
                     × 
                     
                       
                          
                         
                           Gn 
                           ⁡ 
                           
                             ( 
                             m 
                             ) 
                           
                         
                          
                       
                       L 
                     
                   
                 
               
               
                 
                   ( 
                   18 
                   ) 
                 
               
             
             
               
                 
                   
                     
                       θ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       m 
                     
                     = 
                     
                       
                         tan 
                         
                           - 
                           1 
                         
                       
                       ⁡ 
                       
                         ( 
                         
                           
                             Im 
                             ⁡ 
                             
                               ( 
                               
                                 Gn 
                                 ⁡ 
                                 
                                   ( 
                                   m 
                                   ) 
                                 
                               
                               ) 
                             
                           
                           
                             Re 
                             ⁡ 
                             
                               ( 
                               
                                 Gn 
                                 ⁢ 
                                 
                                   ( 
                                   m 
                                   ) 
                                 
                               
                               ) 
                             
                           
                         
                         ) 
                       
                     
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   where 
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       Im 
                       ⁡ 
                       
                         ( 
                         
                           Gn 
                           ⁡ 
                           
                             ( 
                             m 
                             ) 
                           
                         
                         ) 
                       
                     
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     imaginary 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     part 
                   
                   ⁢ 
                   
                     
 
                   
                   ⁢ 
                   
                     
                       Re 
                       ⁡ 
                       
                         ( 
                         
                           Gn 
                           ⁡ 
                           
                             ( 
                             m 
                             ) 
                           
                         
                         ) 
                       
                     
                     ⁢ 
                     
                       : 
                     
                     ⁢ 
                     real 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     part 
                   
                 
               
               
                 
                   ( 
                   19 
                   ) 
                 
               
             
           
         
       
     
     Thus, from the formulas (14), (15), (18) and (19), it is possible to determine the amplitude γm and the initial phase Φmn of the flicker component of each degree by means of formulas (20) and (21) shown below. 
     
       
         
           
             
               
                 
                   
                     γ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     m 
                   
                   = 
                   
                     
                        
                       
                         Gn 
                         ⁡ 
                         
                           ( 
                           m 
                           ) 
                         
                       
                        
                     
                     
                       L 
                       × 
                       
                         sin 
                         ⁡ 
                         
                           ( 
                           
                             
                               ΔΦ 
                               
                                 m 
                                 , 
                                 n 
                               
                             
                             2 
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   20 
                   ) 
                 
               
             
             
               
                 
                   
                     Φ 
                     ⁢ 
                     
                         
                     
                     ⁢ 
                     m 
                   
                   , 
                   
                     n 
                     = 
                     
                       
                         
                           tan 
                           
                             - 
                             1 
                           
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               Im 
                               ⁡ 
                               
                                 ( 
                                 
                                   Gn 
                                   ⁡ 
                                   
                                     ( 
                                     m 
                                     ) 
                                   
                                 
                                 ) 
                               
                             
                             
                               Re 
                               ⁡ 
                               
                                 ( 
                                 
                                   Gn 
                                   ⁢ 
                                   
                                     ( 
                                     m 
                                     ) 
                                   
                                 
                                 ) 
                               
                             
                           
                           ) 
                         
                       
                       - 
                       
                         
                           ΔΦ 
                           
                             m 
                             , 
                             n 
                           
                         
                         2 
                       
                       + 
                       
                         π 
                         2 
                       
                     
                   
                 
               
               
                 
                   ( 
                   21 
                   ) 
                 
               
             
           
         
       
     
     The data length of the DFT operation is made equal to the wavelength of the flicker because, by doing so, it is made possible to directly obtain a group of discrete spectra of integer times of ωo. 
     Generally, FFT (fast Fourier transform) is used for a Fourier transform when processing a digital signal. Nevertheless, however, DFT is used in this embodiment of the present invention. The reason for this is that DFT is more convenient than FFT because the data length of the Fourier transform is not equal to a power-of-two. However, it is also possible to use FFT by processing the input/output data. 
     Under the lighting of a fluorescent lamp, it is actually possible to satisfactorily approximate the flicker component even if the range of the number m is limited to b less than the tenth. Therefore, since it is not necessary to output all the data of the DFT operation, the use of DFT is not disadvantageous for the purpose of the present invention if compared with the use of FFT in terms of efficiency of operation. 
     The DFT block  40  firstly extracts spectra with a DFT operation defined by the formula (16) and subsequently the amplitude γm and the initial phase Φmn of the flicker component of each degree are estimated by means of operations using the formulas (21) and (22). 
     In the flicker reducing section  18  of  FIG. 2 , the flicker generation block  41  computationally determines the flicker coefficient Γn (y) expressed by the above-described formula (2) from the estimated values of γm and Φmn obtained from the DFT block  40 . 
     However, as pointed out above, under the lighting of a fluorescent lamp, it is actually possible to satisfactorily approximate the flicker component if the range of the degree m is limited to be less than the tenth. Therefore, when computationally determining the flicker coefficient Γn (y) by means of the formula (2), it is possible to limit the degree of summation to a predetermined number, for example to the second order so as not to make it infinite. 
     From the above-described formula (1), the signal component In (x, y) that does not contain any flicker component is expressed by formula (22) shown below. 
     
