On an added note:
Some of the mirrors are made in China - they use a cheaper fillament which does not react correctly in the sun. If possible I would recommend testing the unit before purchase. If you cannot test, be leary of the "made in China" knockoffs. With a properly working signal mirror, you can be spotted easily to 50 miles.
A fun way to test them is to take up to a peak and call a friend from the summit. Then allow them to spot you from in town. I have also flashed people on Whitney from the summit of Keynot(Inyos) and other cross range options.
