FR2687265A1 - Electronic image acquisition device with very high dynamic range, and method of acquiring images of very highly contrasted scenes - Google Patents

Abstract

The present invention relates to an image acquisition device including an electronic image sensor (1) converting photons incident on each of its photosensitive points (2) into electric charges, keeping these electric charges until an instantaneous level of saturation of electric charges is reached and transferring, out of the said photosensitive point, the electric charges exceeding the said saturation level. This device is characterised in that it includes a drainage control means (8) able to cause the said saturation level to vary, in the course of image acquisition, between at least two substantially different and substantially non-zero values.

Description

 The present invention relates to a high dynamic electronic shooting device and a method of taking pictures of highly contrasted scenes.

 It applies in particular to cameras and photographic devices with electronic sensors.

 The shooting devices known to date include an electronic image sensor whose dynamics, ratio of the saturation light intensity to the light intensity equal to noise, is limited to about 500.

 Other devices, comprising a flat matrix screen placed in front of the image sensor and whose darkening at each point is controlled as a function of the light intensity reaching this point, has a sensitivity of three to five times lower than traditional shooting systems, the loss of sensitivity being due to the maximum transparency of flat screens, transparency at most equal to 40 percent.

 The present invention intends to remedy these drawbacks by presenting a shooting device having a dynamic range greater than 1000 and a sensitivity approximately equal to that of traditional cameras.

 The object of the present invention is therefore a shooting device comprising an electronic image sensor transforming, at each of its photosensitive points, incident photons into electrical charges, retaining these charges up to an instantaneous saturation level and transferring outside said photo-sensitive point the electrical charges exceeding said level of saturation, characterized in that it comprises a drainage control means adapted to vary said level of saturation, during a shooting, between at least two substantially different and substantially non-zero values.

 The object of the present invention is also a method of taking pictures of highly contrasted scenes with an electronic image sensor comprising photosensitive points, each transforming incident photons into electrical charges up to a level of saturation, characterized in that that it consists of at least a first partial elimination period of electric charges generated on all the photosensitive points of the electronic image sensor, during shooting, said elimination concerning all the electric charges exceeding a substantially lower threshold at the saturation threshold of the image sensor and in that it then comprises, during the same shooting, a second period without elimination of electrical charges up to a value substantially equal to the saturation value of the electronic sensor of images.

 The description which follows, made with reference to the accompanying drawings for explanatory purposes and in no way limiting, allows a better understanding of the advantages and characteristics of the invention.

 FIG. 1 represents a block diagram of a first embodiment of the device according to the present invention.

 FIG. 2 represents a first mode of operation of the device presented in FIG. 1.

 FIG. 3 represents a second mode of operation of the device presented in FIG. 1.

 FIG. 4 represents a third mode of operation of the device presented in FIG. 1.

 FIG. 5 represents a fourth mode of operation of the device presented in FIG. 1.

 FIG. 6 represents a view of the electric charges and the photo-sensitive points of the image sensor presented in FIG. 1 according to the operating mode presented in FIG. 3.

 FIG. 7 represents response curves of the image sensor and of the device presented in FIGS. 1 according to the operating mode presented in FIG. 3.

 FIG. 8 represents densities of electric charges on the photo-sensitive points represented in FIG. 6. FIG. 9 represents an electronic diagram of a first color correction circuit adaptable to the embodiment of the device presented in FIGS. 1 to 8.

 FIG. 10 represents an electronic diagram of a second color correction circuit adaptable to the embodiment of the device presented in FIGS. 1 to 8.

 FIG. 11 represents an electronic diagram of a first automatic dynamic control circuit adaptable to the embodiment of the device presented in FIGS. 1 to 10.

 FIG. 12 represents an electronic diagram of a second automatic dynamic control circuit adaptable to the embodiment of the device presented in FIGS. 1 to 10.

 FIG. 13 represents an electronic diagram of a third automatic dynamic control circuit adaptable to the embodiment of the device presented in FIGS. 1 to 10.

 FIG. 14 represents an electronic diagram of a first matrix correction circuit adaptable to the embodiment of the device presented in FIGS. 1 to 13.

 FIG. 15 represents an electronic diagram of a second matrix correction circuit adaptable to the embodiment of the device presented in FIGS. 1 to 13.

 FIG. 16 represents an electronic diagram of a display circuit adaptable to the embodiment of the device presented in FIGS. 1 to 15.

 FIG. 17 represents a preferred embodiment of the device comprising the elements of devices presented in FIGS. 1 to 16.

 In FIG. 1 are represented an electronic image sensor 1 comprising photosensitive points 2, storage points 3, drains 4, external connections 5, a signal output connection 6 a drainage connection 7, a connection of transfer 11, a drainage control means 8 connected to the drainage connection 7, a transfer control circuit 12 connected to the transfer connection 11, a clock generator 9, an amplification and signal processing circuit 10, an input 13 for controlling the photosensitive dots 2 and a circuit 14 for controlling the voltage of the photosensitive dots 2 connected to the input 13.

