Quote Originally Posted by TMusic
I'd also like an example of an application of mylar "precisely because of its ability to reflect heat!"...
That's easy: mylar thermal blankets. I hear recommendations to people to use these to reflect light all the time. If these didn't reflect heat, they'd be useless for their stated purpose. There's also that stuff that looks like mylar bubble wrap that's used for insulation.

Mylar reflects heat. Therefore, it can create hot spots.

As for aluminum foil, it is an aluminum alloy, not pure aluminum, but it's still at least 93% aluminum. Aluminum doesn't mix well with other metals, so it can't go under that number and still make a useful alloy.

I'm kind of skeptical about using a light meter because light reflects differently off of different surfaces. When lights strikes a surface, one of three things happen. Either it's reflected, transmitted, or absorbed. Metals can't transmit light, so it's either reflected or absorbed. Any light that is absorbed would warm the reflector. I think an ideal experiment would put reflective containers under light and see which one absorbed the most heat.

I'm willing to accept that surface finish has a large impact on reflectivity, since smooth things are shiny. This may be the reason aluminum foil isn't recommended, although it does appear quite shiny. Still, I think it's safe to say the standard answer "aluminum foil is bad because it creates hot spots" isn't really accurate.