Heh, I may do an experiment if I ever get some free time. I realize it's possible to draw some bad conclusions from what I've said, so I'll try and summarize the facts here:

  • Mylar is actually clear, and made from the same type of plastic as 2-liter bottles. The stuff we use is actually "aluminized mylar". Aluminum is deposited on it in order to increase its reflectivity and decrease its gas permeability.
  • Pure, polished aluminum is probably the most reflective thing there is, or close to it. Most grow light reflectors are made from polished aluminum. Aluminum, like other metals, reflects infrared radiation as well as light.
  • There is no fundamental difference between the way aluminum and aluminized Mylar reflect light. Both depend on the high reflectivity of aluminum.
  • Surface finish has a large impact on reflectivity. Aluminum foil is not optimized for reflectivity.
  • Aluminum forms a thin layer of aluminum oxide on it upon exposure to air. This may change its reflective properties.


Anyway, I think it's safe to say that first, aluminum metal can be made to be an excellent reflector, and second, aluminum foil is no more likely to create hot spots than aluminized mylar. Actually, if mylar is a better reflector of light, it's probably more likely to create hot spots.

This doesn't actually change much. Mylar is still easier to work with and probably a better reflector than aluminum foil. But it does mean that people using mylar need to watch out for hot spots, and people using white paint are probably the real winners.