       
         
           
             
               
                 
                   
                     In 
                     ⁡ 
                     
                       ( 
                       
                         x 
                         , 
                         y 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         In 
                         ′ 
                       
                       ⁡ 
                       
                         ( 
                         
                           x 
                           , 
                           y 
                         
                         ) 
                       
                     
                     
                       1 
                       + 
                       
                         Γ 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         
                           n 
                           ⁡ 
                           
                             ( 
                             y 
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   22 
                   ) 
                 
               
             
           
         
       
     
     In the flicker reducing section  18  of  FIG. 2 , arithmetic operation block  30  adds one to the flicker coefficient Γn (y) obtained from the flicker generation block  41  and divides the input video signal In′ (x, y) by the sum [1+Γn (y)]. 
     Then, as a result, the flicker component contained in the input video signal In′ (x, y) is substantially completely eliminated and a signal component In (x, y) that practically does not contain any flicker component is obtained as output video signal (RGB primary color signal or luminance signal with a reduced flicker) from the arithmetic operation block  30 . 
     The system controller  14  of the image pickup apparatus  10  receives the amplitude γm and the initial phase Φmn of the flicker component from the DFT block  40  of the above-described flicker reducing section  18  as inputs and is provided with a parameter control section  14 *, to which the integral value Y(n) and the integral value Y(n−k) of the flicker detecting area of the K preceding field from the integral value retaining block  22  are supplied. 
     The parameter control section  14 * may typically be a parameter control section  14 A having a configuration as shown in  FIG. 4 . 
     The parameter control section  14 A includes an amplitude gain computing section  50  and a multiplier  55 . 
     The parameter control section  14 A receives as inputs the amplitude γm and the initial phase Φmn of the flicker component of each degree determined by the DFT block  40 . 
     The amplitude gain computing section  50  is adapted to output the suppressed gain of the amplitude γm of the flicker component from both the total integral value Y(n) of the current flicker detecting area and the total integral value Y(n−k) of the flicker detecting area of the K preceding field. The amplitude gain computing section  50  includes a subtracter  51 , to which the integral value Y (n) of the current flicker detecting area and the integral value Y(n−k) of the flicker detecting area of the K preceding field are supplied from the integral value retaining block  22 , an absolute value (ABS) circuit  52  connected to the subtracter  51 , a low pass filter (LPF)  53  connected to the ABS circuit  52 , and a gain computing section  54  connected to the LPF  53 . 
     The subtracter  51  computationally determines the difference integral value ΔY(n) of the current integral value Y(n) and the integral value Y(n−K) of the K preceding field. 
     The ABS circuit  52  turns the output value of the subtracter  51  into the corresponding absolute value. When the output value of the subtracter  51  is small, it is possible to presume that there is no moving subject and the amplitude γm is regarded to be highly reliable. If, on the other hand, the output value of the subtracter  51  is large, the amplitude γm is regarded to be lowly reliable because it is possible to presume that there is a moving subject. 
     The LPF  53  is a filter for reducing any excessive fluctuations of the difference integral value |ΔY(n)| output from the ABS circuit  52  that arise due to external turbulences. The LPF is preferably arranged with a time constant that provides a time period good for stably determining if the scene to be shot is under the lighting of a fluorescent lamp or under the lighting of non-fluorescent lamp and prevents the LPF from reacting to external turbulences too sensitively. 
     The gain computing section  54  outputs a value between 0 and 1 according to the output value of the LPF  53  as shown in  FIG. 5 . More specifically, the gain computing section  54  outputs 0 when the output value of the LPF  53  is greater than a threshold value thrB, whereas it outputs 1 when the output value of the LPF  53  is smaller than a threshold value thrA but it outputs a linearly interpolated value when the output value of the LPF  53  is between the threshold value thrA and the threshold value thrB. In short, it outputs 1 when the reliability of the output of the LPF  53  is high but 0 when the reliability is low. 
     Then, the multiplier  55  multiplies the amplitude γm of the flicker component of each degree by the output value of the gain computing section  54 . 
     The gain computing process is executed for each degree of DFT. Basically, the first degree of DFT is the main component. However, it is desirable to computationally determine the gain for higher degrees in a situation where the components of higher degrees arise particularly when a high speed shutter is used. Additionally, there arises no problem if the gain is computationally determined constantly for higher degrees because any spectrum other than that of a fluorescent lamp is shifted far from the phase component of a fluorescent lamp and the gain is suppressed consequently. 
