Looking at the Third Dimension in Surface Measurement
Sometimes the optical system is the best—or even the only—way to measure your surface.
It seems that the subject of 3-D optical surface measurement is all the rage these days. When I work trade shows, a majority of the people who wander into our booth want to know about surface measurement—and a large majority of those want to know about 3-D optical surface measurement. But of those many who are interested, very few are willing or able to actually measure their parts in 3-D, optically or otherwise. The simple fact is that while optical systems have made considerable strides in the past few years, there are still significant barriers to entry for most surface measurement applications.
Optical scanning methods change the way profiles are generated.
This is not to say optical methods are better or worse, but they are different and the basic data set we have to work with is different.
There are a wide variety of different methods and technologies used for optical scanning, including white light interferometry, confocal microscopy, focus variation, digital fringe projection, and many others.
At the same time, there are a rapidly growing number of companies offering optical surface measurement systems, ranging from small two-person shops with grand ideas, to large, full line suppliers who offer optical as part of a broader product line. There are also a number of different ways to measure optically, all of which work on different principles and with varied effectiveness depending on the optical properties of the surface. Some work well on dull surfaces, for example, but their sensors are overwhelmed by highly reflective surfaces. Others rely on a lot of reflective light, but if you scan a darker surface, you don’t see anything at all. And there are significant cost barriers to entry when an optical system is the system of choice. Having said all of that, sometimes the optical system is the best, or even the only, way to measure your surface.
So is optical right for you? In this article, we’ll look at the current state of this technology. We’ll look at applications where it is effective and others where it is not, as well as explore the reasons why. Finally, we’ll offer some practical advice to help you determine if 3-D optical can be a viable solution for your surface measurement application.
The Existential Nature of Surface Measurement
When we measure the diameter of something—a shaft, for example—we arrive at a number that physically represents the diameter of the part. But when we measure the texture of a surface, whether by scanning over an area or tracing a linear distance with a stylus, the result is filtered and processed and has some math applied to it so that the number we get is not a physical attribute of the part but a mathematical representation of the data that we took from the surface. Change the way that data—the profile—is generated or processed and you change the results of the analysis.
With traditional stylus methods of surface finish measurement, we’ve gotten to the point where we pretty much understand the things that might bias our results, like the radius being too large and causing a filtering error. We’ve standardized things so that we can measure parts repeatably and have multiple instruments correlate with each other, and even multiple brands of instruments correlate with each other. We’ve arrived at a methodology that generates a stable number—Ra or Rz for example—that we can be statistically certain represents something about the surface itself.
That’s not to say that at any given spot on the surface the height of the peak or the depth of the valley is the same as the Ra or Rz number. But we can rely enough on the methodology to relate that average number to the manufacturing process that produced the surface. Had optical methods been developed first, and had we been using them long enough to have all the same kinds of issues figured out, they might well be considered the gold standards today.
But that’s not how it happened. After the early days of the thumbnail comparison patches, the next developmental step in technology for surface finish was the use of optical microscopes that allowed a magnified view of surface characteristics. However, measurement was purely visual and comparative and not quantifiable, even though the various degrees of magnification and differing fields of view afforded did lead to the concept of sampling lengths and frequency that are central to surface analysis today.
Seeing in 3-D
Optical scanning methods change the way profiles are generated. This is not to say optical methods are better or worse, but they are different and the basic data set we have to work with is different.
There are a wide variety of different methods and technologies used for optical scanning, including white light interferometry, confocal microscopy, focus variation, digital fringe projection, and many others. Early 2-D optical scanning methods mimicked stylus tracing and collected a series of “linear” data points. More recent 3-D methods collect data from a small area. But regardless of the method used, the smallest increment of resolution available is typically the pixel in the imaging device. With typical systems this resolution might be on the order of approximately two microns square. Within this area, height information is optically averaged to come up with a single value to represent this square. Because of this, peak heights and valley depths may be somewhat flattened. (See Figure 1.)
In contrast, the radius of a diamond stylus is also about two microns (may also be five or 10 microns depending on the specification of the surface to be measured), but the actual point of contact with the surface is much, much smaller than this tip radius. The standards for specifying and measuring surface finish with a stylus based instrument ensure that the tip radius is small enough for the surface that is to be measured. If the tip radius is too large, there can be a filtering of the data due to the large tip, which impacts the measurement. The resolution with a stylus instrument is usually finer than is possible with current optics. The lateral spacing of data points is also closer with these stylus methods, typically 0.25 to 0.5 µm vs. 1 to 2.5 µm for optical. The averaging over the pixels making up this resolution can have a filtering effect which changes the result one can achieve with any given measurement technique. Again, this is not to say one is better or worse, right or wrong. But this resolution is one of the key differences that affect the correlation of parameter data.
Figure 2 shows two trace profiles of a Halle roughness standard, one done with a white light optical system, and the other with a diamond stylus. The lateral sampling interval for the white light scan is about 2.1 µm, and about 0.5 µm for the contact trace. While the profiles appear very similar, there are minor differences in peak height, valley depth, and slope representation that may have important effects on the mathematics of parameter calculation.
