Quality Test & inspection: See the Forest and the Trees

February 1, 2005
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Evolving from simple devices to sophisticated instruments allows manufacturers to see the whole picture.

The vision system responds to the need for more optical resolution and higher capabilities. It has evolved from a simple video-measuring device to a more sophisticated high-resolution scanning instrument. Vision systems use advanced imaging techniques such as extended depth of field and field-of-view stitching. Photo: Nikon Instruments Inc

Recent advances in optical imaging, computers and wireless communications are creating a new digital-to-data revolution that affects how products are manufactured. Digital products are everywhere. Devices such as cell phones, PDAs, jump drives, digital cameras, and the ASIC's that run them, fuel people's ever-increasing appetites for cheaper, better and faster products. To meet these insatiable customer needs, suppliers continuously create new manufacturing processes and products with improved performance at a lower cost. But where does it go from here? When it comes to technology, it starts again, redefined in another form, in another new product.

To support the growth of technology, the use of lenses and optical imaging for manufacturing has been standard on the factory floor for more than 60 years. Early on, manufacturers of close tolerance products depended on a highly skilled technician to acquire noncontact magnified images and take simple measurements. Those applications were analog, the computer was a person's brain, motion was controlled by hand and the image receptor was the craftsman's eye or a film camera. Memory was the notes on the log sheet with a number two pencil. The optical comparator and microscope were the primary tools used to magnify the image of the part feature. The operation of optical instruments almost always follows the applications steps of:

• Image

• Measure

• Analyze

• Move

• Repeat

This sequence has not changed much over time. However, each of these individual steps has been steadily enhanced since the 1980s by the advent of the personal computer (PC). Noncontact video measuring systems (VMS) are one example of a product that has improved these process steps. Video-based products used the PC and computer numerical control (CNC) mechanics to automate these repetitive functions and improve productivity.

Video measuring grew as a hybrid mix of several products including the PC, coordinate measuring machines (CMMs), measuring microscope, optical comparators and the TV camera. The advantage of these instruments is in allowing the PC to do a good job at increasing speed of motion and analyzing video edge detection. Variation induced by human interpretation also is greatly reduced with the VMS, particularly during measuring and analysis. CNC stage and auto focus, also controlled by the PC, increase the speed of image acquisition as well as repetitive moves.

With this automation, optics, computers and CCD devices are programmed to measure parts quickly and accurately. During the past 15 years, the use of these tools in manufacturing has grown exponentially. Growth continues to accelerate because of the rapid advances in computer speed, memory, digital camera and lens designs. Coordinate video measuring and noncontact inspection have been validated by the video generation.



The next level

Manufacturers are producing parts with smaller and more precise features than ever before. To keep pace, the video-based measuring instruments must move to the next level and improve precision, capabilities and digitally communicate larger amounts of high-resolution data.

To create the necessary changes, suppliers of optical and video measuring instruments face the challenge to improve digitized image processing, increase optical resolutions, improve lens and illumination designs, increase mechanical precision and improve Z-axis measuring capabilities. Computers that accompany these tools must have speed, large memory and intelligent software with a graphical user interface capable of connecting people effectively with the data they need.

The optical lens systems for those early video measuring instruments were designed primarily to produce high contrast images so the PC could process a repeatable edge because PC frame-grabber boards often required high contrast to work accurately. This no longer is the case.

The newer frame-grabber image boards have been improving rapidly over time, and the PC now can read low contrast edges and provide great results. Because contrast and the frame-grabber board no longer are such a limiting factor, high resolution becomes the dominating specification to push optical measuring to the next level.

When it comes to optics, contrast and resolution are opposing phenomena; it is impossible to have both at the same time. As resolution increases, contrast will decrease. When designing optical systems, resolution first must be established with the lens system because it can never be improved from its original state. Therefore, resolution is defined as a function of magnification, lens quality, wavelength and numerical aperture, and can be described as a dimension, relative pixel size or line pairs per millimeter. The fine details determine the threshold of what can be imaged. The best optical resolution and smallest details visible in white light imaging are approximately 0.2 micron.



Welcoming vision systems

One new optical instrument responding to the need for more optical resolution and higher capabilities is the vision system. This is a new product class that is evolving from a simple video-measuring device to a more sophisticated high-resolution scanning instrument. Vision systems use the newest advances in imaging techniques such as extended depth of field (DOF) and field-of-view (FOV) stitching.

These new techniques are improving one of the key challenges facing video measuring. During programming, as magnification is increased to obtain better accuracy, the features needing to be measured get zoomed so large that only a fractional part of the whole feature to be measured can be seen. Therefore, as magnification increases, DOF becomes more shallow and FOV significantly smaller. Increasing magnification is among the key ingredients for increasing accuracy and precision, causing conflict for programmers because increasing magnification can be counterproductive by reducing the FOV and DOF.

The FOV stitching and extended DOF techniques have become two of the most valuable new tools for optical imaging because they allow the technician to increase resolution and use the capabilities of ultra-precision Z scanning to stack images for surface profile and form. Because of the built-in mechanical precision of the vision system, high magnification imaging can be used for better sample characterization. Intelligent software adjoins large FOV data files automatically, forming a larger composite image of stitched ultra-high precision coordinate data. To extend the DOF, the optical instrument acquires a high magnification, ultra-thin, DOF image slice at multiple Z-height intervals that span from the bottom to the top of the feature. Because these images are taken at the highest magnification and numerical aperture, it is possible to keep their inherent high pixel resolution data. The image slices are stacked via the software and recombined to produce an accurate image with its three-dimensional height. These optical solutions allow the instrument operator to see both the forest and the trees.



Future use

Vision systems for the future need to be ultra-high quality optical instrumentation. The instrument's precision and functional value will be established from superior mechanics, optics and electronics. This type of product results from scientific collaboration of several new technologies using sophisticated manufacturing materials and processes. Design and manufacturing experience from the digital camera, semiconductor and the lens designs from optical microscopes will provide the vision system with a cutting-edge technology toolbox to offer solutions for challenging new applications. These precision optical instruments must effectively use computer-aided engineered designs and manufacturing to produce the high quality necessary to precisely handle multiple lens systems, cameras, lasers and special illuminators. These measuring instruments must be hefty and robust enough to remove unwanted vibration and deliver results.

Business is changing fast for people in the digital-to-data world. Looking back at the cell phone and consumer digital camera over the past few years, this is evident. A digital camera from just a few years ago will likely be too slow, not have enough memory and have limited zoom capabilities to meet current needs.

Industrial optical and digital instrumentation is changing at a phenomenal rate. New capabilities are advancing precision, and speed and imaging are making products of yesterday obsolete. The technicians using these vision systems clearly understand the power they possess to help their businesses grow by solving problems and measuring things that were once immeasurable.

Because of the growing needs in manufacturing for automation, higher accuracy and better information, the operator of the vision system is better able to turn more of the digital image information into usable data. Vision system technology provides the necessary platform for the technician to zoom deeper into their processes to better characterize their products. Q

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