 The electronic image sensor 1 is of known type, in particular in video cameras. It is, for example, of the type known as DTC, initials of the French words devices à transfert de charges or CCD, initials of the English words "charge coupled device or of the type known as CID. I1 is suitable for deliver on its signal output connection 6, a signal representative of the image formed on its surface by an objective not shown and known to those skilled in the art.

 The photo-sensitive points 2 of the electronic image sensor 1 are elementary points on which a part of the incident photons are transformed into electric charges according to known physical behaviors in particular in photosensitive semiconductors. In each of these photo-sensitive points 2 of the incident photons are transformed into electric charges, the said electric charges being kept up to an instantaneous level of saturation in electric charges and transferred out of said photo-sensitive point 2 for the electric charges exceeding the said level of saturation in electrical charges. The drainage control means 8 is adapted to vary said level of saturation.

 The storage points 3 are elementary points juxtaposed with the photo-sensitive points 2 and partially masked in such a way that the incident light does not affect them. The transfer of electrical charges from a photo-sensitive point 2 to a storage point 3 which is associated with it is controlled by the transfer signal sent by the transfer control circuit 12 on the transfer connection 11.

 The drains 4, the external connections 5, the signal output connection 6, the drainage connection 7, the clock generator 9 and the transfer connection They are known in the production of electronic image sensors. In particular, the drains 4 juxtaposing the photosensitive points 2 receive all the electrical charges exceeding the level of instantaneous electrical charge saturation set by the drainage control means 8.

 It is however good to remember that the drainage connection 7 is suitable for completely emptying the electrical charges of all the photo-sensitive points 2 and that it is, according to the currently known devices used to produce electronic shutters, by performing a drainage complete for the duration of the shooting to reduce the duration of exposure of the photosensitive points 2 which provide electrical charges then used for the formation of the output signal on the signal output connection 6.

 The generations of electric charges on the photosensitive points 2 and their movements on the storage points 3 and the drains 4 are shown in FIG. 6.

 According to the embodiment of the device object of the present invention presented in Figure 1, the electronic drainage control means 8 is connected to the drainage connection 7 and is adapted to control, during a period equal to the transmission of 'an image or a frame by the circuit 10, at least a partial drainage of the electric charges of the photosensitive points 2 by applying to the drainage connection 7 at least one pulse of drainage signal whose voltage is intermediate between the voltage of complete drainage and the voltage emitted outside the pulses. During a shooting, the drainage control means 8 varies the level of saturation in electrical charges between at least two substantially different values and substantially non-zero.

 The drainage control means 8 can, for example, consist of a controlled voltage generator or a function generator or even a pulse generator of controlled values.

 Different operating modes, given by way of nonlimiting examples, of this drainage control means 8 are set out in FIGS. 2 to 5. The production of drainage control means 8 by those skilled in the art, depending signals entering and leaving presented in Figure 1 and Figures 2 to 5, is easy.

 The transfer control circuit 12 connected to the transfer connection 11, is adapted to control a transfer of the charges from the photosensitive points 2 to the storage points 3 at given time intervals, time intervals not necessarily of the same duration.

 The signal amplification and processing circuit 10 is of a type known in video cameras, with analog or digital operation.

 The control circuit 14 for the voltage of the photo-sensitive points 2 is adapted to emit a voltage on the connection 13 equal to that of the photo-sensitive points 2.

 According to this embodiment of the device, the image signal representative leaving the signal output connection 6 is representative of the image formed on the photosensitive dots 2 and comprises, for each of its photo-sensitive dots 2, at least on the one hand, a first component coming from the charges formed on said point before the partial drainage pulse, corresponding to a first non-zero value of the saturation threshold, added to, on the other hand, a second component coming from the charges formed on the said point after the partial drainage pulse, corresponding to a second non-zero value of the saturation threshold, second value different from the first value.

 In each of the photo-sensitive points 2, incident photons are transformed into electric charges, these electric charges are kept up to an instantaneous saturation level and the electric charges exceeding said saturation level are transferred out of said photo-sensitive point 2. The drainage control means 8 is adapted to vary said level of saturation, during a shot, between at least two values which are substantially different and substantially non-zero.

 The taking of a picture and the formation of the representative image signal consist, with an electronic image sensor having a threshold of saturation of the number of electric charges supported for each of its photosensitive points, during the same taking of view and for each photosensitive point, in at least a first period of generation of electric charges and partial transfer of electric charges generated outside said photosensitive point, said partial transfer transferring out of said photosensitive point all electric charges exceeding a first threshold substantially lower than the saturation threshold and in at least a second period of generation of electrical charges and partial transfer of generated electrical charges, said partial transfer transferring outside said photosensitive point all electrical charges exceeding a second threshold substantially equal to the saturation threshold of the electronic image sensor.

 In FIG. 2 are shown signals emitted by the drainage control means 8 and by the transfer control circuit 12.