       FIG. 6  is a flowchart of the sequence of control operation of the parameter control section  14 A. 
     Referring to  FIG. 6 , in the first step, or Step S 10 , the difference between the current integral value Y(n) and the integral value Y(n−K) of the K preceding field is set to difference integral value ΔY(n). 
     In the next step, or Step S 11 , the absolute value of the different integral value ΔY(n) is set to ΔY ABS (n). 
     In Step S 12 , the value obtained by applying an LPF process to ΔY ABS (n) is set to ΔY LPF     —     ABS (n). 
     In Step S 13 , the suppressed gain is computationally determined by means of the function shown in  FIG. 5  from ΔY LPF     —     ABS (n). 
     The present invention can be applied to an image pickup apparatus  100  having a configuration as shown in  FIG. 7 . 
     Referring to  FIG. 7 , the image pickup apparatus  100  is a video camera realized by using an XY address scanning type imaging element, which is a CMOS imaging element  112 . The image pickup apparatus  100  includes an imaging optical system  111 , a CMOS imaging element  112 , an analog signal processing section  113 , a system controller  114 , a lens driving driver  115 , a timing generator  116 , a digital signal processing section  117 , and a luminance detecting section  119 . 
     With this image pickup apparatus  100 , light from a subject enters the CMOS imaging element  112  by way of an imaging optical system  111  and is subjected to photoelectric conversion in the CMOS imaging element  112  so that analog video signals are obtained from the CMOS imaging element  112 . 
     The CMOS imaging element  112  is formed by arranging a plurality of pixels having photodiodes (photo gates), transfer gates (shutter transistors), switching transistors (address transistors), amplifier transistors and reset transistors (reset gates) two-dimensionally on a CMOS substrate along with a vertical scanning circuit, a horizontal scanning circuit, and a video signal output circuit. 
     The CMOS imaging element  112  may be of the primary color system or of the complementary color system and the analog video signals obtained from the CMOS imaging element  112  may be primary color signals of RGB or color signals of the complementary color system. 
     The analog video signals from the CMOS imaging element  112  are subjected to a sample hold process for each color by analog signal processing section  113  that is realized as an IC (integrated circuit) and controlled for the gain by AGC (automatic gain control) before being converted into digital signals by A/D conversion. 
     The digital video signals from the analog signal processing section  113  are processed by digital signal processing section  117  that is realized as an IC and the flicker component of each signal is reduced by flicker reducing section  118  in the digital signal processing section  117  before they are ultimately converted into luminance signals Y and color difference signals R-Y, B-Y and output from the digital signal processing section  117 . 
     The digital video signals from the analog signal processing section  113  are supplied to the luminance detecting section  119 , which luminance detecting section  119  outputs the current luminance level Y(n) and the luminance level of the K preceding field Y(n−K) to the system controller  114 . 
     The system controller  114  is typically realized as a microcomputer so as to control the components of the camera. 
     More specifically, a lens drive control signal is supplied from the system controller  114  to lens driving driver  115  that is realized as IC and the lenses of the imaging optical system  111  are driven by the lens driving driver  115 . 
     Similarly, a timing control signal is supplied from the system controller  114  to timing generator  116  and various timing signals are supplied from the timing generator  116  to the CMOS imaging element  112  to drive the CMOS imaging element  112 . 
     Additionally, the detection signal of each signal component is taken into the system controller  114  from the digital signal processing section  117  so that color signals of different colors are controlled for gain by the analog signal processing section  113  according to the AGC signal from the system controller  114  as described above and the signal processing operation of the digital signal processing section  120  is also controlled by the system controller  114 . 
     As shown in  FIG. 8 , the flicker reducing section  18  arranged in the above-described digital signal processing section  117  includes a normalization process block  120  and an arithmetic operation block  130 , to which digital video signals are supplied from the above-described analog signal processing section  113 , as well as a DFT block  140  connected to the normalization process block  120  and a flicker generation block  141  connected to the arithmetic operation block  130 . 
     The normalization process block  120  by turn includes an integration block  121 , to which input video signals In′(x, y), or digital video signals, are supplied from the above-described analog signal processing section  113 , an integral value retaining block  122  connected to the integration block  121 , an average value computing block  123 , a difference computing block  124 , and a normalization block  125 . 
     In the flicker reducing section  118 , the input video signal In′(x, y) is integrated over a line in the horizontal direction of the image plane to determine the integral value Fn (y) by the integration block  121  as expressed by the above-described formula (4) shown below in order to reduce the influence of the image when detecting flicker. 
