This publication is available free of charge from http://dx.doi.org/10.6028/NIST.IR.8016

NISTIR 8016

A Proposed Interim Check for Field 

Testing a Laser Tracker’s 3-D 

Length Measurement Capability 

Using a Calibrated Scale Bar as a 

Reference Artifact

Vincent D. Lee

Christopher Blackburn

Bala Muralikrishnan

Daniel Sawyer

Mark Meuret

Aaron Hudlemeyer

http://dx.doi.org/10.6028/NIST.IR.8016
This publication is available free of charge from http://dx.doi.org/10.6028/NIST.IR.8016

NISTIR 8016

A Proposed Interim Check for Field 

Testing a Laser Tracker’s 3-D 

Length Measurement Capability 

Using a Calibrated Scale Bar as a 

Reference Artifact

Vincent D. Lee

Christopher Blackburn

Bala Muralikrishnan

Daniel Sawyer

Semiconductor and Dimensional Metrology Division

Physical Measurement Laboratory

Mark Meuret

Aaron Hudlemeyer

Brunson Instrument Company

Kansas City, MO

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.IR.8016

September 2014

U.S. Department of Commerce 

Penny Pritzker, Secretary

National Institute of Standards and Technology 

Willie May, Acting Under Secretary of Commerce for Standards and Technology and Acting Director 
1

A Proposed Interim Check for Field Testing a Laser Tracker’s 3-D Length Measurement 

Capability Using a Calibrated Scale Bar as a Reference Artifact

Vincent D. Lee, Christopher Blackburn, Bala Muralikrishnan, Daniel Sawyer

National Institute of Standards and Technology (NIST), Gaithersburg, MD, US

Mark Meuret, Aaron Hudlemeyer

Brunson Instrument Company, Kansas City, MO, US

Abstract

This paper describes a proposed interim check for field testing a laser tracker’s 3-D length 

measurement capability using a calibrated scale bar as a reference artifact. The tests described 

here are constructed to be sensitive to uncompensated errors in length measurement caused 

by the imperfect geometry and optical misalignments in the laser tracker’s construction. The 

collection of tests described in this paper was specifically selected to be sensitive to all of the 

error sources described in the laser tracker error models developed by the National Institute of 

Standards and Technology, and other researchers. Consequently, successful field test results 

obtained using the methods described here should provide operators with confidence that the 

instrument will perform within the required specification in the field. These tests are not a test 

of the tracker’s ranging system, which is assumed to be individually calibrated and is used to 

establish metrological traceability per ASME B89.7.5 of the reference lengths described in this 

paper. To be of practical use to laser tracker users, the tests are constructed to be quick (less 

than 30 minutes) and easy to perform in the field while still providing a thorough evaluation of 

the instrument’s length measurement performance. This paper describes an analytic method 

for selecting specific field tests, proposes a collection of field tests, and presents a method for 

displaying and analyzing the measurement data.

1

Introduction

Performance testing of laser trackers is an important part of quality control for users of these 

systems. The results of these tests determine if an instrument is working within the required 

performance specifications. Since the introduction of laser trackers, numerous researchers have 

provided publications to help better understand the sources of measurement errors in these 

systems [1, 2]. Much of this information has been used to develop documentary standards (e.g. 

ASME B89.4.19, VDI/VDE 2617-10, ISO/DIS 10360-10) [3-5], which describe standard tests for 

evaluating instrument performance.

 

1 Manufacturers may prescribe other methods for testing their trackers in the field; the tests described here are 

not designed to substitute for the manufacturer’s more extensive evaluations or full parametric compensation. 
2

Tests in the published standards are comprised of a series of radial length measurements, 

transverse length measurements and two-face measurements. Radial length measurements 

test the interferometer (IFM) and/or absolute distance measurement (ADM) ranging system

independently by measuring a calibrated length oriented collinear to the tracker’s beam path. 

Transverse length measurements are performed by measuring calibrated lengths in 

orientations that are not parallel to the beam path. These orientations are also sensitive to the 

angle measuring systems in the tracker. The standards also include two-face tests, which 

involve measuring a stationary target, and then rotating the tracker’s head 180° in the 

horizontal plane and steering the laser beam (in the vertical plane) back toward the spherically 

mounted retroreflectors (SMR) [6]. The results from two-face testing are the distances between 

the two measured coordinates.

