3D Scanning 101

By Debbie Sniderman

Figure 1. This physical object is covered with targets, ready for photographing. Photogrammetry software extracts geometric information of the reflective targets’ centers.

The 3D Imaging and Modeling Metrology Group of the National Research Council of Canada and InnovMetric Software, Inc., maker of PolyWorks software, studied 3D measurement hardware technologies and external frame of reference (EFR) monitoring techniques as a basis for research into developing 3D measurement standards. DE looks at some of their findings.

With multiple digitizing technologies available, it is often not obvious which might best serve your needs. This study looked at principles, characteristics and key advantages and disadvantages of both low- and high-density scanning techniques.

Low-density, Single-point Techniques

Single-point measurement tools target and measure one or more specific points at a time by mechanical contact or optical non-contact methods. There are five key reasons that both mechanical contact and optical non-contact, low-density methods are still relevant and important (see Table 1):

1. When combined with mechanical EFR tracking and controlling software, they can produce automated, rapid, repeated 3D surface measurements.

2. They are suitable for measuring parts with highly reflective or transparent surfaces.

3. Difficult-to-reach areas such as deep holes or underbodies may be easier to measure.

4. They can rapidly measure small objects or those with a small number of features.

5. In general, they provide very accurate measurements.

Mechanical measuring methods involve moving a spherical stylus or probe tip to contact an object’s surface and measuring the (x,y,z) position of the tip’s center.

Optical measuring methods project light generated from a lamp, bulb or laser, onto a surface, a reflective marker affixed to a surface, or other reflectors such as spherically mounted retro-reflectors (SMRs). The absolute or relative distance between the scanner and the object is calculated based on the speed of light and the travel time to and from the object (see Figures 1, 2 and 3).

Figure 2. Simple and coded reflective targets.

Figure 3. Known-length reference artifacts (the black bars and cross) on a surface.

Figure 4. Fixed multi-axis coordinate measuring machine with probe.

Mechanical tracking for single point measuring techniques retains the same coordinate system as the scanned object. Fixed or portable CMMs’ base or zero does not change as measurements are made (see Figures 4 and 5).

Optical tracking for single-point measuring techniques involves either a laser tracker following the center of a reflective probe, or light-emitting or passive targets affixed to a touch probe mount. Again, these tracking technologies remain in the same coordinate system as the object.

Conceptually, this is similar to a camera pivoting on a tripod mount to take multiple photos of an object, but the tripod is not moved to a different vantage point (see Figure 6).

Figure 5. An arm-mounted probe obtains coordinate information from multiple points across a surface.

Figure 6. A laser tracker with a SMR scans multiple reflective targets mounted on a surface.

Figure 7. The basic concept of high-density, non-contact digitizing.

External Frame of Reference

EFR hardware or software monitors the digitizer’s position in real time as it moves around an object, and aligns its many images into a common coordinate system to fully capture and assemble the object’s 3D geometry. EFR is used for precise measurements of large (>1 meter) or complex objects, and can be combined with either low- or high-density digitizing technologies. EFR is required whenever digitizing hardware is moved out of the coordinate system of the object—conceptually equivalent to moving a tripod-mounted camera to another vantage point.

High-density, Non-contact Digitizing

High density, non-contact digitizing methods provide a much faster option for measuring large numbers of points without requiring targets. Since the 1980s, non-contact methods have been preferred for large or freeform surfaces, objects with many features, flexible objects, or fragile objects, because surfaces are not touched by a probe.

Figure 8. Overview of active, high-density, non-contact digitizing technologies.

Figure 9. The basic geometric principle of triangulation-based laser digitizing uses a single static laser beam spot.

Active High-density Scanning

High-density digitizing can be active or passive, but active scanning is the more commonly used method. Conceptually, energy is projected onto a physical part, reflected, and viewed or sensed by an electronic detection device (typically a camera or sensor). Analyzing the position of the reflected energy’s image determines (x,y,z) coordinates of the object (see Figure 7).

Surface 3D digitizers capture an object’s external visible surfaces. Volumetric 3D digitizers capture both exterior visible surfaces and hidden interior geometries.

Destructive Volumetric 3D techniques slowly grind away a small layer of material, digitize what is seen, and repeat until the entire part has been fully digitized and destroyed. Destructive methods are typically used for smaller, low cost, mass-produced parts for which internal structure is crucial, and are rarely used.

Most digitizers used in industrial applications use non-destructive techniques. About 90% of close-range, high-density digitizers belong in the triangulation category enclosed in red (see Figure 8).

Table 2 summarizes principles and characteristics of high-density measuring technologies and associated EFR methods.

