One of the more exciting technologies becoming prevalent in the dimensional inspection industry is industrial computed tomography (CT). This relatively new inspection method utilizes x-ray images to build a three-dimensional model of a part or assembly, allowing inspectors to see inside hard-to-measure parts. Surfaces extracted from the scanned data can then be used to find defects or to inspect geometric dimensions and tolerances (GD&T). But even though CT is an amazing tool, it isn’t right for every job.
An inspection technology that has been more widely adopted is structured light 3D scanning (SLS). SLS works by projecting a pattern of lines onto a part and then one or more cameras take pictures of the part, capturing how the lines deform over the part’s visible surfaces. From these images, 3D geometry is then calculated.
At first glance, it may seem that CT is the superior 3D scanning method since it can collect data on the inside surfaces of parts. However, each inspection method is still used in industry for its own reasons. In this article, we’re going to discuss both the strengths and weaknesses of both inspection methods, specifically in regards to dimensional inspection.
Accuracy
The million-dollar question for most when it comes to inspection equipment is, “how accurate is it?” Thankfully, both inspection methods can be very accurate. Be aware, though, that not all CT and SLS scanners are made for high-accuracy dimensional inspection applications. But the ones that are can achieve accuracies nearly matching CMMs (~5 microns). The accuracy of both of these systems will depend on a number of factors, but the most crucial of these is magnification. If scanning something large, accuracy will go down. If scanning something small, accuracy will go up.
In the case of CT scanning, one of the biggest factors in determining accuracy is the segmentation process. This is the process of deciding where the surface actually lies in the scanned volume. Unless specified in a standard operating procedure, the results may vary significantly from operator to operator.
But a similar operator-dependent accuracy issue exists in 3D scanning. If the part requires a coating prior to scanning because it is transparent, the thickness of the coating applied will vary for each operator. We will discuss this issue further in a later section.
Resolution
Resolution is important because it determines how small your features can be in order to be recognizable in the scanned data. The resolution of industrial CT scanners depend on a number of factors:
- Detector resolution and pixel size
- Source to part distance
- Part to detector distance
- Cone angle
- Source spot size
Because there are so many factors to consider when taking a CT scan, it is crucial that the operator understand the tradeoffs when setting up a CT scan.
The resolution of structured light 3D scanners isn’t as complicated. The primary factors include the following:
- Scanning area
- Camera resolution
- Number of scans
Repeatability
Just as an operator can have a big impact on resolution, the operator can also have a big impact on repeatability. As long as the operator does everything the same way for each part, the results will be repeatable. But if they stray from the path and start relying on manual adjustments throughout the process, the results can vary.
The same is true for SLS, but there are fewer buttons to press. And fewer buttons to press generally allows for better repeatability in the absence of a highly trained operator.
Materials & Finish
It may seems strange to discuss whether a part’s material and finish will affect your measurements, especially if you’re coming from a tactile measurement background. But the truth is that both CT and SLS are both affected by the material and finish (texture and color) of parts.
When it comes to CT, x-rays are absorbed by materials differently. Denser materials, such as metals, tend to absorb x-rays very quickly, while fewer x-rays are absorbed by softer materials, like plastics. This can cause exposure issues in your x-ray images if you’re measuring an assembly containing a wide variety of materials, since it is difficult to expose the image properly. It can also cause issues when two materials in an assembly soak up x-rays at the same rate. Since materials are isolated in CT data using the gray values found in images, if two materials in contact soak up x-rays at the same rate, they will share the same gray value, making it difficult to isolate the materials that need to be measured.
Material and finish also affect SLS, but in a different way. SLS works by projecting a pattern of lines onto the part you are measuring. If the part’s surface is unable to display the image projected by the scanner, good data will not be acquired. Think about it this way, you wouldn’t project a movie onto a shiny black surface. Instead, you would project a movie onto a matte white surface to get a bright, clear picture. The ideal surface for SLS is matte white. Any departure from this color and texture may result in lower quality data. Or in the case of transparent parts, no data at all.
