Laser Tracker

What is a Laser Tracker

A laser tracker is a spherical coordinate measurement system (SCMS). It establishes the position of a point in space from three quantities: a horizontal angle (azimuth), a vertical angle (elevation), and a radial distance. Two precision angle encoders inside a rotating head supply the angles, and a laser-based distance meter supplies the range. The intersection of these three readings is a single 3D coordinate. Because the head rotates to keep the beam on a cooperative target, the operator can move that target continuously and the tracker reports a stream of coordinates, which is the behaviour that gives the instrument its name.

The cooperative target is almost always a spherically mounted retroreflector, or SMR: a glass or air-path corner cube set into a precision steel ball, typically 1.5 inch (38.1 mm) in diameter. The corner cube reflects any incoming beam back parallel to itself, so as long as the SMR stays roughly pointed at the head, the returned light re-enters the tracker and feeds both the distance meter and a position-sensitive detector that drives the gimbal motors to recentre the beam. The optical vertex of the corner cube sits at the geometric centre of the ball, so the measured point is the ball centre regardless of how the operator rolls the SMR against a feature.

The technology was invented in the mid-1980s by Kam Lau and colleagues at the United States National Institute of Standards and Technology (NIST), originally to measure the pose of industrial robots, a task that fixed interferometers handled poorly because of their rigid fixturing. The first commercial trackers reached the market in the early 1990s. The decisive later advance was six-degrees-of-freedom (6DoF) measurement, which added orientation to position and let the tracker drive hand-held probes, scanners, and robot-guidance targets rather than only a bare reflector. That shift moved trackers from final-inspection tools into embedded, in-process metrology.

In application scale, a laser tracker bridges a gap that neither a bench CMM nor a portable arm can fill. A bridge CMM offers length accuracy near 1 to 3 micrometres but is boxed into a working envelope of a few metres inside a climate-controlled room. A portable articulated arm reaches roughly 1.2 to 4.5 metres around its base. A tracker covers a spherical volume from about 10 metres to 80 metres in radius from a single station, and several stations can be bundled together to map an entire aircraft, ship block, wind-turbine nacelle, or particle-accelerator tunnel. The trade is that accuracy is not constant; it degrades with distance, which is why tracker specifications always carry a per-metre term.

Four engineering metrics determine whether a tracker fits a job: the maximum permissible error (the accuracy guarantee, distance-dependent), the working range, the data rate (points per second, which governs scanning and dynamic capture), and whether the system supports 6DoF and surface scanning. These four set the purchase price, which for new mainstream systems generally spans roughly 30,000 to over 100,000 US dollars depending on range, accuracy class, and the probe and software bundle. Understanding how those four interact, rather than chasing a single headline accuracy number, is the core of a sound selection.

Laser Tracker Types and Configurations

Laser trackers do not divide into wholly different machines so much as into configurations layered on a common head: reflector-only systems, 6DoF systems, scanning systems, and automated or fixed-installation systems. A single modern head, such as the Leica Absolute Tracker AT960 or the FARO Vantage, can serve as all four with the right target and software license. The table below summarises the four configurations against the capability that distinguishes each.

Reflector-only operation is the original and still most common mode. The operator carries an SMR to each point of interest, drops it into a magnetic nest or holds it against a surface, and the tracker logs the centre coordinate. This mode delivers the tightest accuracy a given tracker can reach because it involves only the encoders and distance meter, with no auxiliary camera or probe geometry in the error budget. Tooling-ball nests, drift nests, and corner adapters extend the SMR to repeatable reference points and edges.

6DoF probing adds orientation. A powered probe carries an SMR plus a known constellation of LEDs or fiducials that a camera integrated into the tracker head observes. The distance meter and encoders fix the probe position; the camera solves its rotation. This lets the operator reach points the laser beam cannot see directly, for example inside a bore or behind a flange, using a rigid stylus offset from the tracked target. Because the probe geometry and camera enter the error budget, 6DoF accuracy is specified separately and is looser than reflector-only accuracy.

6DoF scanning replaces the touch probe with a hand-held laser line scanner that projects a stripe onto the part and triangulates surface points at high rate, while the tracker keeps the scanner located in space. This produces a dense point cloud over a large volume, suited to surface form analysis, gap-and-flush studies, and reverse engineering of fixtures and tooling. The combined system trades some of the tracker raw accuracy for the ability to digitise free-form surfaces a touch probe could never cover in reasonable time.

Automated and fixed-installation setups bolt the tracker to a stand or robot cell and pair it with a motorised smart target. The tracker then follows a moving machine slide or robot tool and streams live position deviation back to the controller. This underpins robot calibration, machine-tool volumetric verification, and metrology-guided assembly, where the measurement loop runs continuously rather than as a discrete inspection step. The same head used handheld in the morning can run unattended in a cell in the afternoon.

Ranging Technologies: IFM and ADM

The angle encoders in a tracker are broadly similar between makers; the distinguishing technology is how the instrument measures distance. Two principles exist, the interferometer (IFM) and the absolute distance meter (ADM), and modern instruments blend them. Understanding the difference explains why some trackers must return a dropped target to a home nest while others recover instantly, and why datasheets quote distance error as a fixed plus a per-metre term. The table below compares the two ranging principles.

