I did some experimenting with mylar, aluminum foil, and ordinary white paint as reflectors. To my surprise, the mylar was the least effective, the aluminum foil the most effective, but just barely better than white paint. I didn't use any special white paint, just what I had in a spray can at the time. I believe if you take care to get an ultra white paint you will get better results with that than with aluminum foil, and certainly better than with mylar. The differences I found were around 15-20% from best to worst, so the light doesn't look much different, but a PAR meter shows the differences.