     The integral value Fn (y) that is computationally determined by the integration block  121  is stored and retained in integral value retaining block  122  for the purpose of flicker detection in the subsequent fields. The integral value retaining block  122  is so designed as to be able to retain the integral values of at least K fields. Note that K is the number of fields necessary for canceling the flicker component that is obtained from the vertical synchronizing frequency fv and the frequency of the fluorescent lamp fl by means of the above-described formula (6). In the formula (6), GCD is a function for determining the greatest common divisor. 
     If the subject is a uniform one, the integral value αn (y) of the signal component In (x, y) is a constant value and hence it is easy to extract the flicker component αn (y) * Γn (y) from the integral value Fn (y) of the input video signal In′ (x, y). 
     However, a subject generally contains m * ωo component in αn (y) and hence it is not possible to isolate the luminance component and the color component of the flicker component from the luminance component and the color component of the signal component of the subject itself. In short, it is not possible to purely extract only the flicker component. Additionally, since the flicker component of the second term is very small relative to the signal component of the first term in the formula (4), the flicker component is substantially buried in the signal component. 
     Additionally, the flicker reducing section  18  uses the integral value of continuous K fields in order to remove the influence of αn (y) from the integral value Fn (y). 
     More specifically, in this example, when computationally determining the integral value Fn (y) of a line, the integral value Fn−(K−1) (y) of the same line in the (K−1) preceding field and the integral value Fn — 1 (y) of the same line in the immediately preceding field are read out from an integral value retaining block  122  and the average vale AVE [Fn (y)] of K integral values Fn (y), . . . , Fn−(K−1) (y) is computationally determined from the average value computing block  123 . 
     If the subject can be regarded substantially same in the period of the K consecutive fields, it is possible to regard the value of αn (y) same for the K consecutive fields. If the movement of the subject is sufficiently small in the K fields, the above assumption does not practically give rise to any problem. Additionally, when computing the average value of the integral values of K consecutive fields, the signals where the phase of the flicker component is sequentially shifted by −λ0×m×2π are added as seen from the relationship of the above-described formula (3). Thus, consequently the flicker components are cancelled. Therefore, the average value AVE [Fn (y)] is expressed by the above-described formula (7). 
     Note that the average value of the integral values of K consecutive fields is computationally determined in the above description on an assumption that the approximation of the above-described formula (8) shown below holds true. However, the approximation of the formula (8) does not hold true when the movement of the subject is large. 
     Then, it is sufficient that the flicker reducing section  18  provided for a situation where the movement of the subject is large retains the integral values of not less than three fields in the integral value retaining block  122  and computationally determines the average value of the integral values of not less than (K+1) fields including the integral value Fn (y) of the current field. With this arrangement, it is possible to reduce the influence of the movement of the subject due to the effect of a temporal low pass filter that operates. 
     Additionally, in the flicker reducing section  118 , the normalization block  125  normalizes the difference value Fn (y)−Fn — 1 (y) from the difference computing block  124  as it divides the difference value by the average value AVE [Fn (y)] from the average value computing block  123  to computationally determine the normalized difference value gn (y). 
     The normalized difference value gn (y) is expressed by the above-described formula (11). 
     Since the influence of the signal intensity of the subject remains on the difference value Fn (y)−Fn — 1 (y), the level of the luminance change and that of the color change due to flickering can vary depending on the area in the image. However, the level of the luminance change and that of the color change can be equalized over all the areas of the image as a result of the above-described normalization. 
     Note that |Am| and θm that are expressed respectively by the above formulas (12) and (13) are the amplitude and the initial phase of the spectrum of each degree of the normalized difference value gn (y). Thus, by means of the above-described formulas (14) and (15), it is possible to determine the amplitude γm and the initial phase Φmn of the flicker component of each degree shown in the above-described formula (2) when the normalized difference value gn (y) is Fourier transformed and the amplitude |Am| and the initial phase θm of the spectrum of each degree are detected. 
     In the instance of the flicker reducing section  118  shown in  FIG. 8 , DFT block  140  performs a discrete Fourier transform of the data that corresponds to the wavelength (for line L) of the flicker in the normalized difference value gn (y) obtained from the normalization block  125 . 
     If the DFT operation is DFT [gn (y)] and the result of the DFT of degree m is Gn (m), the DFT operation is expressed by the above-described formula (16). 
