Backlight Compensation (BLC) &Wide Dynamic Range(WDR) & S/N Ratio

Backlight Compensation (BLC):

Backlight is the light behind the object of interest in a scene. This can be a major problem, especially in cameras with automatic iris control, because the camera will often adjust to keep the bright background within acceptable levels.

Think about a camera aimed at a door at the end of a dark hallway. When someone opens the door and steps into the hallway, the camera will try to compensate for the sudden bright exterior background. The result will be that the person in the doorway appears silhouetted and detail is lost in “shadow.” In extreme cases, there may be no discernible detail at all. Cameras must have backlight compensation to overcome this situation.

 

Backlight compensation is composed of camera circuitry that samples a scene and makes an assumption that objects in focus are the objects of interest, and that light levels should be optimized for these objects. Extremely high background light levels are selectively shut down while maintaining optimum levels on the objects of interest.

 

Wide Dynamic Range:

A camera’s dynamic range is the difference between the maximum and minimum acceptable signal levels. If part of a scene is illuminated too low, chances are there are not enough photons coming from that area to be converted to a meaningful electronic signal. The detail in the dark won’t be “seen” by the camera. Conversely, if part of a scene is very strongly illuminated (e.g. sunlight streaming through a window), the image from that area of the scene could be washed out. In the worst-case scenario, the scene could, and often does, contain areas of extremely low and high levels of illumination.

 

Dynamic range is the ability of the imaging chip to convert light information. The wider a camera’s dynamic range, the better its ability to deal with these extreme lighting contrasts.

 

Signal-to-Noise Ratio:

The signal-to-noise ratio (s/n ratio) is the ratio of the video signal level to the amount of noise present in the image. Noise in a video image is seen as snow or graininess, resulting in a poorly defined image on the monitor or video recording. The unit for expressing s/n ratio is decibels (dB), but it can also be expressed as a ratio.

A signal-to-noise ratio of 40dB is equivalent to a ratio of 100:1, which means that is the signal is 100 times of the noise level. Conversely, the noise is one hundredth of the signal. At a signal-to-noise ratio of 20dB, the noise is 10% of the signal and would produce an unacceptable picture. The following table provides a guide as to the quality we can expect at various signal-to-noise ratio levels.

 

 

There are many sources of noise, including poor circuit design, heat, over-amplification, external influences, and automatic gain control, as well as transmission systems such as microwave and infrared. The signal-to-noise ratio is an important measure of video quality: the higher the camera’s signal-to-noise ratio, the better the image quality delivered by the camera.

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