Triangulation-based Laser Techniques

Laser triangulation digitizers project a line or a single spot (projected as a line using mirrors) onto an object, which reflects and is imaged by a camera. Knowing projection and collection angles relative to a baseline determines a triangle’s dimensions and the coordinates of a point on the surface (see Figures 9, 10 and 11).

Figure 10. Another form of triangulation-based laser digitizing, with a laser line projected on digitized object.

Figure 11. A handheld laser scanner sweeps across a surface.

These close-range laser techniques offer excellent depth resolution on large parts, and can measure small detail such as borders, edges and cracks. They are more robust to ambient light sources, and less subject to noise from object color or luster.

There is a performance trade-off, however, for technologies with a baseline, such as triangulation. Larger baselines are more accurate, whereas smaller baselines exhibit fewer occlusion effects. The laser speckle effect (see Figure 12) also limits this technique’s accuracy on optically rough surfaces.

Figure 12. An example of the laser speckle effect.

Figure 13. The basic geometric principle of fringe-based projection digitizing.

Fringe-based Projection Techniques

Successive images of fringe patterns are projected onto an object, and one or two high-density cameras capture surface images. Enough fringe patterns are projected until a grid of object coordinates can be formed from intersecting reflections on individual camera pixels (see Figures 13 and 14).

Figure 14. This fringe-based white light projection digitizer is combined with optical tracking EFR.

Detailed measurements can be made by taking a series of photos with different stripe pattern widths (phase-shifting), as shown in Figure 15. This technique is known as white light time-multiplexed pattern projection.

These close-range measurement techniques offer good accuracy and lateral resolution along two axes, and are fast to measure objects with low-curvature surfaces. Important limitations of fringe techniques, however, are that they lack the dynamic range needed to scan shiny finishes, are slow to measure objects with intricate details, and suffer from occlusion effects, requiring additional shots depending on the size of the object.

Figure 15. Object to digitize with interference fringe patterns for coarse and fine depth measurement.

Industrial Computer Tomography (CT) Techniques

These close-range, turnkey techniques produce a complete volumetric point cloud of an object, and in some cases, allow porosity and internal defects to be seen. They are very accurate techniques. Unfortunately, CT can be quite expensive, and calibrating and measuring parts made with different materials is a challenge.

Time-of-flight (TOF)-based Laser Techniques

Pulse-based and phase-shift TOF techniques are co-axial technologies with no baseline and therefore no occlusion effects. They are compatible with conventional surveying tools such as GPS, GIS and LIDAR. Their acquisition speeds are suitable for measuring objects such as cars, planes, trains or buildings. One disadvantage of TOF laser techniques, however, is that they are less accurate than close-range techniques.

Passive High-density Scanning

Also known as dense stereo vision (DSV) digitizing, passive scanners capture high-resolution photographs of surfaces lit with high-lumen external or ambient light, instead of radiating light. DSV techniques use multiple cameras and processing software, and are useful for mid-range scanning. They are simple to set up, have rapid measurement times, and some commercial versions provide automated surface matching. Drawbacks include being less accurate than close-range techniques due to occlusion effects, and only working on parts with texture

Identify Your Needs The following graphics present performance ranges to help you evaluate 3D scanning technologies appropriate for your needs.EFR techniques have their own performance specifications that are indicated separately. According to Marc Soucy, president of InnovMetric, EFR techniques can dramatically influence overall measurement results, and their performances should be taken into consideration as well.Other important factors to consider include:• number of operators required • fixture requirements, system footprint and power • setup, data collection and data processing time • calibration procedures • formats of output files and results • availability of options such as continuous or energy-saving modes,wireless data transfer, remote controlsand the dynamic compensation for thermal expansion, temperature, vibration or other environmental conditions. Accuracy ranges of EFR hardware technologies. The volumetric capacity of the EFR determines the area the digitizer can capture. Accuracy ranges of non-contact digitizing technologies. Range of object sizes measureable by non-contact and EFR technologies. Portability of non-contact and EFR technologies. Range of surface materials, finishes, reflectances and transparencies measurable by non-contact digitizing technologies. Relative measurement flexibility of non-contact and EFR technologies, such as the ability to collect data in deep holes, view surface from different perspectives, offer a large depth of field, or gather islands of data only where needed. Typical minimum resolution of non-contact digitizing technologies, involving minimum lateral spacing between successive points or smallest depth difference needed to detect edge, surface and hole fidelity. All images courtesy of InnovMetric Software, Inc.

More Info:

InnovMetric Software, Inc.

National Research Council Canada

Debbie Sniderman is CEO of VI Ventures LLC, an engineering, manufacturing and R&D consultancy. Contact her at VIVLLC.com.