     Thus, it is possible to determine the amplitude γm and the initial phase Φmn of the flicker component of each degree by means of the above-described formulas (20) and (21). 
     The data length of the DFT operation is made equal to the wavelength of the flicker (for line L) because, by doing so, it is made possible to directly obtain a group of discrete spectra of integer times of ωo. 
     Generally, FFT (fast Fourier transform) is used for a Fourier transform when processing a digital signal. Nevertheless, however, DFT is used in this embodiment of the present invention. The reason for this is that DFT is more convenient than FFT because the data length of the Fourier transform is not equal to a power-of-two. However, it is also possible to use FFT by processing the input/output data. 
     Under the lighting of a fluorescent lamp, it is actually possible to satisfactorily approximate the flicker component if the range of the degree m is limited to be less than the tenth. Therefore, since it is not necessary to output all the data of the DFT operation, the use of DFT is not disadvantageous for the purpose of the present invention if compared with the use of FFT in terms of efficiency of operation. 
     The DFT block  40  firstly extracts spectra with a DFT operation defined by the above-described formula (16) and subsequently the amplitude γm and the initial phase Φmn of the flicker component of each degree are estimated by means of operations using the above-described formulas (21) and (22). 
     In the flicker reducing section  118  of  FIG. 8 , the flicker generation block  141  computationally determines the flicker coefficient Γn (y) expressed by the above-described formula (2) from the estimated values of γm and Φmn obtained from the DFT block  140 . 
     However, as pointed out above, under the lighting of a fluorescent lamp, it is actually possible to satisfactorily approximate the flicker component if the range of the degree m is limited to be less than the tenth. Therefore, when computationally determining the flicker coefficient Γn (y) by means of the above-described formula (2), it is possible to limit the degree of summation to a predetermined number, for example to the second order so as not to make it infinite. 
     From the above-described formula (1), the signal component In (x, y) that does not contain any flicker component is expressed by the above-described formula (22). 
     Thus, in the flicker reducing section  118  of  FIG. 8 , arithmetic operation block  130  adds one to the flicker coefficient Γn (y) obtained from the flicker generation block  141  and divides the input video signal In′ (x, y) by the sum [1+Γn (y)]. 
     Then, as a result, the flicker component that is contained in the input video signal In′ (x, y) is substantially completely eliminated and a signal component In (x, y) that practically does not contain any flicker component is obtained as output video signal (RGB primary color signal or luminance signal with a reduced flicker) from the arithmetic operation block  130 . 
     Thus, the system controller  114  of the image pickup apparatus  110  receives the amplitude γm and the initial phase Φmn of the flicker component from the DFT block  140  of the above-described flicker reducing section  118  as inputs and is provided with a parameter control section  14 *, to which the integral value Y(n) and the integral value Y(n−k) of the flicker detecting area of the K preceding field from the luminance detecting section  119  are supplied. 
     The parameter control section  14 * may typically be a parameter control section  14 A having a configuration as shown in  FIG. 4  and executes a gain computation process for each DFT degree, following the sequence illustrated in the flowchart of  FIG. 6  so as to output the suppressed gain of the amplitude γm of the flicker component from the total integral value Y(n) of the current flicker detecting area and the total integral value Y(n−k) of the flicker detecting area of the K preceding field. 
     The parameter control section  14 * arranged in the system controller  14  of the above-described image pickup apparatus  10  or the system controller  114  of the above-described image pickup apparatus  100  may not be a parameter control section  14 A having a configuration as shown in  FIG. 4  and may alternatively be a parameter control section  14 B having a configuration as shown in  FIG. 9 . 
     The parameter control section  14 B of  FIG. 9  has a filter coefficient computing section  60  and two low pass filters (LPFs)  65   a ,  65   b  whose filter characteristics are variable. 
     In the parameter control section  14 B, the amplitude γm and the initial phase Φmn of the flicker component of each degree as determined by the above-described DFT block  40  or the DFT block  140  are input to the two LPFs  65   a ,  65   b  and the total integral value Y(n) of the current flicker detecting area and the total integral value Y(n −k) of the flicker detecting area of the K preceding field are input to the above-described flicker coefficient computing section  60 . 
     The flicker coefficient computing section  60  includes a subtracter  61 , to which the integral value Y (n) of the current flicker detecting area and the integral value Y(n −k) of the flicker detecting area of the K preceding field are supplied, an absolute value (ABS) circuit  62  connected to the subtracter  61 , a low pass filter (LPF)  63  connected to the ABS circuit  62 , and two coefficient computing sections  64   a ,  64   b  connected to the LPF  63 . 
     In the filter coefficient computing section  60 , the subtracter  61  computationally determines the difference integral value ΔY(n) of the current integral value Y(n) and the integral value Y(n−K) of the K preceding field. 