Both the ASME B89.4.19 and the ISO/DIS 10360-10 (currently a Draft International Standard) 

include a comprehensive set of performance tests. The full set of comprehensive tests outlined 

in the measurement standards require considerable time and effort to complete. Each 

document also includes a non-mandatory appendix describing an abbreviated test procedure 

called interim testing. While comprehensive testing per a measurement standard should be 

done occasionally, interim testing can be used as a quick check of the instrument in the field to 

determine if further testing is necessary. The ASME B89.4.19 interim tests provide a default set 

of measurement tests that are not fully sensitive to the error sources known to exist in laser 

tracker systems. While the interim test recommendations in the ISO/DIS 10360-10 include twoface tests and length tests, no specific guidance for choosing length tests is provided in the 

standard. This can be problematic because it is possible to select positions and orientations that 

are not sensitive to the major error sources in the specific laser tracker under test.

Well-constructed performance tests are sensitive to all the potential geometric misalignments 

in the measuring system under test. The tests in this document are designed to evaluate the 

tracker’s general 3-D volumetric measurement capability. Consequently, radial measurements 

are not included. More precisely, the radial measurements, which are more accurate than the 

measurements that rely on the angular encoders, are performed to calibrate the reference 

length used to test the 3-D measurement performance of the laser tracker. It is assumed that 

the laser tracker’s ranging system has been independently calibrated and has metrological 

traceability to the SI unit of length (the meter) through some other method; this is not 

uncommon.

Selecting interim test measurements

Geometric misalignments of the laser tracker’s internal components exist due to the imperfect

mechanical construction during assembly. NIST and others have developed models that 

describe how these misalignments contribute to measurement errors [7-9]. While the ASME 
3

and ISO laser tracker standards have suggestions for interim tests, no explicit information about 

the tests’ sensitivity to the tracker misalignments is provided. The interim tests proposed in this 

paper are specifically chosen because of their sensitivity to all of the misalignments in the laser 

tracker models. Simulations were performed to determine a set of tests which optimize 

sensitivities to the geometric misalignments, while reducing the number of redundant 

measurements. The interim tests proposed in this paper are a collection of two-face and 

transverse length measurements. 

Two-face tests are included in the proposed procedures because they are fast, easy to perform,

and sensitive to many of the misalignments in the laser tracker. Because of the nature of these 

tests many of the measurement errors reverse in sign so the tests have high sensitivity to the 

misalignments. 

Since two-face tests can be insensitive to certain misalignments in the device, transverse length 

measurements are also proposed. The targets are arranged vertically or horizontally so that

ideally only one rotary axis of the machine is actuated at a time during measurement. Within

these tests a subset of them will be symmetric length measurements, while another will be 

asymmetric. Symmetric length measurements place the tracker such that the targets are 

spaced equal distances away from tracker’s center. Asymmetric measurements will place the

device closer to one target relative to the other for a length measurement. Asymmetric length 

measurements have the appeal of being sensitive to larger number of misalignments than twoface or symmetric length measurements. Diagonal length measurements commonly found in 

the comprehensive performance test of the ASME and ISO standards are excluded because they 

are sensitive to the same errors that are captured by the other proposed tests.

For the interim tests proposed here, the tracker is placed 1 m from the scale bar for all tests. 

This eliminates the need to reposition the tracker, greatly reducing the time required to 

complete the test. A 1 m standoff distance allows for a large portion of the encoders to be 

tested, which increases sensitivity to certain misalignments.

The inclusion of additional tests would provide more information. However, one of the main 

goals is to establish a set of interim tests that allows for a quick but thorough evaluation of the 

tracker’s performance in the field. Other tests can be selected using a similar approach if 

needed. The test described here are merely a recommendation that meet the requirements for 

sensitivity to the misalignments in the models employed. 

Because the tests were designed to create procedures that are consistent from one test to the 

next, there are some redundant measurements. However, consistent procedures reduce the 

likelihood of measurement blunders—such as accidentally repeating or skipping over a required 

measurement.
4

Proposed tests and their sensitivities to laser tracker geometric misalignments

The proposed interim test method utilizes a scale bar containing three kinematic seats that are 

arranged in a collinear fashion. Each kinematic seat holds an SMR such that the laser tracker 

can measure all three while stationary. The distance between the SMRs on the ends is 

nominally 2.3 m with a third SMR located approximately at the center of the bar (Figure 1).

