This article discusses two types of 3D scanning: Structured Light and CT. It will broadly cover the advantages and disadvantages of each and some of the factors to consider in choosing a method for a particular application. Some considerations are scanner portability, characteristics of the objects to be scanned, scanner flexibility and limitations, inspection capabilities, and automation. Although we are looking at these only in a general sense, because there are many different types of CT and structured light scanners, these criteria should apply to the majority of scanners. However, we are just scratching the surface, offering an overview of some of the important points.
Types of Scanned Parts
Transparent objects are a good starting point. Some examples would be a glass bottle or a cookie tray. Structured light scanners rely on projecting a pattern onto a part and then using camera(s) to triangulate the contours of the object. If the light/pattern is transmitted through the material, the camera(s) will not be able to capture the pattern on the external surface. Similar issues occur when the object surface is very shiny and reflective, in which case the light and pattern of the scanner will be refracted or reflected, causing noisy data especially in internal corners of parts. In some cases, ambient or other lighting will affect the data capture as well.
CT scanners do not have these types of problems. For them, density of the material(s) is more important than its finish or transparency. Clear plastics and glass are actually very easy to scan with a CT scanner since their density is quite low. Also, polished and machined surfaces don’t pose a problem for CT scanners as long as it is powerful enough to penetrate the part.
Advantages of CT
One major advantage of CT scanning is that it can capture internal features that can’t be seen when looking at the part from the outside. Structured light scanning is what we call “line of sight” scanning. That means you have to move the part around or spin it in order to see all surfaces or details — structured light won’t pick up all of the data if the part is stationary.
With CT scanning, the X-ray penetrates the object to view internal surfaces that could not otherwise be seen without disassembling or cutting the part — altering some of the features you’re interested in. In fiber-filled materials, the fiber orientation and distribution can be captured, which can be useful for analyzing how the part will hold up under force or pressure. For casting and injection moldings, voids and defects can be found in order to verify whether a part will break under load or whether any additional processing on the part will be affected by the defects. These types of internal inspections will prevent parts that are not just off dimensionally, but also have internal faults, from being sent to the customer or to the next production step.
Part Size and Scanner Portability
Part size and scanner portability go hand in hand. An important limitation on CT scanning has to do with the relatively large amount of power needed to penetrate a part. For denser materials and larger parts, structured light may be a better option for that reason. Certain materials like lead, platinum, and gold are examples of very dense materials that would need to be small in size to work well in a CT scanner. Otherwise you would need a very powerful machine to penetrate the part. Industrial CT scanners are commonly enclosed in lead-lined cabinets or concrete rooms. So, if the part is too large to fit in the room or cabinet, it is also not a candidate for CT scanning.
If a part is too large or heavy to be transported to the scanner, having the ability to take the scanner to the part can be an advantage. Structured light scanners are very portable and can be taken to shop floors and other environments where it makes more sense to bring the scanner to the part, not the other way around.
Multiple parts can be placed into the CT scanner and scanned in one volume to help increase throughput. One CT scan can have as many parts scattered throughout the volume as allows the x-ray to penetrate each part with acceptable resolution of the data. In most cases the resolution of CT scans varies with the volume and size of the scanned object and the power needed to penetrate the material.
With structured light, the resolution is fixed for each field of view (FOV) — scan volume. Some structured light systems have multiple FOVs and the resolution changes for each of the scan volumes. A smaller FOV can be used to keep a tighter resolution but many more scans will be needed to capture larger parts. With larger parts that don’t fit into the CT volume or very dense materials that cause the CT scan resolution to be lower than required, structured light scanning would be optimal for the application.
Last, purchase price and service cost are important factors in choosing the correct scanner. In most cases a structured light scanner will cost less than a CT scanner. In addition to the typically higher purchase price, the maintenance/upkeep costs for the CT scanner will also be higher. If cost is an issue and the advantages of CT scanning aren’t critical, structured light may be the better solution.
CT and structured light scanners both can, and are, being used today in production lines for in-line inspection. Both can be paired with other devices for automated processing. This helps maintain throughput and measurement consistency throughout the process.
Although there are advantages to each type of scanner, many parts are compatible with both. Looking at some of the finer details may cause you to lean one way or another. If not, try both and see which works best for you. Exact Metrology offers many different types of scanners and software because there is not one system that will do everything. Depending on part/environmental characteristics, inspection requirements, and many other factors, certain scanners will work better, or not at all.