Recent advances in electronics technology have made it possible for cameras with High Dynamic Range capability to be used to capture images of welding. This allow fabricators to be able to view the detail of the weld tip, as well as its immediate environment (such as the weld seam and weld pool) to get better control of the welding process. Reliable visualization of the environment around the weld tip is necessary to control and adjust the welding process. However, in order to be able to obtain the best possible image quality of a weld, the selection of the most suitable shutter technology must be made. The camera shutter type determines how and when light will be recorded during a camera exposure, and therefore determines the quality and functionality of the image that is provided. There are two main shutter types used in capturing images of welding: rolling or global.
Rolling Shutter
A rolling shutter captures an image by exposing one line at a time, moving from top to bottom. The rolling shutter capture technique is commonly used in film and can go largely unnoticed in certain applications. However, when applying this form of technology to the welding process, where high amounts of light with pulsing and movement are present, the image quality suffers. The image below demonstrates a rolling shutter exposure of a TIG welding process. As the picture demonstrates, some lines of the image get exposed during a welding pulse where a lot of light is present. These lines appear brighter than the rest. Other lines get exposed when the welding pulse is off when less light is present, these lines appear darker than the rest. Over time, if the pulse frequency does not match the frame rate of the camera, the line pattern will appear to move down the screen, causing a distracting image pattern to the viewer. In some cases, as material and surrounding background move past the rolling shutter, different images and movements would be captured, providing inconsistent results on which to base quality judgements of the weld.
Effects of a Rolling Shutter on an Image of a TIG Weld Process
Global Shutter
A global shutter captures an image by exposing all rows of the sensor to light at the same time. This allows large amounts of motion to be captured at the same time, without blur or distortion. This type of shutter is also easier to pair with external applications such as external triggering or matching the image acquisition to a pulsed waveform of a welding power supply. When partnered with High Dynamic Range imaging capabilities, global shutter image capture is the most accurate and ideal technique for monitoring welding processes that involve motion or rapid changes in brightness. The drawback however, is that a global shutter image provides slightly lower contrast than a rolling shutter due to the fact that there is less time to expose each pixel, so in some situations, the image quality may not appear as vivid to the viewer as those taken with a rolling shutter.
CONCLUSION:
Advanced weld cameras such as the Xiris XVC-O partners both High Dynamic Range imaging capability with the choice of both rolling and global shutter capture modes to provide welding operators unprecedented image quality of their welding processes. By selecting the appropriate shutter technique, image quality can be optimized for the application.
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