Advanced Imaging


Advanced Imaging Magazine

Updated: July 8th, 2008 05:26 PM CDT

Analog Autofocus Improves Inspection Speed

Figure 1: The fast analog autofocus system relies on a shaped infrared beam from an auxiliary optical system.
Figure 2: The shaped beam projects a real image on a bi-cell photosensor. The image shape and position depends on where the optical system is with respect to its best-focus condition. When the microscope objective is too far from the substrate (a)
The image resembles a half moon lying on the plus-sensor side, giving a net positive output voltage. At perfect focus (b)
The image is a straight bar coinciding with separation between the plus and minus sensors, giving a zero output. When too close (c), the image resembles a half moon lying toward the minus sensor, giving a net minus signal.
Figure 3: The analog fast-autofocus sensor works at electronic speeds, so the only appreciable delay is in the mechanical movement.

By Parminder Parmar

Automated optical inspection of a wide range of products requires the camera lens to refocus many times to acquire images at several target positions on the object. The speed at which the production line can run is often limited by the speed at which the autofocus mechanism works. One such autofocus-speed-limited application is flat-panel display inspection.

Substrates for flat-panel displays are now typically two meters across and less than a millimeter thick. The wiring patterns they carry, however, are microscopic. Inspecting them, therefore, requires moving a video microscope mounted on a gantry to a large number of inspection points covering the patterned area. With so many fields to inspect on each panel, the time used up at each stop creates an inspection bottleneck.

With today’s high-speed image-processing engines, actual inspection time – the time to acquire an image and locate any defects in the field of view – has become quite short. It is, in fact, much shorter than the time needed to perform other tasks that are part of the image-acquisition process: slewing to the new inspection-target position, acquiring a new image, and processing the new image. It should be possible, therefore, to reduce or eliminate the inspection-time bottleneck by reducing the autofocus time at each target position.

Traditional Autofocus

Traditional autofocus systems use multiple images acquired at various focus settings to locate the position of best focus. Being essentially digital in nature, they suffer from two ambiguities: distance and direction. That is, they cannot – with a single image – determine how far the camera needs to move to reach perfect focus, nor can they even determine the direction to move to improve focus.

To resolve these ambiguities, they use a method that is essentially digital in nature. They iteratively make a small focus change, then calculate the spatial bandwidth of the acquired image, then compare this bandwidth with previously acquired images to decide whether to make another change. The presumption is that better focus provides finer details that appear at higher spatial frequencies. So, the feedback system attempts to find the focus position that maximizes the limiting spatial frequency by trial and error.

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