However, this doesn’t mean it is impossible to use SLS to measure parts that aren’t matte white. Most matte parts, regardless of color, will still scan well as long as they are completely opaque. And shiny parts will still scan well as long as they are a lighter color. In the event that your part is dark and shiny, or transparent, a temporary coating can be applied to the part. However, the coating will add thickness to the part, and if the person applying the coating isn’t skilled, the coating may be too thick or clumpy.
Geometries
CT scanning will in some ways do a better job at measuring strange part geometries, since it is able to see inside the part. However, parts with larger aspect ratios, like sheetmetal or parts that are flat may not scan well. This is because the thicker areas of the part will soak up more x-rays than the thinner areas, making it difficult to expose properly. In addition to this, the resolution of CT depends on the magnification of the system. If a part has a large aspect ratio, it may not be possible to put the source close enough to the part to achieve the desired magnification, thus reducing the overall resolution of the scan.
Although SLS is unable to see inside parts, it is possible to maintain a high resolution on parts with larger aspect ratios since the scanner can be positioned more freely and does not have to get as close to the part.
Speed
Although industrial CT scanning has historically been a slow endeavor, recent advancements have sped up its process quite a bit. Many CT manufacturers are now starting to add scanning features that take x-ray images while the part rotates continuously. This allows CT scans to be acquired in as little as a few minutes. Although there may be a small tradeoff in quality, the results are satisfactory for most applications.
SLS can take anywhere from 1 minute to 2 hours for a scan depending on the resolution required and the size of the part. If a single plane needs to be scanned for a flatness check, the scan may only take a few seconds. But if a part with a large aspect ratio needs to be scanned, while also maintaining a high resolution, it may take a couple hours to complete the scan.
Reliability
The sad thing about CT scanners is that they slowly destroy themselves over time. The target slowly degrades as it is bombarded with electrons throughout its lifetime. The detector’s pixels slowly die, one by one. The filament within the x-ray tube can go bad. All of these parts are replaceable, but they will cost you.
With structured light 3D scanners, the projector bulb can go out after a few years of use. Although replacing the bulb can cost a few thousand dollars, it is a much cheaper fix than those required for CT.
Cost
Cost is still probably the biggest reason CT scanners are not used more widely in industry. CT scanners made for small part inspection can start at $400,000, while the best systems can cost more than $1,000,000.
SLS, while still quite expensive, is much more reasonably priced than CT. In fact, industrial SLS can be acquired for as little as $5000, while top-of-the-line systems may reach around $100,000. If you add on a robotic positioning system, however, the price can go up even more..
Portability
Although neither CT scanners nor SLS should be moved during operation, an SLS can be moved fairly easily when not in use. In fact, I used to put a Gom Atos Core 80 in my backpack and take it to a factory in China to inspect incoming parts. Small CT scanners can be moved, but you certainly won’t be putting one in your backpack!
Summary
Am I saying that SLS and CT scanners should not be used because of their drawbacks? Definitely not! Both of these technologies are extremely good at their intended purposes. So the next time you’re considering using either of these technologies, make sure to weigh all of the pros and cons so that you can select the correct measurement method.
And if you’re wondering how both of these technologies stack up against CMMS, the table below compares the abilities of CT scanners, SLS, and CMMs in the categories we discussed above.
| Industrial Computed Tomography (CT) | Structured Light 3D Scanning (SLS) | Coordinate Measuring Machine (CMM) | |
| Accuracy | ✅ | ✅ | ✅✅✅ |
| Resolution | ✅✅✅ | ✅✅✅ | ❌ |
| Repeatability | 🟨 | ✅ | ✅✅✅ |
| Ability to measure various materials | 🟨 | 🟨 | ✅✅✅ |
| Ability to measure various geometries | ✅ | ✅ | ✅ |
| Cost | ❌❌❌ | ❌ | ❌ |
| Speed | 🟨 | 🟨 | 🟨 |
| Reliability | ❌ | ✅ | ✅ |
| Portability | ❌ | ✅ | ❌❌❌ |