Usage of WDR
WDR cameras are commonly used in surveillance cameras, and in some medical applications. In surveillance,The WDR allows a camera to filter 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 very common situation in restaurants and stores which have big glass windows.
An example of a wide dynamic range image, and the two different images it is created from, the short exposure image and the long exposure image.
Definition of the problem WDR is solving
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 is narrower than the dynamic range of most scenes, and it is also more limited than that of photographic film.
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 wide dynamic range imaging, which is generally constrained by three factors: the sensor, the signal processing circuits, and the display (or frame grabber).
Common CCD sensors can acquire a contrast of roughly 1:1000 (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. Usually CCD chips are built such that this total maximum charge is about 1000 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 usually require an even wider dynamic range, such as 65-75 dB (1:1800 - 1:5600). 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 typically limit the CCD signal further to a dynamic range of 8 bits (48 dB). This range can be extended to 10 bits, by employing appropriate analog processing and A/D converters; however, this is impractical for most applications. Another alternative type of circuitry employs non-linear transforms, such as logarithmic functions or "knee" curves, to compress the 1:1000 CCD output signal down to an 8 bit signal. While depicting most of the image information, such methods necessarily suppress details in the highlights.
The last limiting factor is the display (or frame grabber). The dynamic range displayable by normal CRT monitors, operating in a lighted room, is limited to about 1:100. An LCD screen is even more limited. The 1:200 or so signal which is generated by the video circuits is further reduced by the display. To optimize the display, the user often needs to adjust the contrast and brightness control of the monitor. A user who wants to display the image at its maximum contrast, usually sacrifices some of the dynamic range.
Common Solutions
There are two basic technological 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, were the sensitivity of the sensor at different illumination levels is different, and enables the capture of a wide dynamic range image in one frame.
There are various combinations of the two technologies. The most common solution is the multi frame solution, where several architectures are used.
Parallel capturing by two or more sensors, using common optical path. In this solution, each sensor captures different part of the dynamic range of the scene by either different exposure time for each sensor, or different optical attenuation in the final optical path, or different sensor sensitivity.
Serial capturing by a single sensor, that captures alternative frames with different exposure times, One frame with Longest exposure time, and following frame with Shorter exposure time.
Since 2003 Fuji have produced cameras with a version (SR) of their Super CCD with double sensors for each location, one more sensitive than the other, achieving the parallel solution. Processing the information on their "S3 Pro" camera produced fairly good HDR JPEG image files but saved as RAW and processed in Photoshop at 16 bits per channel allowed flexible use of the full extended range with inclusion of detail from deep shadow to highlight. The newer cameras have further improvements in the sensors and faster processing, speed being a problem with the S3.
In the serial solution, multiple, usually two images are captured. One image is called "Long" image and refers to the longer exposure time used to capture this image. The "Long" image captures the darker parts of the image and usually the brighter parts of the image would be saturated in this image. The other image is the "Short" image, for the shorter exposure time used. This image captures the brighter areas of the image, while the dark areas are too dark, and contain "noise". The camera Image Signal Processor [ISP] is combining both images, by roughly taking the bright parts from the "Short" image and the dark parts from the "Long" image. The actual combination is a complicated algorithm that creates a smooth and seamless image out of the two.
The first to introduce the concept of taking digital images at different exposures and combining them to a single image with HDR were a group from the Technion in Israel led by Prof. Y.Y.Zeevi. first patent on the concept was published in Israel in 1988 "Wide Dynamic Range Camera (Y.Y. Zeevi, R. Ginosar and O. Hilsenrath), Israel, 1988, Worldwide 1989, U.S.A. Patent No. 5,144,442, 1992." In 1993 first commercial medical camera that performed real time capturing of multiple images with different exposures, and produced an HDR video image was introduced ...
See also
High dynamic range imaging
Multiple exposure
Highlight headroom
References
^ Adaptive Sensitivity. 1993. http://visl.technion.ac.il/research/isight/AS/.
^ Wide dynamic range camera USA Patent 5,144,442. 1992. http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO2&Sect2=HITOFF&p=1&u=%2Fnetahtml%2FPTO%2Fsearch-bool.html&r=30&f=G&l=50&co1=AND&d=PTXT&s1=5,144,442&OS=5,144,442&RS=5,144,442.
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