Figure 1: Scale bar with three kinematic seats holding SMRs

Calibration of this scale bar is typically performed using the ranging system of the laser tracker

as described in section D-3 of ASME B89.4.19-2006. After calibration, the scale bar is measured 

in five different positions using the laser tracker’s ADM system as described below and shown 

in Figure 2:

1. Scale bar in the vertical orientation, nest B at tracker height and the laser tracker at 0° 

azimuth

2. Scale bar in the vertical orientation, nest B at tracker height and the laser tracker at 90° 

azimuth

3. Scale bar in the horizontal orientation, nest B at tracker height and the laser tracker at 

90° azimuth

4. Scale bar in the horizontal orientation, nest B at tracker height and the laser tracker at 

0° azimuth

5. Scale bar in the horizontal orientation, nest B at least 0.5 m above the laser tracker
5

Figure 2: Five test positions to perform interim check of laser tracker

In each of these positions, the laser tracker measures each SMR with a two-face measurement. 

Once completed, 15 sets of two-face measurements, 5 symmetric length measurements and 10 

asymmetric length measurements are calculated. Repeated measurements may also be 

performed. NIST performs three repeated measurements of the SMR in each position. This 

provides information on the repeatability of the instrument. A method for displaying and 

analyzing results is provided in the “Field Test Experiment” section below. The tests described 

above are performed using the laser tracker’s ADM system. If the IFM will be used during actual 

measurements in the field, an additional test of the IFM zero setting should be performed2. To 

test for this error, the user should measure length AC (in IFM mode) with the scale bar in the 

horizontal position without breaking the measurement beam while moving the SMR from one 

end to the other.

Table 1 and Table 2 show the results of a sensitivity analysis for trackers without a beam 

steering mirror. The top row of variable names in these tables denotes each known geometric 

 

2 The zero setting is often referred to as a “bird bath” error. This is the error in the determination of the distance 

from the tracker origin to the apex of the SMR when place in the home position.
6

misalignment parameter in a laser tracker. The parameters are briefly described below with a 

more in-depth explanation by Muralikrishnan et al. [7]:

 x1t and x1m are beam offsets along transit axis and its normal 

 x2 is the transit offset 

 x5 is the vertical index offset 

 x6t is the beam tilt (collimation error) 

 x8 is the transit tilt (squareness)

 x9x and x9y are the horizontal angle encoder eccentricities along x and y

 x10n and x10z are the vertical angle encoder eccentricities 

 x11 is the R0 error 

 x12a and x12b are the second order harmonic scale errors in the horizontal angle 

encoder scale 

 x12c and x12d are the second order harmonic scale errors in the vertical angle encoder 

scale

The sensitivity numbers in these tables indicate how much a given misalignment will propagate 

into a measurement error for a specific position and test. Each sensitivity value denotes a 

certain error ratio. For example, in Table 2, the measurement of length BC with the scale bar in 

the horizontal elevated position has a sensitivity value of -0.7 for misalignment parameter x11.

This indicates that a 1 μm misalignment of x11 produces a -0.7 μm error in the measured 

length. For misalignment parameters that describe angular misalignments, such as x2, a

sensitivity of -0.8 in the same measurement case means that a 1 μrad misalignment will 

translate to a -0.8 μm error in the measurement of length BC. All zeroes in any column means 

the set of tests have no sensitivity to the corresponding misalignments. (See the boxes in Table 

1 and Table 3)
7

Table 1: Two-face test sensitivity matrix for laser tracker without beam steering mirror

Table 2: Length test sensitivity matrix for laser tracker without beam steering mirror

A similar sensitivity analysis was performed for laser trackers with a beam steering mirror 

mechanism; those results are shown below in Table 3 and Table 4. Each geometric error 

parameter for this type of laser tracker is briefly described below and further details can be 

found in a publication by Loser and Kyle [8]:

 x1x and x1y are beam offsets along x and y relative to the mirror’s center of rotation

