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Extended dynamic range

In slow scientific applications such as astronomy, achieving excellent dynamic range is crucial, as details in the dark areas of an image could be just as important as those in the brighter sections and minimum differences in the grey values should be visible. These cameras often deliver true 14 or 16 bits of dynamic range.

For industrial use, a wide dynamic range is for example an issue in the inspection of very bright scenes, where details of the process, as well as a high richness of background details are required. A typical vision camera with 8 or 10 bits of dynamic range would only be able to see either the dark or bright areas with any detail.

Using special support circuits or signal processing, cameras are able to acquire and output an increased dynamic range. One such technique is called multi-slope exposure. This technique allows pixels to be reset at certain stages throughout an exposure, depending on whether or not a predetermined brightness level has been reached. Using this technique, brighter areas of an image will be exposed for a shorter amount of time, leaving the darker areas to expose for longer. The result of this process is an image with details in both the dark and bright areas, making it more useful for subsequent image analysis due to the high contrast.

Other technologies to create HDR images include:

  • 2-chip cameras: 2 images with different exposure times.
  • Dual-line line scan cameras: both lines are operated at different exposure times.

HDR images are either generated by software in the PC or using hardware processing (FPGA) afterwards. The image on the right illustrates the effect when using this technique in a real situation. The whole scene is well exposed, even the filament of the lamp. Without using multi-slope exposure, either the lamp would be overexposed or the rest of the scene would be very dark and contain no details.