Unlike factory applications where lighting conditions can be largely controlled, outdoor imaging applications such as surveillance, situational awareness, aerial imaging, and the like, must cope with light levels that vary almost continuously. This includes gradual lighting changes that occur throughout the course of a day, as well as much more sudden changes that can be caused by weather conditions (passing clouds), surroundings (trees, buildings), and the changing orientation of the camera itself – particularly on vehicle-mounted systems.
Auto-iris lenses are one way of coping with dynamic lighting conditions – providing the camera has a built-in exposure meter and a means to integrate this information with the lens. However, such a solution typically reduces the camera’s performance in low light conditions while also adding mechanical reliability concerns and increasing the size, weight, and power consumption of the overall solution.
Outdoor applications must be able to cope with dynamic lighting conditions, including bright sunlight, cloudy skies, and dusk, dawn, or other low light situations.
It is for this reason that camera manufacturers developed automatic gain control (AGC). The AGC function in cameras compares the pixel values within a metering area to a pre-set desired value and internally varies the camera’s gain level to maintain the desired value. This can extend the camera’s usable response across a range in scene illumination equal to the total gain range available in the particular camera. A relatively basic camera might offer an AGC capability of around 12 dB, while more advanced models can deliver from 15 dB to 30 dB of added gain.
With AGC, users can set the shutter value to suit brighter conditions and then let the AGC function increase exposure by adding gain as the scene darkens. Unfortunately, adding gain also means adding noise to the image. Thus, the use of AGC alone will add noise to the image as soon as conditions begin to darken. By the time the scene reaches dawn or dusk conditions, or when shifting to a shaded area of a daylight scene, the result of the AGC function will often be an unacceptably noisy image.
As an alternative, manufacturers have applied a similar concept to the electronic shutter to create an auto-shutter function, sometimes called EE Shutter. An auto-shutter function allows the camera to adjust to changing lighting conditions without adding any extra gain or noise. Under bright conditions, the shutter automatically changes to a faster setting – typically up to 1/10,000 of a second or higher, if needed. Under dusk, dawn, or cloudy conditions, or when imaging shaded areas, the shutter speed automatically slows down to provide additional exposure to the image.
Unfortunately, use of the auto-shutter function creates two potential issues. First, it is limited by the standard frame rate of the camera. Thus, for a camera that runs at 30 fps, exposures longer than 1/30 of a second are not possible. This greatly reduces the ability to produce good images under low-light conditions. In addition, slowing down the shutter also introduces more motion blur into the image. In many newer systems, the goal is to maintain a shutter speed of 1/60 second or better, limiting low light performance even further.
Automatic Level Control (ALC) provides a solutionTo cope with the challenges of dynamic lighting conditions, JAI has developed a technology called Automatic Level Control (ALC). This advanced auto-exposure function is available in a range of JAI cameras targeted at outdoor vision applications, particularly those in the areas of Global Security Solutions and Traffic Imaging.
The ALC function combines both AGC and auto-shutter, enabling them to work together to achieve proper exposure while minimizing both noise and motion blur. Under dark conditions, the auto-shutter remains fixed at an exposure time selected by the user to avoid unacceptable motion blur. Meanwhile, the AGC function applies as much gain as needed to achieve an acceptable exposure level, up to the maximum amount of gain deemed acceptable from a noise perspective (see diagram below).
ALC integrates AGC and auto-shutter functions to optimize exposure and noise levels as lighting conditions change
As illumination increases, gain is automatically reduced until it reaches zero, thereby eliminating any noise it may have introduced. As conditions continue to brighten, the auto shutter continues to reduce the exposure time until it reaches its minimum. By using this integrated approach, ALC extends the camera’s inter-scene dynamic range to a ratio of illumination levels equal to the camera’s full gain range multiplied by the max/min shutter ratio, typically a greater than 400,000:1 ratio.
User ControlJAI has implemented ALC functionality in a way that gives users substantial flexibility and control over how the function will handle dynamic lighting conditions (specific controls and user interfaces vary from camera to camera). Some cameras offer advanced GUI settings allow users to specify which portions of the frame to use when calculating the desired exposure level. For example, in a vehicle-mounted application, the exposure window could be set so that the effect of the sky could be minimized when adjusting the exposure. Other controls can be used to mange the speed at which the camera reacts to changing conditions, the type of algorithm used (peak, average, log-average), and the maximum amount of gain to apply in low light situations.
Software or RS-232 serial commands allow users to adjust the ALC technology to suit their requirements
Some JAI cameras also include a high sensitivity mode that uses special processing techniques to increase low-light sensitivity when light levels drop below a specified threshold. See the separate Tech Note on High Sensitivity Mode for more information about this feature.
While the ALC function may not quite cover the full range of scene illumination in the most demanding outdoor applications, it provides a clean and cost-efficient solution for most.
If the application can support additional size, weight, and power considerations, the ALC function may be used in conjunction with an auto-iris lens for the widest possible auto-exposure range. In this case, a video level type auto-iris lens with adjustable response time and set-point should be used. The set-point should be set equal to the camera’s internal set-point and the response time should be set slower than the response of the ALC function to ensure a smooth response and prevent oscillations.
For additional information on JAI’s ALC function or other inquiries, please utilize our contact form.
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