 The curve 20 represents on the abscissa the course of time from the start of an electronic image capture by the electronic image sensor 1. The signal 21 represents on the ordinate the signal applied by the drainage control means 8 to the drainage connection 7 with the reference voltage 22 and the maximum applied voltage 23, corresponding to the control of complete drainage. The signal 24 represents the signal applied by the transfer control circuit 12 to the transfer connection 11.

The signal 25 represents a line synchronization signal. The signal 26 represents a frame synchronization signal.

 Signals 20, 24, 25 and 26 are in forms known to those skilled in the art. The values 22 and 23 are also known to those skilled in the art. The signal 21 represents a series of pulses 27 during a first period 28 and then no more pulses during a second period 29. The peak voltage of the pulses 27 is a value substantially greater than the reference value 22 and a value substantially less than the complete drainage control voltage 23. According to this first mode of operation, the electrical charges generated on the photosensitive points 2 of the electronic image sensor 1 between two drainage pulses, signal pulses 21, are partially transferred in the drains 4 to the during each of said pulses but also partially preserved, as shown in Figure 6.

 During the first period 28 are carried out on each of the photosensitive points 2 the generation of electrical charges and the partial transfer of electrical charges generated outside said photosensitive point, said partial transfer transferring outside of said photosensitive point all electrical charges exceeding a first threshold substantially below the saturation threshold.

 During the second period 29, the generation of electrical charges and the partial transfer of generated electrical charges are carried out on each of the photosensitive points, said partial transfer transferring all of the electrical charges outside said photosensitive point exceeding a second threshold substantially equal to the saturation threshold of the electronic image sensor.

 In FIG. 3 are shown signals emitted by the drainage control means 8.

 The signal 30 represents the signal applied by the drainage control means 8 to the drainage connection 7 with the reference voltage 22 and the maximum applied voltage 23, corresponding to the control of complete drainage. This line is represented in correspondence and in a synchronized manner with lines 20, 24, 25 and 26 of FIG. 2, signal 30 represents a series of pulses 27 throughout the duration between the start of shooting and the signal of transfer 26. The peak voltage of the pulses 27 is a value substantially greater than the reference value 22 and a value substantially less than the full drainage control voltage 23.

According to this second operating mode, the electrical charges generated on the photosensitive points 2 of the electronic image sensor 1 between two drain pulses, signal pulses 21, are partially transferred into the drains 4 during each of said pulses but also partially preserved, as shown in Figure 6.

 Figure 4 shows a third mode of operation of the device.

 In this FIG. 4, the signal 26 of FIG. 1 is recalled to indicate the synchronization of the drainage signals, on the one hand, and of the transfer signals, on the other hand.

 The signal 31 represents the signal applied by the drainage control means 8 to the drainage connection 7 with the reference voltage 22 and the maximum applied voltage 23, corresponding to the control of complete drainage. The signal 31 comprises a complete drainage pulse from the start of the shooting then a partial drainage pulse at the end of the shooting.

 FIG. 5 represents a fourth mode of operation of the device.

 The signal 32 represents the signal applied by the drainage control means 8 to the drainage connection 7 with the reference voltage 22 and the maximum applied voltage 23, corresponding to the control of complete drainage. The signal 32 comprises a series of drainage pulses whose peak voltages represent a decreasing sequence from the start of the shooting and reach an intermediate value between the voltages 22 and 23, corresponding to a partial drainage at the end of the shooting .

 FIG. 6 represents a view of the electric charges and the photo-sensitive points of the image sensor.

 In FIG. 6 are represented photosensitive points 2A and 2B, memory points 3A and 3B, and a drain 4, and electric charges represented by points, black on points 2A and 2B and white on points 3A and 3B , whose density is proportional to the density of the electrical charges on points 2A, 2B, 3A and 3B.

 The points 2A and 3A are in correspondence, that is to say that the image transfer signal transfers the electric charges from the photosensitive point 2A to the storage point 3A. Similarly, points 2B and 3B are in correspondence. In FIG. 6, the photosensitive point 2A is considered to receive a light intensity greater than that which reaches point 2B.

 The first part of FIG. 6, above, on the left, represents the state of the electronic image sensor 1 at the start of a shot. The photo-sensitive points 2A and 2B do not carry any electrical charge. Points 3A and 3B may include electrical charges from the previous shot and being transferred to the output of the image sensor 1.

 In the second part of Figure 6, top right, no drainage pulse was transmitted. Electric charges are generated on the surface of the photosensitive spots 2A and 2B in proportion to the incident illumination. Point 2A therefore supports more electrical charges than point 2B.

 The third part of Figure 6, bottom left, shows the drainage of electrical charges during a partial drainage signal. This signal causes the passage of electrical charges in excess with respect to a threshold density of the photosensitive points 2A and 2B towards the drain 4.

In our example, only the photosensitive point 2A has an electrical charge density greater than the threshold value which corresponds to the peak value of the signal 30 in FIG. 3. The charge density of the photosensitive point 2A therefore decreases up to said value threshold. On the other hand, all the electric charges initially present on the photosensitive point 2B are preserved on this point.

