Technical Papers & Journals

CMSC 2019 PRESENTATION - DKIST's Journey of Using a Laser Tracker

This paper presents the journey taken by members of the team at the Association of Universities for Research in Astronomy (AURA) and the Daniel K. Inouye Solar Telescope (DKIST) in using a laser tracker. The construction and alignment of a large telescope...

CMSC 2019 PRESENTATION - Point Density and Fitting Algorithm Interaction in Part Profile Scanning

One of the major differences between tactile and optical profile scanning methods is the sampled point density. Not only do optical methods collect many more points, the point spacing (which is almost uniform in tactile measurement) can be highly variable...

CMSC 2019 PRESENTATION - Using an abbreviated scale bar for abbreviated tests

A scale bar approximately 2.3 m in length is specified in the ISO and ASME laser tracker standards as necessary test equipment to evaluate the measuring performance of laser trackers. A 2.3-m length is chosen as the approximate length that will...

CMSC 2019 PRESENTATION - Developing low cost large volume metrology for next-generation manufacturing spaces

UCL’s 3DIMPact Group has recently completed two major projects to develop large-volume metrology (LVM) for future manufacturing purposes. One is the LUMINAR project (completed 2016) where the group’s task was to investigate ways of mitigating the...

CMSC 2019 PRESENTATION - Creating a new concept in Performance‐built Sport Catamarans

The high-performance boat building industry has forever been focused on hydrodynamics in hull design. New lightweight, outboard-powered catamarans have created a new kind of vessel, which flies as much as it floats during operation. An existing...

CMSC 2019 PRESENTATION - Subtle Sources of Error in Laser Trackers Due to Dispersion in the Internal Optical Elements

The speed of light depends on the index of refraction of the medium in which the light is propagating. In a dispersive medium, the speed of an amplitude modulated wavefront depends on the group refractive index, i.e., slightly slower than the carrier...

CMSC 2019 PRESENTATION - iDOS (Inspection of Double Omega Stringers)

Spirit AeroSystems Inc. has developed a precision inspection system for automatically inspecting the thickness of composite stringers utilizing an array of laser displacement sensors. These parts have complex geometry and many changes in ply layup count....

CMSC 2019 PRESENTATION - Metrology in the R&D for a High Repetition Rate Multi User X-ray Free-Electron Laser Facility

Under development at Argonne National Laboratory is a high-repetition-rate multi user X-ray free-electron laser (FEL) user facility. The machine will be driven by an array of highly efficient compact collinear wakefield accelerators (CWA) where the...

CMSC 2019 PRESENTATION - Part Density Determination Using a Laser Scanner

The purpose of this research is to investigate the accuracy of part density determinations calculated from inspection data gathered by laser scanners via articulating arm coordinate measuring machines (AACMM). Los Alamos National Laboratory (LANL)...

Subtle Sources of Error in Laser Trackers Due to Dispersion in the Internal Optical Elements

David H. Parker

ABSTRACT
It is well known that the speed of light depends on the index of refraction of the medium in which the light is propagating. It is also well known that in a dispersive medium, the speed of an amplitude modulated wavefront depends on the group refractive index, i.e., slightly slower than the carrier light. Corrections for the group refractive index in air are typically made for temperature, humidity, and pressure—without which measurements could be in error by tens of parts per million. The internal instrument optical elements are also subject to dispersive effects, which have heretofore been ignored in the literature—and presumably in the design. Note that this is probably because no commercially available optical design software package models amplitude modulated wavefronts. A thought experiment will illustrate the problem. From Fermat’s principal, a plane wave intersecting a converging lens bends the wave to converge at a focal point. The lens is shaped such that the propagation time to the focal point is the same for all rays. For example, a ray passing through the outer radius of the lens passes through a thinner section of glass, but must propagate a longer distance to the focus. A ray passing through the center of the lens passes through a thicker section of glass, but propagates through a shorter distance to the focus. However, for optical amplitude modulated (OAM) light, the modulated wavefront, which has two sidebands that propagate at slightly different speeds in a dispersive medium, does not reach the focus at the same time! In other words, there is a slight phase shift in the modulated wavefront between the beam passing through the center of the lens, and the beam passing through the outer radius of the lens. This makes the net phase of the modulated waverfont, as received by a detector at the focal point, dependent on the beam geometry—which most likely depends on distance, due to divergence of the beam. At close range, the majority of the received beam passes through the center of the lens, due to the small beam size. At long range, the received beam passes through the entire lens, due to the expanded beam filling the lens. This source of error can be misinterpreted as being due to distance or power level, when in fact it is the optical design. Spherical, or cat’s eye, retroreflectors are also subject to the same source of error. A simple test to measure the errors is proposed.