Extended shutter allows for a multiple slope integration to extend the dynamic range of the camera. The multiple slope operation is controlled using knee points, where each knee point occurs at a separate time during the overall integration time. Each knee point represents a separate reset point for the image where pixels brighter than a certain value are reset to that value. For example the PL-B741 camera can have up to three knee points when operating in extended shutter mode. When the first knee point occurs, all pixels that are over 25% of saturation are reset to 25% of saturation. When the second knee point occurs all pixels that are over 50% of saturation are reset to that value, and when the third knee point occurs all pixels over 75% of saturation are reset to that value. This enables the capture of images where information can be retrieved from both dark areas of the scene that require long integration times, and bright areas that require short integration times.
The value that is set for each knee point is the time from the start of integration to when the knee point occurs. Each knee point must have a greater time than the previous knee point and all knee point times must be less than the overall integration time. The effect in the image is to reset the pixel at that time and to integrate until the end of the overall integration time. Hence, as the knee point time becomes longer, the effect is to shorten the integration time and expand the dynamic range in bright areas of the image.
Note: The value that is set for each knee point is the amount of time before the end of integration where the kneepoint is applied. Each knee point must be smaller than the next knee point (or overall integration time) and larger than the previous kneepoint so that it occurs between the previous knee (or the start of integration) and the next kneepoint.
How to Manipulate the Controls in Capture OEM
- Start PixeLINK Capture OEM and begin the video preview
- Choose an ROI size
- Select an exposure time that reveals detail in the darker regions of your preview, while overexposing bright areas
- If the option is available (not applicable to all cameras), set camera into Global Shutter mode on the External Control tab
- Disable the Flat Field Correction (FFC) on the LUT & FFC tab
- Set Exposure Time for darkest areas
- Add knee points for length of time to expose pixels in brighter areas
- Each subsequent knee point gets only a portion of the remaining grey scale range
These three images were captured of the same target using exposure times of 20ms, 4ms, and 0.04ms. Observe the level of detail visible in the bright and dark areas within the scenes.
- Overall exposure: 40ms
- At 25ms of exposure, all pixels above 50% of VSat (127DN in 8-bit mode) are reset back to this level with 15ms of exposure time remaining
- At 36.5ms of exposure, all pixels above 75% of VSat are reset back to this level with 3.5ms of exposure time remaining