 The last part of FIG. 6, at the bottom right, represents the transfer of charges at the end of the shooting, from the photosensitive points 2A and 2B, to the memory points 3A and 3B, respectively. It is noted that the densities of electric charges on the photosensitive points 2A and 2B have believed, again, since the partial drainage, but that their density is in a lower ratio than before the partial drainage. This ratio, which corresponds to the contrast of the image transmitted by the image sensor, is therefore variable as a function of decreasing illumination of the photosensitive points.

 FIG. 7 represents the response curves of the image sensor and of the device shown in FIGS. 1 and 3. The curve 33 represents the response curve of a traditional electronic image sensor. Curve 34 represents the response curves of the shooting periods of the device according to the invention separating two partial drains. Curve 35 represents the response curve of the shooting period of the device according to the invention separating the last partial drainage and the transfer of electrical charges from the photosensitive points 2 to the storage points 3.

 Curve 36 represents the response curve of the device according to the invention.

 On the ordinate are represented, on the same scale between the four curves, but with non-linear scales, the voltages of the signals representative of image exiting from electronic image sensors. On the abscissa are represented, with the same scale for the four curves, the incident illuminations of the points of the electronic image sensors. The vertical bars on the ordinate axis represent the levels of glare causing significant defects, such as halos or streaks, in the restored images.

 Curve 33 is linear and has saturation for a fairly low lighting value. It also has a relatively low level of glare.

 Curve 34 has approximately the same slope as curve 33 but with an even lower level of saturation, but a higher level of glare. The saturation level corresponds to the level of the partial drainage signal. The level of glare is pushed back because the partial drainage makes it possible to eliminate the excess charges.

 Curve 35 has a slight slope, a high saturation level, and a level of glare as high as that of curve 34.

 Curve 36 is the sum of curves 34 and 35. It therefore has a decreasing slope with an initial slope substantially equal to the slope of curve 33, a high level of saturation and a very high level of glare. The initial slope corresponds to the sensitivity of the image sensor. The saturation level corresponds to the dynamics of the image sensor. The advantages of the device are understandable in Figure 7: it has a high dynamic without decreasing its sensitivity.

 FIG. 8 represents densities of electric charges on the photo-sensitive points represented in FIG. 6.

 The curves 30 and 26 of FIGS. 2 and 3 are recalled in this FIG. 8 to indicate the operating mode of the device and the synchronization of the signals and of the charge densities.

 The curve 37 represents the density of the charges present on the photosensitive point 2A of FIG. 6 which we recall that it receives a high incident illumination. The curve 38 represents the density of the charges present on the photosensitive dot 2B of FIG. 6 which we recall that it receives a weak illumination. We see that curve 37 has a peak before each pulse of curve 30 and a strong growth between two pulses. Each pulse corresponds, on curve 37, a sudden decrease to a threshold given by the peak value of the pulses of curve 30, of the charge density of the photosensitive point 2A.

 Curve 38 shows a regular growth during the whole shooting but with a peak value lower than the value reached by curve 37 after each pulse of drainage of curve 30. We see, again, that the ratio of densities of charges at the end of shooting is lower than the ratio of the initial slopes of curves 37 and 38, this latter ratio being representative of the ratio of incident lightings on the photosensitive points 2A and 2B.

 FIG. 9 represents an electronic diagram of a first color correction circuit.

 In FIG. 9 are represented three memories of values of image points 40, 41 and 42, a first correspondence table 43 connected at its inputs to memories 40 and 42, and a second correspondence table 44 connected at its inputs on the one hand to the memory 41 and on the other hand to the output of the first correspondence table 43.

 The image point value memories 40, 41 and 42 are adapted to keep, for the duration of the output of the signal representative of a point of the electronic image sensor 1, the values which come to them respectively from the image sensor electronic 1, memory 40 and memory 41. These three memories 40, 41 and 42 constitute a means of memorizing the values of the signals representative of the illuminations of the photosensitive points juxtaposing a photosensitive point 2 and carrying a chromatic filter different from the said point photosensitive 2. The correspondence table 44 constitutes a means of combining the values of the signals representative of the illuminations of the photosensitive points juxtaposing said photosensitive point 2 with the value of the signal representative of the illumination of said photosensitive point 2.According to the electronic diagram presented in FIG. 9, the value of the signal leaving memory 41 is corrected by the correspondence table 44 as a function of the values of the points surrounding the point whose value is transmitted by the memory 41. This organization corresponds to an organization of the electronic image sensor 1 in triads, that is to say with, successively on the same point line , colored filters of three colors, for example red, green and blue. In this way, the color of the signal leaving the point carrying a green filter is corrected as a function of the values of the signals leaving the red and blue points joined laterally to said green point. The actual production of a circuit according to the electronic diagram presented in FIG. 9 is easy.

 FIG. 10 represents an electronic diagram of a second color correction circuit.

