Patent GB-1181918-A (GB)
Title: Light-Intensity Control Device for a Television Camera
IPC: H04N5/238
Filed: 1968-09-27
Abstract: 1,181,918. Television. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 27 Sept., 1968 [30 Sept., 1967], No. 46026/68. Heading H4F. A light-intensity control device for a television camera having at least one camera tube 2 includes a light-intensity control element 1, e.g. an adjustable diaphragm or light absorptive element, upon which the light from a scene having variations in its maximum light-intensity is projected, the light-intensity control element 1 being controlled by a motor M for maintaining the maximum light intensity falling on camera tube 2 substantially constant at a level which is nominal for the camera tube. The motor M is controlled by a control signal produced from a measuring signal 6 derived from the picture signal via amplifier and black level clamping circuit 5. Signal 6 is applied to a capacitor in circuit 7 via a unidirectional current conducting element, and current from a current source 8 is also applied to the capacitor. The result of negative charging of the capacitor under influence of signal 6 and discharging under influence of source 8 is indicated by signal 9. In signals 9 and 6 the D.C. white level voltage is represented by the broken line, the ripple voltage in signal 9 being due to the fact that signal 6 exceeds the white level. Signal 9 is applied to an amplifier 10 to which a reference voltage Vr is also applied. Amplifier 10 supplies (as shown at 11) a voltage V which lies above or below the reference voltage Vr for an increased light intensity L + #L or a decreased light intensity L - #L. Voltage V is supplied to two threshold circuits 12a and 12b which open gates 13 and 15 when voltage V is respectively above or below a threshold value indicated by the broken line relative to the reference value Vr, to allow the signal for multivibrator 14 or 16 to be applied to the motor 12 as a control signal. The control signal from multivibrator 16 may be at any one of three selectable frequencies so that the transmission of light by the element 1 may be increased at three speeds until voltage V remains within the threshold zone at reference voltage Vr. Since the capacitor charged by signal 6 in circuit 7 has a given time constant, small zones of great light intensity hardly influence the control. To improve the speed and accuracy of the system the current supplied through source 8 to the capacitor in circuit 7 is increased when the threshold values laid down by circuits 12a and 12b are exceeded. This may be effected for circuit 12a by connecting the circuit along the broken line to source 8. Alternatively further threshold circuits 17a and 17b are connected to source 8. Voltage V from amplifier 10 is supplied to circuit 17a which has a higher threshold value relative to level Vr as compared with circuit 12a. If voltage V exceeds this higher, second threshold value a signal increasing the current supplied is fed to source 8 with the aid of circuit 17a. A similar control is effected through threshold circuit 17b which is connected to circuit 7. To quickly eliminate large light intensities circuit 17a is also connected to multivibrator 14 to select a higher frequency control signal. A second capacitor may be included in circuit 7. This second capacitor has measuring signal 6 applied to it through an element conducting current in one direction, and with current source 18 the voltage across the second capacitor is indicated by signal 19. The ripple voltage at signal 19 is higher than that at signal 9 because the quotient of capacitance and current supplied by the relevant current source is larger for the second capacitor. If the ripple voltage in signals 9 or 19 exceed the first threshold value, then signal 19 provides a more accurate indication regarding the amount by which it is exceeded due to the higher ripple voltage. During the changing of slides, for example, in which for short periods no light comes from the scene, to avoid large adjustments in the light-intensity control element, signal 19 is applied to a threshold circuit 20 which supplies a voltage to block gating circuit 15 when signal 19 falls below a level shown by the chain link line which corresponds to a minimum light intensity for camera tube 2. A detailed embodiment is described with reference to Fig. 2 (not shown) for a colour television camera producing red, green and blue video signals.