and Metrology

The scientists of Samurai Scientists LLC have decades of experience in nondestructive evaluation and testing (NDE) and surface metrology, with affiliates who are subject matter experts (SMEs) in these areas.We developed an NDE system that has been demonstrated to be faster and more accurate in locating subsurface defects than other systems of the same complexity. Other NDE systems we designed and developed include several that combine a variety of methods simultaneously, increasing measurement speed while maintaining or improving accuracy. For specific designs we have implemented NDE 4.0 by adding machine learning and automation, enabling in-line testing of manufactured samples.

We applied statistical sampling to the field of surface measurement metrology, resulting in a technique that led to a Top 10 Downloaded Paper at SPIE, increasing the measurement accuracy of a single-shot hand-held system to approach that of a fixed coordinate measurement machine (CMM). While we also developed interferometric and long-range metrology systems, we concentrate on techniques that are easy to use and do not require special laboratories. Some of our NDE and metrology publications are:

- Kurtz, R.M.; Nesbitt, R. “Enhanced Surface Metrology.”
*Coordinate Metrology Systems Conference*. Phoenix, Arizona, 2011.

There is a constant search for more accurate measurement, which generally leads to higher cost, greater complexity, or devices that do not lend themselves to manufacturing environments. For example, surface metrology can be accomplished by a number of methods, ranging from rulers and visual estimation (cheap, fast, and inaccurate) up to fixed Coordinate Measurement Machines (expensive, slow, and accurate). The tradeoffs involved in selecting metrology methods generally involve these three parameters of cost (initial and operation), measurement speed, and accuracy. We present a method of adding one more tradeoff: measurement precision (perpendicular to the surface) vs. sampling resolution (along the surface). Through application of statistical sampling and curve fitting, we can improve precision by approximately the square root of the amount that we decrease resolution.

We applied this technique to a number of known and unknown targets, using the Cognitens WLS400 by Hexagon Metrology and a custom laser interferometry measurement system. Using the enhancements described in this paper, we were able to improve the measurements significantly. Measurement of a flat reference surface, for example, was enhanced by reducing noise a factor of 11 and improving surface measurement accuracy 2× (limited by the actual surface figure). An application of this technology to a known sphere reduced noise by a factor of eight and demonstrated that the sphere was within its surface and diameter specifications. We used this statistical technique to reduce noise in an interferometry system by 15× and demonstrated that the supposedly flat surface had deviations exceeding 16% over a square region 1 cm on a side. Finally, we modeled the WLS400 to determine its probability of identifying small surface features. Based on this model, we found that statistical noise reduction can improve the minimum resolvable feature height by a factor of five without significant difficulty. - Kurtz, R.M.; Nesbitt, R. “Enhanced Surface Metrology.”
*JCMSC*.**7**(1): 14-19; 2012 Spring.

The constant search for more accurate measurement generally leads to higher cost, greater complexity, and/or devices that do not lend themselves to manufacturing environments. For example, surface metrology can be accomplished by a number of methods, ranging from rulers and visual estimation (cheap, fast, and inaccurate) up to fixed coordinate measuring machines (expensive, slow, and accurate). The tradeoffs involved in selecting metrology methods generally involve these three parameters of cost (initial and operation), measurement speed, and accuracy. We present a method of adding one more tradeoff: measurement precision (perpendicular to the surface) vs. sampling resolution (along the surface). Through application of statistical sampling and curve fitting, we can improve precision by approximately the square root of the amount that we decrease resolution.

We applied this technique to a number of known and unknown targets, using the Cognitens WLS400 white-light stereovision system from Hexagon Metrology and a custom laser interferometry measurement system. Using the enhancements described in this article, we were able to improve the measurements significantly. Measurement of a flat reference surface, for example, was enhanced by reducing noise by a factor of 11 and improving surface measurement accuracy 2× (limited by the actual surface figure). An application of this technology to a known sphere reduced noise by a factor of eight and demonstrated that the sphere was within its surface and diameter specifications. We used this statistical technique to reduce noise in an interferometry system 15×, and demonstrated that the supposedly flat surface had deviations exceeding 16 percent over a square region 1 cm on a side. Finally, we modeled the white-light scanner to determine its probability of identifying small surface features. Based on this model, we found that statistical noise reduction can improve the minimum resolvable feature height by a factor of five without significant difficulty. - Kurtz, R.M.; Nesbitt, R. “Improving the Accuracy of Surface Metrology.”
*Opt Eng*.**50**(7): 73605; 2011 July. (SPIE Top 10).

