Opening 4 : A through-hole in the fire door assembly that can be seen from the unexposed side while looking through the plane of the assembly from a perpendicular position
NFPA 252 Standard Methods of Fire Tests of Door Assemblies, 2003 ed. http://www.nfpa.org/codes-and-standards/document-i nformation-pages?mode=code&code=252, (Std: Meth of Fr Tst of Door Assly - Amer) {271 NFC}