The first item I should mention before proceeding, is that the process to pick a scanner will alter slightly based on what you are scanning. For this discussion I am assuming that mechanical parts are being scanned, and that the maximum size part would be roughly the size of a car door. Therefore, I am excluding long distance scanners, which would be used for entire car, boat, airplane, or building scans for example.
The key categories that you would need to consider are size of the item, type of geometry, material the part is made of, color of the part, accessibility, and your budget limits. Before looking at these factors, a quick note about the two major groups of scanners is important. Fixed scanners have a specific field of view, and commonly are tripod mounted, or on a stand and used in conjunction with a rotary table. A handheld scanner is exactly as the name suggests, held in the hand and moved around the part to collect the scan data. How the scan data is collected will be ignored for now, because we are going to focus on selecting a scanner well matched to its intended use. Although how the scan data is captured has implications, for this discussion we will assume that either laser line or fringe projection systems will produce equivalent quality scan data.
Size
The size of the part will influence the selection process in conjunction with geometry. A scanners field of view (FOV) needs to capture as much of the part as possible. This speeds scan acquisition, and simplifies alignment during processing, as there are fewer scans that need to be merged. The part size (and geometry) will also influence the required accuracy and resolution for capturing the shape with enough detail.
Geometry
For this discussion, geometry is referring to the amount of small details and complexity of the shape. A pine cone would require more detail to capture smaller features. The scan of a car door would be more “organic”, smooth surfaces with not many surface features.
Material
Materials a part is made of often affects how a scanner can “track”, or how well it can follow the surface geometry. Transparent items, very dark black, and shiny surfaces can introduce difficulties for many scanners. Scan sprays are often used to help a scanner “see” the surface more reliably.
Color
A parts color can cause improper tracking because the scan source is being absorbed, or is unable to properly track the surface. Scan sprays can resolve this issue completely.
Accessibility
Some parts might be very large or heavy, and only a handheld scanner might be suitable. Other parts may have large amounts of complex details, and would be better suited to a fixed scanner as long as the part can be readily moved to the scanner.
Budget
One of the more frustrating categories, for sure. Just try to remember that a cheaper scanner that struggles to perform, is never a long-term or valuable solution. This is an area where a more flexible scanner allows you to finish more projects, with less expense in the long term. The goal is to be able to scan the largest variety of parts, with the minimum number of scanners.
In general, every scanner was designed to be used with a defined minimum and maximum part size. Handheld scanners often are not usable below a minimum size, and lose accuracy and have difficulty tracking on very large items. If you routinely scan rings, pendants, small screws, printed circuit boards, or similar items, the detail and resolution of “macro” scanners are unbeatable. These scanners are specifically designed to capture every detail with high resolution. They are, however, not very flexible, and would not be very useful for scanning a large item. They are best used on parts from a few millimeters in size up to about 3 inches.
Handheld scanners typically are used for items starting at 2 inches up to the side of a vehicle in size. Although they can be used for larger areas of scanning, tracking aids (target dots) often have to be used, and this is a time consuming and costly process. Most target dots can only be used once, and need to be removed when you have completed scanning. Optically tracked handheld scanners offer the greatest range of scanning options, especially for scanning larger items, with virtually no set up required. Your budget will determine if this system is even an option, as they are very expensive.
Another option in scanners is what is referred to as “arm-based”. This simply means a scanner has been mounted on a portable measuring arm. They offer the ability to scan, and use touch probes, all with one system. The other big advantage of these systems is the fact they don’t require any tracking aids. The arm is precision calibrated to know where it is in 3D space. Tripod or table mounted generally, so although they are portable, they are more limited than handhelds. Each arm-based model has a circumference it can reach for measurements, and then would need to be moved if the part exceeds that size.
Material and color of parts can challenge some scanners, and offer no difficulty to others. This is due to the method used to capture scan data, type of light used, processing method, and software used. This is generally an area that has to be covered in a more detailed discussion, preferably after some of the above criteria have narrowed the model choices and brands.
Once those 6 categories have been used to pinpoint the needs and requirements, the remaining models need to be explored in greater detail. This is where scanning some test parts to demonstrate capabilities of each system can be very helpful. The intended use of the scans, what software you have, and what personnel will be operating the system also have a large impact. This is where the knowledge and experience using the equipment and software can save you from purchasing the incorrect scanner, and get you the precise system/package to solve specific challenges.