A Closer Look at Optical Inspection
Optically inspecting parts has become smarter, faster and more high-tech than ever.
Optical inspection is vital to quality control in manufacturing, and in breaking down the meaning behind the words, it’s easy to see why.
As described by Matt Novak, director of technology and applications development at Bruker Nano Surfaces, “optical” means “of or pertaining to the interaction of light with matter and also the perception or detection of that light,” while “inspection” denotes “the process of examining objects or the world around us closely for specific (desired or undesired) characteristics.” Thus optical inspection can be defined as the examination of manufactured parts, whether that be through a microscope, laser scanner or other light imaging techniques, to detect possible defects and provide the utmost quality assurance.
“What I see in inspection today is manufacturers checking parts that they’ve either fabricated themselves or that they’ve received from suppliers to make sure tolerance requirements are met,” says Patrick Beauchemin, Ph.D., president of VISIONxInc. “That includes dimensional inspection, determining sizes, as well as quality inspection to make sure there’s no burns or scratches on surfaces. Optical inspection; typically involves cameras, lenses and sometimes illumination systems, certainly nothing such as a touch probe.”
After establishing a basic definition of optical inspection, let’s look back on its history to see how far the industry has come.
“Optical inspection using optical comparators, bright light and image inspection has been around for decades,” explains Novak. “Tactile, comparison inspection was one main predecessor to optical, non-contact inspection.”
“Optical inspection started with traditional optical comparators. The technology was developed in the 1940s and it was the workhorse technology; everybody had optical comparators in their shop,” says Beauchemin. “These were big machines that had no electronics; they were pure optical machines: lenses, prisms, mirrors and projection screens. Traditional comparators magnify the image of the part, typically just by back illumination, to project a black and white image on a big screen—you would see the background as white, and the solid part as black. And the magnification of those machines is straight-forward; for example, a 10x machine magnified a part 10 times, so you could see defects and inaccuracies up close.”
“Traditional comparators have templates, mylars, overlays, all basically the same name for a transparency with lines drawn on it,” adds Beauchemin. “The line is a tolerance band, and you would want to make sure that the outline of your part—that black and white edge— falls in that tolerance band. An operator then scans the projection screen to make sure that the part checks out okay.”
An evolution had to happen at some point; and eventually it did, such as innovation through laser scanners, 3-D imaging and other advanced forms of electronic detection.
Optical Inspection Today
“Use of electronic detection or ‘machine vision’ is now common, due to the availability of powerful of electronics and computers,” explains Novak. “Broad area and lower resolution detection is common for applications in aerospace and automotive manufacturing in general, larger parts with 25 micron tolerances, while smaller area and higher resolution detection is common in semiconductors, electronics and display manufacturing.”
New technology is transforming optical inspection in a variety of ways, providing more efficient, precise and quality-driven results across all industries.
“As electronics, cameras and computers have become more powerful, inspection techniques have become more advanced,” Novak continues. “Now, optical inspection involves the detection of defects industry [with] laser reflectance, uniformity and semiconductor/wafer front end inspection. Optical inspection in automotive and power train manufacturing involves microscopes, macroscopic camera imaging and spectral imaging, to name a few, and comparison to desired form is common using software analysis, image pattern matching and recognition.”
“Today, as miniaturization evolves, particularly in the consumer electronics arena, there is a drive to have more powerful inspection techniques, including 3-D imaging for both lateral and vertical inspections,” says Novak. “Line scanning techniques, laser scanners, optical imaging and 3-D microscopes are powerful tools for examining both geometric and also functional surface characteristics, and are increasingly important as manufacturers face challenges to make smaller and more accurate parts for miniaturized electronics, [such as] MEMS for medical uses.”
The Future of Optical Inspection
With today’s rapidly evolving technology in mind, what can manufacturers and consumers expect from optical inspection in the not-so-distant-future?
“Certainly we’re looking for more accuracy, that’s a big thrust, but I would say the biggest emphasis by far is greater throughput and automation,” says Beauchemin. “People don’t want to sample one part out of a hundred; they want to check a hundred parts, and they want to check them a lot faster. As a result, automation is becoming a bigger deal. We are seeing more people integrating [optical inspection] machines into a complete work cycle. For example, as a robot loads parts into a machine, our optical inspection solution gives a pass/fail result, and the robot picks up the part again to either put it into a ‘bad parts’ bin or the proper container for ‘good parts.’”
“We’re also seeing people going for not-so-objective inspections,” Beauchemin points out. “When you’re measuring a dimension, you might say, ‘This is supposed to measure 1.234 inches, and it’s 1.235.’ That is objective. But how about, ‘I need to make sure there’s not too much overburn on this part, and make sure that it doesn’t have cosmetic defects that would make it unacceptable’? That is maybe in the more distant future, but clearly on the horizon, and something that customers have expressed interest in doing.”
Robots will be an increasingly important part of this process, too.
“More and more people are saying ‘Your machine looks great, it does what we want, but we are looking for ‘lights-out operations,’” asserts Beauchemin. “We want parts made at night with robots loading stock into the machines and unloading finished parts,’ so the bottleneck now becomes inspection. Their robots are working all day and all night; and if your operator is only working a nine-hour shift, they can’t keep up with inspecting these parts.”
And what about the fear that robots will take over, a futuristic anxiety that doesn’t seem so far-fetched in the manufacturing world?
“Technology has historically been creating jobs, not destroying jobs,” says Beauchemin, on an encouraging note. “So what happens is that jobs shift from one area to another. It is important to stay very competitive, and automation is a key part of that. We always need to stay a step ahead.”