Video Camera Iris Control System Improves Dynamic Response

Problem

Modern video cameras offer incredible resolution and sensitivity but the dynamic range of changing light conditions can be very challenging. The application for this particular project involved scanning ground and water from an aircraft flying at several thousand feet above the terrain. Lighting varies from twilight and dark earth to direct sunlight on snow and ice. Water surfaces present a unique challenge because of the extreme contrast between dark wave troughs and sunlight sparkling off of wave peaks.

Numerous manufacturers offer generic electronic iris control but this application has several special requirements. When a high contrast shadow moves across the viewing field an iris control that averages the entire frame to set the iris opening will over- and then under-correct. Many cameras have internal electronic exposure control but the dynamic range is several orders of magnitude less than the requirements for this application. Also, the response time is usually slow which does not work very well in a fast moving aircraft.

Expert Solution

Tracking the extremely dynamic lighting conditions for this application will require a much more sophisticated iris control system than is commonly available.

Figure 1

Figure 1 shows a diagram of the iris control system. The raw video data is interpreted by the microcontroller to yield a processed video output for use by the other components of the system and a precision control signal that is used to adjust the electro-mechanical iris in the camera lens. The solution incorporates hardware and software that provides the dynamic range, slew rate and damping required to successfully yield useable video information.

Background

The iris electro-mechanical system is typically a spring-loaded leaf assembly. The spring holds the leaves in the fully-closed position until a DC voltage is applied which forces open the aperture. The opening size is approximately proportional to the applied voltage. If the lighting conditions change rapidly the iris assembly has a tendency to overshoot the correct opening size. The MCU tries to compensate and overcorrects in the opposite direction. Rapid oscillation and 'hunting' can occur.

Many lenses with DC iris control contain a damping winding to provide a feedback that is proportional to the acceleration of the iris assembly. This design utilizes this information to calculate the inertial position of the iris assembly to provide very precise positional accuracy and damping control. Overcorrection and oscillation are virtually eliminated.

This design was modeled in a popular math simulation tool so that all corners of operation could be predetermined. This information was translated into the hardware and software design of the MCU and A/D. The implementation of most control functions in software allows precise optimization of functionality.

Results

Laboratory tests and practical field test have shown the design to be very functional and stable. The customer is pleased with the performance in all encountered lighting conditions.

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