Mirror Reflectivity Versus Time

Mirror reflectivity was measured using the LED setup (at MIT-Bates) for the glass mirrors as attached to an Aluminum strongback with 3m bonding tape.  This was done before placing the mirrors in distilled water, 1 month after placing them in distilled water, and after 7 months in water.  The following information was taken for the reflectivity (with the caviat that the "430nm" poinbt is a purple LED and the warning that the LED spectrum and camera response may be complicating the 325nm points.  The expected curve does not list an angle, but it is presumed to be close to perpendicular.  45 degrees was chosen for being close to the more common reflected angles.

This appears to show a decrase in reflectivity of about 3% at 350nm after 6 months.  This number is likely an overestimate due to the conditions of the water the mirror was in.  The water had not only the aluminum backing, but also steel bolts and a bronze colored metal stand.  This resulted in the water being filled with flects and leaving a deposit on the mirror.  The mirror also had small back lines in it after 1 month, but these do not appear to have signifigantly increased after 6 months.  To give an idea of the chemistry occuring in the water, the following photos were taken:

By 6 months, the water has a brownish color

The mirror was wiped down prior to testing to get rid of the worst desposits, and this is what came off.

What the mirror looked like pre wiping



Image of the corroded bolt and effect on the nearby mirror surface.