DD,
I know some of the health food stores, strangely enough, sell the "full
spectrum," light bulbs. They look exactly like a regular light bulb, but they
claim that it is more like natural Sun light, and therefore better for
reading, etc.
Another suggestion would be to use the real thing; try blacking out a window
that faces the Sun, and leave a small hole the same size, or a little smaller
than the paper towel tube that you are using. I am sure that if the
experiment is to work, or work out better, that this will be the determining
factor. I am sure you can modify my suggestion to even a more perfect
solution.
Mike