41
Uncertainty Budget Components
Below are the uncertainty budget components for the calibration and measurement of the artifact used in the NIST
Interim Field Test under field conditions. This budget should be modified to reflect your specific use and equipment.
Modifications might increase or decrease overall uncertainty.
The reference lengths are defined as the distances between the centers of any two target nests on the face of the
length artifact. Components of uncertainty in the reference lengths are described below.
Table 2. Uncertainty Budget for the Calibrated Length of the Artifact
APPENDIX I: THE UNCER TAINT Y BUDGE T
2.3m
1.15m
1 Standard Uncertainty (µm)
Component
Symbol
Optical Alignment, Interferometer and
Angular Encoders
Repeatability of Nests
Gravity Loading
Retroreflector
Artifact Temperature
L
tracker
L
repeat
L
gravity
L
retro
L
temp
Uk=2
7.9
5.7
0.6
1.5
2.5
1.4
2.2
0.4
1.5
1.6
1.4
1.1
This component of uncertainty describes the effectiveness of the laser tracker in measuring the distance between
points. There are three components to consider, the accuracy of the interferometer, the performance of the angular
encoders, and the optical alignment. The mirror used in the alignment is flat to λ/10 and is considered negligible. For
one target reflector used in the same orientation in all nests during calibration, the depth and runout errors do not
contribute to alignment errors. If the optical alignment is contained within 0.1 degrees, the projected error due to the
transverse error of the tracker is also negligible. The laser tracker used in this example analysis reported a maximum
permissible error (MPE) of the interferometer accuracy of
MPE
range
= ±0.4µm + 0.3µm/m*L. For the 2.3m length,
MPE
range
= ±1.1µm. Converting from a rectangular distribution to a normal distribution gives
L
tracker
(1σ)= 0.6µm.
L
tracker
