Is Substantially Ambiguous?

A Substantial Question
There's a joke about the differences between mathematicians, physicists, and engineers, the punch line hinging on the precision with which calculations are made and the difference between theory and practice. In preparing a patent application we are all engineers, required to teach someone skilled in the art how to practice our invention. From the engineer's perspective we know that there are no absolutes; a "substantially flat" surface in optics might only allow for 20 Angstrom bumps but a "substantially flat" asphalt runway will have bumps a million times higher. A recent Appeals Court ruling affirmed that a "substantially" flat surface is properly in the eye of the (skilled in the art) beholder.
Recently, Verve, LLC successfully appealed a ruling that claims in their patent (US 4,850,315) were invalid due to indefiniteness. Their invention - a new push rod - was described in the claims as a "tube having substantially constant wall thickness throughout the length of the tube and the tips thereon." T he District Court, in ruling this claim invalid, said "the ambiguity of [the term "substantially"] was demonstrated...by [Verve's] willingness to include great variations in wall thickness..." The court ruled that the claim was indefinite because the meaning of substantially constant had not been defined elsewhere in the patent.
The Appeals Court overturned this ruling, pointing out that the law did not require that the specification and prosecution history be the only source of meaning of words used in the patent. Patent documents, they said, are written for persons familiar with the relevant field and the patentee is not required to include in the specification information readily understood by practitioners. (Otherwise, every patent would be a comprehensive tutorial, instead of a concise statement for persons in the field.)
The question is not whether the word "substantially" has a fixed, unambiguous meaning, but rather how will it be understood by a person experienced in the field, upon reading the patent . Relative expressions like substantially are necessary in patent documents to accommodate the minor variations that are appropriate to the invention at hand. In 1988 the court explained that such terms may serve to describe the invention with precision appropriate to the technology .
When you use terms like substantially, closely approximate, or generally you must not use them as a covert way to avoid "particularly pointing out and distinctly claiming" your invention. You can (and should) use them when they reasonably describe the subject matter so that its scope will be understood by persons in the field of the invention. Your patent attorney can suggest the proper balance between precise definitions and reasonably describing the scope of your invention.

Copyright 2001 TechRoadmap Inc. All rights reserved.
Bruce Horwitz is Founder of TechRoadMap (www.techroadmap.com)