Guest Column | February 19, 2014

IP Cameras 101: WDR And Why It's Important

Axis Train Tunnel WDR Camera

By James Marcella, Director, Technical Services, Axis Communications

What you don’t know can’t hurt you. It’s a common expression that abdicates responsibility for discovering the truth, from one party to another.  For example:  

James: I like your new car. Where did you get the money for that?
Domenic:  What you don’t know can’t hurt you. 

Is it possible to ascribe the same meaning of this idiom to electronics? Can we mitigate risk by burying our heads in the sand using outdated technology?  With regard to video surveillance, the answer to these questions, respectively, is “Yes” and “No.”  

Wide dynamic range (WDR) is a technology pioneered in the mid-1800s by a photographer who wanted to take a picture of the ocean’s horizon. To accurately display the bright sky and the darker water he took two pictures at different shutter speeds — meaning the amount of time a camera’s sensor is exposed to light. Then he combined the two different negatives into one picture. The result was what we term WDR and is a decades-old technology in surveillance cameras. 

Today, WDR cameras take multiple pictures of a scene at different shutter speeds, combine them into one image in micro-seconds, and deliver full-frame-rate video to be viewed or recorded.  The result: security professionals can see all the video’s details across a wide range of lighting conditions from blinding sunlight to shadowy areas and everything in between.   

Some common uses for the technology include any scene where there is a stark contrast in the amount of light in different areas of the video. For instance, most retail stores have a glass façade and doors that permit sunlight to stream into the scene during the afternoon. This results in a strong backlight situation, creating a blinding halo effect around people coming into the store. In other words, the sunlight blinds a traditional camera from seeing the most important detail.

Axis WDR

In this case, a WDR camera would combine a quick exposure of the strong light coming in through the glass and a longer one of the person walking in the door.  With these exposures combined, the security professional can see the details of a person’s face as well as the cars in the parking lot. Without WDR, you would, at best, sacrifice one for the other and, in the worst case, have video that was completely washed out and useless. 

Conversely, as the sun’s rays peek around a building, a traditional camera could see the well-lit area fine, but have trouble looking into a shadowy corner. A WDR camera combines different exposures to see if someone is lurking in the shadows.

What a camera can’t see can hurt you or hinder a security professional’s investigation. If the person coming through the door went on to rob the store, you would be better served by not only having their facial features, but the color, make, and model of their getaway car as well. High-end WDR is not needed for all cameras, but it is a must for scenes that require the camera to be pointed directly at strong backlight, such as toward windows or car headlights. 

Consider yourself enlightened:  That old analog camera without WDR pointed at the front door just won’t cut it anymore. 

For more information, see Axis Communications’ WDR tutorial and white paper, “Wide Dynamic Range: Challenges and Solutions.”