The interferometer splits the laser into a reference path and a measurement path to the SMR, then counts the interference fringes produced as the target moves. Each fringe corresponds to a known increment, roughly half the laser wavelength, so by counting increments the system tracks displacement with sub-micrometre resolution and very high update rate. The limitation is fundamental: an interferometer knows only how far the target has moved from a starting point, not where it is absolutely. If the beam is broken, by an obstruction or by the operator walking through it, the count is lost and the SMR must be returned to a known reference position to re-establish it. Classic interferometric trackers therefore carried a fixed home nest.

The absolute distance meter measures the true distance to the target directly, commonly with a frequency-modulated continuous-wave (FMCW) scheme or a modulated time-of-flight method, without needing any fringe count or starting reference. Its great virtue is robustness: it survives beam breaks, recovers the moment the beam re-acquires the SMR anywhere in the volume, and enables point-and-shoot acquisition where the operator aims at a fresh target and reads its distance immediately. Early ADM units were slower and coarser than interferometers, which is why the two coexisted.

Modern absolute trackers fuse the two. The instruments now generically called absolute trackers combine ADM robustness with interferometric resolution, so the operator gets point-and-shoot convenience and beam-break recovery without sacrificing the fine dynamic resolution needed for scanning and high-rate capture. In practice this means an operator can break the beam to step around an obstacle, re-aim at the SMR, and continue measuring without walking back to a reference nest, while still streaming up to 1000 points per second for dynamic and scanning work. This is the dominant architecture across the current Hexagon, FARO, and API ranges.

The practical consequence for buyers is that the old IFM-versus-ADM trade is largely settled in favour of absolute architectures for general-purpose work, but it still matters at the margins. Pure dynamic applications that capture a fast-moving target benefit from interferometric resolution; applications dominated by frequent beam interruption and multi-target point-and-shoot benefit from ADM behaviour. Read the datasheet to confirm the distance error is specified with both a constant and a per-metre coefficient, because that pairing is what bounds error across the working range.

Targets, 6DoF, and Performance Standards

A laser tracker is only as good as the target it follows and the standard against which its accuracy is verified. Two topics dominate this chapter: the SMR and 6DoF targets that close the optical loop, and the standards ISO 10360-10 and ASME B89.4.19 that define how a quoted accuracy figure is measured. Both are easy to overlook during purchasing and both directly govern the numbers on the datasheet.

The spherically mounted retroreflector embeds a corner-cube retroreflector in a precision steel sphere. The standard size is 1.5 inch (38.1 mm), with 0.875 inch (22.2 mm) and 0.5 inch (12.7 mm) versions for smaller nests and tighter features. The corner cube returns the beam exactly parallel to the incoming ray over a wide acceptance angle, and the manufacturer sets the optical vertex at the ball centre so the measured point is the ball centre in every orientation. SMR quality is graded: top classes hold the centring error to roughly 2.5 micrometres, while economy SMRs are looser. Because centring, sphericity, and dihedral-angle error feed straight into every coordinate, a dropped, dented, or dirty SMR silently corrupts results, so SMRs are treated as calibrated artefacts, kept clean, and re-certified periodically.

6DoF targets extend the system to orientation. Active probes such as the Leica T-Probe and T-Mac, or the FARO 6Probe, carry a retroreflector plus a known pattern of infrared LEDs viewed by a camera in the tracker head. The tracker fixes position from the distance meter and encoders while the camera solves roll, pitch, and yaw, yielding full pose. This enables hidden-point probing with a rigid offset stylus, guided hand-held scanning, and robot tracking. The 6DoF accuracy specification is always separate from and looser than the reflector-only figure because the probe geometry and camera add error terms.

The table below compares the published headline specifications of three representative mainstream trackers. Figures are drawn from manufacturer datasheets; always confirm against the current datasheet for the exact variant, reflector, and verified distance before relying on these numbers.

The performance standards are what make two datasheets comparable. ASME B89.4.19-2006, Performance Evaluation of Laser-Based Spherical Coordinate Measurement Systems, defines point-to-point length tests, two-face tests, and ranging tests for trackers. ISO 10360-10:2016 is the international equivalent within the ISO 10360 series; it specifies acceptance and reverification tests in which the length-measurement error is reported as the radius of the smallest sphere circumscribing the measured point spread. The two-face test, common to both standards, compares a point measured in the tracker front face and back face and is an excellent diagnostic of instrument health. When a datasheet states a figure such as 15 micrometres plus 6 micrometres per metre, it is meaningless without the accompanying standard, reflector class, and maximum verified distance, so those three qualifiers must always be read alongside the number.

Key Specification Parameters

Reading a tracker datasheet means separating the parameters that genuinely drive selection from the marketing superlatives. Seven parameters matter most: maximum permissible error, angular accuracy, distance (ADM) accuracy, working range, data rate, 6DoF accuracy, and environmental envelope. Each is explained below, with the distance-dependent nature of accuracy as the recurring theme.

