Then you should be aware that penumbra means "partial shadow" not "underlying principle."
That's one of the dictionary definitions. I'm talking Supreme Court definitions. FYI:
http://www.nationalreview.com/levin/levin200503140754.asp
http://www.law.umkc.edu/faculty/projects/ftrials/conlaw/griswold.html
Note Justice Douglas' use of the word "penumbra":
In NAACP v. Alabama we protected the "freedom to associate and privacy in one's associations," noting that freedom of association was a peripheral First Amendment right. Disclosure of membership lists of a constitutionally valid association, we held, was invalid "as entailing the likelihood of a substantial restraint upon the exercise by petitioner's members of their right to freedom of association." Ibid. In other words, the First Amendment has a penumbra where privacy is protected from governmental intrusion. The right of "association," like the right of belief (Board of Education v. Barnette, 319 U.S. 624), is more than the right to attend a meeting; it includes the right to express one's attitudes or philosophies by membership in a group or by affiliation with it or by other lawful means. Association in that context is a form of expression of opinion; and while it is not expressly included in the First Amendment its existence is necessary in making the express guarantees fully meaningful.
The foregoing cases suggest that specific guarantees in the Bill of Rights have penumbras, formed by emanations from those guarantees that help give them life and substance. Various guarantees create zones of privacy. The right of association contained in the penumbra of the First Amendment is one, as we have seen.