 In FIG. 10 are shown the signal amplification and processing circuit 10, a luminance circuit 45, a chrominance circuit 46 and a correspondence table 47 whose inputs are connected on the one hand to the luminance circuit 45 and on the other hand to the chrominance circuit 46. The luminance circuit 45 is adapted to supply a signal corresponding to the luminance of the image leaving the circuit 10 according to known embodiments. The chrominance circuit 46 is adapted to supply a signal corresponding to the chrominance of the image leaving the circuit 10 according to known embodiments. The correspondence table 47 constitutes a means of combining the luminance and the signal representative of the illumination of each point of the image sensor having a chromatic filter. According to a variant, the correspondence table 47 constitutes a circuit for combining the luminance and chrominance suitable for correcting the ratios of the signals representative of each color as a function of the luminance signal.

 It is understood that, according to the diagram shown in FIG. 10, the value of the luminance of each point of the image representative signal leaving the circuit 10 is corrected as a function of the chrominance of said point.

 FIG. 11 represents an electronic diagram of a first automatic dynamic control circuit.

 In FIG. 11 are shown the signal processing and amplification circuit 10, a low-pass filter circuit 50, a subtractor circuit 51, a rectifier circuit 52 and a low-pass filter circuit 53 connected to the control means. drainage 8.

 The low-pass filter circuit 50 has a time constant greater than the total shooting time, it emits an analog signal corresponding to the weighted average over time of the signal leaving the processing and amplification circuit 10. The average signal is received by the subtractor circuit 51 which also receives the signal leaving the processing and amplification circuit 10. The signal leaving the subtractor circuit 51 is the difference of the two signals it receives. This signal is transmitted to the rectifier circuit 52. The circuit 52 rectifies the signal coming from the subtractor circuit 51. The low-pass filter circuit 53 has a time constant greater than the total shooting time, it emits an analog signal corresponding to the weighted average over time of the signal leaving the rectifier circuit 52. This average signal is received by the drainage control means 8 and fixes the peak value of the drainage pulses presented in FIGS. 2 to 5. In this way , the response curve of the device varies directly as a function of the average difference between the signal leaving the circuit 10 and its average value. The circuit, the diagram of which is presented in FIG. 11, constitutes a means for measuring deviation receiving the signal leaving the electronic image sensor 1 and providing at least a weighted difference of the signal representative of the image leaving the electronic image sensor with a weighted average of this signal. This deviation measurement means is connected to an input of the drainage control means 8.

 The entire circuit presented in FIG. 11 is easy to understand and carry out for those skilled in the art. It should be noted that other circuits operating on the standard deviation, for example, or operating digitally, conform to the spirit of the invention. It should also be noted that the circuit presented in FIG. 11 is compatible with the automatic gain control circuits, with the automatic lens diaphragm control or with the automatic shooting time controls, also called automatic electronic shutters, or auto-iris.

 FIG. 12 represents an electronic diagram of a second automatic dynamic control circuit.

 In FIG. 12 are shown the signal processing and amplification circuit 10, a peak value extraction circuit 55, a threshold circuit 56, an integrator circuit 57 and the drainage control means 8.

 The peak value extraction circuit 55 is of known type. I1 receives the signal leaving circuit 10 and emits a signal equal to the maximum voltage of the signal coming from circuit 10. Threshold circuit 56 emits a threshold crossing signal when the signal leaving peak extractor circuit 55 exceeds a value of predetermined threshold. The integrator circuit 57 receives the threshold crossing signal leaving the threshold circuit 56 and by its integration function, it increases the voltage of the signal which it emits by means of the drainage control 8. It also has a constant of very high time but not zero so that the value of its decree output signal when the integrator circuit 57 does not receive a threshold overshoot signal. The voltage value of the signal leaving the integrating circuit 57 fixes the peak value of the drainage pulses emitted by the drainage control means 8, pulses described in FIGS. 2 to 5. In this way, the voltage of the drainage pulses is automatically dependent of the peak value of the signal leaving circuit 10.

 It should be noted that other electronic diagrams operating digitally make it possible to perform the functions presented in FIG. 12 and also make it possible to avoid oscillations of the peak value of the drainage pulses for an invariable scene.

 The electronic circuit, the diagram of which is given in FIG. 12, constitutes a peak value measurement circuit adapted to receive the signal representative of the image leaving the electronic image sensor 1 and to supply a signal representative of the maximum illumination of the said signal by drain control 8.

 According to a variant, the device according to the invention comprises a circuit for measuring the number of photosensitive points of the electronic image sensor generating a number of electrical charges greater than a fixed threshold adapted to receive the signal representative of image leaving the sensor d electronic images 1 and to provide a signal representative of said number by means of drainage control 8.

 According to this variant, the drainage control means 8 performs a control of said number.

 FIG. 13 represents an electronic diagram of a third automatic dynamic control circuit.

 In FIG. 13 are shown the drainage connection 7, a current measurement circuit 60, an integrator circuit 61 and the drainage control means 8.

 The current measurement circuit 60 is adapted to measure the current flowing through the connection 7, a current which is a function of the number of charges transferred from the photosensitive points 2 to the drains 4.