Maximum permissible error (MPE) is the manufacturer guarantee that error will not exceed a stated bound under stated conditions. For trackers it is almost always written as a constant plus a per-metre term, for example a representative angular term of 15 micrometres plus 6 micrometres per metre, so a 10 metre measurement is bounded near 15 plus 60, about 75 micrometres. Because MPE grows with distance, two trackers with the same headline constant can diverge sharply at range. Manufacturers typically note that real-world results run near half of MPE, but procurement specifications should be written against MPE, not typical figures.

Angular accuracy reflects the encoders and gimbal and is the term that scales with distance, since an angular error of a few arc-seconds projects into a larger linear error the farther the target sits. It is the dominant contributor to overall uncertainty at long range, which is why long-range work demands either a higher-grade head or shorter sightlines from multiple stations bundled together.

Distance accuracy is the ADM or interferometer term. It is often far tighter than the angular term over short to medium range and may itself carry a small per-metre component. In a well-set-up measurement the radial direction is the most accurate axis and the two angular directions the least, which is why operators prefer geometries that put critical dimensions along the beam.

Working range is the maximum SMR distance at which the MPE is verified, and it is the basis of variant naming: the Leica AT960 SR, MR, LR, and XR variants cover roughly 10, 20, 40, and 60 metre radius respectively, while the FARO Vantage S6 Max reaches up to about 80 metres (60 metres with a 1.5 inch SMR) and the E6 Max about 35 metres. Range and accuracy trade against each other, so buying more range than the part needs can mean paying for a looser per-metre figure than necessary.

Data rate, quoted in points per second or hertz, governs dynamic and scanning capability. Mainstream absolute trackers stream up to 1000 points per second, enough to capture a moving target or feed a hand-held scanner. For static point-by-point inspection the rate is almost irrelevant; for scanning, robot tracking, and dynamic capture it is decisive.

6DoF accuracy and environmental envelope round out the list. 6DoF figures are separate from and looser than reflector-only figures and must be checked if probing or scanning is planned. The environmental envelope, for example a FARO Vantage operating range of roughly minus 15 to plus 50 degrees Celsius with an IP52 rating, and the instrument weight near 13 to 14 kilograms, determine where and how the tracker can be deployed on a real shop floor or outdoor site. Below is a consolidated key-specification comparison.

Selection Decision Factors

To convert the preceding chapters into a specific model, follow the decision sequence below. Most tracker selection errors come not from a single wrong parameter but from sizing the range and accuracy against the wrong distance, or from forgetting the targets, software, and service that turn a head into a working system. These eight steps can serve as a fixed RFQ template.

1. Volume and sightlines: Map the largest part and the worst sightline, then size the working range so the farthest critical point sits well inside the verified distance. Plan station moves and bundling for parts larger than one station can cover, since accuracy at the edge of range is the binding constraint, not the headline figure.
2. Accuracy at working distance: Compute MPE at your actual maximum distance using the constant plus per-metre formula, not the constant alone. A tracker that looks tight at 2 metres may be loose at 30. Match that computed value against the tightest tolerance the part demands, with margin.
3. Ranging architecture: Choose an absolute (ADM-based) tracker for general work with frequent beam breaks and point-and-shoot. Confirm it also delivers interferometric-grade resolution if you will scan or capture moving targets.
4. 6DoF and scanning needs: Decide whether reflector-only suffices, or whether hidden points, small features, or surface digitising require a 6DoF probe or a hand-held scanner. Budget the separate, looser 6DoF accuracy and the probe and scanner licenses, not just the head.
5. Targets and accessories: Specify SMR sizes and grades (1.5 inch standard plus 0.875 and 0.5 inch as needed), nests, tooling-ball and corner adapters, and how many SMRs the crew needs. SMR class directly caps achievable accuracy, so do not pair a precision head with economy reflectors.
6. Environment and thermal control: Confirm operating temperature, IP rating, and vibration tolerance against the deployment site. Plan for refractive-index compensation via the built-in weather station and for scaling part coordinates to 20 degrees Celsius using measured material temperature and the correct expansion coefficient.
7. Software and standards reporting: Verify the analysis software supports your workflow (alignment, GD&T, surface comparison, reporting) and that the system is certified to ISO 10360-10 or ASME B89.4.19 for the variant you buy, so audit and customer requirements are met.
8. Total cost of ownership (TCO): Add head, targets, probes, scanner, software, training, and annual calibration to the purchase price. A tracker spends years in service; calibration cadence, spare-part lead time, and software maintenance often outweigh the initial price difference between brands.

One last commonly overlooked dimension is manufacturer serviceability: local calibration laboratories accredited to the relevant standard, field-service response time, loaner availability during recertification, and long-term software support. These seem secondary at purchase but determine downtime years later. Hexagon (Leica), FARO, and API all maintain calibration and service centres in China, North America, and Europe, which makes them safe choices for large, long-lived programmes; smaller and integration-focused suppliers such as Brunson and VMT serve specific niches well but should be checked for regional service coverage before commitment.