 x2 is the transit offset 

 x3 is the mirror offset 

Two face Sensitivity matrix w/o beam steering mirror

Targets A z x1t x1m x 2 x 5 x6t x8 x9x x9y x10n x10z x11 x12a x12b x12c x12d

Scale bar A 0 2 2 1.5 3 3 2.3 2 0 2.3 0 0 0 0 0 0

in vertical B 0 2 2 0 2 2 0 2 0 0 0 0 0 0 0 0

orientation C 0 2 2 1.5 3 3 2.3 2 0 2.3 0 0 0 0 0 0

A 90 2 2 1.5 3 3 2.3 0 2 2.3 0 0 0 0 0 0

B 90 2 2 0 2 2 0 0 2 0 0 0 0 0 0 0

C 90 2 2 1.5 3 3 2.3 0 2 2.3 0 0 0 0 0 0

Scale bar in A 90 2 2 0 3 3 0 2.3 2 0 0 0 0 0 0 0

horizontal B 90 2 2 0 2 2 0 0 2 0 0 0 0 0 0 0

orientation C 90 2 2 0 3 3 0 2.3 2 0 0 0 0 0 0 0

A 0 2 2 0 3 3 0 2 2.3 0 0 0 0 0 0 0

B 0 2 2 0 2 2 0 2 0 0 0 0 0 0 0 0

C 0 2 2 0 3 3 0 2 2.3 0 0 0 0 0 0 0

Scale bar in A 0 2 2 0.6 3.2 3.2 1 2 2.3 1 0 0 0 0 0 0

horizontal B 0 2 2 0.9 2.2 2.2 1 2 0 1 0 0 0 0 0 0

elevated C 0 2 2 0.6 3.2 3.2 1 2 2.3 1 0 0 0 0 0 0

Length Sensitivity matrix w/o beam steering mirror

Lengths A z x1t x1m x 2 x 5 x6t x8 x9x x9y x10n x10z x11 x12a x12b x12c x12d

Scale bar A C 0 0 0 0 0 0 0 0 0 1.5 0 -1.5 0 0 2 0

in vertical A B 0 0 0.3 0 0 0 0 0 0 0.8 0.3 -0.8 0 0 1 1.1

orientation BC 0 0 -0.3 0 0 0 0 0 0 0.8 -0.3 -0.8 0 0 1 -1.1

A C 90 0 0 0 0 0 0 0 0 1.5 0 -1.5 0 0 2 0

A B 90 0 0.3 0 0 0 0 0 0 0.8 0.3 -0.8 0 0 1 1.1

BC 90 0 -0.3 0 0 0 0 0 0 0.8 -0.3 -0.8 0 0 1 -1.1

Scale bar in A C 90 0 0 -1.5 0 0 0 1.5 0 0 0 -1.5 2 0 0 0

horizontal A B 90 -0.3 0 -0.8 0 0 0 0.8 0.3 0 0 -0.8 1 1.1 0 0

orientation BC 90 0.3 0 -0.8 0 0 0 0.8 -0.3 0 0 -0.8 1 -1.1 0 0

A C 0 0 0 -1.5 0 0 0 0 1.5 0 0 -1.5 -2 0 0 0

A B 0 -0.3 0 -0.8 0 0 0 -0.3 0.8 0 0 -0.8 -1 -1.1 0 0

BC 0 0.3 0 -0.8 0 0 0 0.3 0.8 0 0 -0.8 -1 1.1 0 0

Scale bar in A C 0 0 0.5 -1.5 -0.8 0 0 0 1.5 -0.2 0.7 -1.4 -2 0 -0.4 0.6

horizontal A B 0 -0.3 0.2 -0.8 -0.4 -0.1 -0.2 -0.3 0.8 -0.1 0.4 -0.7 -1 -1.1 -0.2 0.3

elevated BC 0 0.3 0.2 -0.8 -0.4 0.1 0.2 0.3 0.8 -0.1 0.4 -0.7 -1 1.1 -0.2 0.3
8

 x4t and x4n are the cover plate offsets (caused by refraction of the beam exiting body of 

laser tracker through an aperture)

 x5 is the vertical index offset 

 x6x and x6y are the beam tilt terms 

 x7 is the mirror tilt (collimation error) 

 x8 is the transit tilt (squareness)

 x9x and x9y are the horizontal angle encoder eccentricities along x and y

 x10n and x10z are the vertical angle encoder eccentricities 

 x11 is the R0 error 

 x12a and x12b are the second order harmonic scale errors in the horizontal angle 

encoder scale

 x12c and x12d are the second order harmonic scale errors in the vertical angle encoder 

scale

Table 3: Two-face test sensitivity matrix for LT with beam steering mirror

Two face Sensitivity matrix w/ beam steering mirror

Targets Az x1x x1y x2 x3 x4t x4n x5 x6x x6y x7 x8 x9x x9y x10n x10z x11 x12a x12b x12c x12d

