News & Events

 January 13, 2014

Prof. Ellis publishes book on displacement measuring interferometry

ellis bookDisplacement measuring interferometry is widely used in industry and academia.

But up until now, if you looked for a handy source of information about this topic, you would be out of luck.

"You would find an amalgamation of papers and chapters in books," explains Jonathan Ellis, Assistant Professor of Mechanical Engineering and of Optics.

Thanks to Ellis, that is no longer the case. SPIE has just published his Field Guide to Displacement Measuring Interferometry, available at

Displacement measuring interferometry "uses light as a very precise ruler to measure the displacement of objects," Ellis said. "This is commonly used in calibrating positioning systems. So if you have a stage that needs to move from point a to point b, it determines how accurate that is."

For example, machines that measure a car body or aerospace parts need to be calibrated to ensure that the performance of those machines meets specifications. Typically, laser interferometers would be used to do that.

Prof. EllisEllis hopes his field guide will be useful to at least a couple of audiences. For example, it can be read as a refresher if "you've taken a course on displacement measuring interferometry, or seen it in some form in a book chapter, for instance, and now you want to refer back to it because you forgot an equation, or you want to know if there is a better way to make a measurement."

It can also be read as an introduction for people who know nothing about the topic, but are interested in learning how to use light to make displacement measurements.

His field guide could also be useful in university classrooms and labs. "There are some things in here that I directly teach in my graduate class," Ellis said. And, instead of having to explain the basics of beam alignment to every student who joins his research team, he can now direct them to a 15-page section on that in his book.

Ellis earned his PhD at TU Delft in the Netherlands, where he researched displacement interferometry for dimensional stability measurements.  The field guide draws upon "the core background from my Ph.D. research," Ellis said.

His research group, which includes 10 graduate students and six undergraduates, is looking for new approaches and refinements to displacement interferometry. "We're trying to build more compact systems than are now commercially available," Ellis said. "We're trying to develop fiber-delivered, fiber-detected systems, and we're trying to implement multi-axis sensing, so you can not only measure a linear displacement, but also determine if a stage has pitch or roll or yaw or straightness errors."

Ellis said it took about 18 months to write the field guide. He was responsible for producing not only the text, but the equations and literally hundreds of highly technical drawings – two per page.

"There were many days I was getting up at 4 in the morning to write for two or three hours before anyone else got up," Ellis confided. He is especially grateful to Steven Gillmer, one of his graduate students, to Vivek Badami, a friend, colleague and researcher at Zygo, and to his wife Kate Medicus, an optical metrologist at Optimax Systems, for their help in reviewing and proofing the book.

This will not be Ellis' last word on the subject of displacement measuring interferometry, however. "This was meant to be short, sweet and to the point," he explains. "I'd like to do a full, formal textbook at some point in the future."