There is some science to this... I thought I would post a couple links, including one which has some interesting DIY alternatives...
From the University of New Hampshire:
http://mvh.sr.unh.edu/mvhtools/psd_filters.htm
NASA has looked into this as well:
http://www.sti.nasa.gov/tto/spinoff1996/50.html
The Turf Tec site Dale mentioned:
http://www.turf-tec.com/catalog-n.html#TSDG1-N
Maybe wearing purple shades is not such a silly idea after all...