VIDEO CAMERA WITH AUTOMATIC INTENSITY CONTROL

Abstract: A video camera with automatic intensity control, provided with an imaging system and an image transducing apparatus (3) having an entry window (2), on which images of the pictures to be taken can be formed by this system, and is adapted to transform said images into electrical signals delivered to an output connection (6), wherein the amplitude of the image signal derived from the output (6) of the image transducing apparatus (3) and corresponding with the light intensity of an image portion is transformed into a control signal for a control means (8, 17) of a collecting electrode (5) or of a liquid crystal plate (15), which, synchronously with the image line scanning, can influence the image signal in such a manner that the sensitivity of the scanned image point is decreased at a large amplitude and is increased at a small amplitude.

Description: V Video camera with automatic intensity control The invention relates to a video camera provided with imaging system and an image transducing apparatus having an entry window, on which images of the pictures to be taken ca be formed by this system, and is adapted to transform said images into electrical signals delivered to an output connec tion. In the current video cameras an iris diaphragm in the optical system is used for adapting the light intensity of the image formed on the entry window, which diaphragm is controlled by means of a control system responsive to the light intensity in such a manner that a substantially con¬ stant average light intensity is maintained. Such a camera has several draw-backs. A first draw-bac is that the mechanical control of the diaphragm has a given inertia, so that a lag in the light intensity control arises, which, in particular in the case fast light changes, e.g. with a fast camera movement from light to dark and vice versa, will lead an objectionable image transition. A second draw-back is that such a control acts on the whole image, so that in the case of large light contrasts and depending on the manner of intensity measurement, some image regions will be over- or underexposed, with, as a consequence, reproduc¬ tion losses in the image parts of these regions. Although, for the first draw-back, possibly a solution might be found by increasing the speed of the diaphragm drive, there is no solution for the second objection in the case of a diaphragm-type light control. It is an object of the invention to provide a camera o the above-mentioned kind, by means of which these objections can be avoided. To that end the camera according to the invention is characterized in that the amplitude of the imag signal derived from the signal output and corresponding with the light intensity of an image portion is transformed into control signal for a control means for an additional image influencing element which, synchronously with the image line scanning, can influence the image signal in such a manner that the sensitivity in the image point in question during the image scanning is decreased at a large amplitude and is increased at a small amplitude. Since, now, the sensitivity in any image point, or at least in groups of closely adjacent image points, can be adapted to the local light intensity, large light contrasts can be levelled and a good reproduction of the image portion can be obtained in the very bright and dark image portions. The control should, of course, be so that not all the light contrasts will be suppressed, but only an adaptation t transitions in the light intensity will be obtained. In a video camera with a transducer tube with electron beam scanning and a common electrode arranged behind the image window, the connection of which is, at the same time, the signal output, the voltage at this electrode can be con¬ trolled, according to the invention , in correspondence with the amplitude of the image signal during scanning. It also possible, and in particular in so-called CCD transducers, to obtain such a control by arranging,,in the light path towards the transducer, a liquid crystal element or similar light control element, to which a HF voltage can applied, the frequency thereof, and, thus, the transmittanc of said element, being controlled in correspondence with th scanning. Sometimes ligh intensity peaks will remain which cann be completely suppressed by this control. In order to sup¬ press also these peaks, it is possible, according to the in vention, to send the image signal, if required after a pre- amplification, towards a threshold circuit which only passe the signal portions exceeding the chosen threshold, which portions, if required after adaptation of the amplitude thereof, will be subtracted from the original image signal order to reduce the brighter portions accordingly, and, in particular, before said threshold circuit an inverter can b provided, so that, than, the output signals of the threshold circuit can be directly added to the image signal. The invention will be elucidated in more detail by reference to a drawing, in which Figs. 1 and 2 show two embodiments of the video camera according to the invention with associated electric circuits in a highly simplyfied schematic way. The camera according to Fig. 1 comprises an optical image forming system 1 forming images of the pictures to be taken on an entry window 2 of a video picture tube, which screen 2 is provided, inthe usual manner, with a light sensi¬ tive layer which, under the influence of irradiation, can produce photoelectrons which, when scanning said layer by an electron beam 4, will influence the intensity of the elec- tron current accordingly. The current modulated in this man¬ ner is collected by a collecting electrode 5 on this screen, which gives rise to an image signal at a signal output termi¬ nal 6 connected with said electrode 5. The signal derived from the terminal 6 is sent towards a pre-amplifier 7 for a further processing. A DC voltage applied to the electrode 5 having a suita¬ ble magnitude (and polarity) in respect of the cathode of the tube 3 ensures the collection of the beam 4, which voltage is adjustable by means of an adjustment circuit 8. In the known camera tubes of this kind the voltage at the terminal 6 is not changed anymore after a suitable adjustment thereof. According to the invention, the amplitude of the signal derived from the terminal 6 is (in particular after a pre- amplification thereof in the amplifier 7) constantly deter- mined in a measuring stage 9, which, in particular, is adap¬ ted to detect a sudden and considerable variation in the am¬ plitude level of the image signal. This measuring stage 9 produces, when detecting such a variation, a control signal which is supplied to a control input 10 of the voltage con- trol circuit 8, and this in such a manner that, at a sudden and large brightness increase or decrease, the voltage ϋ at the electrode 5 is changed in the sense of a sensitivity decrease or increase respectively. Such a change can detected very quickly, so that within a very small number of image points of an image line a sensi¬ tivity adaptation can already be obtained. This has as a con¬ sequence that nowhere in the picture over- or underexposure will take place, and that as well in the very bright as in the very dark image portions a good reproduction of the imag portions will be obtained. This is especially clearly visible in the case that th image shows a sharp transition between bright and dark por¬ tions. This control appears to work so fast that, while a clear brightest contrast is maintained, the reproduction of the image portions at both sides of the shadow boundary is excellent. This is especially also the case during movement of the camera from bright to dark portions and vice versa. With the current cameras with diaphragm control this i not possible. In the first case, dependent on the manner of light measurement, the reproduction of the image elements in the bright or dark portions will be very poor, or, in the case of an adjustment on the average, will be insufficient i both portions. In the second case it will take some time before the diaphragm has been adjusted on the new condition, and, in the mean time, a poor image reproduction will be ob¬ tained. Of course the voltage control by means of the stages 8 and 9 should be so that normal brightness contrasts in the image will not be suppressed. At a over-all satisfactory adjustment it can occur that, in particular in the very brig image portions, the reproduction of finer image elements wil be insufficient. In order to improve this additional circuit elements are used which are diagrammatically shown in the drawing. The output of the amplifier 7 is connected with the input of an amplifying stage 11, which can be a part of a current image signal amplifier, and, at the same time, with an auxiliary stage 12 which is adapted to amplify the same signal by a desired factor and, at the same time, will sub- ject it to a threshold operation in which only those signal portions which exceed the threshold will be passed. The out¬ puts of the stages 11 en 12 are connected with the inputs of a subtracting stage 13 in which the normal image signal at the output of the amplifier 11 is decreased by the signal portions passed by/the stage 12. The image signal thus obtai ned appears at the output 14 of the latter stage 13, and is supplied to subsequent circuit elements of the current type. Instead of using a subtracting stage 13, it is also possible to use an inverting stage for the signal supplied t the circuit 12, after which the output signals of the stages 11 and 12 can be added as such. Fig. 2 shows a modified embodiment which is, in parti¬ cular, suitable for use in a CCD camera comprising a special image cell matrix in which charges collected in the various image points can be shifted towards output registers, but this embodiment can also be used in an image tube according to Fig. 1. Between the image forming system 1 and the image trans ducer 3, which is, in particular, a CCD transducer but can also be an electron-beam tube according to Fig. 1, a liquid- crystal plate 15 is provided having a control electrode 16 which is connected with a frequency generator 17. The trans- mittance of this plate depends, for instance, on the frequen cy of the applied voltage. The frequency of the generator 17 can be controlled, and, to that end, the output voltage of the stage 9 of Fig. can be applied to the control input 18 thereof, this in suc a manner that a control of the above-mentioned kind will be obtained. It will be clear that instead of a liquid-crystal pla light transmittance can be controlled by means of an electric signal. It will be clear that the element 15, as shown in Fig. 2 interrupted lines at 15', can also be arranged before the image forming system 1. Moreover it will be clear that this element not only be constructed as a separate element, but can also be integrated in a suitable way into the system 1 or the image transducer 3.

