Dynamic Analysis of Working Process of Surveillance Cameras

In an interlaced scanning system, a full image is made up of two fields: one containing the odd-numbered lines and the other the even-numbered lines. This means that 25 odd fields and 25 even fields are displayed each second, totaling 50 fields per second. During this process, the CCD accumulates charge. If the charge transfer can be paused for a few fields, the charge inside the sensor can be temporarily stored. This allows multiple exposures to accumulate before the charge is transferred, increasing signal strength and enhancing screen brightness. However, this multi-field accumulation reduces the camera’s frame rate, leading to dropped frames. As a result, such cameras are typically used in scenarios where motion is minimal or static, making dropped frames less noticeable. **High-speed electronic shutter** A high-speed electronic shutter enables the camera to capture fast-moving objects with clarity. Standard cameras often produce blurred images or trailing effects when filming high-speed subjects. This happens because the exposure time of the CCD is too long, allowing the object to move across the sensor during the exposure period. The accumulated charge on the sensor is controlled by the DSP, which results in overlapping positions of the object being captured as a smeared image. To prevent this, the electronic shutter must be fast enough so that the exposure time in each cycle remains short—typically around 1/50th of a second. When the exposure time is brief, the object is only imaged at one position on the CCD, resulting in a sharp, clear image. A frame rate of at least 15 frames per second is generally sufficient for smooth motion perception. In PAL systems, the standard frame rate is 25, ensuring continuous visuals. High-speed cameras, often used to film bullets or missiles, rely on this technology to capture precise, motion-free images. **Light compensation** Increasing the electronic shutter speed significantly reduces the exposure time of the CCD, leading to insufficient charge accumulation and a dark image. This creates a conflict between speed and brightness. To resolve this, light compensation techniques are employed. These methods involve adding external lighting to enhance the amount of light reaching the sensor within the limited exposure window. **Traffic monitoring camera** Traffic surveillance cameras often feature a high-speed electronic shutter. This ensures that moving vehicles are captured clearly without blurring or trailing. However, due to the reduced exposure time, the image may appear dim. To counteract this, a fill light is triggered when a vehicle is detected by a sensor. The fill light activates instantly, providing additional illumination. This allows the CCD to collect more charge, resulting in a bright, clear image of the license plate. This is why you often see flashing lights at intersections. The fill light can be controlled automatically, activating only when needed to ensure optimal image quality. **Automatic iris effect** The automatic electronic shutter can also simulate the function of an automatic aperture. If the aperture is small, the shutter slows down to increase exposure time, improving brightness but potentially causing motion blur. Conversely, if the aperture is large, the shutter speeds up to reduce exposure, preventing overexposure while maintaining balance. **Wide dynamic range camera** Modern wide dynamic range cameras use the CCD to capture images multiple times. When there is a strong contrast, the camera first uses a slower shutter speed to properly expose darker areas. Then, it captures a second image with a faster shutter speed to correctly expose brighter areas. Using DSP processing, these two images are combined into a single frame with balanced brightness. While effective for scenes with high contrast, such as entryways at night, these cameras are not ideal for traffic monitoring due to their slower shutter speeds, which can cause motion blur.

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