There is a constant search for more accurate measurement, which generally leads to higher cost, greater complexity, or devices that do not lend themselves to manufacturing environments. We present a method of using statistical sampling to improve metrological accuracy without these undesirable effects. For metrology of flat surfaces and steps between flat surfaces, this method demonstrated precision improvement up to a factor of 55, and accuracy increase of at least a factor of 10. The corresponding precision and accuracy improvements on a spherical surface were both factors of eight. Since this accuracy improvement can be implemented in software, it does not affect the speed of measurement or the complexity of the hardware, and it can be used to improve the accuracy of a wide range of metrology systems. - Kurtz, R.M. “Directed Acoustic Shearography.” In: Kazemi, A.A.; Kress, B.C.; Chan, E.Y., editors.
*Photonics in the Transportation Industry: Auto to Aerospace III*. Orlando, Florida, USA:**SPIE**; 7675, 2010. p. 76750B.

Modern vehicles use modern materials, including multiple metallic layers, composites, and ceramics. This has led to significant improvements in quality, reliability, and lifetime, at the cost of significantly increased complexity. It is particularly difficult to test these modern materials for buried defects such as internal corrosion, glue/weld failures, and disbonds, yet these defects can lead to damage and even failure of the part. As one tool in the array of nondestructive evaluation (NDE) technologies, we report on Directed Acoustic Shearography (DAS), which combines the sensitivity of shearography with the speed of ultrasonic imaging, and adds improved depth resolution. We show that DAS is particularly useful in detecting buried defects in modern materials, how it lends itself to automation, and present early tests of DAS detecting buried defects as small as 1/32 inch in a multilayer aluminum structure. - Kurtz, R.M.; Piliavin, M.A.; Pradhan, R.D., et al. “Reflection Shearography for Non-Destructive Evaluation.” In: Gerhart, G.R.; Shoemaker, C.M.; Gage, D.W., editors.
*Unmanned Ground Vehicle Technology VI.*Orlando, FL:**SPIE**; 5422, 2004. p. 532-540.

Conventional nondestructive evaluation (NDE) techniques include visual inspection, eddy current scanning, ultrasonics, and fluorescent dye penetration. These techniques are limited to local evaluation, often miss small buried defects, and are useful only on polished surfaces. Advanced NDE techniques include laser ultrasonics, holographic interferometry, structural integrity monitoring, shearography, and thermography. A variation of shearography, employing reflective shearographic interferometry, has been developed. This new shearographic interferometer is discussed, together with models to optimize its performance and experiments demonstrating its use in NDE. - Kurtz, R.M.; Pradhan, R.D.; Aye, T.M., et al. “Long-Range Phase Conjugate Interferometry.”
*Spaceborne Sensors*. Orlando, FL:**SPIE**; 5418, 2004. p. 115-126.

The most accurate method of measuring distance and motion is interferometry. This method of motion measurement correlates change in distance to change in phase of an optical signal. As one mirror in the interferometer moves, the resulting phase variation is visualized as motion of interferometric fringes. While traditional optical interferometry can easily be used to measure distance variation as small as 10 nm, it is not a viable method for measuring distance to, or motion of, an object located at a distance greater than half the coherence length of the illumination source. This typically limits interferometry to measurements of objects within <1 km of the interferometer. We present a new interferometer based on phase conjugation, which greatly increases the maximum distance between the illumination laser and the movable target. This method is as accurate as traditional interferometry, but is less sensitive to laser pointing error and operates over a longer path. Experiments demonstrated measurement accuracy of <15 nm with a laser-target separation of 50 times the laser coherence length. - Kurtz, R.M., inventor; Samurai Scientists, LLC assignee.
**Directed Acoustic Shearography**. Patent US008596128B2. 2013 Dec 3. 5 p.

A nondestructive evaluation system, consisting of a metrology device and a directable acoustic transducer, is used to measure the variation of the surface when stress is applied. The directable acoustic transducer selects the location of the applied stress. The difference between the surface during stress application and in the absence of stress detects both surface and buried defects along the location of the stress. By scanning the beam from the acoustic transducer, the location of the stress can be adjusted, enabling the device to located the detected defect in three dimensions.

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