 The integrator circuit 61 is adapted to integrate the value of the current flowing through the connection 7 during a complete shooting. This value is also a function of the charges passing through the drain 4.

 The integral value of the current flowing through the connection 7 fixes the peak value of the drainage pulses emitted by the drainage control means 8 described in FIGS. 2 to 5. In this way, the control acts on the number of electrical charges passing through l 'set of drains 4 and cancels it.

 It should be noted that a differentiating circuit can be interposed between the integrating circuit 61 and the drainage control means 8, this differentiating circuit making a difference between the value reaching it from the integrating circuit and a fixed value, possibly adjustable. The servo then operates in such a way that the number of electrical charges evacuated by the drains 4 is constant.

 The electronic circuit, the diagram of which is shown in FIG. 13, constitutes a circuit for measuring the number of electric charges generated on the photosensitive points and leaving the drains 4, the output of this circuit being connected to the input of the drainage control means 8 and thus fixes the value of at least one instantaneous saturation level.

 FIG. 14 represents an electronic diagram of a first matrix correction circuit.

 In FIG. 14 are shown the signal processing and amplification circuit 10, an image memory 65, an adder circuit 66, a second image memory 67 and a multiplier circuit 68.

 The image memory 65 permanently stores information for each of the photosensitive points of the electronic image sensor I, information corresponding to the non-uniformity of the black signal, commonly called in English DSNU, abbreviation of dark signal non-uniformity. This information is additional factors to add to the signal emitted for each photosensitive point to obtain a constant black signal.

 The image memory 67 permanently stores information for each of the photosensitive points of the electronic image sensor 1, information corresponding to the non-uniformity of response of said points, commonly called in English PRNU, abbreviation of pixel response non-uniformity. This information is multiplying factors to be assigned to the signal emitted for each photosensitive point to obtain a constant signal for constant lighting. This information is representative of shooting defects corresponding to the point irregularities of a lens, for example its vignetting, to the point irregularities of transfer in the drain 4 and to the irregularities of uniformity of response.

 The image memories 65 and 67 are synchronized, point by point, with the output signal from the electronic image sensor 1 in such a way that for each point of the sensor 1, the signal leaving the processing and amplification circuit 10 arrives at the adder circuit 66 simultaneously with the signal corresponding to this point and representing the information stored, for this point, in the image memory 65 and in such a way that the signal leaving the adder circuit 66 arrives at the multiplier circuit 68 simultaneously with the signal stored for this point, by the image memory 67.

 The adder circuit 66 realizing the addition of the two signals which reach it and the multiplier circuit 68 realizing the multiplication of the two signals which reach it, the output signal of the multiplier circuit 68 is calibrated, for each point, for all the values of illumination up to the saturation level.

 It should be noted that the circuit presented in FIG. 14 can operate in an analog, digital or partially analog and partially digital manner according to electronic diagrams and with known electronic components.

 FIG. 15 represents an electronic diagram of a second matrix correction circuit.

 In FIG. 15 are shown the signal processing and amplification circuit 10, an image memory 70, an adder circuit 71 whose inputs are connected on the one hand to the signal processing and amplification circuit 10 and on the other hand to the image memory 70, the drainage control means 8, a comparator circuit 72 whose inputs are connected on the one hand to the drainage control means 8 and on the other hand to the adder circuit 71 and a switch 73 whose data inputs are connected on the one hand to the adder circuit 71 and on the other hand to the signal processing and amplification circuit 10 and whose switching control input is connected to the circuit output comparator 72.

 The drainage control means 8 provides a value representative of a saturation threshold.

 The image memory 70 is identical to the image memory 65 presented in FIG. 14. It is synchronized with the electronic image sensor 1.

 The switch circuit 73 is adapted to display on its output links one of the two values it receives on its data inputs as a function of the signal leaving the comparator circuit 72. If the value supplied by the drainage control means 8 is greater than the value supplied by the adder circuit 71, the value displayed on the output links of the switch 73 is that coming from the adder circuit 71.

 FIG. 16 represents an electronic diagram of a display circuit.

 In FIG. 16 are shown the signal processing and amplification circuit 10, an analog-digital converter 75, a memory 76, an incrementation circuit 77, a clock 78, a counter 79, an adder circuit 80, a memory 81, a conversion table 82 and a digital-analog converter 83.

 The analog-digital converter 75 is of known type. I1 receives at its input the analog signal representative of image leaving the processing and amplification circuit 10 and provides a digital signal corresponding to the analog signal. The digital signal is transmitted to an address bus of the memory 76 and of the correspondence table 82.

 The memory 76 has at least as many addresses as there are different values that can exit from the analog-digital converter 75. The memory 76 is adapted to supply the incrementing circuit 77 with the value corresponding to the address provided by the analog-to-digital converter 75 and memorizing at the same address the value leaving the incrementing circuit 77.

