| EP0409161A2 | 1991-01-23 | |||
| US5030984A | 1991-07-09 | |||
| FR2665318A1 | 1992-01-31 |
| 1. | An autoiris system for a video camera characterised in that it comprises means (6) to detect movement between frames of a video signal and means (8) to vary the response time of a control system for the autoiris in dependence on the amount of movement in a scene. |
| 2. | An autoiris system according to claim 1 in which the response time is most rapid at low levels of motion. |
| 3. | An autoiris system according to claim 1 or 2 in which the movement detection means (6) produces a picture movement signal (PMS) from at least one selected area of a scene. |
| 4. | An autoiris system according to claim 3 in which the PMS is derived substantially from a central area of a scene. |
| 5. | An autoiris system according to claim 1, 2, 3 or 4 in which the response time of the control system (8) decreases above a predetermined level of movement in the picture. |
| 6. | An autoiris system according to any of claims 1 to 4 in which the response time of the autoiris system is proportional to the amount of movement in the scene. |
This invention relates to auto-iris systems for television or video cameras which are used for controlling the exposure of video frames.
The auto-iris systems of television cameras in current use have a control signal for the iris which is formed by applying a non-additive mix of the raw red, green and blue video components to individual peak and average signal detectors. The outputs of these signal detectors are combined as required before low pass filtering to produce the final control signal for the iris. A manual control signal which provides an override for forced over or under exposure of a video frame can also be combined with these signals. To ensure that the exposure is taken from the central area of the picture the edges of the picture may be gated out in producing the iris control signal. The speed of response of the system is dependent on the low pass filtering function and has to be reasonably quick for good performance of the system.
The maximum speed of response is a subjective limit set by the effect on the system of changes in picture content. For example, where a person wearing a light shirt walks into a previously well exposed shot, thereby causing the auto-iris control system to reduce the exposure somewhat the viewer will see a disturbing, unwarranted change of scene illumination if the system's response time is too quick.
This type of effect is sufficiently disconserting for the typical response time on auto-iris systems to be set to be slower than would otherwise be ideal. If a camera fitted with such a system is switched quickly from a bright scene to a
dark static frame the automatic opening of the iris can be observed. A faster response time would be appropriate for cases where there is an extremely rapid change of picture content.
The slow response time of current systems is particularly objectionable when a change in illumination of a face or of text on white paper is involved. There is no movement in such changes in illumination to mask the effect of the slow response time.
The present invention seeks to vary the response time of the auto-iris system in dependence on the amount of motion in a scene.
The invention will now be described in detail by way of example with reference to the accompanying figure which shows a block diagram of an embodiment of the present invention.
In the figure a video camera 2 views a scene through a lens 4 included in which is an iris control system responsive to an iris control signal. The camera produces a video output signal which can be stored for future use.
A movement detector 6 receives the output video signal and detects the amount of motion by comparing consecutive frames of the video signal in a well known manner. Preferably the movement detector looks for changes between selected picture areas so that iris control can be made dependent on movement in " the main area of the scene being shot by the camera.
The output of the movement detector is a picture movement signal (PMS) which is input to an iris control circuit 8. The iris control circuit produces the control signal for the
auto-iris in the lens 4 in dependence on the amount of movement. It is arranged such that the response time of the ■ iris control system in the lens 4 and the iris control circuit 8 is some function of the PMS. In its simplest form it might be made to be proportional to the PMS.
The system is arranged such that when there are low levels of movement in the picture the response time of the auto-iris system is rapid. The system can be arranged such that above a certain threshold the response time can be made to decrease thereby ensuring that at extremely high rates of change between consecutive frames the masking effect of significant movement levels in a scene (mentioned above) acts to conceal the slow response of the iris.
In practice changes in scence illumination can cause effects which appear to the movement detector to be movement in the scene. For example the formation of shadows when the sun comes out. The system proposed will treat these as movement, entirely appropriately, and will therefore cause the auto-iris system to control the iris setting accordingly.