The 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 then heighten the details visible within the area of view.
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Examples where WDR functionality is most needed include:
The challenge in attaining consistently high-quality surveillance video is that the camera needs light to generate an image. But too much light and the image is blown out; too little light and the image is 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 adjust for the optimal amount of light. Hence, WDR techniques have been developed to improve the speed of iris adjustment and thus enhance video quality and integrity.
There are two basic techniques commonly used to successfully provide WDR capability:
Multi-frame imaging – In this technique the camera captures multiple frames of the field-of-view; each frame having a different dynamic range. The camera then combines the frames to produce one improved WDR frame (see example below).
Non-linear sensors – These are typically logarithmic sensors, where 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 real-time, and secondly it can process moving objects more quickly than non-linear sensors. Multi-frame imaging is also 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 in popularity in the near future.
Wide Dynamic Range (WDR)
Frame 1: Short Exposure
Frame 2: Long Exposure
When utilizing 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 your system investment.
A Display Note: Have you ever noticed that every monitor you see in your office has a different display look? 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, you will need to 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 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/images will display.
Nighttime Use: WDR functionality is not optimal for nighttime use. Where possible, disable the camera’s WDR at night in order to maximize low-light performance if this is not an automatic feature of your camera. WDR cameras utilizing the multiple-exposure technique typically restrict the minimum shutter speed which in low-light conditions can result in motion blur. Not all WDR cameras have the option disabling the function at night, so it’s important to consider camera placement and lighting conditions prior to installation.
Motion Blur: This is a common issue for any type of camera. Motion blur occurs when the elements in the field-of-view being recorded change, due to either rapid movement in the scene or the length of the 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. Do not consider them for general use.
WDR Goes Mainstream
Over the last several months, we’ve seen a number of companies introduce models with WDR functionality or add 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, we are seeing WDR offerings in an expanding 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 what the most important thing is you are trying to achieve. If facial recognition is the crucial element, place your camera at a height and angle that will result in best image quality given the variable or fixed lighting conditions you anticipate. 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/images. There is no “one-size-fits-all” magic technology out there that will produce perfect video in every scenario. At least not yet…