"Angular determination of toolmarks using a computer generated virtual tool," J. Forensic Sci., (2015)

[73]  R. Spotts, L. S. Chumbley,  L. Ekstrand*, S. Zhang, and J. Kreiser, "Angular determination of toolmarks using a computer generated virtual tool," J. Forensic Sci., 60(2), 303-315, 2015;doi:10.1111/1556-4029.12759

Abstract

A blind study to determine whether virtual toolmarks created using a computer could be used to identify and characterize angle of incidence of physical toolmarks was conducted. Six sequentially manufactured screwdriver tips and one random screwdriver were used to create toolmarks at various angles. An apparatus controlled tool angle. Resultant toolmarks were randomly coded and sent to the researchers, who scanned both tips and toolmarks using an optical profilometer to obtain 3D topography data. Developed software was used to create virtual marks based on the tool topography data. Virtual marks generated at angles from 30 to 85° (5° increments) were compared to physical toolmarks using a statistical algorithm. Twenty of twenty toolmarks were correctly identified by the algorithm. On average, the algorithm misidentified the correct angle of incidence by
6.12°. This study presents the results, their significance, and offers reasons for the average angular misidentification. 

"Autoexposure for three-dimensional shape measurement with a digital-light-processing projector," Opt. Eng. (2011)

L. Ekstrand* and S. Zhang, "Autoexposure for three-dimensional shape measurement with a digital-light-processing projector," Opt. Eng. 50(12) 123603, 2011; doi: 10.1117/1.3662387

Automatically adapting the camera exposure time is crucial for industrial applications where minimum human intervention is usually desirable. However, it is very challenging to realize such a capability for a conventional fringe projection system where only a finite increment of the exposure time is allowed due to its digital fringe generation nature. We study the generation of sinusoidal fringe patterns by properly defocusing binary ones, which permits the use of an arbitrary exposure time. This provides the potential to adapt the exposure time automatically. We present the principle of an automatic exposure technique and show some experimental results.

"Phase error compensation for 3-D shape measurement with projector defocusing," Appl. Opt., (2011)

Y. Xu*, L. Ekstrand*, J. Dai, and S. Zhang, "Phase error compensation for 3-D shape measurement with projector defocusing," Appl. Opt., 50(18), 2572-2581, 2011; doi: 10.1364/AO.50.002572

"Three-dimensional profilometry with nearly focused binary phase-shifting algorithms," Opt. Lett. (2011)

L. Ekstrand* and S. Zhang, "Three-dimensional profilometry with nearly focused binary phase-shifting algorithms," Opt. Lett. 36(23) 4518-4520, 2011  (Cover feature); doi: 10.1364/OL.36.004518

This Letter investigates the effects of different phase-shifting algorithms on the quality of high-resolution three-dimensional (3-D) profilometry produced with nearly focused binary patterns. From theoretical analyses, simulations, and experiments, we found that the nine-step phase-shifting algorithm produces accurate 3-D measurements at high speed without the limited depth range and calibration difficulties that typically plague binary defocusing methods. We also found that the use of more fringe patterns does not necessarily enhance measurement quality.