The incrementing circuit 77 is of known type. I1 is adapted to increase by 1, therefore to increment, the value which it receives from memory 76 and to supply it with the resulting value. The incrementation circuit 77 can, for example, consist of a digital value adder and a flip-flop permanently displaying on one of its input ports the value "1", the other input port receiving the numerical value leaving memory 76
Clock 78 is of known type. It is suitable for emitting pulses at a frequency sufficient for the number of different addresses of the memory 76 divided by the duration between two shots to be less than this frequency.

 The counter 79 is reset to zero at the end of shooting and it supplies, between two shots, the number of the clock pulse it counts. This number is transmitted on the address bus linked on the one hand to memory 76 and on the other hand to correspondence table 82.

 The memory 76 is reset to zero for each of its addresses, but the digital value stored at this address is previously transmitted to the adder circuit 80.

 The adder circuit 80 is of known type and is suitable for adding, to each pulse of the clock 78, the digital values which reach it on the one hand from the memory 76, and on the other hand from the memory 81 and to supply the resulting numerical value on the one hand at the correspondence table 82 and on the other hand at the memory 81.

 The memory 81 is adapted to keep the digital value from it from the adder circuit 80 and to restore this digital value to it, at each pulse of the clock 78.

 The correspondence table 82 is of a type known by the English name of LUT, abbreviation of the English words "look-up table": it is a memory which receives data, here those which come during the taking of view of the analog-to-digital converter 75 and between two shots of the counter 79, on its address bus. The correspondence table 82 is in writing mode during shooting, that is to say that it transmits previously stored digital values and in reading mode between shots, that is to say that it stores for each of the addresses supplied by the counter 79 the value leaving the adder circuit 80.

 The digital-analog converter 83 is of known type.

 According to the electronic diagram presented in FIG. 16, the memory 76 keeps, at each of the addresses corresponding to a value leaving the analog-digital converter 75, the number of photosensitive points 2 of the electronic image sensor 1 which emitted the signal corresponding to this address. The memory 76 therefore keeps a histogram of the image received by the electronic image sensor 1. The adder circuit 80, associated with the memory 81, adds all the digital values stored in the memory 76 up to the address value equal to the value leaving the counter 79 and supplies this sum to the correspondence table 82, when the latter is in read mode. The transfer function of the correspondence table 82 thus has for each value of number of electric charges a slope proportional to the number of photosensitive points of the electronic image sensor having this number of electrical charges. In this way, a histogram equalization is performed for each image. The circuit presented in FIG. 16 is easily achievable by those skilled in the art.

 Figure 17 shows a preferred embodiment of the device.

 We find in Figure 17 elements of the previous figures which provide the same functions as in their first presentation.

 It should be noted that, according to the spirit of the invention, the variation of the saturation threshold during a shooting can also be obtained by varying the control voltage of the photosensitive points, the control voltage of the points or the control voltage for transferring the electrical charges from the photosensitive points 2 to the memorization points 3.

 According to a first variant, the drainage control means 8 is connected to a connection giving the relative operating voltage of the photosensitive points and the drainage control means 8 varies this relative operating voltage during each shot.

 According to a second variant, the drainage control means 8 is connected to a connection giving the relative operating voltage of storage points 3 juxtaposing the photosensitive points 2 and the drainage control means 8 varies this operating voltage relative to the course of each shot.

 According to a third variant, the drainage control means 8 is connected to a connection giving the control voltage for transferring the electric charges generated on the photosensitive points to the storage points 3 juxtaposing the photosensitive points 2 and the drainage control means 8 varies this transfer control voltage during each shot.

 The embodiments of devices which are the subject of the present invention and which use these voltage controls are easily achievable by those skilled in the art.

 It should be noted that the word "drainage11" used in the description above and in the claims below designates both the transfer of electrical charges from the photosensitive points 2, 2A and 2B of the electronic image sensor 1 as well as the transfer of these charges in other elements of the electronic image sensor 1 as soon as this transfer corresponds to a level of saturation in electrical charges of at least one photosensitive point of the image sensor and when this level of saturation is controllable.

Claims (18)

