Michael Daly wrote:

Full sun is about 10,000 footcandles. We can see in 0.1 FC,
ie we can "see through" something that's 99.999% reflective.

I can see thru some aluminized mylar in a dimly lit room. My
guess is that these are no more than 50% reflective.

You might see 0.1 FC in a 5 FC room through a 2% transmissive film.

High-reflective aluminized mylar is harder to get and more expensive.
If you're going to recommend Al mylar, you have to be specific
about the quality and reflectivity.

I like Duraflex from Graphic Arts Systems in Cleveland.
About 10 cents/ft^2 in 0.002"x54"x100' rolls.

It's a myth that mirror finishes (metallic coatings, polished
metal etc) are great reflectors of heat and light. Good quality
white paint can outdo mediocre mirror finishes easily and are
often a lot cheaper.

Duane Johnson gets great results with Mylar heliostat mirrors.

Nick