As an engineer you don't have much of a career path, unless you want to go into management. (I advise against it.) So as a way of recognizing professional seniority and competence the role of Principal Engineer was created.
In this context, “Principal” implies head or most senior. Every so often though, I see jobs advertised for “Principle Engineer”. “Principle” has an entirely different meaning. A principle is a rule or fundamental doctrine. So I assume a Principle Engineer creates rules. I imagine they could be machine vision rules, in which case he or she would be a Machine Vision Principle Engineer, but this doesn't sound like the kind of work a senior or highly experienced machine vision specialist would be engaged in.
What's the takeaway? If you should be writing a job description for a highly experienced machine vision specialist, title it “Principal Engineer” and I might apply. I am not however interested in being a “Principle Engineer.”