     The ABS circuit  62  turns the output value of the subtracter  61  into the corresponding absolute value. When the output value of the subtracter  61  is small, it is possible to presume that there is no moving subject and the amplitude γm and the initial phase Φmn are regarded to be highly reliable. If, on the other hand, the output value of the subtracter  61  is large, the amplitude γm and the initial phase Φmn are regarded to be lowly reliable because it is possible to presume that there is a moving subject. 
     The LPF  63  is a filter for reducing any excessive fluctuations of the difference integral value |ΔY(n)| output from the ABS circuit  62  that arise due to external turbulences. The LPF is preferably arranged with a time constant that provides a time period good for stably determining if the scene to be shot is under the lighting of a fluorescent lamp or under the lighting of non-fluorescent lamp and prevents the LPF from reacting to external turbulences too sensitively. 
     The coefficient computing sections  64   a ,  64   b  output a value between 0 and 1 according to the output value of the LPF  63  as shown in  FIG. 10 . More specifically, they output 0 when the output value of the LPF  63  is greater than a threshold value thrB, whereas they output 1 when the output value of the LPF  63  is smaller than a threshold value thrA but they output a linearly interpolated value when the output value of the LPF  63  is between the threshold value thrA and the threshold value thrB. In short, they output 1 when the reliability of the output of the LPF  63  is highest but 0 when the reliability is lowest. 
     The LPF  65   a  executes an LPF process on the phase Φmn of the flicker component of each degree by means of the filter coefficient indicated by the coefficient computing section  64   a.    
     The LPF  65   b  executes an LPF process on the amplitude γm of the flicker component of each degree by means of the filter coefficient indicated by the coefficient computing section  64   b.    
     As shown in  FIG. 11 , each of the LPFs  65   a ,  65   b  has a weighting circuit  651  for weighting with weight a, an adder  652 , a delay circuit  653  for producing a delay quantity of Z −1  and a weighting circuit  654  with weight  1 -a. The current detection value is heavily weighted when the coefficient a is large, whereas the current detection value is lightly weighted and the past detection value is heavily weighted when the coefficient a is small. With this arrangement, it is possible to make a right correction and eliminate correction errors by using a past detection value if a moving subject comes into the scene. 
     The coefficient computing process is executed for each degree of DFT. Basically, the first degree of DFT is the main component. However, it is desirable to computationally determine the gain for higher degrees in a situation where the components of higher degrees arise particularly when a high speed shutter is used. Additionally, there arises no problem if the coefficient is computationally determined constantly for higher degrees because any spectrum other than that of a fluorescent lamp is shifted far from the phase component of a fluorescent lamp and the gain is suppressed consequently. 
     A long time constant may be selected for the LPF  65   a  for the phase regardless of Y(n) and only the LPF  65   b  may be made variable as a function of Y(n) to reduce correction errors due to the subject. However, a slow following performance may take place in a transient situation from the presence of lighting of a non-inverter fluorescent lamp to the absence of lighting of a non-inverter fluorescent lamp and vice versa. 
       FIG. 12  is a flowchart of the sequence of control operation of the parameter control section. 
     Referring to  FIG. 12 , in the first step, or Step S 20 , the difference between the current integral value Y(n) and the integral value Y(n−K) of the K preceding field is set to difference integral value ΔY(n). 
     In the next step, or Step S 21 , the absolute value of the different integral value ΔY(n) is set to ΔY ABS (n). 
     In Step S 22 , the value obtained by applying an LPF process to ΔY ABS (n) is set to ΔY LPF     —     ABS (n). 
     In Step S 23 , the filter coefficient is computationally determined by means of the function shown in  FIG. 10  from ΔY LPF     —     ABS (n). The function shown in  FIG. 10  retains threshold values respectively for the amplitude and the phase and the computation for determining the filter coefficient is conducted for the amplitude and also for the phase. 
     The parameter control section  14 * arranged in the system controller  14  of the above-described image pickup apparatus  10  or the system controller  114  of the above-described image pickup apparatus  100  may alternatively be a parameter control section  14 C having a configuration as shown in  FIG. 13 . 
     The parameter control section  14 C of  FIG. 13  has a computing section  70 , a low pass filter (LPF)  75   a  whose filter characteristics are variable, and a multiplier  75   b.    
     In the parameter control section  14 C, the amplitude γm and the initial phase Φmn of the flicker component of each degree as determined by the above-described DFT block  40  or the DFT block  140  are input to the LPF  75   a  and the multiplier  75   b , while the total integral value Y(n) of the current flicker detecting area and the total integral value Y(n−k) of the flicker detecting area of the K preceding field are input to the above-described computing section  70  from the normalization process block  20  or the luminance detecting section  119  described earlier. 
