Dynamic range compression for surveillance video cameras with wide dynamic range

The latest security cameras take full advantage of recent advances in digital imaging technology and increasingly incorporate intelligent algorithms based on motion and object detection to automate the surveillance process. However, we may still ask the following question: Why is it that even the best surveillance cameras often produce rather washed-out and low contrast images under some conditions?

A related question relates to the performance of Wide Dynamic Range (WDR) functions, which are invoked to allow the camera to handle conditions of extreme illumination:

In judging WDR performance, is a higher "WDR value" dB measurement necessarily better?

The answers lie in the difference between what is captured by the video camera, and what can be displayed or processed. Understanding this distinction, and proposing how both parts can be optimised in conjunction, is the goal of this article.

If the ultimate aim of a security camera is to replace a human observer at the scene, a WDR capability is essential. Real environments present a very wide range of illumination levels and the human visual system is highly effective at extracting information in all but the most extreme conditions. A camera must do the same, or better, if its performance is not to vary strongly depending on its location, the time of day, and the ambient lighting conditions.

While a combination of low noise CCD or CMOS sensors and an intelligent auto-gain algorithm is readily able to adjust camera sensitivity between day and night conditions, it is scenes in which very bright and dark areas co-exist that present the most serious challenge. These occur frequently in real situations: at dawn and dusk; in direct sun with deep shadows; under backlit conditions found frequently in entrance areas; at night under directed artificial illumination.

In such environments, a camera needs to perform the following functions:

• Capture as much of the original scene as possible
• Produce a natural-looking image
• Enhance detail in shadow and highlight areas
• Ensure that all relevant information is preserved on standard 8-bit output

Effective performance is the result of both the sensor technology and the signal processing which is applied to the raw captured image stream. Often, this performance is characterised by a simple measure of dynamic range measured in decibels (dB). However, as we will show below, this number is only part of the story and does not tell us how useful the WDR images will be for later analysis, whether human or automated.

Next: Capture vs display