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Exposure Zones and Backlight

The real strength of MOBOTIX cameras are their freely configurable exposure zones which are unique worldwide. Variable exposure windows (= exposure zones) can be defined in the image for automatic exposure control. In this case, exposure time is controlled exclusively on the basis of the exposure zones specified by the user. This allows the main image area settings to be optimized in backlit scenes, irrespective of the intensity of the backlight exposure.

In order to generate top-quality images in all weather and lighting conditions, the raw images produced by the MOBOTIX camera sensor are enhanced independently. For this purpose, the following automatic functions are integrated into the cameras:

Exposure time: The camera automatically selects this between the minimum and maximum settings (1/8000s - 1s) and executes it on a purely electronic basis, without mechanical components.
Pre-amplification: The actual camera selects the pre-amplification in the digital image sensor, irrespective of the lighting conditions and the exposure time.
White balance: The white balance is performed automatically for each image and can be limited to specific reference regions within the image. This results in the MOBOTIX cameras' high degree of color brilliance and precision.
Auto contrast: In order to ensure that image contrast remains high in dull weather conditions (rain, mist), the MOBOTIX cameras automatically boost each image so as always to make use of the entire brightness range from 0 to 100 percent.
Sharpness: The high quality of image delivered by MOBOTIX cameras is attributable, among other things, to the integrated software sharpness filter, which also compensates greatly for ageing and dirt on the lens.

In MOBOTIX cameras each image passes through all the automatic functions, which means that every live or stored image is always regulated to optimum effect.

The original shot of the bank scene clearly shows the light coming in through the front of the window which would cause glare on a normal video camera and make the faces in the foreground dark, depending on how bright it was and the position of the blinds. This problem has been solved in the sample scene by setting up an "exposure zone" in the bottom half of the image. As a result, the top half of the image, which includes the window, no longer affects the exposure control.

Advantage of CMOS Sensors
Since MOBOTIX cameras don't possess an auto iris lens, brightening a dark area is not adversely affected by a lens which closes the shutter. This is where the new CMOS sensors have a great advantage over conventional CCD image sensors. At the same time, exposure intensity under backlighting conditions is a unique selling feature of MOBOTIX cameras. A further advantage of this design is that there are no moving parts in the lens which could freeze up when operated in winter.

The picture above shows a typical situation: Everyone entering the room from a darker room has to be recorded. Outside the room the lighting conditions may vary greatly because of the influence of daylight. The image was recorded with an exposure window covering the whole area. Given the sharp differences in light, the bright areas are slightly bloomed, and the dark walls of the room are under-exposed.

However, if the two exposure windows (shown here in green, second picture) are on the inner walls of the room, they are perfectly exposed. By contrast, the scene in the lobby can be exposed with precision by means of the exposure window in the door area.

The setting which is selected depends upon the task at hand. If the aim is to record a person in the lobby which is flooded with daylight, the middle vertical exposure window needs to be selected (bottom picture). However, if the face of a person standing at the door needs to be recorded, it is given the same illumination as the room. In this event, the exposure windows on the right/left inside the room should be selected (middle picture). Even if the lobby is dark and the person shines a torch at the camera, no incorrect exposure occurs because the torch in the door frame is not picked up by the outside exposure windows.

Variable Exposure Zones

The exposure zones can even be selected to such a degree of freedom that the individual street lights in the street scene can be excluded from affecting the exposure control.

A weighting can be set in order to regulate the exposure inside and outside the exposure zones with precision. The automatic exposure function first of all determines the two possible settings within and outside the exposure windows in order to determine their average in accordance with the specified weighting (0-100 percent).