     The computing section  70  is adapted to output the LPF coefficient of the amplitude γm of the flicker component from the total integral value Y(n) of the current flicker detecting area and the total integral value Y(n−k) of the flicker detecting area of the K preceding field. The computing section  70  includes a subtracter  71 , to which the total integral value Y (n) of the current flicker detecting area and the total integral value Y(n−k) of the flicker detecting area of the K preceding field are supplied, an absolute value (ABS) circuit  72  connected to the subtracter  71 , a low pass filter (LPF)  73  connected to the ABS circuit  72 , and a coefficient computing section  74   a  and a gain computing section  74   b  connected to the LPF  73 . 
     In the computing section  70 , the subtracter  71  computationally determines the difference integral value ΔY(n) between the current integral value Y(n) of the current flicker detecting area and the integral value Y(n−K) of the flicker detecting area of the K preceding field. 
     The ABS circuit  72  turns the output value of the subtracter  71  into the corresponding absolute value. When the output value of the subtracter  71  is small, it is possible to presume that there is no moving subject. The amplitude γm and the initial phase Φmn are regarded to be highly reliable, therefore. If, on the other hand, the output value of the subtracter  71  is large, the amplitude γm and the initial phase Φmn are regarded to be lowly reliable because it is possible to presume that there is a moving subject. 
     The LPF  73  is a filter for reducing any excessive fluctuations of the difference integral value |ΔY(n)| output from the ABS circuit  72  that arise due to external turbulences. The LPF is preferably arranged with a time constant that provides a time period good for stably determining if the scene to be shot is under the lighting of a fluorescent lamp or under the lighting of non-fluorescent lamp and prevents the LPF from reacting to external turbulences too sensitively. 
     The coefficient computing sections  74   a  outputs a value between 0 and 1 according to the output value of the LPF  73  as shown in  FIG. 10 . More specifically, it outputs 0 when the output value of the LPF  73  is greater than a threshold value thrB, whereas it outputs 1 when the output value of the LPF  73  is smaller than a threshold value thrA but it outputs a linearly interpolated value when the output value of the LPF  73  is between the threshold value thrA and the threshold value thrB. In short, it outputs 1 when the reliability of the output of the LPF  73  is highest but 0 when the reliability is lowest. 
     The gain computing sections  74   b  outputs a value between 0 and 1 according to the output value of the LPF  73  as shown in  FIG. 10 . More specifically, it outputs 0 when the output value of the LPF  73  is greater than a threshold value thrB, whereas it outputs 1 when the output value of the LPF  73  is smaller than a threshold value thrA but it outputs a linearly interpolated value when the output value of the LPF  73  is between the threshold value thrA and the threshold value thrB. In short, it outputs 1 when the reliability of the output of the LPF  73  is high but 0 when the reliability is low. 
     The LPF  75   a  executes an LPF process on the phase Φmn of the flicker component of each degree by means of the filter coefficient indicated by the coefficient computing section  74   a.    
     The multiplier  75   b  multiplies the amplitude γm of the flicker component of each degree with the output value of the gain computing section  74   b.    
     The LPFs  75   a  has a configuration as illustrated in the above-described  FIG. 11 . The current detection value is heavily weighted when the coefficient a is large, whereas, when the coefficient a is small, the current detection value is lightly weighted and the past detection value is heavily weighted. With this arrangement, it is possible to make a right correction and eliminate correction errors by using a past detection value if a moving subject comes into the scene. 
     The coefficient computing process is executed for each degree of DFT. Basically, the first degree of DFT is the main component. However, it is desirable to computationally determine the gain for higher degrees in a situation where the components of higher degrees arise particularly when a high speed shutter is used. Additionally, there arises no problem if the coefficient is computationally determined constantly for higher degrees, because any spectrum other than that of a fluorescent lamp is shifted far from the phase component of a fluorescent lamp and the gain is suppressed consequently. 
     A long time constant may be selected for the LPF  75   a  for the phase regardless of Y(n) and only the multiplier  75   b  may be made variable as a function of Y(n) to reduce correction errors due to the subject. However, a slow following performance may take place in a transient situation from the presence of lighting of a non-inverter fluorescent lamp to the absence of lighting of a non-inverter fluorescent lamp and vice versa. 
       FIG. 14  is a flowchart of the sequence of control operation of the parameter control section. 
     Referring to  FIG. 14 , in the first step, or Step S 30 , the difference between the current integral value Y(n) and the integral value Y(n−K) of the K preceding field is set to difference integral value ΔY(n). 
     In the next step, or Step S 31 , the absolute value of the different integral value ΔY(n) is set to ΔY ABS (n). 
     In Step S 32 , the value obtained by applying an LPF process to ΔY ABS (n) is set to ΔY LPF     —     ABS (n). 
     In Step S 33 , the filter coefficient is computationally determined by means of the function shown in  FIG. 10  from ΔY LPF     —     ABS (n). 
     In Step S 34 , the suppressed gain is computationally determined by means of the function shown in  FIG. 5  from ΔY LPF     —     ABS (n). 