Regardless of which trace may be the more “realistic” representation of the surface, and hence, the more “accurate” calculation of whichever characteristic is being measured, the point is, the results do not always correlate. In fact, of the three surface parameter types, amplitude parameters correlate best—usually to within 20%—while hybrid and spatial parameters are more problematic.
Surface finish parameters have become steadily more sophisticated in terms of assessing surface functionality, especially since the widespread application of digital and computerized methods. One of the key expectations of 3-D optical systems—and one of the assumptions current in the marketplace—is that they would dramatically improve this link between a parameter and the desired surface functionality. Surfaces do exist and function in the real world in three dimensions, after all, so being able to represent them in their “native” form has to improve our ability to assess their ability to function, right?
Well, hopefully. But until recently, the 3-D parameters available were direct corollaries of what could be done in 2-D. Ra, for example, or average height, transformed into Sa, which is just the average height over the 3-D area. Rz, which is 10 point height, became Sz which is 10 point height in 3-D. There’s Rq and Sq, and so on.
Beginning in 2010, the International Organization for Standardization (ISO), published a new Geometric Product Specification, ISO 25178, based on an areal method of analyzing surface texture. The standard defines a few “native” 3-D surface texture parameters, along with many of the 3-D corollaries to the 2-D parameters, and the associated specification operators. It also describes the applicable measurement technologies, and calibration methods, together with the physical calibration standards and calibration software required.
Additional research is also being done on how to quantify and characterize 3-D surfaces, and how to develop mathematical algorithms to incorporate three dimensional surface data, but it’s not as easy as it might seem. For example, in the various Rz parameters (there are several), a “peak” is defined in relation of a mean line. In two dimensions, this is fairly straightforward, but when you add a third dimension, that mean line becomes a plane extending in an infinite number of directions and mathematically determining what differentiates a peak from a sub-peak (or a rock on the side of a hill!) is tremendously more difficult.
Just the amount of data that needs to be acquired and processed is intimidating. At 0.25 µm point lateral spacing, a typical 5.6 mm 2-D trace would generate 22,400 data points. To generate a 5.6 x 5.6 mm square field of view with this data density, a 3-D optical system would have to generate 501,760,000 data points. This would necessitate a 500 megapixel camera, not to mention a computer capable of processing the data in a timely manner. Even at 0.50 µm data point spacing, the 2-D contact trace would have 11,400 data points, while square field of view with this data density would require 125,440,000 data points, which still is not practical with today’s technology.
Because of this, most optical systems look at fields of view smaller than this, or use lateral point spacing greater than this, or both. Technology continues to develop, and it may be possible in the not too distant future to achieve higher resolutions, but it may be many more years before it becomes affordable to the average user of surface metrology systems.
So what does this mean? Are 3-D optical surface texture measurement systems simply ‘not there’ yet? Not necessarily. It depends on the application. For an application that was specified and initiated using optical methods, there will probably be no problem at all. However, for parts that have a long data history of contact measurement, the lack of correlation may cause issues. And even in cases where studies have been conducted to verify process parameters with optical systems and establish a correlation, any change in the process—switching to a new grinding wheel composition, for example—may alter the process equation.
Thus, in industries like automotive, where parts are manufactured in disparate locations and where there is a long history of stylus generated surface data, change to optical at this stage would seem to offer few benefits. And even if (when) technology advances to allow equivalent data density, it still may be more economical to measure certain parts with traditional 2-D stylus tracings.
In other industries, however, such as the computer industry, where manufacturing process specifications seem to be rewritten every few years, optical may already be the best choice. There are also areas where we are creating these complicated 3-D surfaces that need to function in 3-D. This is where we really see that the world needs to get to a 3-D measurement method and real 3-D parameters to assess how a surface might function in 3-D.
But the largest barrier to 3-D optical surface measurement is cost: 3-D optical surface measurement systems are orders of magnitude more expensive than traditional, stylus-based systems. For example, the lowest cost entry-level optical system will run in the neighborhood of $60,000. An entry-level stylus system will run about $2,500.
Thus for now, the people who really can’t, or don’t want to touch the part will be the ones most likely to use the optical technologies. In the optics industry, for example, contact methods simply won’t work: they have to measure optically. Medical implants are another area where the tiniest scratch can result in a rejected part. In certain areas of nuclear research, also, the risk of contamination from touching parts is so great there is no option but optical. And who knows what the future holds, either in applications or technology?
Is Optical Right for You?
With the cost of optical systems, the number of suppliers and the differences in technologies, it is not something you want to jump into. There are simply too many variables, and too many unknowns. If you feel you need to go optical, you need to choose the “right” optical method for your application.
First do your homework on the systems available. Then work with a number of manufacturers of those systems. Send them your parts and have them measure them to your specifications on their equipment. Even if parts look the same to the eye, they may not measure the same optically. Two optical systems using different technology may “see” the surfaces differently. They may have different resolutions, or one sees reflected light differently.
If the results are good, and the price is right, you too can start seeing things differently.