Mylar For Light Proofing?

so ive been having some trouble lightproofing my box, its gunna be in a dark basement so any light leaks will be noticable...is mylar opaque or solid? will light penetrate through it

Most mylar is almost opaque - probably blocks 99.5% of light. I think they make those glasses for viewing solar eclipses out of mylar.

Should handle average light leak OK, but 2 layers would definately do it.

I use some cheap mylar and it is NOT opaque, you could read by the light it lets through. Yet strangely, I am content with the reflective properties...