TY - RPRT TI - Letter Circular 549: information on ultraviolet transparency of window materials and fabrics (supersedes Letter Circular 235, 429, and 511) PY - 1939 PB - National Institute of Standards and Technology CY - Gaithersburg, MD SN - LCIRC 549 DO - 10.6028/NBS.LCIRC.549 ER -