On Wed, 21 Aug 2013 20:49:32 -0500 in rec.crafts.metalworking,
"Lloyd E. Sponenburgh" lloydspinsidemindspring.com wrote,
Yeah... way smaller than 6". Consider -- you can light a fire with a 2"
magnifying glass. They pass less than 80% of the heat-producing
wavelengths.

A properly aluminized mirror reflects over 97% of all visible
wavelengths, and most IR.

And the reflector focuses all those different wavelengths at the
same distance.