     The parameter control section  14 * arranged in the system controller  14  of the above-described image pickup apparatus  10  or the system controller  114  of the above-described image pickup apparatus  100  may alternatively be a parameter control section  14 D having a configuration as shown in  FIG. 15 . 
     The parameter control section  14 D of  FIG. 15  has a delay quantity switching section  80  and two delay circuits  85   a ,  85   b , each having a delay quantity that can be freely switched. 
     In the parameter control section  14 D, the amplitude γm and the initial phase Φmn of the flicker component of each degree as determined by the above-described DFT block  40  or the DFT block  140  are input to the LPF  75   a  and the multiplier  75   b , and the total integral value Y(n) between the current flicker detecting area and the total integral value Y(n−k) of the flicker detecting area of the K preceding field are input to the above-described delay quantity switching section  80 . 
     The delay quantity switching section  80  controls the operation of the delay quantity of the amplitude γm and that of the initial phase Φmn of the flicker component from the current integral value Y(n) and the integral value of the K preceding field. The delay quantity switching section  80  includes a subtracter  81 , to which the integral value Y (n) of the current flicker detecting area and the integral value Y(n−k) of the flicker detecting area of the K preceding field are supplied, an absolute value (ABS) circuit  82  connected to the subtracter  81 , a low pass filter (LPF)  83  connected to the ABS circuit  82 , and a switching control section  84  connected to the LPF  83 . 
     In the delay quantity switching section  80 , the subtracter  81  computationally determines the difference integral value ΔY(n) between the current integral value Y(n) and the integral value Y(n−K) of the K preceding field. 
     The ABS circuit  82  turns the output value of the subtracter  81  into the corresponding absolute value. When the output value of the subtracter  81  is small, it is possible to presume that there is no moving subject. The amplitude γm and the initial phase Φmn are regarded to be highly reliable, therefore. If, on the other hand, the output value of the subtracter  81  is large, the amplitude γm and the initial phase Φmn are regarded to be lowly reliable because it is possible to presume that there is a moving subject. 
     The LPF  83  is a filter for reducing any excessive fluctuations of the difference integral value |ΔY(n)| output from the ABS circuit  82  that arise due to external turbulences. The LPF is preferably arranged with a time constant that provides a time period good for stably determining if the scene to be shot is under the lighting of a fluorescent lamp or under the lighting of non-fluorescent lamp and prevents the LPF from reacting to external turbulences too sensitively. 
     The switching control section  84  outputs a value equal to 0 or a value equal to 1 according to the output value of the LPF  83  as shown in  FIG. 16 . More specifically, it outputs 0 when the output value of the LPF  83  is greater than a threshold value thr, whereas it outputs 1 when the output value of the LPF  83  is smaller than the threshold value thr. In short, it outputs 1 when the reliability of the output of the LPF  83  is high and 0 when the reliability of the output of the LPF  83  is low. 
     The delay circuit  85   a  sets the value indicated by the switching control section  84  for the amplitude γm for the flicker component of each degree in the delay circuit  85   b.    
     The delay circuit  85   b  sets the value indicated by the switching control section  84  for the phase Φmn for the flicker component of each degree in the delay circuit  85   b.    
     As shown in  FIG. 17 , each of the delay circuits  85   a ,  85   b  has a delay element  851  with a delay quantity of Z −k  and a changeover switch  852 . The changeover switch  852  selects signal A when the switching control signal ctrl is  1 , whereas it selects signal B when the switching control signal ctrl is 0. In short, it selects the current signal A when the reliability is high but selects the signal of the K preceding field to use the detection value of the past that is reliable. 
     The coefficient computing process is executed for each degree of DFT. Basically, the first degree of DFT is the main component. However, it is desirable to computationally determine the gain for higher degrees in a situation where the components of higher degrees arise particularly when a high speed shutter is used. Additionally, there arises no problem if the coefficient is computationally determined constantly for higher degrees, because any spectrum other than that of a fluorescent lamp is shifted far from the phase component of a fluorescent lamp and the gain is suppressed consequently. 
       FIG. 18  is a flowchart of the sequence of control operation of the parameter control section. 
     Referring to  FIG. 18 , in the first step, or Step S 40 , the difference between the current integral value Y(n) and the integral value Y(n−K) of the K preceding field is set to difference integral value ΔY(n). 
     In the next step, or Step S 41 , the absolute value of the different integral value ΔY(n) is set to ΔY ABS (n). 
     In Step S 42 , the value obtained by applying an LPF process to ΔY ABS (n) is set to ΔY LPF     —     ABS (n). 
     In Step S 43 , the delay quantity of the delay circuit  85   a  and that of the delay circuit  85   b  are switched according to the function shown in  FIG. 16  on the basis of ΔY LPF     —     ABS (n). 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.