Scale bar A 0 0 0 3.7 0 2 2 3 0 0 2.1 0.7 2 0 2.9 0 0 0 0 0 0

in vertical B 0 0 0 2.8 0 2 2 2 0 0 2.8 2 2 0 1.4 0 0 0 0 0 0

orientation C 0 0 0 1.4 0 2 2 3 0 0 5.7 5.3 2 0 1.1 0 0 0 0 0 0

A 9 0 0 0 3.7 0 2 2 3 0 0 2.1 0.7 0 2 2.9 0 0 0 0 0 0

B 9 0 0 0 2.8 0 2 2 2 0 0 2.8 2 0 2 1.4 0 0 0 0 0 0

C 9 0 0 0 1.4 0 2 2 3 0 0 5.7 5.3 0 2 1.1 0 0 0 0 0 0

Scale bar in A 9 0 0 0 2.8 0 2 2 3 0 0 4.3 3 2.3 2 2.2 0 0 0 0 0 0

horizontal B 9 0 0 0 2.8 0 2 2 2 0 0 2.8 2 0 2 1.4 0 0 0 0 0 0

orientation C 9 0 0 0 2.8 0 2 2 3 0 0 4.3 3 2.3 2 2.2 0 0 0 0 0 0

A 0 0 0 2.8 0 2 2 3 0 0 4.3 3 2 2.3 2.2 0 0 0 0 0 0

B 0 0 0 2.8 0 2 2 2 0 0 2.8 2 2 0 1.4 0 0 0 0 0 0

C 0 0 0 2.8 0 2 2 3 0 0 4.3 3 2 2.3 2.2 0 0 0 0 0 0

Scale bar in A 0 0 0 3.2 0 2 2 3.2 0 0 3.8 2.2 2 2.3 2.6 0 0 0 0 0 0

horizontal B 0 0 0 3.4 0 2 2 2.2 0 0 2.4 1.2 2 0 1.9 0 0 0 0 0 0

elevated C 0 0 0 3.2 0 2 2 3.2 0 0 3.8 2.2 2 2.3 2.6 0 0 0 0 0 0
9

Table 4: Length test sensitivity matrix for LT with beam steering mirror

The preceding sensitivity analysis was performed for a laser tracker measuring the artifact 

shown in Figure 1 and for the positions shown in Figure 2 and is only valid for this case. 

Manufacturers typically specify two-face tests due to their sensitivity to a large number of 

errors. However, observations of the data presented in Table 1 and Table 3 above, show that 

two-face tests are not sensitive to the second order harmonics in the horizontal and vertical 

angle encoders (highlighted by the boxed in area). Additionally, in the case of a laser tracker

with a beam steering mirror, two-face tests are not sensitive to beam offsets and beam tilt 

errors (x1x, x1y, x6x, and x6y) as shown in Table 4. The addition of length measurements 

captures the effects of geometrical misalignments that do not manifest themselves in two-face 

tests (boxed in portions of Table 2 and Table 4). The collection of both of these tests is sensitive 

to all the misalignments described in [7-9] and comprises of a thorough evaluation of a laser 

tracker’s performance. 

Field Test Experiment

This section provides a sample of results obtained from performing the proposed interim tests. 

These tests were performed at NIST in a controlled environment (humidity controlled to 50 % 

RH, and temperature to (20 ± 0.5) ° C), and represent the tracker’s performance under ideal 

measurement conditions. The results presented may not be achievable under significantly 

different environmental conditions. The measurements were taken using a single laser tracker 

without a beam steering mirror. This data was collected over the course of two weeks and 

shows the variation in the device’s performance over this time interval. The data is presented 

graphically for ease of interpretation. Two separate charts are provided simply for ease of 

readability. A chart for the two-face test results and a chart for the length test results are 

Length Sensitivity matrix w/ beam steering mirror

Lengths Az x1x x1y x2 x3 x4t x4n x5 x6x x6y x7 x8 x9x x9y x10n x10z x11 x12a x12b x12c x12d

Scale bar AC 0 0 0 0 -1.2 0 0 0 0 0 0 0 0 0 0.6 0.6 -1.5 0 0 0 1.5

in vertical AB 0 0 0.3 -0.3 -0.7 0 0.3 0 0 0 0 0 0 0 0.2 0.4 -0.8 0 0 -0.3 0.8

orientation BC 0 0 -0.3 0.3 -0.5 0 -0.3 0 0 0 0 0 0 0 0.4 0.2 -0.8 0 0 0.3 0.8

AC 9 0 0 0 0 -1.2 0 0 0 0 0 0 0 0 0 0.6 0.6 -1.5 0 0 0 1.5

AB 9 0 0.3 0 -0.3 -0.7 0 0.3 0 0 0 0 0 0 0 0.2 0.4 -0.8 0 0 -0.3 0.8

BC 9 0 -0.3 0 0.3 -0.5 0 -0.3 0 0 0 0 0 0 0 0.4 0.2 -0.8 0 0 0.3 0.8

Scale bar in AC 9 0 1 0 -1.5 -2.1 0 0 0 -1.5 0 0 0 1.5 0 0 0 -1.5 2 0 0 0

horizontal AB 9 0 0.5 0.6 -0.8 -1.1 0.3 0 0 -0.8 -0.3 0 0 0.8 0.3 0 0 -0.8 1 1.1 0 0

orientation BC 9 0 0.5 -0.6 -0.8 -1.1 -0.3 0 0 -0.8 0.3 0 0 0.8 -0.3 0 0 -0.8 1 -1.1 0 0

