Quality Test & Inspection: Find the Right Optical Option

May 1, 2005
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Whether it's a simple magnifying glass or a fully automated system, understanding optical inspection helps the manufacturing process.

Today's global economy demands that manufacturers remain productive and keep costs down without sacrificing quality, and advances in optical inspection are helping them to do just that. From visual inspection with a simple magnifying lens to production line inspection of product variations to automatic inspection of semiconductor wafers at UV wavelengths, optical inspection plays a key role-from product development through production-across a wide range of manufacturing industries.

A broad field with many areas of specialization, optical inspection helps meet multiple requirements in the manufacturing process. When assessing optical inspection options, it is helpful to understand a few fundamental concepts.

First, any inspection process requires visualization. A human or electronic eye must be able to "see" the item being inspected. Although simple inspection can be conducted with the unaided eye, parts usually are magnified by an optical system of some kind. Optical systems can be fixed or variable and include zoom optics, which cover a range of magnifications. An optical microscope is a good example of an inspection system for human interpretation of magnified images. Here, the operator resolves finer details at selectable magnifications.

There is an important consideration with magnification for optical inspection. As magnifications increase, the fields of view decrease. To see more detail, less of the part is seen. In addition, both the distance from the optics to the part and the depth of focus typically are inversely proportional to the magnification. These facts limit inspection of handheld parts to lower magnifications. At lower magnifications the part is in focus over a range of positions in front of the lens without being too close to the lens. High magnification inspection requires that the part be in a fixed position, and not handheld.

Visualization with an electronic eye usually is a camera that displays the image of the part on a monitor or converts it into electronic signal levels for further analysis and processing.

Optical inspection usually requires more than ambient lighting. Illumination sources improve the image contrast of the part being inspected. Variable lighting sources can emphasize edges, highlight texture or better define outer perimeters. White light can aid color perception while colored light can highlight some details while suppressing others.

The equipment used for optical inspection ranges from simple units that bolt to a workbench to modular units that can be attached to a coordinate measuring machine (CMM) to stand-alone manual and automated systems.

An example of a self-contained optical inspection system is one that attaches to a workbench and is mounted on an articulating arm. It includes motorized zoom optics, illuminators and color liquid crystal display (LCD). It has a large working distance and depth of field that allows handheld parts to be inspected at relatively high magnifications. A push-button remote control allows magnification change without diverting the operator's eyes from the monitor.

Although a zoom lens makes magnified inspection easier than rotating between fixed magnification lenses, at times it is inconvenient to reach for a control to change magnification while inspecting a part. An inspection device with a lens system that varies magnification with distance from the lens can be helpful. With such a system, a part being inspected is held beneath the optics with two hands while viewing a magnified image on a video monitor. At a large distance from the lens, a broad area can be seen at once. While scanning the surface, a feature or defect can be magnified simply by moving the part closer to the lens. A large depth of focus allows this system to retain crisp imaging across its magnification range. Magnification can vary more than 10X over a 6-inch range in front of the lens.

Simple optical inspection systems can be quite compact. This allows them to be used on CMMs. Some small units can be positioned like a touch probe by adding the capability of visual inspection. Self-contained surface lighting and zoom optics allow magnified visualization of any area on a part that the CMM probe can access.



Beyond optical inspection

Optical inspection provides a suitable means for part and feature validation, but typically lacks true measurement capabilities. Simple visual measurement within the field of view compares the size of magnified features to a scale reticle in the eyepiece, relying on inspectors to quantify the results. Using perceived tolerances and inclusion results, inspectors determine objectively whether a specific part meets or exceeds documented tolerances without quantitative objective data reinforcement. True part measurement requires measurement of highly magnified features across a large part-beyond a single field of view.

Video measurement systems are optical inspection devices that combine high magnification optics with positioning stages which track the distances between features throughout the system's full range of travel. Very simply, a video measurement machine uses magnified images of the part to identify dimensional characteristics and relationships. To achieve good measurement accuracy, these systems typically are built on sturdy platforms and use high quality optics and video monitors for the display of magnified images.

Several levels of optical measurement systems are available. One level simply requires that the operator place a vernier on a stage holding the part. The operator finds a feature and aligns it to a reference in the eyepiece, zeroes out the vernier, translates the stage to another feature and reads the dimension between the features on the vernier. Moving one level higher, the vernier is replaced with a scale and readout.

