3D Optical Metrology
Most use of optics in mechanical metrology involves two-dimensional measurements; i.e., one measures the length and width of an object. Optics provides a convenient way to perform non-contact measurements. In 3D metrology, one wants an accurate determination of dimensions in depth, that is, along the optical axis, as well as along the transverse dimensions. This is a much more challenging job.
My interest in this field was initiated by considering the problem of making simple and inexpensive, but highly accurate, measurements of a few discrete dimensions on an object being observed through an endoscope. This is inherently a three-dimensional problem, because one generally does not have control of the distance to or the spatial orientation of the object. Of additional interest is the fact that endoscope optical systems in general exhibit a large amount of geometric distortion.
There are a few basic techniques that one can use to perform 3D optical metrology. I refer to methods which effectively measure the transit time of light (i.e., lidar) to determine a depth dimension as active methods. Active methods tend to be expensive and complicated. Simpler are what I refer to as passive methods: geometrical perspective, structured light, and focus based depth measurements. All of these simpler methods have been used in commercially available endoscopic measurement systems.
My contributions, as represented by a number of patents, both issued and pending, have been directed toward increasing the accuracy and decreasing the cost of geometrical perspective based methods. One overall goal of these efforts was to devise methods and apparatus for making accurate measurements using any standard endoscope, rather than requiring a special purpose measurement scope. By “accurate” measurements, I mean that the systems I invented have been arranged to reduce the errors in the measurement to the maximum extent possible. When compared using equivalent optical and video subsystems, my measurement systems are typically 10 to 20 times more accurate than those previously commercially available.
As part of this work, I studied the limits to measurement accuracy presented by the performance of the human visual system. I determined the requirements on the measurement apparatus necessary to make optimum use of the capabilities of the human observer, and showed how to meet these requirements very inexpensively.
As another aspect of this work, I determined what the requirements were on an optical system to maintain the accuracy of measurements when the focus of the system is adjusted. As one product of this effort I invented a new class of optical systems that have exactly the properties desired in this application, as well as broader applications.
For more details, see the list of my papers and patents, or my available intellectual property.
|
Copyright © 2002, David F. Schaack. All Rights Reserved. |