1 / Shooting device comprising an electronic image sensor (1) transforming, in each of its photo-sensitive points (2.2A, 2B), incident photons into electrical charges, keeping these electrical charges up to a level instantaneous saturation with electrical charges and transferring outside said photo-sensitive point the electrical charges exceeding said saturation level, characterized in that it comprises a drainage control means (8) adapted to vary said saturation level, at during a shooting, between at least two substantially different values and substantially non-zero. 2 / Device according to claim 1 characterized in that the drainage control means (8) is connected to drains (4) juxtaposing the photosensitive points (2,2A, 2B) and receiving all the electrical charges exceeding the saturation level and in that the instantaneous saturation level is a function of the potential difference between the drains and the photosensitive points. 3 / Device according to claim 2 characterized in that it comprises a measuring circuit (60,61) of the number of electric charges generated on the photosensitive points and leaving the drains (4) and in that the output of this circuit is connected to a saturation level control input of the drainage control means (8). 4 / Device according to claim 1 characterized in that the drainage control means (8) is connected to a connection giving the relative operating voltage of the photosensitive points and in that the drainage control means (8) varies this relative operating voltage during each shot. 5 / Device according to claim 1 characterized in that the drainage control means (8) is connected to a connection giving the relative operating voltage of storage points (3.3A, 3B) juxtaposing the photosensitive points (2.2A , 2B) and in that the drainage control means (8) varies this relative operating voltage during each shot. 6 / Device according to claim 1 characterized in that the drainage control means (8) is connected to a connection giving the control voltage for transferring the electric charges generated on the photosensitive points to the storage points (3.3A, 3B) juxtaposing the photosensitive points (2,2A, 2B) and in that the drainage control means (8) varies this transfer control voltage during each shot. 7 / Device according to any one of the preceding claims, characterized in that it comprises a storage means (40,41,42) of the values of the signals representative of the illuminations of the photosensitive dots juxtaposing a photosensitive dot (2) and carrying a filter chromatic different from said point and in that it comprises means for combining (44) said values with the value of the signal representative of the illumination of said given photosensitive point. 8 / Device according to any one of claims 1 to 6 characterized in that it comprises a combination circuit (47) of the luminance and the chrominance adapted to correct the ratios of the signals representative of each color as a function of the signal of luminance. 9 / Device according to any one of claims 1 to 6 characterized in that it comprises means for combining (47) the luminance and the signal representative of the illumination of each point of the image sensor having a chromatic filter . 10 / Device according to any one of the preceding claims, characterized in that it comprises a measurement means (50,51,52,53) of deviation receiving the signal leaving the electronic image sensor (1) and providing the minus a weighted deviation of the image representative signal leaving the electronic image sensor with a weighted average of this signal and in that this deviation measurement means is connected to an input of the drainage control means (8). 11 / Device according to any one of claims 1 to 9 characterized in that it comprises a peak value measuring circuit (55,56,57) adapted to receive the image representative signal leaving the electronic image sensor (1) and in providing a signal representative of the maximum illumination of said signal by means of drainage control (8). 12 / Device according to any one of claims 1 to 9 characterized in that it comprises a circuit for measuring the number of photosensitive points of the electronic image sensor generating a number of electrical charges greater than a fixed threshold adapted to receive the signal representative of image leaving the electronic image sensor (1) and providing a signal representative of said number by means of drainage control (8). 13 / Device according to any one of the preceding claims, characterized in that it comprises an image memory (65) and an adder circuit (66), said image memory being synchronized with the electronic image sensor and supplying for each photosensitive point of the electronic image sensor an additional coefficient of correction to the adder circuit, said adder circuit also receiving the signal representative of images leaving the electronic image sensor (1). 14 / Device according to any one of the preceding claims, characterized in that it comprises an image memory (67) and a multiplier circuit (68), said image memory being synchronized with the electronic image sensor and supplying for each photosensitive point of the electronic image sensor a multiplication coefficient of correction to the multiplier circuit, said multiplier circuit receiving, moreover the signal representative of images leaving the electronic image sensor (1). 15 / Device according to any one of the preceding claims, characterized in that it comprises an image memory (70), a comparator circuit (72), a switch (73) and an image combining means (71) , said image memory being synchronized with the electronic image sensor (1) and said image combining means being adapted to combine for each point of said sensor on the one hand the signal leaving the corresponding image memory at said point, on the other hand, the image representative signal corresponding to said point and in that the comparator circuit is adapted to compare the signal leaving the image combining means with a signal leaving the drainage control means ( 8) and to provide a comparison result to the switch, said switch receiving on the one hand the image representative signal leaving the electronic image sensor and on the other hand the signal leaving the image combining means and providing his exit one of these two signals depending on the comparison signal. 16 / Device according to any one of the preceding claims, characterized in that it comprises a means of producing a histogram (76,77) of the numbers of electric charges generated on the electronic image sensor adapted to provide, for each number of electrical charges generated, a number of photosensitive points of the electronic image sensor which generated said number of electrical charges. 17 / Device according to any one of the preceding claims, characterized in that it comprises a transfer function calculation means (80,81) of histogram equalization adapted to fix the transfer function of a conversion table (82), said transfer function having for each value of the number of electric charges a slope proportional to the number of photosensitive points of the electronic image sensor having this number of electric charges. 18 / Method for taking pictures of highly contrasted scenes with an electronic image sensor (1) comprising photosensitive points (2.2A, 2B) each transforming incident photons into electrical charges up to a saturation level, said electronic image sensor having a saturation threshold of the number of electrical charges supported for each of its photosensitive points, characterized in that it consists, during the same shooting and for each photosensitive point, in at least a first period (28) of generating electric charges and of partial transfer of electric charges generated outside said photosensitive point, said partial transfer transferring outside of said photosensitive point all electric charges exceeding a first threshold substantially lower than the saturation threshold and at least a second period (29) of generation of electric charges and of partial transfer of electric charges born, said partial transfer transferring outside said photosensitive point all the electrical charges exceeding a second threshold substantially equal to the saturation threshold of the electronic image sensor.