AUTOMATIC LIGHT CONTROL FOR LOW LIGHT LEVEL TELEVISION CAMERA

• US 3,691,302 A
• Filed: 02/25/1971
• Issued: 09/12/1972
• Est. Priority Date: 02/25/1971
• Status: Expired due to Term

• Current Assignee: Jack H. Jones, Rolf Gaebele
• Original Assignee: Jack H. Jones, Rolf Gaebele
• Inventors: Gaebele, Rolf, Jones, Jack H.
• Primary Examiner(s): Murray, Richard
• Application Number:
  US05/118,660
• Time in Patent Office: 565 Days
• Field of Search: 178/7.2,DIG.29
• US Class Current: 348/217.1
• CPC Class Codes: H04N 23/72 Combination of two or more compensation controls

Abstract

The video signal in a low light level television camera is fed to a first operational amplifier which continuously integrates this signal to produce an indication of the average value of the intensity of light in a viewed scene. The video signal is also applied to a pair of circuits that detect and store the peak values of the video signal in alternate fields formed by the scanning electron beam in the camera pick-up tube. During formation of one field, the peak value of the intensity of light in the viewed scene is detected and stored in one circuit while the stored indication of the peak intensity of the light detected by the other circuit during the preceding field is integrated in a second difference amplifier. The output voltage of the second amplifier is a measure of the difference between the stored peak voltage applied thereto and a reference voltage. This difference voltage is passed by a diode switch to a summing amplifier where it is combined with the average value signal from the first amplifier if the peak value signal indicates that the intensity of light from the viewed scene exceeds a prescribed peak reference light intensity. The combined signal is applied to a voltage translator which produces signal voltages of the correct polarities for varying the gains of image intensifier and vidicon pickup tubes of the camera inversely with respect to changes in the intensity of light from the viewed scene.

Claims

1. In a low light level television camera including a camera pick-up tube and an image intensifier tube for increasing the intensity of light from a viewed scene that is incident on it and for producing an image of the viewed scene in the pick-up tube which is responsive to control signals for alternately scanning the image in odd and even fields to produce a video signal having an amplitude that is a time varying function of the intensity of the image and thus the intensity of the light from the viewed scene, an automatic light control (ALC) circuit comprising first means responsive to a video signal for producing a first output signal having an amplitude that is an indication of the average value thereof and thus is an indication of the average value of the intensity of light from the viewed scene, second means responsive to the video signal for producing a second output signal having an amplitude that is an indication of the peak value thereof and thus is an indication of the peak value of the intensity of light from the viewed scene, and third means for automatically selectively combining the first and second output signals for producing third and fourth control signals which are applied to the pick-up tube and intensifier, respectively, for automatically adjusting the gains thereof as a function of the intensity of light from the viewed scene.

2. The ALC circuit according to claim 1 wherein said second means, during a current field:

is detecting and storing an indication of the peak value of the video signal produced during the current field for storage during the subsequent field; and

, is operating on a stored indication of the peak value of the video signal which was obtained during the previous field for producing the second output signal.

3. The ALC circuit according to claim 2 wherein said second means comprises odd and even field peak detection and storage circuits, first coupling means alternately coupling the video signal to said odd and said even field peak detection and storage circuits during consecutive fields, each of said detection-storage circuits detecting the peak value of the video signal during the field this signal is connected thereto and storing this detected signal during the subsequent field, and second coupling means alternately coupling a stored signal from said even and said odd field peak detection and storage circuits during consecutive fields.

4. The ALC circuit according to claim 3 wherein said first and second coupling means are each responsive to control signals from the camera for operating out of phase with each other during the same field for causing said first coupling means to couple the video signal to one detection-storage circuit while said second coupling means couples a stored signal from the other detection-storage circuit.

5. The ALC circuit according to claim 4 wherein said second means comprises fourth means establishing a prescribed threshold level, and said third means comprises a summing circuit receiving the first and second output signals and fifth means detecting when the amplitude of a stored signal coupled from one of said detection-storage circuits is greater than the prescribed threshold level, said summing circuit combining the first and second output signals during a current field only when a stored signal coupled from an associated peak detection-storage circuit exceeds the prescribed threshold level.

6. The ALC circuit according to claim 5 wherein said second means comprises a pair of dump circuits, each of said dump circuits being coupled to a different one of said detection-storage circuits and responsive to control signals from the camera for dumping at the start of a current field the signaL stored in the peak detection-storage circuit having the video signal coupled thereto.

7. The ALC circuit according to claim 6 wherein said fourth means comprises a source of variable reference voltage, and said second means comprises a differential integrator circuit for averaging the difference between the reference voltage and the stored signal passed by said second coupling means for producing the second output signal, said fifth means passing the output signal of said differential integrator circuit to said summing circuit where it is combined with the first output signal only when the amplitude of the stored signal passed by said second coupling means exceeds the prescribed threshold level.

8. The ALC circuit according to claim 7 wherein said fifth means comprises the diode switch which is cut off by an output signal of the differential integrator circuit when the amplitude of the stored signal passed by said second coupling means exceeds the prescribed threshold level for coupling the former signal to said summing circuit.

9. The ALC circuit according to claim 8 wherein said first and second coupling means each comprises a pair of field effect transistor switches and each dump circuit comprises a field effect transistor switch.

10. The ALC circuit according to claim 9 including the series combination of a NAND gate and an inverting amplifier coupled to the gate electrode of and driving each of said transistor switches.

11. The ALC circuit according to claim 10 wherein said third means comprises a voltage translator for causing the first and second control signals to be of the opposite polarity.

12. In a low light level television camera including a camera pick-up tube and an image intensifier tube for increasing the intensity of light from a viewed scene that is incident on it and for producing an image of the viewed scene in the pick-up tube which is responsive to control signals for alternately scaning the image in odd and even fields to produce a video signal having an amplitude that is a time varying function of the intensity of the image and thus the intensity of the light from the viewed scene, an automatic light control (ALC) circuit comprising first means responsive to a video signal for producing a first output signal having an amplitude that is an indication of the average value thereof and thus is an indication of the average value of the intensity of light from the viewed scene, second means responsive to the video signal for producing a second output signal having an amplitude that is an indication of the peak value thereof and thus is an indication of the peak value of the intensity of light from the viewed scene, and third means for automatically combining the first and second output signals for producing a third signal which is applied to one of said tubes for automatically adjusting the gains thereof as a function of the intensity of light from the viewed scene.

13. In a television camera including a motor driven iris arranged for passing light through a variable size aperture therein to the intensifier, the ALC circuit according to claim 12 wherein said third means produces a fourth control signal which is applied to the motor driven iris for adjusting the size of the aperture therein.