SPONSORED
MAKING THE MOST OF
LINE SCAN IMAGING Line scan imaging systems are becoming faster and more powerful. Jools Hudson from Gardasoft explains how using dedicated lighting controllers and timing controllers can improve your line scan operations and increase versatility. Line scan imaging is used extensively for high-speed, continuous inspection of materials such as sheet steel, paper, textiles and packaging film. Line scan systems lend themselves to versatile imaging techniques by utilising high line resolutions, multi-line configurations, or using dedicated controllers to provide timing and lighting control. A typical system might look like figure 1 which utilises a lighting controller and a timing controller. Lighting controllers can help overcome many of the shortcomings of traditional line scan imaging systems and provide some significant benefits, including: • Reduced cost • Higher performance • More reliable inspections • Novel imaging configurations • More compact systems Line scan cameras will require precise control in most applications. For example, the camera line rate must be synchronised to the speed of the moving object to avoid image distortion, and precise control of lighting intensity is essential since the fast line rates permit each pixel very little light.
OPTIMISING LIGHTING INTENSITY Stable lighting is a crucial factor in all machine vision systems which need reliable, reproducible inspections. All LED lighting deteriorates in efficiency over time due to aging, so the ability of lighting controllers to compensate for this is of critical importance. Most line scan applications use continuous illumination and aging will be most problematic if the light is being run continuously at 100%
34
of its rating. The solution is to use an LED with a higher rating than necessary and drive it via a lighting controller. When the system is first set up, the controller can be used to set the brightness to 70% or 80% of maximum. As the light gets older, the brightness can be adjusted upwards to compensate for the loss of efficiency and extend the useful lifetime of the light. With Ethernet connectivity, these adjustments can be carried out remotely. Lighting controllers can also be used to ensure uniform illumination over the full length of the line sensor to compensate for geometric variations. For line lights with a uniform brightness, images tend to be darker at the ends for geometrical reasons. This can be overcome by splitting the light into segments and using a lighting controller to modify the brightness of each segment to create uniform illumination across the length of the sensor. This gives true flat field correction, which will perform better than a software-based compromise.
REDUCING THE NUMBER OF CAMERA STATIONS Since line scan cameras run at increasingly high line frequencies, there are more opportunities to be creative in their use. Applications that previously required two or more camera stations can now often be combined into a single camera station. This can lower costs as well as reducing the overall size of the system. Typically, a combined station will
