Opening 6 : For the purpose of this standard, a through-hole in the fire window assembly that can be seen from the unexposed side while looking through the plane of the assembly from a perpendicular position
NFPA 257 Standard on Fire Test for Window and Glass Block Assemblies, 2007 ed. http://catalog.nfpa.org/NFPA-257-Standard-on-Fire- Test-for-Window-and-Glass-Block-Assemblies-Prior-Y ears-P4186.aspx, (Std: Fr Tst for Window and Glass Block Assly - Amer) {274 NFC}