'Numen Lumen' is popularly held to mean 'God our Light' (not a literal translation); it is the motto on the UW seal. The UW seal may be seen at Memorial Library; it is inlaid in the second floor, just north of the Information Desk. For more information contact University Archives at 425 Steenbock Memorial Library
(550 Babcock Dr., 262-5629); they have a 'Numen Lumen' file.
From "In the Loop" by Melanie Conklin in Isthmus, 8/23/96:
Q: What does the UW motto Numen Lumen mean?
A: Chancellor J.H. Lathrop translated it as "God our light" when he proposed the alleged Latin phrase for the university seal in a letter dated Feb. 11, 1854. A number of scholars have since challenged that interpretation. Some could find no example in classical Latin of the two words standing together to make a phrase, and concluded that it was untranslatable. Others defined lumen as "light" and found in numen such variations as "Providence" and "the quality of divinity in a deity," resulting in such clumsy translations as "The divine within the Universe, however manifested, is my light." Small wonder the big steroid W has eclipsed Numen Lumen on Badger T-shirts and letterhead in recent years."
According to the Wisconsin Alumni Association,
The phrase "Numen Lumen," developed as part of the University of Wisconsin-Madison's seal and motto, is Latin and can be translated to mean, "God, our light." But, according to a University Communications News Library article, the UW's first chancellor, John Lathrop, who assisted in the creation of the motto, interpreted it to mean, "The divine within the universe, however manifested, is my light."
This credo was the UW's first seal and was chosen to reflect the religious beliefs and values of Wisconsin citizens. And apparently, the UW was not the only university to use such strong religious words and objects in its motto — at the time, many other universities used similar references. Although "Numen Lumen" is still an important seal at the university, it is not as heavily used in representations of UW-Madison today.
Picture of the seal can be found here: http://photos.news.wisc.edu/photos/12710/view