AC 0 0 1 -1.5 -2.1 0 0 0 0 -1.5 0 0 0 1.5 0 0 -1.5 -2 0 0 0

AB 0 -0.6 0.5 -0.8 -1.1 0.3 0 0 0.3 -0.8 0 0 -0.3 0.8 0 0 -0.8 -1 -1.1 0 0

BC 0 0.6 0.5 -0.8 -1.1 -0.3 0 0 -0.3 -0.8 0 0 0.3 0.8 0 0 -0.8 -1 1.1 0 0

Scale bar in AC 0 0 1.3 -2 -2.4 0 0.5 -0.8 0 -2.1 0 0 0 1.5 -0.6 0.4 -1.4 -2 0 -0.7 -0.2

horizontal AB 0 -0.7 0.6 -1 -1.2 0.3 0.2 -0.4 0.7 -1 0.1 0.1 -0.3 0.8 -0.3 0.2 -0.7 -1 -1.1 -0.4 -0.1

elevated BC 0 0.7 0.6 -1 -1.2 -0.3 0.2 -0.4 -0.7 -1 -0.1 -0.1 0.3 0.8 -0.3 0.2 -0.7 -1 1.1 -0.4 -0.1
10

provided below. Each line on the charts represents one full run of the interim tests on the day 

specified and each data point represents the largest error of three repeats.

Figure 3 shows the results of the two-face tests. Because the results of these tests are 

distances, the errors are always positive. It is interesting to note that with the scale bar 

mounted vertically there are large and repeatable errors with the SMR measured in the lowest

nest (nest C in Figure 2). Table 2 shows that this test is sensitive to a number of misalignments 

so the source of these large errors cannot be determined. For the testing described in this 

document it is only important that the tests are sensitive to all the misalignments—

compensation using other methods may be required if the errors are unacceptably large.

Figure 4 shows the results of the length tests. Unlike the two-face results, these test results 

may be positive or negative. The chart shows measurement results that are somewhat biased 

in the positive. Table 3 shows that the length measurements are sensitive to x11, which is the 

R0 or birdbath error. Furthermore, the sensitivities have the same sign for all of the tests. This 

implies that, if present, this error source could be responsible for some of the bias in the data. 

Further testing is needed to determine if x11 is the source of the bias.

Figure 3: Test results for two-face measurements

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

A B C A B C A B C A B C A B C

0 degrees 90 degrees 0 degrees 90 degrees above tracker

vertical horizontal

error, mm

Two-Face Test Results

24-Apr-14

25-Apr-14

4-May-14

8-May-14
11

Figure 4: Test results for length measurements

By periodically performing this test and plotting the data, the instrument user can monitor the 

instrument’s performance over time. As more data is collected, trends can be established to 

denote the overall health of the laser tracker. Should the device start to deviate from the user’s 

performance requirements corrective actions can be taken. This data can also be used to 

determine the required calibration intervals, which may be very different for different 

measurement tasks. 

Conclusion

The interim check described in this paper provides a method for users of laser trackers to

quickly check that their device is performing within required specifications. The various 

measurements in the proposed interim tests were chosen based on simulations using NIST’s

and established laser tracker models, and are designed to expose internal mechanical 

misalignments in the form of length and position measurement errors. These tests measure a 

scale bar in five different orientations relative to the laser tracker. The battery of tests involves 

a series of length measurements and two-face tests. A commercially available laser tracker 

system was tested with the interim test procedure, and a method for collecting and displaying 

the data was provided. The proposed interim tests can typically be completed in 30 minutes or 

less by an experienced user.

-0.01

-0.005

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

AC AB BC AC AB BC AC AB BC AC AB BC AC AB BC

0 degrees 90 degrees 0 degrees 90 degrees above tracker

vertical horizontal

error (mm)

Length Test Results

24-Apr-14

25-Apr-14

4-May-14

8-May-14
12

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