Wide dynamic range
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Wide dynamic range (WDR) describes an attribute of an imaging system that can record greater scene details, from shadows to highlights than normal. Having "wide dynamic range" is a relative term that can describe both the capability of a sensor or other imaging system, as well as the contents of an image file containing such data. The amount of dynamic range in an image which would qualify as "wide" changes over time as new systems are developed. WDR is desirable for photography and video-based systems due to its greater latitude in manipulating images.
Cameras with wide dynamic range capability are commonly used in surveillance cameras, video systems, and some medical applications. In surveillance, WDR is intended to provide clear images even under backlighting, where the intensity of illumination varies a lot—namely when there are very bright and very dark areas simultaneously in the camera's field of view. WDR allows an imaging system to correct for the intense back light surrounding a subject and thus enhances the ability to distinguish features and shapes on the subject. WDR cameras are usually recommended for situations where light enters a premise from various angles such as a multi-window room. A camera placed on the inside of the room will be able to see through the intense sunlight or artificial light coming in. If an indoor security camera is pointed towards a window or an entrance door, you will see the background washed out during daytime. This is a common occurrence in restaurants and stores with large glass windows.
Issues solved by WDR capability
The (intraframe) dynamic range of a camera is usually defined as the ratio of the brightest point of an image to the darkest point of the same image. It is also called the maximum contrast of that image. Unfortunately, the dynamic range of most electronic cameras is severely limited. It usually is narrower than the dynamic range of most scenes.
Imaging applications often deal with situations in which lighting conditions are far from optimal. In particular, these may include objects positioned against strong back lighting, in which case the objects details become too dark, since the camera adjusts itself to the high average brightness. In some situations there will be many spots with steep gradations of brightness, which are hard to handle by standard cameras. Other situations depend on the dynamic behavior of the camera; abrupt changes of illumination will cause profound transition effects on the overall system.
All the above situations call for WDR imaging, which is generally constrained by three factors: the sensor, the signal processing circuits, and the display (or frame grabber).
Common charge-coupled device (CCD) sensors accumulate charge in a "potential well" that is proportional to the number of photons that struck the sensor in that pixel. The size and depth of the "potential well" determines the dynamic range capability of the sensor. It is not unusual that a sensor can acquire a contrast of roughly 1:1,000 (60 dB) dynamic range of intensities. The darkest signal is constrained by the thermal noise, or "dark current", of the sensor. The brightest signal is limited by the total amount of charge that can be accumulated in a single pixel. Image sensors are built such that this total maximum charge is greater than 1,000 times the charge generated thermally. This dynamic range can be substantially enhanced, in special applications such as scientific or astronomical cameras for still imaging, by cooling the sensor and by employing special readout circuits. However, such methods, in addition to being very expensive, are inappropriate for real-time applications. As described above, many applications require an even wider dynamic range, such as 65–75 dB (1:1,800–1:5,600). When imaging such a scene with a 60 dB imager, either details in the darker areas get lost in the noise ("cut off"), or details in the brighter areas are lost in saturation, or both.
Charge readout circuits, analog amplifiers, and A/D converters for real-time video sometimes limit the CCD signal further to a dynamic range of 8 bits (48 dB). This range can be extended to 10 bits or 12 bits by employing appropriate analog processing and A/D converters; however, this is expensive for some applications.
An alternative type of circuitry employs non-linear transforms in the chip or as a follow-up process, such as logarithmic functions or "knee" curves to compress the 1:1,000 CCD output signal down to an 8-bit signal.
The last limiting factor is the display (or frame grabber). The simultaneous dynamic range displayable by normal cathode ray tube (CRT) monitors, operating in a lighted room, is limited to about 150:1. An LCD screen is even more limited often to a 1,000:1 (sequential contrast from black to white). The signal which is generated by video circuits may not be fully able to be shown on a display. To optimize display, users need to adjust the contrast and brightness controls for the monitor.
There are two basic technical solutions that are used to provide WDR cameras:
- Multi-frame imaging – where the camera captures more than one frame or "version" of the field of view, each frame has different dynamic range, and the camera combines the different frames to produce one WDR frame. This technique is also known for still images as HDR imaging
- Non-linear sensors – usually logarithmic sensors, where the sensitivity of the sensor at different illumination levels is different, and enables the capture of a WDR image in one frame