WDR is Everywhere Lately

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WDR is Everywhere Lately

WDR is EverywhereThe term wide dynamic range (WDR) is used to describe the function of a camera designed to produce clearer, more actionable images and/or video in circumstances where back light and intense illumination can vary excessively, especially when both very bright and very dark areas are simultaneously present in the camera’s field-of-view. The wider the dynamic range, the better the camera’s sensor can accurately capture varying light intensities and heighten the details visible within the area of view.

Some examples where WDR functionality is needed include:

  • areas with rapid and constant changes in illumination, like entryways and exit points;
  • where lighting condition in the field-of-view moves from a well-lit area into a darker area;
  • city transportation centers, such as parking garages, tunnels or train stations, where people and vehicles enter and exit (commonly with bright daylight outside and low levels of light inside);
  • scenarios such as toll plazas, gas stations and anywhere vehicles with bright headlights are driving directly towards the camera; and
  • environments with intense light reflection off banks of windows, such as office buildings and shopping malls, or for areas with water features.

The challenge in attaining consistent high-quality surveillance video is the light cameras need to generate an image. With too much light, the image is blown out; too little light causes the image to be dark and unusable. Often, a scene has a wide range of light variation and the camera simply can’t adjust its iris settings or shutter speed to properly accommodate the optimal amount of light. However, WDR techniques have been developed to improve the speed of iris adjustment, and thus, enhance video quality and integrity.

WDR Techniques

Two basic techniques are commonly used to successfully provide WDR capability:

Multi-frame imaging. With this technique, the camera captures multiple frames of the field-of-view, each frame having a different dynamic range. The camera combines the frames to produce one improved WDR frame.

Non-linear sensors. Typically logarithmic sensors are the sensitivity of the sensor, at different illumination levels varies, enabling the capture of a wide dynamic range image in a single frame.

Of the two techniques, multi-frame imaging is the most commonly used by manufacturers supporting WDR capability for two main reasons. First, it is superior at capturing images in realtime; and secondly, it can process moving objects more quickly than non-linear sensors. Multi-frame imaging is more cost-effective and integration-friendly, because it is far more common in the marketplace. While non-linear sensors do perform well, they have not been widely-adopted and current trends do not suggest this technique will overtake multi-frame imaging popularity in the near future.

WDR Tips

When using cameras with WDR capability, there are a few things to keep in mind during set-up and installation to ensure the best possible output from the system.

A display note. A limiting factor rarely considered is the display used to view images or video.

The dynamic range that can be displayed by normal CRT monitors is limited to approximately 1:100. An LCD screen is capable of even less. The approximate 1:200 signal that is generated by the video circuits is further reduced by the display.

To optimize a display, adjust the contrast and brightness control of the monitor. Setting a display with the image at its maximum contrast will result in sacrificing some of the dynamic range, but this will produce a better image. When considering a display for surveillance use, keep in mind that the better the contrast ration, the better the video and images will display.

Nighttime use. WDR functionality is not optimal for nighttime use, so whenever possible, disable the camera’s WDR at night in order to maximize low-light performance if this is not an automatic feature of the camera. WDR cameras, using the multiple-exposure technique, typically restrict the minimum shutter speed, which in low-light conditions can result in motion blur. Because not all WDR cameras have the option to disable this function at night, it’s important to consider camera placement and lighting conditions prior to installation.

Motion blur. A common issue for any type of camera is motion blur, occuring when the elements in the field-ofview being recorded change due to rapid movement in the scene or the length of exposure time. This is where camera positioning and lighting considerations play a key role in obtaining clear, usable video images. As with any camera set-up, the more uniform the lighting, the better. A poorly-lit scene, requiring longer exposure times, will create conditions for motion blur to occur. Keep in mind, the purpose of WDR technology is to optimize video and images in scenes with simultaneously very bright and dark areas. WDR is not designed to optimize solely low-light or extremely bright scenes. Only use WDR cameras where appropriate, and do not consider them for general use.

WDR Goes Mainstream

Over the last several months, a number of companies have introduced models with WDR functionality or added WDR capability to existing camera models. Consequently, the prices of WDR security cameras have dropped to more affordable levels. And, because early demand has been strong, WDR offerings are expanding the number of form factors to meet the needs of both indoor and outdoor applications.

As with any security installation, the key to success is determining the most important goal you are trying to achieve. If facial recognition is the crucial element, place the camera at a height and angle that will result in best image quality, given the anticipated variable or fixed lighting conditions. Capturing license plates or other vehicle identification elements will require different set-up and consideration. Do your research; each scene and camera location will be different and may require varying functionality to produce the most usable video and images. There is no “one-size-fits-all” magical technology out there that will produce perfect video in every scenario… at least not yet.

This article originally appeared in the August 2013 issue of Security Today.

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