Claims: CLAIMS 1. A video camera with automatic intensity control, provided with an imaging system and an image transducing apparatus having an entry window, on which images of the pictures to be taken can be formed by this system, and is adapted to transform said images into electrical signals delivered to an output connection, characterized in that the amplitude of the image signal derived from the output (6) of the image transducing apparatus (3 ) and corresponding with the light intensity of an image portion is transformed into a control signal for a control means (8, 17) for an additional image influence element (5, 15), which, synchro¬ nously with the image line scanning, can influence the image signal in such a manner that the sensitivity in the image point in question during the image scanning is decreased at large amplitude and is increased at a small amplitude. 2. The video camera of claim 1 with a transducer tube wit electron beam scanning and a common electrode arranged behin the image window, the connection of which is, at the same time, the signal output, characterized in that the voltage a this electrode (5) can be controlled in correspondence with the amplitude of the image signal during scanning. 3. The video camera of claim 1, characterized by a liquid chrystal element (15) or similar light control element arranged in the light pass towards the image transducing apparatus (3), to which element a control signal can be ap¬ plied for controlling the transmitivity of said element (15) in correspondence with the scanning. 4. The video camera of anyone of claims 1..3, characteriz in that the image signal, if required after pre-amplificatio (11), is sent towards a threshold circuit (12) which only passes the signal portions exceeding the chosen threshold, which portions, if required after adaptation of the amplitud thereof is subtracted (13) of the original image signal in order to reduce the brighter portions accordingly. 5. The video camera of claim 4, characterized in that before said threshold circuit, an inverter stage is arranged and in that the output signals of the threshold circuit are directly added to the image signal.

Inventors: WIJNEN ARIE MARINUS, WIJNEN, ARIE, MARINUS
Applicants: COPYGUARD ENTERPRISES, COPYGUARD ENTERPRISES S.A.
Application year: 1989
Main classification: H04N5
IPC codes: H04N5