For example, a digital readout (DRO) can display the values for multiple axes, as well as conduct conversions between measurement units. Stages on such systems usually include coarse positioning for large moves across the part and fine control for precise location at each feature. For advanced manual measurement systems, software can be installed to record a measurement sequence. These measurement routines can later "guide" an operator from feature to feature on the part to be measured.

Yet another level moves from a manually operated stage position

to motorized control. A joystick-

controlled stage can duplicate what is done manually far more quickly and usually in more than one axis at a time. Driving motorized stages through software allows positional changes to be programmed so they can be precisely repeated.

There is more to video measurement than part positioning. Optical measurement systems are most accurate when using consistent, controlled lighting. Several methods of back and surface lighting using incandescent and LED illuminators are available.

A popular surface light is a segmented ring light. Concentric rings of LEDs divided into 6 or 8 segments can be selected for optimal imaging of every feature. With this advanced lighting solution, the operator has control over the illumination angle and light direction. Measurement software on such systems retains all of the light settings so they are duplicated exactly on subsequent measurements of each feature.

On-axis lighting provides illumination directly from above while backlighting can improve contrast of through-holes and edges of the part. Any or all of these lighting techniques can be used in a measurement sequence.

The key to good video measurement is the system's metrology software. More than automating a sequence of motions determined by an operator, metrology software adds intelligence. It includes algorithms to improve edge detection, to consistently provide autofocus and to compute measurements based on data points and relationships between those data points. It includes diagnostics and calibration routines to ensure measurement accuracy over time. And it is flexible enough to allow manufacturers to measure parts as they are redesigned.

Fully automatic video measurement systems can be set up once and run many times. Using this kind of system, all variables are controllable and definable within a program that is written once, stored and then run by many operators with the same accurate results. The stage movements are automated, as are the magnification range and illumination requirements. The operator simply places the part in a fixture and runs the program. The automated system performs all the necessary calculations, compiles data reports results to the system monitor and transfers them as necessary for further analysis or statistical process control.

The more automated the measurement process, the less variability occurs from operator to operator, leading to enhanced productivity. Rather than dedicate people to manual inspection, they can perform other functions while an automatic measurement system measures a part or a series of parts. In a work-cell environment, a machine operator can take newly manufactured parts to an automatic measurement system, start the measurement routine and return to other tasks in the work cell.



The future of optical inspection

Because vision measurement systems use monitors to display images of the parts and features being measured-unlike optical microscopes and other optical inspection devices without cameras-these systems can be used for inspection, teaching and measurement. Operator fatigue is reduced by using an automated vision system to achieve repeatable, quantifiable results each time while still allowing for manual optical inspection at increased magnifications.

It is important to consider the range of capabilities desired from optical inspection. Although it generally is true that the more complex automated systems come at a higher price than simpler manual systems, the total cost can be less than one might expect. When selecting an optical inspection system, remember to include the costs of the time that operators use the system, as well as the training costs of skilled operators to accurately interpret measurements vs. those for simply loading parts into a fixture and initiating a measurement program.

Also consider the value of the breadth of measurements that can be achieved using an automatic system. Video measurement is more than measuring the distance from point A to point B. The more that is known about the parts made, the better the processes can be controlled-and the quality.

With many variables in manufacturing, customer demands for ever-increasing quality and the continual downward pressure on costs, optical inspection and measurement can be used at many places in the manufacturing process to make companies competitive.

Companies should avoid being "penny-wise and pound-foolish" when making optical inspection decisions. The more you know about the parts-and the earlier in the process is it known-the less the risk of defective parts. Q

Mark Glowacky is president of RAM Optical Instrumentation Inc. He can be reached at info@ramoptical.com.



Tech tips

• Whether performing simple visual, production line or automatic inspection, optical inspection plays a key role in many manufacturing industries.

• With an optical microscope, the operator can resolve finer details at

higher magnifications while trading off a smaller field

of view.

• The distance from the optics to the part typically is inversely proportional to the magnification. Optical systems include fixed, variable or zoom lenses.

• Optical inspection usually requires more than ambient lighting. White light can aid color perception while colored light can highlight or suppress details.

• The equipment ranges from simple units that bolt to a workbench to modular units that can be attached to a CMM to standalone automated systems.

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